genek

I am using a ST 90 w/ 13/6 and 14/6 props. After flying for a while when I move the throttle to idle it takes a few seconds for the rpms to drop. The linkage is fine. Tank placement is correct. The engine is stock and so is the muffler. The last time I taxied on the runway it did it again. Any ideas? Thankyou, Gene

BillS

My ST 90’s return to idle slightly slower than other engines. Chances are your low speed is slightly rich. When correct the 90 will have a slightly lean sound as it passes through mid RPM returning to idle.

Bill

Sport_Pilot

Sure sign of a rich mixture (especially idle) and a too hot glow plug. The rich mixture cools off the glow plug the timing is retarded. As the fuel burns off the glow plug gets hotter, the timing retards, and the idle slows down. Start with leaning out the idle, but going with a colder plug and leaning the high speed will also help.

genek

Thankyou to both of you. I am going to try it this weekend, the engine and tank are out of the plane now. If you see this reply may I ask: If the engine is running at idle speed can a pinch test on the fuel line show the rich idle mixture? Thanks ; Gene

KidVermin

My Feedback: (3)

One suggestion not mentioned yet besides previous.
On my ST engines, 45 to 3250, I've taken off the carb and put a bead of Permatex high temp gasket seal around the O ring. This prevents air bleeding in below the carb and stopped the run on.

Sport_Pilot

Pinching the line will help show up the problem, but may not help show how much you will have to lean it. If your glow plug is way to hot it won't show that.

GoNavy

My Feedback: (15)

I am told that our engines carry a lot of fuel in the crankcase when flying, and some more than others, and that it takes awhile to reduce that resevoir when slowing the engine.
I haven't had a supertigre yet that didn't pass through that long slowdown to idle phase on final approach: Throttle back, slight slowing, then some "burbling", finally resuming normal sound at the desired speed. Running the ST up slightly after being at idle does not create the crankcase fuel reserve, hence they return to idle quickly. Just my humble opinion.

Sport_Pilot

Actually the fuel crankcase "loading up" does happen. This happens when the full speed mixture is rich and idle is lean, and the engine slows down before it speeds back up, just the opposite of what we are talking about. This is when a rich mixture causes fuel to condensate out and drain to the bottom of the crankcase, when the mixture is corrected there is a delay because the pooled fuel must evaporate. But in any engine including Supertigre will not do this with the right low speed mixture, correct glow plug, and fuel.

Meschmidt

My Feedback: (15)

Like rreiss said, seal the carb and also the engine backplate with hi temp silicone sealer. I have some older ST 60's and 51's that started to run inconsistently. Sealed them up, readjusted the carbs and haven't had to tune them at all now. Try it as it worked on a newer engine too! Also don't run the spraybar slot directly into the bottom of the carb bore, angle it slightly towards the cylinder. This greatly improves the idle and midrange. Paul

GoNavy

My Feedback: (15)

Also: Make sure the carb barrel is not geting hung up by the screw which runs in the barrel slot. I had a ST 51 that intermittent problem returning to idle. I put a washer under the head of the screw and problem solved.