Ok I am about nuts over this. I have had lots of experience with engines. I have a Super Tigre 90 in a Great Planes Extra 300. Starts fine. Runs great for about 5 min. Then it throtles down to just above idle and the plane comes down. I have made perfect landings and the plane is sitting on the runway idleing. Thinking the tank was getting vapor lock I dismantled it and it did the same thing. If it is overheating will it idle down like this or just shut off? Guys at the field are all scratching their heads too. Anyone want to take a shot at this?

What prop are you running on the engine? Just curious.

Try a flight without the cowl and see what happens. Should answer your cooling question.

I had problems with this engine running the rec prop (13-8) in the Tiger Man. Switched to a 14-6 and the engine loved it . . . I can not see how a 13-8 prop on this engine is close to large enough . . .but then again you did not mention that you were running a 13/8 thus my advice may be of little use.

Actually, the recommended prop for this engine is a 13x6 (at least it was in my instruction book).

The APC 13x6 prop I put on worked great, and I don't intend to go bigger. The engine likes the higher RPMS. This is no 4 stroke.

Question, When it lands & is just sitting there idling, Is the carbuatoer at full open? or is it shut? Sounds like you might have a servo, or linkage problem. I had a battery holder break on me a few years ago, & it did all kinds of weird stuff till I found it. Just a thought. good luck to ya & hope you get it figure'd out............................MILDBILL

I run mine on a 14x8 APC. Works fine in the Midwest CAP 232, which is about the same size plane. I changed to a Magnum carb and have fewer problems. But none of this has anything to do with your problem.

As someone mentioned above, make sure the carb barrel is still open. I fought a fluke like yours on a pattern plane a number of years ago.....it would cut down at a predictable spot about 30 seconds after takeoff. Drove a whole bunch of us nuts. If I cut the throttle, and then went back up, I'd be good for another 30 seconds. So I'd chop the throttle on the back side of each split S and be good for another manuever in front of me. Changed engines, tanks, etc. Back home one day after flying, I opened the carb, poured in some after-run oil, and sat there absent-mindedly flipping the prop, when all of a sudden the carb closed. Reopened, and timed it. Took 30 seconds to close again. This was on a JR 9 PCM, and the fail-safe system had a glitch in it. I disabled the battery fail-safe option and the problem went away.

I doubt if your problem is that, but it is worth a look to see if your carb stays open.

Clair

I have run 13x8APC and 14x6 MA on mine with no trouble. Did you use Loctite on the exhaust presure fitting? I have seen Loctite form a skin and close the fitting off. I also drill mine out to the next size. Maybe your clunk line has a hole part way down?!?!?

Let us know what the cure is.......David

I have ruled out any radio, servo linkage ect problem. This has gone on for a long time. My next move is fly without the cowl. I have had the fuel tank out and apart 2x so after the cowl it gets replaced if that doesn't fix it. Last flight was a 12x8 and it sounded and flew great( for about 5 min). I have had a 13x6 on it with the same problem. The carb is opening ok it sounds fuel starved at high end. Why!!!!!!!. I am on a mission now. I will figure it out this weekend if it kills me!

I WILL NOT BE BEATEN BY THIS SILLY ENGINE!
Maybe it is a reincarnation of my ex wife? That would explain alot!
I'll keep ya posted
Thanks

I agree, it sounds like a servo problem. It doesn’t make sense that an engine with the throttle wide open could idle. I doubt that your engine, at an idling RPM (2500), would be getting the correct fuel mixture. This lack of vaccum would cause your engine to starve for fuel and quit running.

My theory is that the vibration while in flight is causing the potentiometer within your throttle servo to misread its location? Try swapping out the throttle servo.

About the lean condition at high RPM. Could your low speed needle setting be interfering with your high speed needle? The low speed idle should be fat when you adjust your highspeed then you set you low speed.

I got tired of fooling with my G90 carburetor and put on a Perry (4501). It took less that a minute to tune, great idle, transistion and 700 extra rpm on top. I turns an APC 13x6 at 12,700 now verses 11,900/12,000 before. I wore holes in two pairs of jeans and wasted two weekends trying to get my Super Tigre engine tuned - not worth it.

Good Luck...

Of course, without seeing the engine, we'd not know what the difficulty may actually be. One way to diagnose this would be to remove the engine from the model and put it onto a test fixture. Make sure all of the fuel plumbing is good, and the tank's centerline goes through the center of the carb. Attach the stock muffler. Use a 13 x 8, 14 x 6, or any other prop of about the same size. run the engine. Use a wire for the throttle link, and make sure it's restrained so that you can position the throttle at any position. Run the engine.

Do you still get the problem? Then something in the engine is the likely culprit. Maybe it's had an overheat at one time, and now, when it warms up, it loses compression. Overheats are common, and mostly completely unintentional and hte modeler may be completely unaware it happened.

If you don't get the problem on the test stand, it's not the engine, but something in the entire engine/model/mount/fuel system. That includes the receiver and servos.

Are you using a Futaba PCM receiver? Is there any chance at all that the battery pack's marginal and you're getting "throttle failsafe" activation? When the engine "throttles down", can you retard the stick on the transmitter and get throttle action back?

These are all things to investigate as part of the debugging procedure. You've undoubtedly thought of some or all of these. Just hoping to help you get your engine running well again.

Only 2 things can make an engine come back to idle, one is the servo closed the venturi or the venturi closed by itself. Bax is right about getting the engine out of the plane and on a test stand...it is very possible the engine is over heating. There may be a small piece of crap in the spray tube...very hard to say but getting the engine on a test stand or remove the cowl and run it on the ground for 5 minutes to see what happens. I have owned several ST .91's and don't recall having to many problems...I find the Perry carb suggestion interesting if a 700RPM gain is what I can look for. You will figure it out soon no doubt...let us know what you find. Good Luck.

One more thing to think about, so simple it is easy to overlook....make sure the needle valve is not slowly turning in flight to a leaner position. I have seen this happen. Index it someway so you can tell if it is the same at the end of the flight as at the beginning.

Clair

I have pretty much ruled out the radio. I've changed 2 servo to be sure. Last flight was with new 9c and receiver. I have narrowed it down to the cowl. I believe it would shut off competely if this was thew culprit. My main idea is a cut in the clunk line near the tubing. When the fuel gets to that level it sucks some air and leans out some. I'll let you guys know the results this weekend.
Thanks for all the input.