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Race Bike Carbs... Questions #487862 04/29/13 4:44 am
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pushrod tom Offline OP
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Mike and I headed home to NJ early today (bad weather at Willmington)and decided to stop at the AMA Museum. We had a chance to get a close up look at some genuine machines from their glory days. While examining the Dick Hammer 1967 Daytona bike I got Mike to use his fancy telephone flashlight to check out the GP carbs. They said 1 5/16". So, if that is a 500 and Mr. Hele put those carbs on for Daytona, how come I'm using 1 5/32 GP's on my 650? Does anyone have any direct info about these matters?
The 1 5/32's have worked pretty good for me as have the Mikuni 34mm TM's. Maybe, for what I'm doing, bigger is better? Maybe a certain Mr. Parker is correct in his reports? Hmmmm....perhaps! Comments are invited. Cheers, PRT

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Re: Race Bike Carbs... Questions [Re: pushrod tom] #487900 04/29/13 1:27 pm
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John Healy Offline
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There is a concept which seldom is discussed by Real men called ride ability. The horsepower we all talk about is a function of Torque and RPM divided by a number calculated by wizards. Some thing like Torque x RPM / 5252.

So think about this. In the scheme of things, that little 500 racer you were looking at makes very little Torque so to get that 50 hp Doug Hele was looking for what did he have to do? Scream the guts out of that little engine where 9,000 was typical and 10,000 rpm wasn't unheard of. Think about that...

A while ago I made the statement that the bigger Triumph made the ports and valves the slower the Bonneville became. Put a 1963-1964 Bonneville head, with smaller carbs, smaller ports and smaller valves and the bike became what its predecessors were: a 110mph motorcycle. Remember the story of the T110's name came about.

Here is a PM i got from Alan Gill only days before he had his posts removed.

"John
Just wanted to pass on a thank you for mentioning to me (possibly over 12months ago)that the smaller port heads went better - while at the time I was considering making carbs and ports much larger.

Even though the bike is (was, last time I rode it) far from its correct jetting, its a real buzz to ride now and has better low down torque than anything I have ridden to date - including 900cc yamaha twins. It really does leave me having to check the tacho, as im dropping into sharp corners at no great speed and the bike pulls as though I dropped 2 gears - the revs are low, and I feel like I might be lugging the motor - although it doesn't pull like its lugging.

Thanks again."

There is such a thing as "ride ability", and if you plan to increase the carbs, ports and valves with the resultant decrease in Torque you better consider what you plan to do with the motorcycle. Not many of us get to pass a Harley on our Daytona at 9,000 rpm at 130 mph on the Daytona banking. Ride that same Daytona around town between 2,500 and 5,000 rpm and you would be bored to tears.
John


Re: Race Bike Carbs... Questions [Re: pushrod tom] #487924 04/29/13 5:53 pm
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the 1966,7 and 8 BSA and Triumph 500 twins used 1 3/16 GP2s

Re: Race Bike Carbs... Questions [Re: pushrod tom] #487939 04/29/13 7:19 pm
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John Healy Offline
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Those pesky American distributors couldn't leave anything alone. clap


Re: Race Bike Carbs... Questions [Re: pushrod tom] #487946 04/29/13 8:00 pm
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Keep in mind that the bike in question is a purpose built land speed racing engine that will be running at high rpm and WOT with its sole purpose, punching through the air at the highest rate of speed possible.

Last edited by M. Goni; 04/29/13 8:04 pm.
Re: Race Bike Carbs... Questions [Re: pushrod tom] #487960 04/29/13 8:52 pm
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John Healy Offline
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While you and I know that the bike at the AMA is a purpose built machine, people reading this can, and will assume, that if the factory used these carburetors they could also. Even if their bike is a commuter or tourer. There is a disconnect with many people who read these posts assuming what was good for Dick Hammer at Daytona will be good for them. I am not so sure it would be for a Bonneville Record attempt - Just how much torque are you developing at that High RPM with those carburetors. Bonneville isn't Daytona where you are almost below sea level.

That is what Allan experienced. Over a period of several years on this web site he got caught up with the bigger is better crowd, not realizing that for his purpose it wasn't.


Re: Race Bike Carbs... Questions [Re: John Healy] #487973 04/29/13 9:51 pm
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pushrod tom Offline OP
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Thanks for all the feedback so far. Mike posted a couple of videos on the Ohio LSR thread. Max RW torque is 44.8 at 5,500-6,000. Peak hp is at about 6,800. He's not letting that thing fall off the curve once he gets the go sign! We were just starting to think about the next evolutionary move with the carb thing. If we go big it will be next season. PRT

Re: Race Bike Carbs... Questions [Re: pushrod tom] #487974 04/29/13 10:09 pm
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Thank you John! Oh and to add, we are racing the east coast LSR tracks so elevation will be from sea level to 1100 feet above sea level.

Last edited by M. Goni; 04/29/13 10:49 pm.
Re: Race Bike Carbs... Questions [Re: pushrod tom] #488214 05/01/13 12:00 pm
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The only way to know for sure is to try out bigger carbs....


79 T140D, 96 900M Ducati ,2001 Sportster....On a bike you can out run the demons..
Re: Race Bike Carbs... Questions [Re: John Healy] #488702 05/04/13 1:10 pm
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Allan Gill Offline
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Originally Posted by John Healy
While you and I know that the bike at the AMA is a purpose built machine, people reading this can, and will assume, that if the factory used these carburetors they could also. Even if their bike is a commuter or tourer. There is a disconnect with many people who read these posts assuming what was good for Dick Hammer at Daytona will be good for them. I am not so sure it would be for a Bonneville Record attempt - Just how much torque are you developing at that High RPM with those carburetors. Bonneville isn't Daytona where you are almost below sea level.

That is what Allan experienced. Over a period of several years on this web site he got caught up with the bigger is better crowd, not realizing that for his purpose it wasn't.



Cheers John, and thankyou again for helping me look towards the light.

to add to that (PRT has heard this before, so I appolgies for regurgitating this again)

I have 2 heads, one which was in nice nic which I had modified a little and gave an almost parallel port of 30mm x 30mm down to the bend near the guide (A65 heads have a tight spot in them in the inner section) in general with this the bike pulled strong and accelerated well. Max rpm was 8000 in second (7k with air filters) but topped out at 6k in top gear.

The doner head, which had its own underlying issues was of a different casting, it had a fractionally smaller port from the start of 29mm, it received the same flow treatment as per head one, but then had the ports reduced, this port became tapered and had a port height of 20mm just before the guide. The shape isn't a perfect elipse as the floor is flat bottomed so giving an exact area before the valve isn't easy.

The mixture became too rich and a drop in the main jet and needle position was required, Max rpm became 7200 and I could achieve 7000 in 3rd, being a road bike I didn't get the luxury of changing into top gear before having to slow down, but max speed was already better. If I could reach those rpms in top id be a very happy bunny indeed.

Whilst I was in belgium we had a few rides, (the head which I had hoped to cure the wierd leak had returned pee-ing out as bad as it always did and compression on the left pot had reduced - I didn't calculate it, but on the kick you knew which piston was on compression and which was not) I was maintaining speed between 30-50 mph, a nice bimble pace, and changing into 4th was doable from about 25mph (comfortably) after 75 miles of riding, i brimmed the tank and still only used less than 4 litres - it averaged out at 83mpg at those speeds.

acceleration had also improved, and made the bike real fun to ride.

I'll be doing a similar mod to the other head, I will be using bigger valves, which may make zero difference, and ill be using a port height of 22mm instead of 20mm, in the quest of trying to achieve a higher rpm. This will still be a tighter port than what the Lightning clubman 465(466) heads were using. I will loose a little off the very bottom, but it shouldn't be too much to notice.

I think a lot of this is black magic, and although there is calculations which will give you X sized port for Y rpm, it doesn't tell you what you will loose in the middle, and regless of throttle position, if your gear changing isn't hitting Q rpm to maintain torque, you won't be making any great shakes fast (talking about a stock 30mm port)

just my 2c


beerchug
Re: Race Bike Carbs... Questions [Re: Hillbilly bike] #488879 05/05/13 7:07 pm
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[
quote=Hillbilly bike] The only way to know for sure is to try out bigger carbs.... [/quote]


Bingo , spot on. went to the dyno with 4 sets of carbs and got the ex gas spot on with each and did runs. was shocked to see the 40mm mkIIs made the best hp and all around best curve. this was on my 750 road racer. even tried some that were not within the rules but when they didnt cut it, I was pleased to see the ones that worked the best were ;-)


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Re: Race Bike Carbs... Questions [Re: pushrod tom] #488882 05/05/13 7:31 pm
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John Healy Offline
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Quote
I was pleased to see the ones that worked the best were ;-)


That's a no brainer... the ones on the bike.

What would interest me is port shape and diameter. Time is a factor and you need to be able to get flow started as quick as possible and keep the air velocity at a the point you get maximum fill. No good to have the intake closing before the air flow gets up to speed.

Then you have to match the power curve to the available gear ratios. You want the most torque under the curve for the gear ratios that are available. No good to have a lot of horse power in a narrow rpm band when the gear ratios drop you hundreds, or thousands, of rpm's below it on each shift.

But the 40mm MKII's aren't 40mm. They are 39mm.

Illegal carbs: Smooth Bores / GP's are not going to do anything for your type of racing! If they were only legal, flats slides would be worth a try.



Re: Race Bike Carbs... Questions [Re: pushrod tom] #489168 05/07/13 3:19 pm
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To: John Healy.

John, while reading these posts about carb sizes, I must ask:
What was the purpose Triumph changed to the bigger carbs in 1966
when the smaller ones performed better?

Was it just a "bragging rights" marketing ploy?

Re: Race Bike Carbs... Questions [Re: pushrod tom] #489180 05/07/13 4:53 pm
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John Healy Offline
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Irish: Did you ever wonder what the Triumph guys were thinking when they saw BSA with the decal on the gas tank - Big Valve Super Rocket? This is America - bigger is better!

The bigger Triumph valves, ports and carburetors got, the slower the bikes became. We had 1970 Bonnevilles you couldn't get over 85 mph unless you put an aluminum spacer between the head and carburetor tapered down to 28 mm.

What good is it to have 50 plus horse power if you have to keep the bike over 5,000 rpm to get it. The gear spacing on the 4 speed just won't support this narrow a power band and have any real performance. He!!, I can't convince people to stop shifting at 3,500 rpm, but they read these posts and have to have those carburetors the Big Boys have.

It wasn't until they increased the bore for the 750 to where there was enough swept volume to get the air velocity back to where it should have been. In fact if you look at a 750 inlet port there is a small restriction just as the air enters the port. The 5 speed does help the 750 in this regard to be a more useable motorcycle.

While Tim is using Big Carburetors, he has a lot of skill and big balls. Because he is able to maintain an awful lot of corner speed, he is able to keep the bike in a narrow power band. A lessor rider, or a rider with a different style, would probably find his bike a dog and difficult, if not impossible, to ride...

For example, Jerry Wood who I tuned for, in an effort to keep corner speed at the maximum would slide the front tire as he was leaned over and braked coming into the corner. Because you need immediate, but smooth, throttle response to control the bike doing this to keep from high siding, throttle response always took preference over absolute hp. If you ever watched him in the infield at Daytona you would understand what keeping up corner speed can do.

Carburetor, valve, port size and shape, etc. should be thought of as just a couple of factors to consider when looking for performance. But first you must define what performance means to what you are going to do with the motorcycle. Triumph was looking at sales and what the competition was doing. Of course the bikes put out for road test were heavily massaged before the Press could get there hands on them and know the truth.


Re: Race Bike Carbs... Questions [Re: John Healy] #489189 05/07/13 6:21 pm
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I been talking to guys racing at Bonneville and larger than "normal" carburetors are often there and other LSR events. Same for drag racing where the engine is well above peak torque all the time.Higher RPM's also needs more carburetor.
On street auto engines carburetor size can be figured out with a cheap vacuum gauge hooked up to the intake manifold.Run along in second gear at about 3000 rpm and then open the throttle wide..The vacuum will drop to zero and then slowly climb up as the engine nears peak power.If the gauge reads above 3-4 inches you can probably use more carburetor.Of course this is a very generalized statement....
I have no idea if this will work on a Brit Bike,maybe John has done it?


79 T140D, 96 900M Ducati ,2001 Sportster....On a bike you can out run the demons..
Re: Race Bike Carbs... Questions [Re: pushrod tom] #489201 05/07/13 7:41 pm
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You would be far better getting something like a 928 carb and oval boring it. The bigger you go, you loose one hell of a lot mid-range with a typical carb. Big valves are fine if they let the mixture past the valve and fill the chamber- as opposed to the mixture trying to slide accross the chamber wall. I think unless you have a cam with a high enough lift - this is what it is going to do unless you have a cam with decent lift.

The speed of air through a carb will also not increase (much if any) just by going bigger.

IMO, your much better going for torque ( which will give good acceleration) and then gear the bike up, if you can do 7 grand in each gear, go higher until top gear starts tailing off. Slow riding might suffer a tad but not as much as trying to port the bike to rev at a harmful rpm.

Each to their own, but I found shrinking the ports gave more power, more torque and harder acceleration in any gear from any speed than a stock setup. I also found the last bit of throttle movement made no difference.

More air must be drawn through the carb meaning a smaller jet will pass more fuel than a bigger jet on a bigger carb. ( the number on the jet is the maximum it can flow.

When my bike is built and tuned up, I'm going to try filling the slide base in the attempt to improve flow (mk1 Concentric).

2c


beerchug
Re: Race Bike Carbs... Questions [Re: pushrod tom] #489217 05/07/13 8:38 pm
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John Healy Offline
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Quote
Big valves are fine if they let the mixture past the valve and fill the chamber- as opposed to the mixture trying to slide accross the chamber wall.


You aught to explore the concept of attaching the flow to the "chamber wall." Think of a rocket. Without the cone outlet from the tube, the flow just comes out and literally stops moving as it disperses into the atmosphere. Put a simple cone on the end of the tube and flow attaches to the sides of the cone giving it direction and flow. If the rocket didn't have the cone there would be little forward movement. It would just meander about.

This one of the reasons people who work the area around the valve seat find so much power. And from what I have seen it isn't intuitive.


Re: Race Bike Carbs... Questions [Re: pushrod tom] #489223 05/07/13 9:53 pm
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So over sized valves ARE a good thing? Now I am confused. I can see with the Rocket theory, the output is forced (like the exhaust valve).

I do remember Mark Parker mentioning that he had an improvement (in the midrange?) just by fitting (Jaguar) bigger valves.


beerchug
Re: Race Bike Carbs... Questions [Re: pushrod tom] #489230 05/07/13 11:19 pm
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John Healy Offline
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Quote
I do remember Mark Parker mentioning that he had an improvement (in the midrange?) just by fitting (Jaguar) bigger valves.


Fair enough, but you cannot look at these things as an abstraction. The end result is the sum of the parts. This is why you need to make one change at a time. Then check the results on a dyno and verify on the track (street).

A tuner could make ten changes and get fantastic results. He can share all, but one of them with you (that is if he understands what he did) and you be no farther ahead than when you started. Or, the larger valves might have corrected something else he did, and that he or you are not aware of.

There is a lot of science available, and some basic tricks, but in the end a lot this is about hard work. We have a local racer who is responsible for the Honda 350-450 road race revolution here in the states. His name is Todd Henning. Todd turned the head of a lot of the UK Gold Star and Matchless guys in our AHRMA classic racing at Daytona with a 450 Honda. He spent thousands of hours running the bike on a dyno making subtle changes and recording the results until he could beat anyones G50 or GS. You can say he was obsessed! It was too bad that he fell off in California and suffered a pretty bad brain injury.

In the end they were taking 450 heads and welding them up solid and starting from scratch with port and combustion chamber design. They had several of these. The welding alone cost $1000 20 years ago. Then it was almost a day in a multi axis CNC machine. They would make one change. Take pile of the ancillary parts and run the bike on the dyno. They had various exhaust diameter pipes they could vary in length and change the megaphone for various lengths and tapers. Try different carbs from smallest to largest with each exhaust set-up. Vary the inlet track length. etc., etc., until exhaustion...



Re: Race Bike Carbs... Questions [Re: John Healy] #489249 05/08/13 1:00 am
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pushrod tom Offline OP
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John, Let's hear a hearty Amen to your statements. I will not claim to be the brightest bulb but I have been working on this for about 6 years and have made progress but don't feel that I am near the end of the rainbow. It is definitely the total package that counts and knowing what the specific mission you are trying to achieve is a good place to start. Example: For me, it is trying to achieve the greatest speed in a standing mile within the class specs. PRT

Re: Race Bike Carbs... Questions [Re: John Healy] #489406 05/08/13 10:40 pm
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Originally Posted by John Healy
Irish: Did you ever wonder what the Triumph guys were thinking when they saw BSA with the decal on the gas tank - Big Valve Super Rocket? This is America - bigger is better!

The bigger Triumph valves, ports and carburetors got, the slower the bikes became. We had 1970 Bonnevilles you couldn't get over 85 mph unless you put an aluminum spacer between the head and carburetor tapered down to 28 mm.

What good is it to have 50 plus horse power if you have to keep the bike over 5,000 rpm to get it. The gear spacing on the 4 speed just won't support this narrow a power band and have any real performance. He!!, I can't convince people to stop shifting at 3,500 rpm, but they read these posts and have to have those carburetors the Big Boys have.

It wasn't until they increased the bore for the 750 to where there was enough swept volume to get the air velocity back to where it should have been. In fact if you look at a 750 inlet port there is a small restriction just as the air enters the port. The 5 speed does help the 750 in this regard to be a more useable motorcycle.

While Tim is using Big Carburetors, he has a lot of skill and big balls. Because he is able to maintain an awful lot of corner speed, he is able to keep the bike in a narrow power band. A lessor rider, or a rider with a different style, would probably find his bike a dog and difficult, if not impossible, to ride...

For example, Jerry Wood who I tuned for, in an effort to keep corner speed at the maximum would slide the front tire as he was leaned over and braked coming into the corner. Because you need immediate, but smooth, throttle response to control the bike doing this to keep from high siding, throttle response always took preference over absolute hp. If you ever watched him in the infield at Daytona you would understand what keeping up corner speed can do.

Carburetor, valve, port size and shape, etc. should be thought of as just a couple of factors to consider when looking for performance. But first you must define what performance means to what you are going to do with the motorcycle. Triumph was looking at sales and what the competition was doing. Of course the bikes put out for road test were heavily massaged before the Press could get there hands on them and know the truth.
well thanks john but, I too would think it wouldnt carburate good, but its so good my mother could ride it and it idles better then my streets bikes ever did. [***] luck maybe. :-)


Tim Joyce
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Re: Race Bike Carbs... Questions [Re: pushrod tom] #490685 05/19/13 7:45 pm
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Hi Timmy..! Yes, your bikes are very well tuned indeed...!! And as I am still learnig, the key to good carburetion is not running AMAL's.. There are not enough adjustments.. My bike now will pull 3/4 throttle cleanly in top gear from an idle on the dyno.! What a difference a decent fuel curve makes, and that is with a 40MM carb and an open megaphone. Back to the garage, the bike is supposed to ship out next month to the IOM..

If you ever decide to race out west, I got a bike you could throw a leg over.. that would be an honor.!
Cheers.!
Ron smile

Re: Race Bike Carbs... Questions [Re: pushrod tom] #493964 06/13/13 1:45 pm
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Using the same basic head on a 650 750 and 883 with 38mm carbs and 44.5 and 39mm valves I've found they can all run with a sweet power curve dependant on the exhaust, the smaller motors rev more, and are sweeter with smaller dia headers. I think the 750 with 79.5 x 74mm bore and stroke with std cam profile has a lot of potential, with bigger header pipe dia it can have lots of top end once past 5,500rpm (I think this would suit Tom, because a LSR run could stay above that rpm. But then it's not just the pipes it's the combination, the pipes wouldn't do the same thing on a std head or on a motor fitted with a hotter cam. The bigger motor can run the bigger pipes and have a nice at any rpm power curve, but it still has a bit of a step at 4,500 which is nice. The bigger motor 883 with std cam was getting peak power at around 6,000 putting offset rocker buttons in it raised that peak up around 7,000. More lift again would possibly raise the rpm and HP, neither of which I need on the road, and take away from how nice it pulls at lower rpm.


mark
Re: Race Bike Carbs... Questions [Re: pushrod tom] #494644 06/19/13 11:24 am
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Back to Tom's quest for speed. A question for Tom; at what RPM does the Hornet make max HP? Is it 7,000-7,500? See if this is bad reasoning? If you were running a Trident for top speed where would you aim for with your max HP rpm? I would be aiming for 8,500 or higher because I know they can make good power there, plus by peaking high up the range HP will be more and speed should be higher? The A65 isn't a long stroke like a Triumph twin it's 74mm stroke is closer to that of the Trident, its the same as the bevel drive Ducati 750 - 860 twin so why not tune for max power at 8500 or so? Then there should be more power and more speed.
I'm sure you can do that using the std cam, just by changing the head, carbs and header pipes. The Ducati 750 beat works triples by using 40mm carbs and tuning for power at a similar RPM to the triple.
I'd love to see what a big valve head like I use with 38mm carbs TMs or VMs and a set of big dia header pipes and X connector(I have in the shed trying to rust)would do on the Hornet. What HP they might make on the dyno and speed it might do. I could fix up a head and send it and pipes if you wanted to try it out, I don't have spare carbs though. I think it would be a fun interesting thing to do. The zoom with this set up on a big bore 750 started around 5,500RPM so on a 650 it may be a bit higher.


mark
Re: Race Bike Carbs... Questions [Re: pushrod tom] #494975 06/22/13 1:32 am
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Alex Offline
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Offline
BritBike Forum member
Joined: Sep 2002
Posts: 7,728
Dang...i love these discussions, but you guys are killing me. Man I miss racing.


A smattering:
'53 Gold Flash
'67 Royal Star
'71 Rickman Metisse
'40 Silver Star
'37 Rudge Special
sixtyseventy Lightboltrocket road racer...and many more.
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