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ET: Engine (Twin) Big Carburettors


vibrac

Well Known and Active Website User
VOC Member
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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VOC Member
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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VOC Member
VOC Forum Moderator
....... 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

Well Known and Active Website User
VOC Member
Sort of like Rollie’s Lightning being ridden at Bass Lake in 1980 on petrol until the throttle was cracked.
 

Nigel Spaxman

Well Known and Active Website User
VOC Member
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

Well Known and Active Website User
VOC Member
Contary to the known unsaid make, one can really tune a mikuni to an optimum.
At any given throttle position and speed
 

Cyborg

Well Known and Active Website User
VOC Member
It would be interesting to see what the intake tract looks like... what diameter the intake valve is etc etc. and how well it atomizes fuel above 3/4 throttle while below 10,000 rpm.
 

Magnetoman

Well Known and Active Website User
VOC Member
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.
At the risk of going off topic, on a ride through Texas last year my Catalina Gold Star shed its silencer. Once back home I installed a replacement but the bike ran horribly. Making a long story short, with the air cleaner removed the bike ran fine, but a visible cloud of air/fuel pulsed from the inlet at idle. That ejected pulse had traveled over the spray tube on its way back out and drawn additional fuel into the mixture, but enough of the rich mixture escaped into the open air that when the remainder of the cloud was sucked back in (diluted with fresh air), and despite the third pass over the spray tube, the mixture was lean enough (or, not-incredibly-rich enough) that the bike ran OK. But, with the air cleaner attached the over-rich ejected cloud wasn't able to be diluted and the bike ran horribly. Once I discovered the problem I solved it by drilling holes in the baffle of the replacement silencer to reduce the back pressure that was pushing the mixture back out the carburetor during the large overlap of the inlet and exhaust cam.

Since we're already a bit off topic, some of you might be interested in some recent flow bench results. My search for data on spray tubes or flow bench data on Amal carburetors in general turned up nothing so I believe this is the only flow bench data of its type is to be found in print or on the web. It's relevant for anyone thinking of using a 1000-Series Concentric since most are in 2-stroke configuration.
 

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