1973 cb 750 starts but only 2 and 4 seem to fire

[Bjørn Iversen]

I have been having some issue getting my bike started but it is able to start and stay running.  My issue is that after running, number 1 and 3 exhausts are still cold to the touch.  Doesn't that seem odd seeing as they should be firing with 2 and 4 which seem to be fine?  what can cause this?

[brcree]

  I had the same problem with my cb 450, (dirty fuel bowel), clogged up jets..Check the bowels for crud and clean out.......clean jets as well.....no gas getting to pistons.

[Re-run]

I would have to agree with br. See, 1&4 fire on the same coil/points and 2&3 are the same way. As you have issues with plugs coming from different coils/points, the issue is not likely ignition. Although it could be bad plugs. Easy way to check that is to just swap the plugs and see if the issue moves. If it does, new plugs are in order.

Otherwise, it is carb clean time. Make sure you pull the idle and mains. Also clean the passages well.