Hi Here's a problem I am unable to solve with my son's YS FZ110. After a crash last fall (no aparent damage to engine) and sitting thru winter and spring, engine runs at too-high speed at idle, even with throttle barrel ENTIRELY closed. As throttle is advanced above idle, rpm drops and engine stumbles. With patience, throttle can be moved to full-throttle and engine awakes to full-throated roar. Going back to idle initially produces idle rpm. But soon speeds up and back to before. Low-speed adj has no effect even if completely closed. Hi-speed needle affects high speed normally. Pressure regulator screw has no aparent effect. Fuel (with lotsa bubbles) can be seen going to carb with throttle at idle (to be expected because rpm is probably 3500 and regulator is opening). New CoolPower 30% heli fuel; OS 4-stroke plug.

I have replaced the diaphragm-the old one was stretched (BTW, does stretched d affect running?).

Seems to me that threre must be an air leak, preventing throttle from choking down the air supply thru the carb to slow the engine. But I can't find a leak around the carb, manifold, air box, or valve cover. We are still using the original, thin YS paper gaskets.

Am I on the right track with my diagnosis? Or is there something else that might be causing the problem? What should I do next?

BTW, I have read online YS info and believe that I understand YS 4-stroke operation. Except one thing. At idle, pressure system and regulator are not supposed to be supplying fuel to carb. Engine is running on suction, not pressure. There should be no fuel going thru fuel line from regulator to carb. Ok, so how does fuel get to the carb during idle???

Thanks for the help. Regards. -Gary

check the push rod tube o-rings and the valve cover gasket to make sure they are sealing and also check the fual line from the regulator to the carb fo a nick.

Also the LS needle is an airbleed adjustment so closing it all the way richens the idle, although whay you describe is a lean condition at idle.

As for the regulator and fuel flow at idle, the regulator is not like a gate valve, it allows a little fuel to get past at idle.

Also I wonder if there's any chance of damage to the fuel line inside the tank?

It sounds like a typical air leak somewhere in the engine. I would replace all the gaskets and O-rings with new ones and give her another try. Re-using old gaskets often leads to air leaks that can be hard to track down.
Paul

Thanks Petec and gaRCfield Close inspection now shows one push-rod cover not centered in upper O-ring. Probably slightly bent in crash last fall. I'll bet this is the prob! I missed seeing this until your suggestions. I'll check it out later today. Regards. -Gary

I just want to re-iterate what 42etus wrote about reusing gaskets. I tried reusing the gasket under the valve cover and engine didn't run right due to small leak that was difficult to see. Since these engines are supercharged, any air leak is detrimental to the performance of the engine.

Good news is that you can get gasket material from the auto parts store and make some of the simpler ones, like the one under the valve cover. Very effective and easy to do.

Glad to help.

One thing that does help with the valve cover is to lap it flat. I use 400 grit and a plate of glass on mine.

Hi Y'all- Success! And I learned something too. Ends of push-rod tubes have rather large radiused surfaces that contact O-ring seals, top and botom. Radius is too large for tube ends to fit nicely into O-rings. Tube ends can be pushed aside, leaving a gap along an edge. Aparently that's what happened in my son's crash last fall. Irony is that I could have re-centered the one push rod end with my thumb and saved myself a lot of putzing around, such as with the regulator diaphragm!

Runs almost right now. But I am not out of the woods yet. Won't idle down to 2000 rpm, even with throttle barrel entirely closed. About 2800 is minimum. Also won't stay idling and low-speed adj does not have as much effect as it should. Must still have an air leak somewhere. I have ordered new gasket set and O-rings.

IMO, YSs are Great! when they are running right. And Not! when they're Not!!!

Thanks for your help.

Regards. -Gary