34mm Mikunis on Rapide

Nigel Spaxman

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I have been running my bike in and adjusting the carburetors using my seat of the pants dyno. My bike is a 48 Rapide with 8:1 pistons, Terry Prince MKII 105 cams, and a straight through Toga muffler.

The carburetors are set up with these parts
6DH3 needle clip in center slot.
159 P4 needle jet
35 pilot
2.5 slide
170 main jet

Finally it runs really well from idle to red line.

I am really surprised by the small size of the main jet. I think I might possibly be able to use one size smaller than this but one size bigger results in poor running at higher RPM I have two Triumph 750s with Mikunis, one with 32s and the other with 36s and they use 230 and 250 main jets. I am running air cleaners with paper elements, Wassel ones the same type as used on Triumphs (maybe they have too much restriction)

I guess the air cleaners must contribute to the small jet size, maybe the cams too, but I am still surprised. It took me a long time to decide to use this small a jet because I could not believe it was possible. The bike seems to have a lot of power. I have taken it up to 90 mph and it cruises most smoothly at 80.

Does anyone else use a main jet that small?

Nigel
 

Gene Nehring

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Nigel,

Both Robert Watson and Michael Vane-Hunt are running mikuni carbs.

My comet with a 34 and 8:1 is running a 200 main currently and like you I am fettling at the moment.

Best Eugene.
 

Albervin

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34mm equates to about 1 3/8" I think. That is massive for a Rap even with 8:1 and TP cam.
 

greg brillus

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The temptation to use too big a carb is nothing new, hotroders have done it for decades, but you can easily turn a nice engine into a pig. A Vincent is not a high revving engine, so large carbs are a complete waist of time. Lots of "Experts at the track" tell me the 32 mm TT's I'm running are way too small, but I can rev it easily to 6200..............Funny they never seem to overtake me down the straight...........;)
 

Albervin

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The temptation to use too big a carb is nothing new, hotroders have done it for decades, but you can easily turn a nice engine into a pig. A Vincent is not a high revving engine, so large carbs are a complete waist of time. Lots of "Experts at the track" tell me the 32 mm TT's I'm running are way too small, but I can rev it easily to 6200..............Funny they never seem to overtake me down the straight...........;)
Precisely. The key is "slow revving". No big venturi effect required so the big carb essentially drops torque which is a Vincent's big plus.
 

Gene Nehring

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The late John McDougall set up a lot of bikes with VM34 mikuni's. His own black shadow was running 36's. According to all that knew him it was the one of the fastest road going shadow's in North America.

I am no expert and I do fin the Carb set a little bit of a black art.
 

davidd

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Carbs are like tires, pick the one you need for the job. Short roadracing tracks would like a 32mm, while Daytona likes a 36mm. Many 500 singles run 45mm at Bonneville. 4000' elevation and 7000 rpm steady state running requires the carb to process as much air as possible. Smooth running low down is not an issue.

David
 

Bill Thomas

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My 36 mm Dellorto's work well on my Special, I like the proper starting jet, But I have took off the pumps now I am only running 9 to 1 pistons, And Somerton cams with radius followers. Cheers Bill.
 

Nigel Spaxman

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John's bike was fast. I rode it once. He said "be careful it is pointed in a straight line if you open it up". His bike had lots of torque as well. John said "it's a Vincent don't worry it will have plenty of torque". John said that Phil Irving had told him the reason they used such small carburetors is that they were the biggest ones that Amal sold at the time apart from the racing ones. On the Lightnings they did use 1 3/8 carbs which is about 36 mm. Those engines weren't high revving either. My Ducati 900ss has 40 mm carbs. It is higher revving than a Vin but the engine is only 864cc, it has low end power too, but not like a Vincent. I think the main thing is not to mess with the ports. The shape around the valve is already very good and is pretty large so the important thing is just to open the port out to match whatever carb you are installing but only for about 1" into the head. On my bike I opened the port in the head to 30mm. The manifold tapers from 32-30 mm. I left a sudden step in the manifold just after the carburetor. The step is something I believe works well. It is well known that BSA Goldstars had this step. (Goldstars by the way use 1 1/2" carburetors!) I think maybe the step helps the large port to act more like a smaller one. Probably if you never take the engine over 4000 RPM the 28mm carbs are better than the bigger ones.
 
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