I have a new TT91 fs that I have run about 4 -12 oz tanks of fuel through. Low speed idle is great as well as 3/4 to full throttle. From about 1/3 to 3/4 throttle, engine is rough and pulsing. Have tried low speed needle adjustment, but does not improve. Using 10% Nitro fuel. Any suggestions..Fred123[&:]

The engine probably isn't broken in yet. Most of the TT 4 strokes like a fuel with 15% nitro and make sure you are using the OS type "F" plug that is designed for the engine. The amount of fuel you have through the engine is about half of what is needed for break in. Hopefully the fuel has some castor in it to help the break in of the conrod bushings.

A high fuel level (Tank too high) will cause the mid range to be rich when the high end and low end are adjusted properly.

Enjoy,

Jim

Fuel is 50% Castor and 50% Synthetic. Checked valve adjstment last night and found them about .006 or so. Put them back to about .002. Don't know if that would make any difference. fred123

PS I am using the "F" plug

Is that a fact for non-pumped 4-strokes? No wonder I was having hard time getting some Saitos mounted inverted to idle correctly.

Seany,

You can allow for the fuel pressure with the needles but typically higher fuel level will leave the mid range rich.

Study your plane carefully to determine your fuel tank position?

The same plane would as likely have a lean midrange when inverted and the engine, therefore, upright?

Many inverted engine situations present a high fuel tank to the engine. This is especially so for a plane set up with a upright engine in mind in the beginning.

Four strokes mounted on their side present less of a problem.

It seems like a sport pattern trick to mount the engine on its side?

Enjoy,

Jim

Would a tank mounted with the outlets level with the needle valve on an upright 4stroke be considered too high? Fred123[&:]

Did you leave the choke assembly off? If so it could be causing your rich mid range because you would need the main needle opened further to compensate for the slower movement of the air past the needle valve at wide open throttle. The choke assembly has an air reduction and a venturi built onto it to increase the air velocity past the needle valve.

Sorry,

I forgot about the fabled venturi on the TT 91's. They don't run good without it.

The center line of the tank itself should be centered on the needle valve outlet.

Enjoy,

Jim

MrMotor: No, the choke assembly is installed. Took it out this afternoon after adjusting the valves last night, and I believe it ran worse. Will re-check the valve adjustment tonight..Fred123[&:]

P.S. Just checked the valves, and found one of the locking nuts had come completely off and the other was loose. I called myself tightning them, but since the nuts and threads are so small I was afraid of stripping them.. I tightned them much more this time.. Will run again tomorrow and re-check. I am surprised it even ran at all.. This four stroke is sure a new and different animal . Fred 123[&:]

The tank centerline should actually be below the carb spraybar centerline by about 1/4 to 1/2 an inch. But on the centerline shouldn't be so bad that it runs rough. It should smooth out after break in. Sometimes (I think) a weak plug will cause a lean idle setting that will cause a rough midrange. The plug could have been fouled on the initial break in.