32 mm carbs on a 750

Drummer99 · Apr 19, 2018

Can I use 32 mm amal carbs off an 850 on a 750 commando that normally runs with 30 mm amals ? Thx Drummer 99

Craig · Apr 19, 2018

Should be no problem as long as the 750cc head intakes are 32 mm as well .... others with more knowledge will soon let you know stay tuned .... .... (edit ) whoops I see you may be saying 30mm intake on the 750cc ..... guess you better stay tuned

oldmikew · Apr 19, 2018

Mine was supplied with 30mm carbs as standard .. it has an rh1 head..fitting 32 mm carbs was an instant performance boost.. you might have to fit larger main jets. Some 850s however were supplied with cut spray tubes - how that would work out am not sure

bill · Apr 19, 2018

these manifolds 062819 should give you the proper transition for 32 mm carbs to 30mm head

ewgoforth · Apr 19, 2018

I believe Dyno Dave's bike equipped with 32 mm carbs and a 30 mm port head made more power on a dyno than anyone elses bike at an INOA rally in the nineties. I don't know if he had the 30 mm intakes or 32 but they weren't the tapered ones you would expect to run. From what he said I think this is something he just through together with what he had laying around, not some secret performance technique.

acotrel · Apr 19, 2018

As long as there is no step in the inlet tract - taper the manifold.

Stephen Hill · Apr 19, 2018

Sometimes a step adds turbulence and can be beneficial. The story goes that when the factory put an oversize carb on a BSA Goldstar, leaving a step in the intake manifold produced more power than tapering the manifold.
If you already have the carbs, try it and see how it goes.

Stephen Hill

SteveA · Apr 20, 2018

Search this site, you will find stuff......not really an innovation...and no real need to run the whole discussion again...you could even bore the 32s to 33mm and find lots of folks had done it before you on a 750, like the works Nortons! Look at some of Jim Comstock's flow work on a thread on here (Comnoz), he showed the effects of different manifold designs, steps and all....

Nortoniggy · Apr 20, 2018

My original Combat 32mm ported head self destructed when one of the exhaust valve seats came out and pounded the alloy of the combustion chamber. I managed to obtain a replacement older head with 30mm ports. I didn't do anything to the inlet ports other than clean them up as I'd read about the 30mm ports being a better shape than the Combat ones. I just bought 30mm heat insulating washers and tapered them so they were 32mm one side to match the inlet manifolds. The bike runs well and pulls strongly.

32 mm carbs on a 750

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