Engine Trouble Engine dies at idle once warmed up.

[jmden]

Hello motoredbikes.com! First post here and hope to get some answers.

Have a 66cc China girl with NT carb--Flying Horse kit from Bikeberry. Installed a SBP jackshaft kit which is great! But, it has a bad habit of dying when I let off the throttle once it's warmed up. That's the only thing that's keeping this thing from running well. My idle screw is in all the way, so the slide is about 1/3 the way up from the bottom when looking in where the air filter would go. Either air or fuel issue, I guess. Have a CNC velocity stack on the carb. Intake and exhaust ports are matched. Speed Demon expansion chamber/muffler.

I've got a .080 jet in it as that's where it ran the best. Too much less than that and it's not getting enough gas at WOT and starts to lose power. Tried jets from .060 on up. The bigger I got the better it ran and just rocketed at .080, so stayed there.

Any ideas as to why it's dying at idle once warmed up?

Thanks!

 

[TheBaldKiwi]

Hello motoredbikes.com! First post here and hope to get some answers.

Have a 66cc China girl with NT carb--Flying Horse kit from Bikeberry. Installed a SBP jackshaft kit which is great! But, it has a bad habit of dying when I let off the throttle once it's warmed up. That's the only thing that's keeping this thing from running well. My idle screw is in all the way, so the slide is about 1/3 the way up from the bottom when looking in where the air filter would go. Either air or fuel issue, I guess. Have a CNC velocity stack on the carb. Intake and exhaust ports are matched. Speed Demon expansion chamber/muffler.

I've got a .080 jet in it as that's where it ran the best. Too much less than that and it's not getting enough gas at WOT and starts to lose power. Tried jets from .060 on up. The bigger I got the better it ran and just rocketed at .080, so stayed there.

Any ideas as to why it's dying at idle once warmed up?

Thanks!

Try adjusting your needle setting. I found that adjusting my needle affected my idle quite a bit, with leaner giving a higher idle and richer giving a lower idle. My needle setting is currently the second from the top as it gave the best results.

 

[zippinaround]

sounds to me like your choke is stuck on it can come loose so is always on check it.

 

[gary55]

If it runs great accept for the idle the choke is a unlikely cause. The needle is more likely, but I think he would have thought of that having done so much jet work.
Having to run a #80 jet on a nt is unusually rich. Did it idle very high when it was jetted at around #70? I could be off on this, but my thinking is a small air leak could be corrected with jetting explaining the need for a #80. If the leak were in the intake and corrected through jetting it would most likely run good through it's entire range, but if it were at the crank seal at higher rpm it may run fine while at low rpm case pressure may not be high enough to deliver a good charge and die.
So the needle height if not yet addressed is the starting place. Move the clip all the way to the top and see if this helps.
I still think their is a problem some where causing the need for a #80, but if I'm wrong you might want to run a little hotter plug NGK b6hs
It wouldn't hurt to look for a possible intake leak and a look at the crank seals, especially the one behind the rotor to see if it's pushed out a little. They should be flush to the case.

 

[jmden]

Try adjusting your needle setting. I found that adjusting my needle affected my idle quite a bit, with leaner giving a higher idle and richer giving a lower idle. My needle setting is currently the second from the top as it gave the best results.

Have tried all 4 settings to no avail, but possibly should try it again.

 

[jmden]

sounds to me like your choke is stuck on it can come loose so is always on check it.

No. I've checked that. If I choke it when warm up at all it dies immediately.

 

[crassius]

try it again - every time I see an idle screw all the way in, I always drop the needle leaner and it clears up - on a new motor, it may sometimes need to be set back a bit richer once the rings get a good seat

 

[gary55]

try it again - every time I see an idle screw all the way in, I always drop the needle leaner and it clears up - on a new motor, it may sometimes need to be set back a bit richer once the rings get a good seat

Have you ever seen a new engine need to use a #80 jet with a nt carb. to run right? Sounds like something else besides the needle is off.

 

[crassius]

that is strange, but there are intake & exhaust mods in the mix - #80 was specific to running WOT, so might throw everything off down lower

 

[jmden]

Good points. I'll mess with the c-clip setting some more to double check. It does idle ok for awhile 'til it gets hot, so that makes me wonder if I get an air leak once it warms up, right? But I've taken a spray bottle of water and misted it all over the place and nothing changes--that's when it still hasn't warmed up. Kinda hard to do that when it's warmed up, 'cause it dies. But maybe I should ask someone to hold steady throttle when it's hot and I can spray it. I have been very careful at gasket junctions to make sure both surfaces are flat before joining with the gasket.