Misc: Carburettors Big Carburettors

vibrac

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I have never been a fan of huge carbs perhaps its because the rules kept the pre 49 to TT carbs and that meant 1-3/16 at best I have opened a road bike to 32 and would goto 34 on a standard head with a Mikuni after that I think you risk making a hole in the head to the spring housing (but I have never tried it)
What prompts my thread is that last weekend at a CRMC meeting with Bens flying bricks we were camped next to a guy with a NOR Vel, and his 500 Velo push rod engine had the most enormous Mikuni. On discussion he said its 44mm thats almost 1-3/4 in real money! it was mounted very close to the head and had no belmouth (in fact all the singles on the minivation racing team in the nex bay had no belmouths on their Mikunis either)
I was a bit taken aback at the size but then I was even more perplexed as he started it up on the rollers and warmed it up not sitting on and going Blip...blar in the time honoured fashion, oh no! once running he popped it in neutral on the stand and let it tick over unattended, sounding for all the world like one of those stationary engines at a steam fair where you can hear every stroke, then after 5 to 6 miniutes he jumped on and rode off to the start as smooth as silk.
I think modern carburettors like Mikuni may have changed the game while I was not paying attention.....
 

bmetcalf

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Ducati tuner Reno Leoni is supposed to have been running 40mm Del Ortos on racing 250 Ducs in the '70's, but they probably didn't idle.
 

BigEd

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....... I was a bit taken aback at the size but then I was even more perplexed as he started it up on the rollers and warmed it up not sitting on and going Blip...blar in the time honoured fashion, oh no! once running he popped it in neutral on the stand and let it tick over unattended, sounding for all the world like one of those stationary engines at a steam fair where you can hear every stroke, then after 5 to 6 miniutes he jumped on and rode off to the start as smooth as silk.
I think modern carburettors like Mikuni may have changed the game while I was not paying attention.....
Did you notice what ignition system was being used? Something with a good adv/retard would make a difference to how well it ticked over.
 

bmetcalf

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Sort of like Rollie’s Lightning being ridden at Bass Lake in 1980 on petrol until the throttle was cracked.
 

Nigel Spaxman

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I agree with Magnetoman. For idling the carb size makes almost no difference. When the slide is closed the small bore carb and the big one are the same. I have a DBD34GS Goldstar (so does Magnetoman) They have a 1 1/2" GP carb. If you remove the bell mouth the gas sprays out onto the ground. I think that is mainly because of the very late closing of the intake valve, not because of the carburetor. The GP idles well as long as you add a screw to hold the slide up to fix the idle speed. Ducati 500 Pantahs had 40 mm carbs.
 

Vincent Brake

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Contary to the known unsaid make, one can really tune a mikuni to an optimum.
At any given throttle position and speed
 
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