YS 91 Problem
 

Just got my YS91 back from the shop. Had the cylinder replaced after braking off the muffler in a crash.

Installed it and turned the needle valve out 2 1/2 turns. Started the engine and then turned in the needle quite a bit. Got it running well... then I heard a loud CRACK and fuel came out from under the cowling and the engine stalled. I had turned in the needle 1 1/2 turns (so it was now 1 turn out).

Upon inspection, I noticed fuel coming out from one of the push rod covers.

What happened? What should I do?

How many turns on the needle valve should I have started with?

YS 91 Problem
 

I am by no means a YS guru, but I am running two of them; a 91FZ and 63FZ.

When they stop running due to a lean condition, they make a HORRIBLE noise- sounds like something broke big-time. I have no clue what this is, but it is very disconcerting; particulary on the 63.

Still, they fire right back up and run fine at a richer setting. As to the fuel leak, that doesn't sound good. My 63 blew it's rear cover gaskets, but Dave replaced them.

One thing I have noticed is that you cannot go by the recommended needle settings all the time. My pal's 63 is happy at just a brunette over one turn out on the main needle. Mine won't run for long at less than 2.5 turns. He has a 91 too, and both of them like 1.5 turns with the APC 16x4W. He is running a 20% nitro home brew, and I'm using Cool Power 15%. I think they (YS engines) are happier with higher nitro, but I'm satisfied with what I'm getting on 15%.

Moral of the story; these are finicky engines, and very individualistic. I make that observation after many years or running model engines, two and four stroke, in airplanes, helicopters, and cars/trucks.

Once you find the right combo with a YS four stroke, nothing runs better or stronger. But getting there can be aggravating. You just need a bit of patience.

Still, if your engine is still leaking, just send it back to Performance Specialties. It will NOT run right with a leak, of any sort.

Steve