opjose
Posts: 5279
Joined: 11/26/2005
From: Poolesville,
MD, USA
Status: offline
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quote:
ORIGINAL: Bratpak
Wow this is quite a thread !!!
I have a GMS 47 on a Twist and it has about a gallon of fuel ran through it.
Every flight has been a deadstick. 
Lots of things to try here, I wish the weather was better.
The engine idles and runs great, lots of power and rpm when in flight, but after 5 minutes of flying the engine quits. It doesn't matter what kind of flying either.
It has the Tower type muffler on it. I took the fuel inlet nipple off and the hole in the brass bar is NOT misaligned. Checked all fuel lines etc. and everything is ok.
Has anyone tried to cut the length down on the muffler to see if that makes any difference in the pressure the muffler is sending to the tank??
Mark
Forget the muffler, that shouldn't be your problem.
The muffler provides a good amount of pressure if your seals are tight... More than you may imagine!
The first thing you should do is fire up the plane with only about 1/3 of a tank of fuel in it.
Run it up to warm up the engine, then let it idle for 4+ minutes.
Make SURE that the idle stays at the same speed.
If it starts slowing down, the engine is too rich, which may cause deadsticks when you have the plane in a nose down attitude.
Once it idles at a constant speed, do "quick" pinch test at idle.
You should get a barely perceptable increase in speed. If not, richen. If so leave as is.
Now do a nose up test again performing the pinch test nose up.
When you set the plane nose up, make sure that it stays idling, though the rate may increase. Likewise it should behave the same way when you pinch test it.
Level it out, and do a longer pinch test. Does the engine continue to run for 3-5 seconds when you pinch off the fuel at idle?
If so good, if not, too lean.
Now nose down pinch testing and idling.
The idle speed will decrease somewhat, but the engine should NOT bog down nor shut off even after many minutes.
This is also a good check to see if your clunk is providing fuel at lower tank levels...
Now do all of the same at full speed.
If you've made ANY adjustments to the high speed needle, go back and start over with the low speed needle, from the beginning.
Remember you SHOULD see some smoke at full throttle... but you should not see a large increase during transition.
I had exactly the same problems as you, and it turned out my low speed was too rich, while my high speed was too lean.
I went crazy looking for fuel problems, muffler pressure problems, etc.
Heck even with a Pitts style muffler the GMS produces more than ample pressure, enough to pump the fuel up a longer run to the engine at full nose up.
Make SURE that there is no way your clunk can come close to hitting the back of the tank!
During acceleration the tubing may be stretched pulling the clunk backward a bit, while cutting off flow... found this out the hard way.
Carefully tighten all of the fittings, and the cap screws.
The cap screws ARE loose NOW if you've not tightened them after you've run the engine for a bit.
It is EASY to strip or break the fittings. I had a problem with one T.H. engine, as I had cracked the fitting trying to tighten it...
Changed it out and my problems were gone...
The nipple fittings must be tight to get a good seal, but "tight" on the nipples requires FAR FAR less force than say the cap screws... the cap screws must be fairly tight... think ALMOST prop nut tight.
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RE: GMS Engine Tuning Problem - 
