The magazine for classic Japanese motorcycles

Q&A: Please help!

The carbs are VB52C jetted Primary 68 Secondary 75. I tried a set of VB42A jetted 68/102 but it ran terribly. I re-jetted them with the original jets from the VB52C. It runs better but still spits with the odd backfire and won’t idle below 1700rpm. New coils made no difference. It has the standard airbox with a new filter and the compression on all four is between 135 and 150psi. I have been told that the engine is not original so maybe it has the wrong carbs anyway. As a long time subscriber to your magazine I am pleading for help.

Pat Lackey
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A: The main jets you had fitted to the VB42 carbs, i.e. 68 primary and 102 secondary are correct. The fact that the plugs are actually wet when you take them out shows that the relevant cylinders cannot have been firing. The spitting and backfiring is often a sign that the mixture is too weak, but I think the problem here is probably ignition-related. If you could get the Colortune fitted you would see what was going on in the cylinders, in terms of weakness or intermittent firing, and this would give you a clue about the cause of the problem. Otherwise, just make sure
the points are correctly adjusted and clean, and that the HT leads are in
good condition.