JohnS

Hope I can get some help out there folks.

I have a TT .91FS installed in a 60 size 4-Star. This engine always ran fairly well in this plane , but with the main needle screwed in to about 1 turn (which I always thought was too much). Today, in relatively high humidity, I had to turn the main needle in to about .5 turn and still it was too rich. I was afraid to screw it in any more. Had 2 deadsticks out of 3 flights. Using Magnum 20% fuel. Choke is installed. Engine mounted upright. OS F plug.

Could this have anything to do with valve adjustment which I haven't done in a long while, or is there a problem with the needle valves in these engines?

pikebishop

it's possible vibration is causing the needle to slowly wear creating a richer mixture. do you have an extension on the needle? lots of times the extensions seem to increase needle wear.

JohnS

Yes, I do have an extension on the needle valve. I was thinking about getting a new needle valve from Thunder Tiger anyway so I will try it without the extension (or a very short one if needed).

Thanks for the info. JS

TNRabbit

If you must use an extension, place a piece of fuel tubing over it to help dampen vibration. Any shortening you can do to the extension will also change the harmonic frequency at which it vibrates.

Sport_Pilot

Did you switch to a lower nitro fuel? This would cause the engine to run rich.

JohnS

One thing I did find out is that if I screw the needle all the way in and blow into the inlet with some fuel tubing, the flow is not shut off. This doesn't happen with my TT46Pro. That one shuts off.

William Robison

My Feedback: (3)

John:

This would tend to indicate you did indeed have needle vibration, and the seat is now wallowed out. You will have to replace the spray bar, if it can be done. Otherwise it's time for a new carb.

Bill.

Flyboy Dave

My Feedback: (21)

Check the low end screw as well, and make sure it is not vibrating out.

FBD.

mentorman

My Feedback: (2)

I had to get a spraybar and neddle assembly from E-Hobbies.com. It was like 15.oo dollars with shipping. Look up that site and you can call them great people. They stock everything for thundertiger.

JohnS

Thanks everyone for your great inputs. I think I will just get a new needle and spraybar assembly and start zeroing it in from there.[8D]

apalsson

My Feedback: (1)

John,

Another thing to consider when you overhaul your engine - you might want to check the valve clearances.
I had some isues with my TT91 and after adjusting the tappets, the motor was like a different unit.

I agree with the comments on the spraybar / needle but I would also check this.

Basil Yousif

My Feedback: (3)

I am also having simular problems. I can't get reliable high end RPM's. My needle valve had bent on a crash and I bent it back. It looks straight now but the end of the needle valve is flat and doesn't come to a pin point. I had done the valve adjustment- before I adjusted them I got absolutely no compression from the TT 91 FS and now it's got excellent compression. My engine has only been through 3 gallons of 15% PM fuel. I have had two dead sticks after I adjusted the valved and got the comptression back. I think I'll order the needle and spray bar assembly also.