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Posted By: Hillbilly bike Mikuni TM's and velocity stacks - 02/08/14 8:41 am
I have the new 34 MM Mikuni's mounted on the 650 Triumph. I tried a total intake length from the carb intake to valve of 13 inches....No good, The large TM carbs foul the inside of the rider's knees. The rider has to get his knees out of the wind and being tucked in gives the rider a secure feeling at 120 plus MPH
I was using the same intake length with the 32 MM Concentrics....But shorter intake, and a 3 inch non tapered velocity stack cleared the rider's legs. The bike was competitive set up this way....The Concentrics are crude but very forgiving about intake length and stacks...
Ok,I have no experience with Mikuni's and how they react to velocity stacks...
Anyone have any experience? The bike is going on the dyno next month ...I have jets, different length intakes...But need to get this velocity stack business figured out...And like everything else, I'll have to fabricate the stacks in my little shop of horrors...

[Linked Image]
Posted By: Allan G Re: Mikuni TM's and velocity stacks - 02/08/14 9:07 am
For the 1 cent value of 2c I would be thinking on a different line. (well semi different)

I would personally fit the stacks, a good set of stacks makes a big difference at top end when jetted up to accommodate it.

I would look at making a large airbox under that seat of yours. As large as you can, have the velocity stacks fitted into that. The achieved effect will be the air has a larger area to draw from which isn't being affected by the draft which surrounds it. Otherwise with with just the stacks in open air, the faster you go... the smaller area (which is not effected by the draft) that the carb can draw from.

Alternately have a stack fitted inside a HUGE air filter for each carb.

HTH
Posted By: Hillbilly bike Re: Mikuni TM's and velocity stacks - 02/08/14 9:39 am

I do plan on using air filters, the bike ran the same speed with or without K&N filters in the past....
I think your ideal of shorter intakes, long velocity stacks with filters being easier to "adjust" the length for tuning....And is easy for me to fabricate..
Posted By: Allan G Re: Mikuni TM's and velocity stacks - 02/08/14 9:54 am
AFAIK its the length between the carb-to-valve which has the effect with torque/power etc... anything before the carb is a plenum to store air, the more air stored the more you can make use of, the shape of the stack is all that is required (have a gander at Julian's A10 off roader)
Posted By: Mark Parker Re: Mikuni TM's and velocity stacks - 02/08/14 10:01 am
Set them up closer to the head and next month try them close and also out where they are now and see what power difference there is. They might get better power in closer with a shorter inlet run, and you might not need to solve this problem.
Posted By: Rickman Re: Mikuni TM's and velocity stacks - 02/08/14 10:08 am
HB,
I believe it's too late in the year to think about this, but, NEXT year, at the beginning of the re-build, re-fix, re-think, end-of-race-season, you might think of doing a Lilo, like he did to his bike build, and re-position those central frame down tubes? Get the area where the carbs need to be, free of those down tubes, and out of the way of you being tucked in?

..... Uhmmm, I think the term for the air in the airbox that Allan is talking about, is something like; undisturbed air?

... Just thinkin' out loud here...

I have the GP on my GS, and an OLD mik remote float carb.

The velocity stack on the GP is long-ish, and OH so smooth, with it's long taper.
The mik's velocity stack is MUCH shorter, not as definitively centerable, as it is clamped on, and the 'taper' is more of a bell-mouth, as I understand the term...
Posted By: Hillbilly bike Re: Mikuni TM's and velocity stacks - 02/08/14 10:57 am
I'm still listening to you guys.... This photo from above shows what goes on and the narrowed seat rails.
It's an OIF so I can't change the main frame....I do have 60's frames but that means a whole new build ....
Anyways, I can add about a 5 degree downward elbow near the head. This will drop the carbs down about 1-1/2 inches and clear the back of the riders knees.
I'm thinking 5 degree elbow will have little to no effect on airflow? Ya think the radius matters? I can do any radius...

[Linked Image]
Read this article http://www.emeraldm3d.com/articles/emr-adj-length-intake/ It is spot on... where the carb is does not matter for power, but closer to the head makes tuning a bit easier at part throttle. Also from the picture it appears that if you move the carb closer will allow the rider to tuck his knees in better.

Oh yes I think your exhaust is too short as well... the intake and exhaust need to be matched up together for max everything...

Ron
Posted By: Hillbilly bike Re: Mikuni TM's and velocity stacks - 02/08/14 2:01 pm
Originally Posted by Ron - in California
Read this article http://www.emeraldm3d.com/articles/emr-adj-length-intake/ It is spot on... where the carb is does not matter for power, but closer to the head makes tuning a bit easier at part throttle. Also from the picture it appears that if you move the carb closer will allow the rider to tuck his knees in better.
It will allow a better rider position.
Oh yes I think your exhaust is too short as well... the intake and exhaust need to be matched up together for max everything...

Ron

With the previous Amals the carbs were closer to the head. Then I screwed on non tapered 3 inch long stacks with K&N's and picked up 2 MPH....
Ron, the auto race tuners on Speed Talk forums agree with a shorter intake and long velocity stacks...They say it makes for a more constant fuel curve throughout the RPM band.
The Mikinis have a 2-1/4 inch bellmouth. I assume the stacks should be the same diameter?

I have several lengths to try out the the exhaust and it's easily changed.
Some photos of Tim Joyce's road race Triumph show a exhaust very similar to the one you're looking at on my bike. There's no consistency with exhausts in LSR racing. Some fast machines have short pipes, some long, some individual, some collector styles. Same for drag racing.....
I'll see what happens during the dyno pulls and rural back road runs...Thanks
Posted By: Rickman Re: Mikuni TM's and velocity stacks - 02/08/14 2:27 pm
Seeing that pic from above......

How much longer would the intake tubes need to get, for the carbs to be behind the down tubes?

That way, you could tuck the carbs in closer to the centerline of the frame? Maybe get 'em farther behind the rider's knees and legs?

After all, the most time this bike is used [ ? ], it IS at WOT, right???
Posted By: Hillbilly bike Re: Mikuni TM's and velocity stacks - 02/08/14 5:12 pm
Originally Posted by Rickman
Seeing that pic from above......

How much longer would the intake tubes need to get, for the carbs to be behind the down tubes?

That way, you could tuck the carbs in closer to the centerline of the frame? Maybe get 'em farther behind the rider's knees and legs?

After all, the most time this bike is used [ ? ], it IS at WOT, right???


Yeah, it's wide open for 35 seconds but the RPM does vary from about 5500 to 7500 rpm as it goes through the gears and then then nearly a half mile in high gear and crosses the line at 7200 rpm.
I'm going with 4 inch intake manifolds , the carb and then a velocity stack...Now I can tune the length without worrying about throttle cables and fuel lines.
That sounds like a great idea.. once you get the length figured out you can make it all fit best for the bike and rider.

Remember to support the back end of those carbs or stacks, too much stress on the intakes can cause failure.

For grins I ran the numbers from that guys experiment, same power at 7,500 but 19 more H.P at 5,500. All that without retuning the fuel mixture or ignition timing. So you may get about half of that.. but an extra 5 H.P. at 5,500 is a bunch..!

Good Luck..!

Ron
Posted By: David Dunfey Re: Mikuni TM's and velocity stacks - 02/08/14 8:59 pm
I also thought the intake tract length began at the valve and ended at the carb intake, but Stuart Hooper disabused me of that idea. He told me it ends at the bell mouth and it does not matter where the carb is. The intake is tuned just like the exhaust. The negative pulse in the intake reverses as it moves the end of the bell and turns positive and heads back down the intake tract. The intake tract length needs to be tuned so the positive pulse hits the intake valve just before it closes. The pulse weakens each time it reverses, so the second pulse is the strongest. Unfortunately the second pulse needs the longest intake tract.

To hit the middle of your range using 6500 RPM the second pulse would indicate a 20.3" tract, third pulse 14.9" and the fifth pulse is 11.4". These are just starting points for the dyno work.

David
Posted By: Hillbilly bike Re: Mikuni TM's and velocity stacks - 02/09/14 8:05 pm
I reworked the intakes so the carbs are 4 inches from the head. Velocity stacks can be made from readily available .060 wall by 2-1/4 inch diameter aluminum tubing.
10 inch stacks will look pretty wild...
Posted By: David Dunfey Re: Mikuni TM's and velocity stacks - 02/10/14 2:48 pm
I cannot remember what Stuart's are, but he told me he started out with two foot long stacks with slip rings on the dyno. I thought he said he settled at 8", but the photo is below:

[Linked Image]

David
Originally Posted by Hillbilly bike
I'm still listening to you guys.... This photo from above shows what goes on and the narrowed seat rails.
It's an OIF so I can't change the main frame....I do have 60's frames but that means a whole new build ....
Anyways, I can add about a 5 degree downward elbow near the head. This will drop the carbs down about 1-1/2 inches and clear the back of the riders knees.
I'm thinking 5 degree elbow will have little to no effect on airflow? Ya think the radius matters? I can do any radius...

[Linked Image]

That looks about right to me.Long stacks on the atmosphere side of the carb can mess up your mixture at some speeds.If you do add stacks,they should match the carb venturi (34 mm ID,with about 1/4" radius at the atmosphere side).

Use a big air box if you fit filters (3-6 times the cylinder volume).15" to the bell mouth,or end of radius on the carb entry (if you fit no stack) is almost perfect at 6000 rpm.Refer to Blair's formula which is temperature dependent.It's good at 25 degrees C.You might want something shorter at higher rpm.
13.222" would be good at 6,800 rpm at 25 C ambient temperature.I hope your intake port diameter is very close to 1-1/8",which is about ideal for a 650 at 7000 rpm.
I guess I could of commented on the "famlily reproduction prevention device" just in front of the seat.. smile

Ron
Posted By: Allan G Re: Mikuni TM's and velocity stacks - 02/11/14 5:55 pm
Originally Posted by Ron - in California
I guess I could of commented on the "famlily reproduction prevention device" just in front of the seat.. smile

Ron


X marks the spot wink
Posted By: Hillbilly bike Re: Mikuni TM's and velocity stacks - 02/11/14 8:12 pm
Originally Posted by Allan Gill
Originally Posted by Ron - in California
I guess I could of commented on the "famlily reproduction prevention device" just in front of the seat.. smile

Ron


X marks the spot wink


Humans dressed in animal skins riding vibrating machine at triple digit speeds isn't exactly safe no matter what....That's why it's fun...

Ok...we have two groups here, those who say add the intake length between carb and head. And the others who say keep the carb closer to the head with a velocity stack for length.
The article Ron provided looks like a good push for velocity stacks. And it fits my bike better than the long intakes...
But I believe clever tuners can make either system work nicely.
Posted By: David Dunfey Re: Mikuni TM's and velocity stacks - 02/11/14 9:16 pm
The math that I was using as well as the idea that the carb position is not critical to the acoustics is from D. William Denish from his V-Twin tuners handbook (2000). It is concerned with the math of racing. It is hard to find and is listed on Amazon for between $200 and $425. A friend who drags Harleys had a copy. Stuart told me it was his main source of information for the build. He did 146 mph for the record at Bonneville, but went 160mph in one practice run. Last year the blown version did 171 mph at Lake Gairdner for a record. That's my best shot!

David
Oh no.. I am not saying one is better than the other overall.. I am saying getting the carbs out of the rider's way is more important. Plus either way the carb needs some careful support to keep it from breaking something.

Sometimes asthetics do count..!

Posted By: Hillbilly bike Re: Mikuni TM's and velocity stacks - 02/20/14 8:36 am
Ok, the Mikuni 34 mm TM's are moved in close for rider clearance,9 inches from the carburetor airhorn to intake valve. Any closer and the wide carb body hits the frame parts.
The velocity stacks might be around 3-5 inches.I will make them from thin wall steel tube and hammer form the rolled entry ...
Question, the Mikuni air horn is 2-1/4 inches in diameter. Should the stacks be the same diameter or smaller diameter with a flared end to fit the airhorn? There's air metering openings around the edge of the air horn that need to be considered...The tuning link by Ron states length is more important than diameter.
I got the ignition disaster temporarily patched up.The Tomasselli dual cable throttle pulls the carbs wide open with a touch more than 1/4 turn.....Out of the box I changed the pilot jets to a #20 , lowered the needle to the leanest position,#240 main jets. The bike starts one or two kicks cold, one when warm, and settles down to a lopey but steady 1000 rpm idle. And I never touched the idle mixture screws. Blipping the throttle is instant response without black smoke or orange flames of a rich mixture. This is not an indication of proper tuning but at least it's a reasonable starting point.
The ID of the stacks should be approximately the venturi size of the carb (34mm),or just slightly bigger.
If you made them 2-1/4 ID,the wave would be returning from the carb entry,and not from the end of the end of the stack.
Posted By: Hillbilly bike Re: Mikuni TM's and velocity stacks - 02/20/14 9:55 am
Originally Posted by Pete R
The ID of the stacks should be approximately the venturi size of the carb (34mm),or just slightly bigger.
If you made them 2-1/4 ID,the wave would be returning from the carb entry,and not from the end of the end of the stack.


Here's what the 2-1/4 inch airhorn looks like. Notice the air bleeds . 1-1/2-1-5/8 inch diameter stack will have a large flare end to fit the airhorn...I have no experience with this but wonder if the air flow will be disturbed in a bad way. I suppose actual testing might be necessary ...

[Linked Image]



Posted By: Pete R - R.I.P. Re: Mikuni TM's and velocity stacks - 02/20/14 10:34 am
The air bleeds will just be outside the stack and the stack will blend into the venturi,just like the proper good Amal stacks.I don't see a problem.
It is,of course,easier to just have the carb on the end of a longer tube,with no stack attached.That seems to be difficult,because of space considerations.
I guess it depends on what AMAL you mean.. I have seen both methods used, bleeds inside and outside. It will be difficult to get a stack that fits the venturi correctly, and it may yield better results, I do not really know for sure. But it would make for a smaller diameter thus possibly fit the bike better. What is really important is to make sure the rider does not foul the air intake.
Ron
Posted By: Hillbilly bike Re: Mikuni TM's and velocity stacks - 02/21/14 8:02 am
Using available formulas and plugging in my engine specification comes up with different results.

Engine Pro - Intake Port Length is 14.02 inches
Gordon Blair Formula - Intake Length is 10.69 inches
Simplified Chrysler Formula - Intake Length is 11.67 inches
Simplified Chrysler Formula (with bigger cam) - Intake Length is 11.15 inches
Simplified Formula - Intake Length is 11.00 inches
Simplified Formula 2 - Intake Length is 13.60 inches
Simplified Formula 3 - Intake Length is 12.83 inches

Obviously a formula is only a quess and time spent on a dyno and at the track is the only way to know for sure.
There appears to be two things happening at once from intake and or velocity stack length.
Longer length appears to move peak HP to a higher rpm and moves the torque lower and flatter at the same time.
Shorter length does just the opposite.
For my racing I need both torque and HP at higher rpm...
This is the reason for the vast differences in intakes and exhaust on very similar engines on LSR bikes. There is no absolutes here, you run what works best for your rider and machine.
And the new exhaust will need tuning along with carb and intake....
A lot of trial and error.....My recent ignition fiasco really sucks because it's still winter here and dyno operators are begging for business....
but I'll get it done...
Good job H.B..!! You did some homework and found out all those books and computer programs only get you so close.! To fine tune, you have to do the work yourself. We found that even 1/4" on intake length made a noticable difference.

Same with exhaust. When my guy made the new exhaust header pipe (to take a bunch of bends out of it). All the lengths were to be the same. Well, the dyno knew better.. The dyno instantly found it and told us all our settings (carb) were now off and the power was down, even after changing the jetting. A quick tape measurement showed it to be almost 2" short. Added the 2" and voile, it went back to where it was.

And you are correct, tuning the intake tunes more than one thing.. That length also accounts for cam specs, and exhaust lenght, and of course desired result.

I should make a separate WEB site with all the pictures I have of a bunch of race bikes, no two are the same on either intake or exhaust. If you see an up pipe that ends near the seat, that is darn short..! So a good example is look at a BSA Gold Star with a Clubman's pipe, then look at a Gold Star with a Scrambles pipe. No tape measure is needed to see the difference..! Tuning can be frustrating or it can be fun, but it does take a lot of time.
Ron
Using the Gordon Blair formula,and plugging in an ambient air temperature of 25 C,and 10.69",I come up with 8411 rpm and 6562 rpm.
You could be better off going for 5850 rpm and 7500 rpm,with a length of 11.988"

Any formula that doesn't account for air temperature is worthless,especially if they've been using it on car engines with under-bonnet temperatures of 100C or more.Your average intake temperature,between the carburettor and the valve,will be slightly less than ambient air temperature.
Posted By: Hillbilly bike Re: Mikuni TM's and velocity stacks - 02/22/14 7:03 am
Last year the intake length was 12.5 inches using 3 inch non tapered velocity stack and 9.5 inches of carburetor and intake . The bike ran 120 mph with 32 mm Amals, a stock Bonneville head, 37 inch long straight pipes and 4.76 overall gearing with a 27 inch tall tire. The bike was still accelerating at the end of the mile according to the rider.
Reworked head this year along with 34 mm flatslides several exhaust systems to try out and 4.66 overall gearing, same tire...
Probably start with a 12 inch total induction length
Posted By: Mark Parker Re: Mikuni TM's and velocity stacks - 02/22/14 7:07 am
This is what I've done, could be neater but works. The air bleeds are blended, maybe that has an effect on how they work as air bleeds but I cannot tell. These are TM38s
[Linked Image]
Not sure how long the total inlet tract is but the carb is visible in this photo and gives an idea, good power from 2,500 - 7000 but I doubt the length of that is the main thing. A small change to the exhaust can have a much bigger impact, as can head work. I think the shape of the bellmouth is important to what it can flow and may be more benefit than such a long run as you have in the photos trying for a little gain with optimum length tuning which may be a bit elusive anyway. I'd be seeing what the power difference is with them at their longest practical length compared to the long set up in your photo, shorter may actually work better.
[Linked Image]
Posted By: Hillbilly bike Re: Mikuni TM's and velocity stacks - 02/22/14 8:06 am
Mark...As I said before,the fastest in class LSR OHV bikes have what appears to be nothing much done on intake tuning . And every exhaust system is different...
Maybe I should do no tuning...exactly what I did last year grin
Posted By: konon Re: Mikuni TM's and velocity stacks - 02/22/14 8:57 am
Hope it all fits together for you HB.You think your doing the right thing, and you get to Ohio and it goes slower. Sometimes there's no ryhm or reason.
Posted By: Hillbilly bike Re: Mikuni TM's and velocity stacks - 02/24/14 11:26 am
After many discussions, looking at what many other top of the heap lSR bikers have done....There appears to be no parallel twins ( or Harleys) running intake and carb combined length longer than about 9-10 inches.
So I believe the tuning is done with exhaust and jetting to work with whatever cam profile is being used in their engine...
Posted By: Allan G Re: Mikuni TM's and velocity stacks - 02/24/14 1:11 pm
Originally Posted by Hillbilly bike
Mark...As I said before,the fastest in class LSR OHV bikes have what appears to be nothing much done on intake tuning . And every exhaust system is different...
Maybe I should do no tuning...exactly what I did last year grin


I wouldn't alter anything until you have tested the mods you have done so far, you could be gaining with one thing an loosing on the other.
Originally Posted by Hillbilly bike
There appears to be no parallel twins ( or Harleys) running intake and carb combined length longer than about 9-10 inches.
So I believe the tuning is done with exhaust and jetting to work with whatever cam profile is being used in their engine...
Have a look at what johnm is using on his Norton.You'll never be the fastest by copying what others are using.
Peter is exactly correct. I just had an ahem "debate" with some supposed engineer.. He told me.. "this is how it has been done for over 50 years, why do you want to change it..?" I then replied, well if I do only what others have done, then my bike can not be any faster than theirs.. He never got it.

Most if not all bikes I look at, show no signs of doing this kind of testing.. they all seem to take the easy route and live with it. And some go darn fast that way..! But, the very best will spend the time and money to find a bit more....

Ron
Posted By: Hillbilly bike Re: Mikuni TM's and velocity stacks - 02/24/14 2:23 pm
Originally Posted by Pete R
Originally Posted by Hillbilly bike
There appears to be no parallel twins ( or Harleys) running intake and carb combined length longer than about 9-10 inches.
So I believe the tuning is done with exhaust and jetting to work with whatever cam profile is being used in their engine...
Have a look at what johnm is using on his Norton.You'll never be the fastest by copying what others are using.


At the E.C.T.A track in Ohio we hold the class record. grin .But guys better than me are gonna be chasing.. I asked all the questions here during the original engine build and pretty much did what you and a few other suggested.
I know how to use tools and fabricate parts, but I am no more than an amateur tuner...And Johnm's expertise is far ahead of me...So I would be just coping his build?



I have some dyno time coming up... that and the limited track time is all I got...
I look at racing like making love to a woman. Have fun and smile and try to please. Take yourself too seriously and it becomes anxiety..........
Posted By: Julian Re: Mikuni TM's and velocity stacks - 02/28/14 5:20 am
Originally Posted by David Dunfey
I also thought the intake tract length began at the valve and ended at the carb intake, but Stuart Hooper disabused me of that idea. He told me it ends at the bell mouth and it does not matter where the carb is. The intake is tuned just like the exhaust. The negative pulse in the intake reverses as it moves the end of the bell and turns positive and heads back down the intake tract. The intake tract length needs to be tuned so the positive pulse hits the intake valve just before it closes. The pulse weakens each time it reverses, so the second pulse is the strongest. Unfortunately the second pulse needs the longest intake tract.

To hit the middle of your range using 6500 RPM the second pulse would indicate a 20.3" tract, third pulse 14.9" and the fifth pulse is 11.4". These are just starting points for the dyno work.

David


Absolutely right David, I agree with Stuart,
The only real factor of carb positioning is that close to the head you don't get the same fuel "dropout" at low revs that you would if the carb was mounted a long way out and needing high gas speed to keep the fuel particles suspended.
total length is what is important.
However Trial and error will be the key as with any theory. Sometimes on some applications of theory you need a different theory :o)

Julian

Cool looking bike by the way!
Posted By: Dennis M Re: Mikuni TM's and velocity stacks - 03/24/14 12:52 am
Total length from the velocity stack to the exhaust outlet is what should be kept in mind. During testing any intake length change should also be tested with exhaust length and style changes.
Posted By: johnm Re: Mikuni TM's and velocity stacks - 03/24/14 5:07 am
I posted this graph several years ago.

http://www.britbike.com/forums/ubbthreads.php?ubb=showflat&Number=268898&page=1

It compares the calculated lenght for Blair and Vizard.


There are two things I would add from my experience.

Firstly as several others have said the inlet tuning needs to match the exhaust. For road racing I think you should tune the inlet to fill in the low points on the hp curve from the exhaust lenght. For example - tune the exhaust for a 6800 rpm and the inlet for 6300. This produces a smooth curve.

Secondly possibly the most important thing of all. Vibration of the carbs. With the long inlet tracts you will probably find the carbs vibrate badly. They are out on the end of a long tube and can really buzz at certain rpm. You must mount them on good quality Mikuni type rubber connectors and support the carbs so they can move but not buzz! If you find you are getting a misfire at the end of a long straight with the throttle wide open you are probably getiing frothing in the carbs and fuel starvation from vibration. (I assume you have already tested to ensure you have sufficient fuel flow through the lines fuel taps etc.)

This is really important to check this or all your tuning tests may be misleading!

Anyone wanting do this stuff must read the back to basics paper here. It is excellent!

http://www.profblairandassociates.com/RET_Articles.html
Posted By: Hillbilly bike Re: Mikuni TM's and velocity stacks - 03/24/14 6:47 am
This bike was clocked at 131 mph on run at the Bonneville BUB meet last year. The fastest LSR speed by a naked frame modified Triumph 650 on gas.It's in altered class allowing very low seating which should help top speed . It has a Triumph 2 valve head and pre unit cases. I have discussed engines with this guy (Astek racing)....The engine has short intakes, about three inches. When I asked him about the long exhaust he said it was chosen because it looks "cool". He's a bit secretive about the engine build but claims the engine has never seen a dyno, the bike has no tachometer. My guess it about 7500 rpm to make the power needed?

[Linked Image]

This was Tim Joyce's race bike,or one of them. Very highly modified of course and a 750 ...Notice the intake length and exhaust length. 8500 rpm?
Circuit racing is far different than LSR racing but I'm thinking Tim's bike will run quite fast on top end at an LSR track.
I think these examples show relationship between exhaust and intake lengths

[Linked Image]

OK, an update here. I finally got some dyno testing yesterday.. Learned a lot..! Unfortunately I had to make several changes at one time, thus it makes it more difficult to get to where I want quickly. New carb, with a totally different velocity stack design, and different jet designs, and some cylinder head changes. This is far less than ideal, where I like to make a single change at a time.

The first two dyno runs were aborted due to an extreme lean conditions. Made a 15 point main jet increase that made minimum difference. Quickly discovered an error in the main air correction jet. (new carb and old carb used different types of air jets and did not cross reference properly). Once the air jet was changed to a smaller size the jetting went rich, like it should. The air correction jet will be tested later to match that air jet to the needed fuel characteristics. Once we got our first full dyno run we noticed an increase in power at 6K, but slightly less at 7.5K. Aha..! This showed that we are either too long of too short on intake length. One of the changes with the new carb/stack set up is I went long (about 1.5") to start on purpose. Much easier to remove metal than add it on. Shortened the stack by 1/4" and the power peak started to move up, as expected. Ran out of time.. today will continue to shorten the stack until the power peaks, then starts to fall off. Also note that the 1/4" change in length required a main jet change. First change the length, then change the main jet. Each jet size (in my case) produces almost 1 H.P. So, it is impossible to make any meaningful judgement on intake length unless the jetting is also optimised for each change.

What is also interesting is that the more power we make, the cleaner the engine runs, emissions wise. And even more interesting is that the engine heat does not always increase with power increase. The change in engine heat may later allow for ignition timing changes as well. Currently I dyno with the timing slightly retarded until we get a good state of carb tune, then test for max power with ignition timing, while monitoring the engine heat. I finalize the ignition curve last. However I will likely do an initial ignition map later today, as the current readings tell me the timing is way off at some ranges. Being able to program the ignition curve in 500 RPM increments adds a lot of power at some steps.

Later..

Ron
Posted By: JBMorris Re: Mikuni TM's and velocity stacks - 04/02/14 9:16 pm
Not too sure about all the math involved but the Art and Design Department says to change out the pink overflow tubes to clear!:
[Linked Image]
Maybe some engine turning?

[Linked Image]
Quick update.. finally got back to the dyno... we got the best ever set up..! More power than ever, everywhere... But this was with my old carb. The new one is not as good.. yet. Tomorrow we start tuning the intake length.. we know it is off with the new set up. It is either too long or too short.. we will see. Worst case use the old carb.. we know it is awesome overall. What is amazing is that the power only tapers off a few tenths from peak to 8K... unlike before where it would drop a bunch at above peak RPM. All good so far... Oh yes and the engine is not running hot anywhere..!

All the hard work is paying off...

Ron
Posted By: Hillbilly bike Re: Mikuni TM's and velocity stacks - 04/18/14 4:21 pm
Refresher.............your old and new carbs are ?
Sorry, both are Del Orto's... PFM. The main difference is how the velocity stack is attached. The other difference is the new carb has an enrichener circuit the old one has a non-functioning tickler, so it can be a chore to fire up cold..

Ron
Posted By: Hillbilly bike Re: Mikuni TM's and velocity stacks - 04/21/14 6:52 am
I'm familiar with PHF's on my 1000 Guzzi rider. Dellorto carbs seem to be quality devices . Models with accelerator pumps can help lower speed throttle response on engines very large carburetors. I was considering them on the lSR bike but the lower priced Mikuni TM's got my attention.
Mikuni TM's are very good... just can't use them for racing... Even the Del Orto's have to be a cerain version to be legal.. no pumps allowed, can't block them off, must have factory non-pump version. Luckily they are readily available.. Just got a new needle to test.. the current needle was too lean at 3/4 throttle and slightly lean at 1/2 throttle and of course rich at lower openings... the new needle should fix this.

Ron
Posted By: Hillbilly bike Re: Mikuni TM's and velocity stacks - 04/22/14 11:22 am
....LSR class rules follow the SCTA and there are no restrictions in modified classes about anything on or inside the engine other than displacement,2 or 4 stroke, fuel type, supercharged/nitrous and pushrod or non pushrod. Production classes must look like a stock bike but anything goes for engine modifications so long as the stock airbox, if equipped, and stock muffler outlet size.
Posted By: Allan G Re: Mikuni TM's and velocity stacks - 04/22/14 12:29 pm
Originally Posted by Hillbilly bike
Production classes must look like a stock bike but anything goes for engine modifications so long as the stock airbox, if equipped, and stock muffler outlet size.


So a A65 engine in an A10 frame with Alu forks won't cut the mustard then, even if it has the other bits?
Posted By: Hillbilly bike Re: Mikuni TM's and velocity stacks - 04/22/14 2:15 pm
Originally Posted by Allan Gill
Originally Posted by Hillbilly bike
Production classes must look like a stock bike but anything goes for engine modifications so long as the stock airbox, if equipped, and stock muffler outlet size.


So a A65 engine in an A10 frame with Alu forks won't cut the mustard then, even if it has the other bits?


No,it would be classified as Modified Production ...Engine and frame from same manufacturer but otherwise never came that way from the factory. The rest of the cycle parts can be any brand or home built. The engine cradle has to be stock but the rest of the frame can be modified within the rules.1955 and earlier parts can be run as Vintage. Vintage engines are not supposed to use electronic ignitions or electronic injection... Send for a SCTA rule book to see all the rules...
The ECTA track relies on the honor system, there are no protests, no trophies. If you want a pretty trophy girl to kiss ,bring your own....
Posted By: Allan G Re: Mikuni TM's and velocity stacks - 04/22/14 5:10 pm
Originally Posted by Hillbilly bike
If you want a pretty trophy girl to kiss ,bring your own....


laughing

Thanks for that HB, suppose it makes sense but it was worth clearing it up. I'm guessing the completion gets even harder when you start entering modified frames, when you go to that, you may as well get something lighter still.
Quick update, as I am headed to the race track.. time to see what we can do..! Did a quick dyno check after going over everything and making a carb needle change.. Aha..! found a bad spark plug cap. You can not easily find stuff like that any other way.. note I said easily.. (with a twin plug head it won't misfire, just lose power). Once I removed the cap, there were the carbon tracks.. all the power is there and a bit more..! Geesh.. that was almost too easy..! I will be short on gearing, as my one transmission sprocket got ruined last week.. but I suspect we will make do OK.

We are going to have some great competition this weekend.. two G50's. two Goldies, a BMW, and one sneaky fellow on an Aermacchi.. could be more, but those are the preregisterd bikes. A couple of those guys are good pals... should be a lot of fun..!

Ron
Posted By: dommie90 Re: Mikuni TM's and velocity stacks - 05/13/14 9:24 am
If you get hold of "MIKUNI carburetor tuning by victory libary" i has all the information on mikis.
including velosity stacks and jetting its a very detailed book excellent reading
baz
Posted By: ken sak Re: Mikuni TM's and velocity stacks - 05/10/15 4:36 am
Originally Posted by Hillbilly bike
Using available formulas and plugging in my engine specification comes up with different results.

Engine Pro - Intake Port Length is 14.02 inches
Gordon Blair Formula - Intake Length is 10.69 inches
Simplified Chrysler Formula - Intake Length is 11.67 inches
Simplified Chrysler Formula (with bigger cam) - Intake Length is 11.15 inches
Simplified Formula - Intake Length is 11.00 inches
Simplified Formula 2 - Intake Length is 13.60 inches
Simplified Formula 3 - Intake Length is 12.83 inches

Obviously a formula is only a quess and time spent on a dyno and at the track is the only way to know for sure.
There appears to be two things happening at once from intake and or velocity stack length.
Longer length appears to move peak HP to a higher rpm and moves the torque lower and flatter at the same time.
Shorter length does just the opposite.
but I'll get it done...


with the pro program what dia did it suggest
and what cam ex lengths did you run Hb?
Posted By: Hillbilly bike Re: Mikuni TM's and velocity stacks - 05/10/15 6:40 am
Originally Posted by ken sak


with the pro program what dia did it suggest
and what cam ex lengths did you run Hb?


Shorty after this discussion last year I spend hours tuning the bike on a chassis dyno. None of the formulas mentioned produced worthwhile results....
Highest peak power with the broadest torque spread was made with no velocity stacks, the carbs as close as practical to the head (about one inch longer intake manifold than stock).The exhaust was two 1-1/2 x 38 inch exhaust pipes.
The combination made record breaking speed at the track....What works best on my 650 Triumph with Sifton #390 cams might not be best on another engine.
Posted By: ken sak Re: Mikuni TM's and velocity stacks - 05/10/15 8:41 am
thanks for the info Hb. congratz on your record too, i was thinking of just running the stock 78 750 manifolds with flatside mikuni's and 2 750 inlet cams, only a couple of degrees here and there and much the same lift as the sifton #390
i'm oldish and have looked up the t100 tuning kit to get an idea of lengths of pipes-finish at the back of primary , short megas to about 4" finish about the back axle.
what's your low end power like, with the 390, another site says it's not streetable but I've put radical cams
in cars and put up with it
cheers, Ken
Posted By: Hillbilly bike Re: Mikuni TM's and velocity stacks - 05/11/15 7:16 pm
Originally Posted by ken sak
thanks for the info Hb. congratz on your record too, i was thinking of just running the stock 78 750 manifolds with flatside mikuni's and 2 750 inlet cams, only a couple of degrees here and there and much the same lift as the sifton #390
i'm oldish and have looked up the t100 tuning kit to get an idea of lengths of pipes-finish at the back of primary , short megas to about 4" finish about the back axle.
what's your low end power like, with the 390, another site says it's not streetable but I've put radical cams
in cars and put up with it
cheers, Ken


My 650 with 34 MM flat slides ans Sifton 390 cams idles reliably at 1000 rpm on the electric tach. Off idle is ok at part throttle but use full throttle and there's a bit of reversion at around 4000 rpm. Once past that the throttle response is fantastic....I think in a 750 the #390's would be ok with the proper gearing in a light bike. Perhaps messing with part throttle jetting, intakes and pipes might get rid of the reversion for limited street use..
Cams for vintage bikes like Triumphs are mostly older grinds and lack the sophisticated profiles of modern car cams.Triumph guys jockey the cam timing around to find power, but on something like a Chevy V8 if you need to alter timing by more than two degrees, it's better to select another cam from the endless list of grinds available.
Originally Posted by Hillbilly bike
Using available formulas and plugging in my engine specification comes up with different results.

Engine Pro - Intake Port Length is 14.02 inches
Gordon Blair Formula - Intake Length is 10.69 inches
Simplified Chrysler Formula - Intake Length is 11.67 inches
Simplified Chrysler Formula (with bigger cam) - Intake Length is 11.15 inches
Simplified Formula - Intake Length is 11.00 inches
Simplified Formula 2 - Intake Length is 13.60 inches
Simplified Formula 3 - Intake Length is 12.83 inches

Obviously a formula is only a quess and time spent on a dyno and at the track is the only way to know for sure.
There appears to be two things happening at once from intake and or velocity stack length.
Longer length appears to move peak HP to a higher rpm and moves the torque lower and flatter at the same time.
Shorter length does just the opposite.
For my racing I need both torque and HP at higher rpm...

Here's what I have as the Gordon Blair formula, and it offers 4 alternative lengths for any given ambient air temperature.

Gordon Blair gives the following empirical formula for induction lengths (see: “Design and Simulation of Four-stroke Engines”). He maintains that it is accurate to about 3%.
The induction wave is reflected up to five times, and the equation for the length is:
L = (aC /N)
Where L = Induction length from valve head to end of bellmouth in mm.
a = reference speed of sound at 340 at 15 degrees C, 346 at 25 degrees C
C = Dimensionless intake ramming factor
* for first ramming peak C = 8900, second C= 6600, third C= 5150, fourth C= 4150.
N = rpm at which you want the torque improvement.
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