Hi guys, Fiat x1/9 1500cc running keihin zx750r carbs. Runs okay under throttle, but misfire at idle and off the throttle. Was misfiring at 3000+ revs but blocked air correction jets has temporarily sorted this. It must have been running lean at this point. Also we are getting backfire sometimes after engine turned off. We are running 1.6 main jets. Can anyone shed any light on how to sort this? Thanks

[Edited on 3/9/16 by Marx175]

[Edited on 3/9/16 by Marx175]

Are they balanced
Imbalance causes all sorts of issues
If youve not got a balancer use a length of mig wire as a feeler gauge under each butterfly so that they are equal , then start and adjust idle
I got mine almost perfectly balanced that way

Will have to get them checked but pretty sure they are as the local garage had a quick go at them before.

What colour are the plugs
Is there one a different colour that might indicate an air leak
What fuel pump are you using

Hi, using the motorcycle fuel pump from the same model as crabs and the plugs are all a nice light brown colour.

Main jet only controls top 1/4 throttle, ie: wide open running so don't worry about that until you have the bottom running good.

Is it possible we have issue with lower end jets?

Its an idle jet for low revs then a progression between the two via the shape of the needle
All the way in and then three turns out to start and adjust them equally to get a nice even tickover
Should get a nice smooth idle with them
Is the issue hot and cold ?

Yes hot and cold

I would try idle mixture screws at 2-1/4 turns out drop throttle needles to one notch below top
I had fuel leaking up through closed needles so changed needles and that sorted it but I did have a few sets of carbs so had a few spares

Thanks.