hello,
i have what i believe to be a zongshen 250cc engine on my atv.
i have some problems with the running of it:
it will only start on full choke, and when it does the revs roar out and after a few minutes it will backfire out the exhaust and cut out.
if its running, and i reduce it to half choke the revs will remain high but sound a bit smoother. if i then take choke off completely it dies every time.
at one stage, while running on half choke i pushed the trottle in fully, and this actually brought the revs down to what you could almost call a reasonable idle speed.
so really seems like a getting too much fuel issue.
i have took the carb appart, cleaned out the jets with carb cleaner and reassembled. i then replaced all the intake gaskets with silicone gasket. no help.
i followed the instructions on this forum saying how to adjust the carb, turned the mixture screw in fully and then 2 full turns back out, no help. adjusting the idle screw while this is going on makes no difference to the engine speed at all.
heres a picture of the engine:




any insight or ideas of what the problem could be would be greatly appreciated. thanks in advance.

 

choke on running is the idle jet. I know you cleaned the carb but did you verify you could see light thru the idle jet. It's very hard as its a very small opening.

 

HomelessManatee can you tell your exhaust pipe inside and outside diameters, just curious because yours is much bigger than mine. Thanks.

 

thanks for your reply. i did see light through the jet, and really cleaned it loads of times with carb cleaner, so i would be 99% that the jet is clear, it is possible that something got in it while it was going back in the carb or maby from the fuel i suppose.
i can check it again and see, but i think there are more issues here.
i noticed today that there is a slight fuel weep which is i think from around the mixture screw on the bottom of the carb. not sure if that could make a difference. could any valve issue contribute to the problem?

do you want the pipe going back from the engine? or the muffler? i will check tommorow or friday for you.

 

From engine, about 10cm from engine block.

 

had a quick look there, the outside diameter is 32mm, i hadnt time to take the pipe off but i know from the past that the wall of the pipe is definitely no more then 2mm, so inside diameter around 30-31mm.