w8ye

My Feedback: (16)

The GP and the PRO use the same muffler

http://www.bissonmufflers.com/en/muf...ing_id=114085#

Volante24

I have the very same strange behavior with two of three GP-42 we have (the third one is simply still NYB probably, would show the same otherwise). Very pronounced! Yet the engine usually would not die in the air, just drops down to low and uneven speed. Then after working at low throttle for a while seemed to be fine until ran at full throttle.

First there was a clear correlation when weather was on the colder side and, in particular, wet with high moisture in the air. No such problems when warm or dry. It seemed that richening the mixture helped. Since the problem showed up at colder temperatures, we started to use a kind of a small light aluminum screen for the head when cold. But then later the problem escalated. Following the advice gotten on this Forum I tried to switch to 10x5 APC (before we flew with 10x6 APC). I liked it more since lower thrust on idle makes it easier to land. Alas, the problem with full throttle aggravated so much that I had to put back 10x6. The engine could half-die, or completely, on takeoff! Now this behavior is observed already at relatively warm temperature... And richening mixture within reasonable limits does not seem to help.

Something to consider as possible hints: originally the engines had a rather pronounced peak over the needle mixture position. This is not so anymore, although I regularly flush the carb assembly with the gas through the fuel line. (Originally clogging of the needle seat apparently caused problems at some regular intervals).

I am flying on the fuel containing 8% Klotz, 10% Castor, 2.5% acetone and 79.5% methanol (the numbers may be more approximate, but I'm pretty confident the components sum up to 100%!) The plug is Tower Hobby one; for unknown reason they have to be replaced from time to time, otherwise the engine refuses to work stable... Otherwise the engine is fine. Very tight when cold (it is difficult to crank cold below freezing), starts well (but does not like priming it is easy to flood it).

jeffie8696

Have you tried any different fuels ? I think first thing to suggest would be a 5% nitro popular brand fuel and work from there. My GP 42 likes up to 15% nitro without problem. And it makes some excellent power. Although I may need to increase the timing a little at the crankshaft intake opening.

Volante24

No, I did not try. Somebody at the field said modern engines are designed for fuel with nitromethane, but the manual for GP states it can be run Ok with no nitro as well. Where we flew there is a problem to get nitromethane, and any type of branded fuel you get from the RC stores would cost four times and up more at once...

It would be good to understand the cause of the problem anyway. Otherwise a situation is quite possible that the engines will run with 5% nitro without problems for some time, but then it appears necessary to raise it to 10% etc....

jeffie8696

I suggested it so you may be able to eliminate the fuel as the source of the trouble. I have been victim of a bad jug of fuel before to the tune of an engne that wouldn't.

Volante24

That's right, but I'm not sure in my case this will be a conclusive test. I expect if I try the nitro fuel I will not have these problems for a while: indeed, based on the symptoms, it looks like something is "marginal" when I ran it presently. E.e., normally things work but when something comes to extreme the engine kind of quits. In this situation using a more effective fuel may shield the problem for a while. If I switched completely to nitro fuel and watched for the symptoms for an extended time, this would probably be more conclusive...

Nick

jeffie8696

Do you feel it may be a little too tight and when it heats up good it is siezing? Perhaps too rich and running too cold or too lean and running hot?

Zippi

My Feedback: (10)

Does anyone have the latest info on the new TT F-130S ? I haven't seen a price listed anywhere.

Volante24

I'm not sure how would it look like when seizing. It used to be a few times when the (first) engine was very young that it stopped around the top dead point as if no compression were there and the air blew through between the piston and the cylinder but it was at small throttles. Now it seems to me, if anything related to compression at all, that rather compression goes low there, for the motor sounds to run "easily" after such drops. But it is only a feeling, I do not have real experience of this sort with model engines. Let me say again, there originally were strange combinations in the observed behavior: the problem seemed to show up mostly at lower temperatures (still above freezing) when air was humid, like with mist or after rain. And at the same time richening mixture originally helped to some extent.

Maybe I should try to partially block the muffler back hole (reduce its diameter), to relatively boost up back pressure on high RPMs? I recall once early on we had similar symptoms which turned out to be caused by the loose bolt which tightens the two muffler halves together. We found this out when landed the plane with motor still working, apparently the fit was sufficiently tight with the muffler cold, while the pressure was progressively dropping when aluminum expanded upon heating. But since then we always had this thing under scrutiny, the bolt is really tight now.

What else I can try to do? I have an IR temp meter and can try to watch the cylinder head temperature but, of course, not in flight. What it should be? Also, I can accurately measure the glow plug temperature (more precisely, of the spiral). In this way, actually, two years ago I was able to find out why my Tiger Shark .56 was refusing to run stable on the plain methanol/castor mixture, and made some changes that made it to work flawlessly. But, again, this is all limited to the bench-like tests, even if in the plane.

Nick

jeffie8696

I think you are on the right track. Head temps are another diagnostic aid , I can not personally tell you what they should be but perhaps one of the other members will provide that.
If all else fail you can ship it to me and I will do some comparisons to my own and let you know what I find out.