erratic runaway Thunder tiger BX

[khalif]

Thunder tiger 12BX



I have just gotten hold of a S/H replacement engine for the old Kyosho GS11 It is a Thunder tiger 12BX Pro, I managed to shoe horn it into the car (Buggy) without any probs. It was in clean condition and came as a spare with another car. It started Ok but was very erratic and would not idle at all and would suddenly rev very high. I removed it from the car and stripped the carb which had a fair few bits of crud in it, cleaned it all and refitted it, but it was impossible to adjust the idle needle in the car (for physical reasons could not get to the screw). I removed the motor and rigged it up on the bench. I was able to set the Idle quite well and the motor responds quite well to the throttle but every so often it goes crazy and the revs go sky high even though the throttle is barely open (0.5mm). If I pinch the fuel line when it does this, the motor slows (and stops if I keep it pinched), release it and it goes sky high again. I have tried it with the carb completely closed (0mm) when it goes doodle alley but it is the same. I have also tried sucking the fuel line with the carb slide closed and it does not leak. The tank and exhaust do not leak. The backplate is sealed and the motor sucks in fuel with the carb open and venturi blocked with a finger. I have had quite a few aircraft and heli motors and they were all easy to set up but this is a real crock. I took the carb off a brand new (30mins running) Magnum 25 and fitted it, It works OK. Fairly stable just doesn't go crazy like before and is quite useable but just a bit lacking in max revs. Just to try, I took the Tiger carb and put it on the Magnum works fine and the idle is superbly slow with a 9x6 prop. No gitches or weird behaviour at any revs.
Makes no sense..............Any ideas anyone?

 

[Rolex]

Possibly just a bad O-ring around the needle or a worn or broken tip.
The prop provides some resistance and a steady load, so it wouldn't be able to rev the way you described, as it did in the car.

 

[khalif]

I could see nothing wrong with the needle. I will get the new O rings on the needle/spray bar assy next Wednesday. I sealed the ones already there with silicone grease the second time I stripped and cleaned the carb. The Carb O ring is OK and is also Si greased. The magnum is superb with the TT carb and will sit at any rev setting without a change in note once hot. The TT is also very stable now with the magnum carb but the top end is a bit lacking. I guess I could fit a flywheel to the Magnum instead of a prop and provide some cooling. Anyway first the O rings.
Thanks
KH

 

[Nitrotrout]

Is this a used engine? If it's not the carb and you say there's no leaks, sounds like your sleeve needs to be repinched.

These are the TTR tuning instructions for the pro .12-18 They're a bit vague but might help you (or not).

NEEDLE VALVE ADJUSTMENT
This is used to adjust the high speed fuel mixture. Optimum setting is
described in the BREAK-IN/RUN-IN PROCEDURES.

MIXTURE METERING SCREW ADJUSTMENT
This is used to obtain a smooth transition and stable idle.
1) After the needle valve has been set, keep running at the highest speed
for about 10 seconds then abruptly close the throttle to the lowest
speed. Run at idle for about 5 seconds. Then quickly open the throttle.
2) If the engine hesitates before picking up speed, and there is plenty of
smoke and un-burned fuel coming out from the exhaust, the mixture
is too rich. Screw in (clockwise) the mixture metering screw about 1/8
turn.
3) On the other hand, if it speeds up before quitting, the mixture is too
lean. Screw it out about 1/8 turn.
4) If this is difficult to observe, then allow the engine to idle slightly longer
before opening the throttle. Repeat procedure 1) patiently for few times
to ensure the best running is obtained.

THROTTLE-STOP SCREW ADJUSTMENT
This is used to set the minimum idle speed. If the engine runs too fast at
the lowest throttle opening, turn the throttle stop screw counter-clockwise
to reduce the throttle opening.

The mixture metering screw of your
carburetor has been pre-adjusted at the
factory. If it is screwed in or out too far, reset
it as follow:
Screw in or out the mixture metering screw
until its end is flush with the throttle. Then
screw the mixture metering screw out about
1 turn. Now it is the standard position.
1 turn
Mix. Metering

 

[khalif]

The piston/sleeve seems OK, when cold it feels like the piston is gripped at TDC. I used the procedure from TT to set the carb. There may have been an air leak into the fuel line at the carb. I will bring the fuel line over the carb such that any air that returns into the line at start-up will remain in the highest part, at the moment with the Magnum carb it is like this. I will get the O rings midweek, re-clean the carb and try again.
Thanks for the advice.
KH
Just an update.- I removed the O rings there are six of them. I think it will be difficult to find them locally given the dimensions.