gx390 idling problem

[Jeepracer427]

I have a stock gx390 with a gx270 head on it. I cant get the carb adjusted to idle. Does anyone know what the problem is?

 

[anderkart]

If your running a 390 carb, you might need to jet smallar to better match the smaller valves/ports in that 270 head.

If your running a 270 carb, you'll probably need to jet richer to better match the 390's larger displacement.

 

[Jeepracer427]

do you know by any chance what size jet i should use? Im using the 390 carb

 

[anderkart]

Not realy but I'd think you'd need to go about 25% smaller. Is your spark plug all black and sooty looking? You've got kind of an oddball combonation there no one else has probably ever tried, so your probably just going to have to exsperiment with jet sizes and plug readings to get everything dialed in right.

 

[Jeepracer427]

makes sence. what is the main jet size on the gx390 carb