I've just fitted a single Mikuni VM34 to my 750 Commando, jetted IAW the american NOA notes. I could not get it to run properly at tickover. I noticed that with the throttle stop screw right out, the slide was open at the engine side about 1mm ( 0.040") and the slide was sitting on top of the needle jet. I turned 0.020" off the top of the needle jet to allow for some further adjustment and it is much better. Anybody come across this problem before, ie throttle slide not going down far enough?
Dave Evans
The last 850 I had I conve…
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I've sorted it now. The s…
I've sorted it now. The slide was a 2.5 previously fitted to a twin carb 850 setup. Settings advice in the USNOA notes gives a no. 3 slide for the single carb 750. Bingo, so far so good. I didn't think there would be much difference between a 2.5 and a 3 slide. on a Mk1 Amal the difference in cutaway is about 1mm between 2.5 and 3. On the Mikuni it's 4mm. Whilst the Amal tuning guide gives rough settings for the effect of each aspect of the carb, ie pilot jet, cutaway, needle, needle jet and main jet, there is in fact a large overlap. The cutaway starts at just above tickover with maximum effect at 3/16 to 1/4 throttle but still is having an effect at 1/2 throttle. more details of the overlap at Captain norton's notes, vol 3 sect 1 - Amal Carburetters where you can see a list of figures taken from "The Complete Guide to Motorcycle Repair and Maintenance" By Neil Shultz. A basic book but good for those not weaned on castrol R since birth and looking for some basic knowledge. (in american)
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The last 850 I had I converted to a single Mikuni. It woked pretty well but the bike woudl not rev past about 4,500 revs as the carb could not get enough petrol.
The guy on the dynomometer where we were testing it, suggested fitting vacuum fuel pump.
Otherwise I had no real problems!