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#567286 - 10/12/14 10:26 am Mikuni Carb Mount Question  
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Blown Income Online content
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Edgewater, Md
Hi All,

I am trying to mount up my TM 34 Mikunis on my 72 Lightining LSR bike. Does anyone know if there is a mounting spigot that is 34mm (carb end) that tapers to 32mm (cylinder head side, needs to have the AMAL flange to mount). I have found some that are 34mm straight thru but not tapered.

Any help is appreciated.

Thanks


1955 BSA Bantam D1 Plunger
1956 BSA A10RR Street and LSR Bike
1961 BSA C15S
1966 BSA spitfire
1969 Triumph T100C
1970 Triumph TR6R
1972 BSA Lightning LSR Bike
1974 Triumph T150V
#567294 - 10/12/14 10:55 am Re: Mikuni Carb Mount Question [Re: Blown Income]  
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Allan Gill Offline
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Rotherham - S. Yorkshire
I'm not overly familiar with the mikuni carbs, however if the fitting for the hose is the same on a 32mm as a 34mm then I would just buy a stub to suit a 32mm and taper it to suit.

Which class is this running in? 650 or long stroke/big bore 750 ??


beerchug
#567325 - 10/12/14 1:59 pm Re: Mikuni Carb Mount Question [Re: Blown Income]  
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DavidP Online content
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Gnashville
The OD of the engine side of TM32 is the same as TM34 (same with the VM32/34.) AFAIK, the bodies are identical save for the bore. Thus, you should be able to buy 32mm adapters and taper out to 34mm at one end. I got the adapters for my VM32's from MAP, and I buy straight hose at NAPA to mount them (cut to length with a hacksaw.)
For some odd reason, the allen bolts which come with the MAP adapters are not CEI threads. I had to buy them elsewhere (BSA head.)


Stepping on others doesn't make you stand tall.

71 A65L "Zelda"
92 BMW K100rs "Gustav"
#567374 - 10/12/14 8:37 pm Re: Mikuni Carb Mount Question [Re: Blown Income]  
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Blown Income Online content
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Edgewater, Md
Allan, This bike will be running MPS 750/4 class. I had the crank stroked and punched the cylinder to 3" bore.

DavidP, Thanks I will check MAP for the adapters.


1955 BSA Bantam D1 Plunger
1956 BSA A10RR Street and LSR Bike
1961 BSA C15S
1966 BSA spitfire
1969 Triumph T100C
1970 Triumph TR6R
1972 BSA Lightning LSR Bike
1974 Triumph T150V
#567411 - 10/13/14 7:05 am Re: Mikuni Carb Mount Question [Re: Blown Income]  
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Hillbilly bike Online content
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Running from demons in WNY
Chris, Triumphs are different... But.. to mount 34 MM flats I used stock screw in intakes with a the flange cut off and a short length of 1.5 inch tubing welded on. Then the interior tapered with a hand held die grinder as best as possible.
There's a lot of info on air flow and intakes...But it seems air doesn't flow exactly where you want it to go so I don't know how "exacting" the tapering has to be.


650 Triumph modified production LSR record holder 133.1 MPH... Twin 650 engine Triumph LSR that goes sorta fast...
#567418 - 10/13/14 8:10 am Re: Mikuni Carb Mount Question [Re: Blown Income]  
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Blown Income Online content
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Edgewater, Md
HB, I have a pair of the aluminum triumph screw in Mikuni mounts but there small on both ends, just under 30mm on the head side and 32mm on the carb side plus I would need to weld the flange on the head side. I think I will try the MAP 32mm set and taper bore the ID to match the carb. Time to get this project wrapped up and tested as the May event will be here before we know it.


1955 BSA Bantam D1 Plunger
1956 BSA A10RR Street and LSR Bike
1961 BSA C15S
1966 BSA spitfire
1969 Triumph T100C
1970 Triumph TR6R
1972 BSA Lightning LSR Bike
1974 Triumph T150V
#567454 - 10/13/14 1:45 pm Re: Mikuni Carb Mount Question [Re: Blown Income]  
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Allan Gill Offline
Allan Gill  Offline



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Rotherham - S. Yorkshire
The popular bsa small port A65 head completes its taper within about 3/16" of the carb mounting the head the steep taper works quite well.


beerchug
#567456 - 10/13/14 2:03 pm Re: Mikuni Carb Mount Question [Re: Blown Income]  
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JER.Hill Online content
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Forfar, Scotland
Hi

You could also look into some of the old TZ mountings, these were rubber flanged mountings and take the standard Mikuni carb. You may need to re-space the flange PCD! Benefit of this is keeping the track length short (if this is the aim).

John

#567589 - 10/14/14 11:17 am Re: Mikuni Carb Mount Question [Re: Blown Income]  
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Ron - in California R.I.P. Offline
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I would use the 34 MM ones and not worry..

Ron

#567606 - 10/14/14 1:06 pm Re: Mikuni Carb Mount Question [Re: Blown Income]  
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Edgewater, Md
Ron, You dont think there will an issue with a 2mm difference from the adapter size to head head port? Wouldnt the step cause an issue with the A/F speed and flow?


1955 BSA Bantam D1 Plunger
1956 BSA A10RR Street and LSR Bike
1961 BSA C15S
1966 BSA spitfire
1969 Triumph T100C
1970 Triumph TR6R
1972 BSA Lightning LSR Bike
1974 Triumph T150V
#567657 - 10/14/14 7:04 pm Re: Mikuni Carb Mount Question [Re: Blown Income]  
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DavidP Online content
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Gnashville
I put 32's on a stock '71 A65 head without modifying the ports. Seems to work well, but I'm not racing.


Stepping on others doesn't make you stand tall.

71 A65L "Zelda"
92 BMW K100rs "Gustav"
#567904 - 10/16/14 9:45 am Re: Mikuni Carb Mount Question [Re: Blown Income]  
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Ron - in California R.I.P. Offline
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California
It is done all the time... just look at anyone with intake stubs and a hose connection. And what is interesting is that the Gold Star came from the factory with a mis-match. It was thought this increased power.

Best / easiest.? way to check is with a flow bench.

Ron

#567950 - 10/16/14 2:45 pm Re: Mikuni Carb Mount Question [Re: Blown Income]  
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Allan Gill Offline
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Rotherham - S. Yorkshire
I do t know about using a flow bench for this purpose, what it would show is the larger port will flow more, and not the gains of the reduction in carb - reduction benefits.

When the larger 1 3/8 carb was initially (accidently) fitted it wasn't checked against the head size and they found more power was gained from that bike, on the strip down they found the carb was not the 1 3/16 as they planned to fit and thus opened the head out accordingly. This dropped more in power to a point less than if the bike was run with a 1 3-16 carb. They sleeved the port back to standard and power was restored.

I believe later on they gave a sharp taper to the ports which improved running further

( I can add the Roland Pike link if anyone wishes to read it, as he worked on the competition bikes at the time)

What you really could do with is giving the bike some dyno time, try the same stubs with two different carb sizes and have some tapered stubs with you which you can swap over. But what Ron says is what I would also say is that the size difference will give the better performance.

My best advice would be to look for a small port head, reduce the ports further and stick to the 30mm carbs.

My best 650 modified lightning head is a small port with the floor raised 4mm, ( which gives a tighter choke point at the guide than my 30mm head with the floor raised 10mm.
It revs harder than when it was stock with the same valves and springs and the torque is greater than anything I've modded to date.

The use of a flow bench would tell me if the ports are equal.

You could use a flow bench on this head though! And use it by raising the port floor until it effects flow.


beerchug
#567990 - 10/16/14 6:26 pm Re: Mikuni Carb Mount Question [Re: Blown Income]  
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Edgewater, Md
Thanks Guys,Looks like I have some options to explore. Dyno time is a must for this bike. The carbs and mounts should be here tomorrow. Almost ready to fire her up grin Just got to finish the painting of the body work and tank, hopefully in the next few days.


1955 BSA Bantam D1 Plunger
1956 BSA A10RR Street and LSR Bike
1961 BSA C15S
1966 BSA spitfire
1969 Triumph T100C
1970 Triumph TR6R
1972 BSA Lightning LSR Bike
1974 Triumph T150V
#567999 - 10/16/14 7:18 pm Re: Mikuni Carb Mount Question [Re: Allan Gill]  
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Hillbilly bike Online content
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Running from demons in WNY
Allan I've never used head flowing equipment but....When the port is smaller the flow might be less but the pressure is greater. A law of physics, reduce flow and pressure increases, yes? Is port velocity the same as pressure? I'm thinking state of the art flow benches can measure flow and pressure.

And the raised port floor does more than reduce flow. The air flow needs to be straightened out before it gets to the valve head so it can flow around the head in an equal cone effect.The raised port floor give a longer short side port radius before the air hits the valve and a straighter flow. Form what experts say,not me of course,1/4 inch can make a noticeable difference.


650 Triumph modified production LSR record holder 133.1 MPH... Twin 650 engine Triumph LSR that goes sorta fast...
#568042 - 10/17/14 2:21 am Re: Mikuni Carb Mount Question [Re: Blown Income]  
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Allan Gill Offline
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Rotherham - S. Yorkshire
There should be a point where you can fill the floor without effecting flow rate, the flow bench will tell you where that would be.

On the latest head it raised the maximum rpm point by at least 500 rpm over stock, just by raising the floor and nothing else.

For the terms which we are talking within I would say that pressure is the same as velocity, as the valve is opened there is less space ( like the teacup effect on a carb slide) that requires filling ( pressure equalising) before the mixture ends up in the cylinder. I would like to believe also that because the flow rate is accelerated it is still drawing a greater amount of mixture before the valve closes. The first thing you need to do jetting wise is to lean it out! Fitting bigger carbs may do that, (but I've found 928's can flow really well) and leaner needle jets and main jets must be used. Even running a tad too rich, the improvement in pull is noticeable.


beerchug
#568059 - 10/17/14 7:05 am Re: Mikuni Carb Mount Question [Re: Allan Gill]  
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Running from demons in WNY
Raising the port floors on my 650 Triumph added 5 HP and raised the power peak about 300 rpm. The engine on the dyno has a flat power curve and it lessened reversion. But at the same time I went from 32MM Amals to 34MM Mikuni flat slides.....Dyno testing showed the engine like short intakes with the changes.
Car guys have been raising port floors for 30 years and modern high output engines have a longer distance from the inside port radius to the valve seat.


650 Triumph modified production LSR record holder 133.1 MPH... Twin 650 engine Triumph LSR that goes sorta fast...
#568087 - 10/17/14 11:34 am Re: Mikuni Carb Mount Question [Re: Blown Income]  
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John Healy Online content
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Joined: Mar 2005
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Boston, Massachusetts
You raise the ports partly because a column of air starts to loose significant velocity when it is moved through a bend of more than 15. This is one reason often see the 30, 45, 60 3 angle valve seat. There is also something that is not well understood, but you do not want the flow to stall as it exits the inlet port. You want it to continue to into the cylinder and not loose forward momentum. You do this by attaching the flow to the inside of the cylinder head and cylinder. Think about how a rocket gets forward motion and just doesn't flounder around.

The volume of the port, and resultant air velocity it can support, is important because with port size and design you are dealing with a timed event. Large ports can move a lot of air, but because air has Mass it can take a time to get it moving and the optimum arrival time in the cylinder could have long past. People doing modern head work are looking to approach sonic speeds .4 to .5 Mach. I am told by people who do this with a Triumph head are lucky/happy to see port velocities of sonic speeds of .025 to .3 Mach.

You are also relying on air velocity to create a pressure wave up and down the intake runner as the intake valve closes, timed so that peak pressure is located just behind the intake valve as it starts flowing air just as the intake valve starts to open. And because of the high velocity continues flowing air into the cylinder long after the piston has started moving back up the cylinder. We want to avoid, "Things at rest want to stay at rest," and benefit from, " Things in motion want to stay in motion."

If you were looking for an bad example of intake and exhaust port design both BSA and Triumph heads would fill the bill. Especially Triumph, as the valve spring galley is situated directly above the ports. Without making a new casting you can only raise the port a 1/4" or so. On the other hand the Norton head is a more modern design with the potential for moving a lot more air. The late Kenny Augustine took a Gold Star head, made a taller casting and raised the exhaust port giving significant increases in performance. He was working on a Triumph head doing the same thing using "D" shaped exhaust ports and was coming up with some pretty good numbers.

For those thinking about improving their bike spend your money on a good multi-angle valve job. If you are thinking about actually changing the shape of the ports themselves you need to start to understand the effect of bore and stroke and related rod length, cam timing as it relates to the available port velocity and rod length, length of the intake port runner and how it relates to the resonate frequency of the exhaust system. relating all this to the gear set ratios available, etc. and be prepared to find and then loose HP only to find it again.

To really be successful you need to spend a lot of time on a dyno. People are enthralled with the performance of the Honda 350 and 450 used in AHRMA racing but forget that Todd Henning spent thousands, yes thousands of hours on the dyno to find it and for the engine to be reliable.




#568133 - 10/17/14 4:47 pm Re: Mikuni Carb Mount Question [Re: Blown Income]  
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Allan Gill Offline
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Rotherham - S. Yorkshire
John you always have an excellent and detailed way of describing this sort of thing.

I quite like the standard A65 spitfire cam for the 650, it doesn't give any great boost of power like some of the Johnson cams might but it is quite workable with different port configurations and because of the narrower gap between inlet and exhaust lobes you can give the cam a few more degrees retard without the risk of clattering valves.

I'd think that the 750 swept volume, using the same cam the ports speeds would be fantastic. I also don't think anything bigger than a 40mm inlet valve will do anything for you.

I tried raising the whole port on one of my A65 heads and just broke into where the spring seat retainer sits. Will require sleeving now.

What I can never understand is how the triumph motors (T120) were quicker than the A65's. Although the A65 seemed to be first to jump with larger porting, the shorter stroke and better combustion chamber design should make the BSA a quicker bike .... In theory


beerchug
#568145 - 10/17/14 6:55 pm Re: Mikuni Carb Mount Question [Re: Blown Income]  
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Ron - in California R.I.P. Offline
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My comments had nothing to do with port size. And FYI, the late Gold Star had a 1 1/2" GP Carb.

Yes you can spend a lot of time and money on the dyno... But, talk to someone that not only has a flow bench, but knows how to use it. Except that the head needs to be off the engine, it is easier and a LOT cheaper to use a flow bench to get this data. Basicaly the head gets flowed sans everything, and an "air straightener" is used. This gives max flow of the intake at your max valve lift. Then add each type of carb mount and repeat the flow test. You want it to be the same (put the air straightener in to the carb mount). Then add the carb and retest.

You will most likely see a drop in flow once the carb is put on. The issue is that carb size is not a measurment of flow. So if you had every brand and type of 34 MM carb, you would find different flow rates of some or many of them. Round slide carbs will differ between AMAL, Mikuni, etc. Then you have smooth bore and flat slides. For LSR you want the flow through the carb to be close to the flow without the carb. Some tuners will say equal, some will say "X" percent less. Then go to the dyno.

But as I originally stated, there are a whole lot of guys using a hose between the carb and the head, and they go pretty darn fast. I was told the step due to the hose is not seen by the engine, due to the velocity. Obviously the porting discussion is a good but a different topic.

Oh yes, I have also found that the overall intake length is critical, small steps can make a big difference. Tuning is a factor of time and money.. I wish I could find a rich widow that likes motorcycles..

Ron

#568268 - 10/18/14 9:31 pm Re: Mikuni Carb Mount Question [Re: Blown Income]  
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Blown Income Online content
BritBike Forum member
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BritBike Forum member

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Edgewater, Md
Thanks for all the input guys. Almost ready to install the head (need to get some valve spring shims and assemble). The head has a 3 angle valve job. I don't have access to a flow bench and the closest machinist that does is months out with taking new work. I think I will get the bike up and running and will be spending some time on the dyno for tuning and make adjustments as needed.


1955 BSA Bantam D1 Plunger
1956 BSA A10RR Street and LSR Bike
1961 BSA C15S
1966 BSA spitfire
1969 Triumph T100C
1970 Triumph TR6R
1972 BSA Lightning LSR Bike
1974 Triumph T150V
#568465 - 10/20/14 3:52 pm Re: Mikuni Carb Mount Question [Re: John Healy]  
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Running from demons in WNY
Here's some info that may apply on extending the distance from the port bend radius and the valve seat.
Of course there no valve or guide in the way but it shows the gain from extending the distance. As a reference a straight pipe flowed 293 CFM. These examples might also apply to exhaust pipes.

This flowed 259 CFM


slight extension flowed 290 CFM




650 Triumph modified production LSR record holder 133.1 MPH... Twin 650 engine Triumph LSR that goes sorta fast...
#568560 - 10/21/14 8:25 am Re: Mikuni Carb Mount Question [Re: Blown Income]  
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Mark Parker Offline
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Bega NSW Australia
Wow that's interesting. All I can think is that the extra length is reducing turbulence, that the length after the turn is settling the flow down around the turn. That's not exactly what I've been doing with my BSA head, though it does make the short turn slightly longer. I've just filled the inside of the turn as the gas will be running on the outside radius and the inside radius is meant to be a dead flow area, that will slow velocity for no gain anywhere. Filling it is definitely effective, it put top end power up a fraction and midrange and bottom end up quite a bit, which I'd put down to higher gas speed. I'm sure the limitation I now have is using valve lift of .410"/10.4mm which is only 23.5% of inlet valve dia (44.5mm)I have a friend who's experimenting with some very special roller rockers with .5" lift on his 914 A65.


mark
#568578 - 10/21/14 11:25 am Re: Mikuni Carb Mount Question [Re: Mark Parker]  
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Pete R - R.I.P. Offline
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You can guarantee that valve lift is a limitation.
Even on a standard "big valve" head, I found it needed somewhere around 1/2" to 9/16" valve lift to be able to flow as much as a 29mm round port can.

#568603 - 10/21/14 4:01 pm Re: Mikuni Carb Mount Question [Re: Blown Income]  
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Allan Gill Offline
Allan Gill  Offline



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Rotherham - S. Yorkshire
I think HB's experiment is very interesting, however it needs to be taken into consideration how this if effective when placed over a cylinder.

The tighter turn radius which is the same as what Mark and myself are doing has a very positive effect, if you take a standard thunderbolt head ( for example) or many of the big port heads which many engineering shops will do has the port more like the straight pipe ( along the ports floor) and has little to no turn radius. Whist the flow down the port may be ok I believe it's ability to fill the cylinder lacking and possible that some mixture is lost down the exhaust.

That said many standard heads ( including both small and large port) A65 heads have a good shape at the outer radius.

Mark, if you ever find the opportunity, it would be interesting to see your results using a standard 30mm head, lightly filled at the port floor and tapered only at the fist 1/2" to take a larger carburettor.


beerchug
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