OS 91SII starting Problem
 

I have an OS 91SII 4 stroke engine. The engine or tank has not been changed since the last flight. It has always started very easy and zero problems with it. Same fuel, changed glow plug, but no difference, checked gap on valves and they are fine.

Problem: Remove the fuel line and fuel flows through the line as it should. If you choke it and get fuel into the carb, then it fires and quickly runs the prime through and stops. Without choking, it is more difficult to fire. While it is running on the prime, if I advance the throttle, just slows down and quits. It just seems like a fuel flow problem, but can not find the problem.

Any ideas or help?

RE: OS 91SII starting Problem
 

Sounds lean. But, without more details, its only a guess. You say "engine or tank have not been changed since last flight" I ask, "when was this last flight? July, Aug., Sept. The temps & conditions from those days are different then conditions here in December. The needles need to be richen'd (opened). If it was in December that your last flight was, then it still sounds lean, but might be dirt or something plugging, or blocking the fuel flow into the engine from the spray bar. You also say, "remove the fuel line & fuel flows through the line as it should" What is being done here? Fuel line just removed & nothing else, but fuel flows from the line? Is the vent line for the tank hooked to the muffler for pressure? Are you spinning the engine over with a starter & is that "when it flows as it should"? Just hand flipping it over? holding finger over exhaust ? Again, this sounds like it is lean, or not getting any, or proper fuel. Any & all things that can restrict fuel flow, or allow extra air, "vacuum leaks" will cause this kind of a condition, "runs on prime, & then stops". Check to make sure all gaskets & O-rings are in good shape & that all the bolts are tight. If all this looks good, just open the needle a turn & see how it reacts to that change.

RE: OS 91SII starting Problem
 

Using electric started to spin motor. I have opened up the needle value, but no difference. I flew this plane about one month ago. I usually do not have to change needle valve more than two or three time a year and that might only be a click or two. This motor has been that solid. I pulled motor and took at look at o rings and made sure bolts were tight and they were. I agree it seems like it is lean, but only prime fuel would make it seem that way. Pulling tank and taking another look. It is something I am overlooking

You would think that the spinning of the prop with the electric started would draw fuel up. That being said, I am not sure it is.

Thanks for the information

RE: OS 91SII starting Problem
 

Pull the high speed needle and with the fuel line to the carb connected, gently blow into the vent line until you see fuel come ot of the hole the needle goes in. Replace the high speed and set it open to about 2.5 to 3 turns. Remove the idle jet and again blow through the vent until fuel escapse there.Reset it to factory starting point, I"m not sure what the settings are for the 91 SII engine. This will tell you that your fuel is moving ok, and will flush most crud out of the carb. The engine should start and run, but will need a full carb tune now.

Check for a cracked or pin hole in the supply line also. Usually though the engine will start and run, but not run well with a fuel line issue. Don't forget to check the clunk line. A while back, my engine would start, but die by the time I carried the plane out to the flight line. Cart it back and get it running again and it died again. Then it wouldn't start. The clunk line had broken clean off the brass tube. All I was getting for fuel was what splashed up onto the brass tube.

Don

RE: OS 91SII starting Problem
 

Don

Tank and clunk looks good, but will replace all fuel tubing. Will take your suggestion and clean out parts where the needle valve and idle screw are located.

RE: OS 91SII starting Problem
 

If it doesn't work this time, you might try the fuel in a different engine?

RE: OS 91SII starting Problem
 

I had the same problem with an SC four stroke. It turned out that it was 1/2 to 1 turn too lean on the low speed needle. Richend it all up bench ran it and reset the needles and all seems fine.

RE: OS 91SII starting Problem
 

Thanks for all the support on my 91 starting problems. I went through the engine last night and covered all the bases that you guys listed. Put it back together this morning. Fired it up and it runs great. Not exactly sure what the problem was since I checked all the areas before firing up.