RE: GMS Engine Tuning Problem
 

I have a GMS 1.20 it stops running when I back down on the throttle in flight. I changed the lines any Ideas?

RE: GMS Engine Tuning Problem
 

Put an 11x7 APC on it. Works well on my GMS47, which has the same muffler you mentioned.

I run 10% fuel and rarely touch the needle. Takes a bit to kill the engine with the throttle cut but it will stop eventually.

RE: GMS Engine Tuning Problem
 

As per my response on the other thread:

At higher speeds the pressure from the muffler increases the fuel flow into the carb.

As you back off, that pressure remains.

If your low speed needle is too rich, the engine can shut off as the glow plug cannot overcome the cooling effect of all of the fuel.

Conversely if the mixture is too lean as you back off, the engine can stall out for lack of fuel.


How much fuel have you run through the engine?

The GMS don't settle down until you've run about a gallon of fuel at varying throttle settings.

RE: GMS Engine Tuning Problem
 

Hi, Can someone please help me. I purchased 2 GMS 2000, 47 engines. Both run like crap. Most of the engine experts in my club have had a go at adjusting high & low end, to no avail. The problem is twofold:
(1) Engine starts fine, idles well, taxi to the flight line, give it full throttle & it dies. Around the third attempt, it may condescend to run for takeoff.
(2) In flight move from mid range to full and anything can happen, engine will most times die. Which makes it bloody good when you are attempting doing a low level pass or training on takeoffs & touch & goes?

Now for the fun thing both engines are new. Both have been ground & air (attempted) run in. Both have the shinny large mufflers and both are working on our club premixed fuel. 18% performance (blue) oil, 10% nitro. We have changed plugs around & it seems OS 8's seem best. I have dismantled one engine & note burn mark is only on one side of the head (strange) & makes me wonder if a longer glow plug may work. Tried 10x6 & 11 x 7.5 props - no difference. On my grandson's trainer I have now exchange fitted an OS46AX, which runs as smooooth as silk on the same premix fuel and an OS 8 plug and it doesn't seem to matter about prop size.
There has to be an answer to the GMS engines, other than me returning both to the manufacturer under their 2-year warranty.
Be obliged for help

Bluey

RE: GMS Engine Tuning Problem
 

Also make certain to seal the engine exhaust flange, and the seams of the muffler. If you
are losing pressure it will lessen the fuel pressure to the carb, which doesn't help. ;)

Stick with the 10-6, or a 9-7. I use a 9-7. Make sure you have castor in the fuel mix for
cooler running.

FBD. :D

RE: GMS Engine Tuning Problem
 

Wow have I been here!

Same problem... even though the engine ran PERFECTLY on the ground and when the plane was held at any attitude and would also run up and down w/o problems.

In my case it turned out that I had the clunk to close to the back wall of the fuel tank.

The clunk was capable of contacting the back of the tank ( unbeknown to me ). The "slits" in the clunk permitted fuel to flow. But when the engine was run up the clunk would be drawn against the back wall greatly restricting fuel flow and dynamically leaning things out.

Changing out the fuel system and in one case adding a one way pressure valve cured my problem.

Also check out Flyboy's post above.

It pays to go over the engine well just to be sure...

Remember to tighten down the cap screws too, as they come loose once the engine has been run.

RE: GMS Engine Tuning Problem
 

Wayne this engine runs like hell, I fixed all my 3 GMS with this thread suggestions and they sure have improved, I was afraid they would consume much more fuel but not at all.

For all of you who have brand new GMS´s I followed this procedure for breakin, take a look at this link, it worked out perfectly for my engines, believe me.

http://www.georgiacombat.com/ABCEngineBreakIn.html

Regards.
Rafa

RE: GMS Engine Tuning Problem
 

FOr the people who is Having trouble with the GMS engines, why don't you check for the fuel restriction in the carb FIRST... as experience the experience of the users has showed that most engines have the fuel restriction at the carb.........

RE: GMS Engine Tuning Problem
 

To quote Flyboy

"...some of the early GMS carbs (like mine) were miss-drilled for the inlet of fuel aprox.
40% restriction. This lead to a leaning out in flight, and deadstick. Most of them would
not respond to the main needle....you could open them up 7-8 turns and nothing would
happen due to the restriction."

So there are a couple of indicators that point to this

1 - An earlier engine

2 - No effect when you turn the needle OUT.

That suggests that the carb needs to be check if either or both of the conditions are met.

RE: GMS Engine Tuning Problem
 

This does not apply to all GMS engines. From .61 and up, the pressed-in brass jet that requires drilling on the smaller models is different. It has a square groove machined around the periphery, and it has 2 holes drilled into the bottom of this groove to allow fuel into the area where the needle valve meters it. So it makes no difference if the holes are or are not lined up with the fuel inlet nipple.

RE: GMS Engine Tuning Problem
 

I would hope that the problem was identified and remedied last year. Surely, the warranty
claims exposed the incorrect drilling of the fuel inlet nozzle. ;)

All in all, I have not regretted the 65 bucks I spent on the engine....I like it more and more. :D

FBD. :D

RE: GMS Engine Tuning Problem
 

Yes I have a GMS61 no problems at high end but starting is a problem, lack of fuel until you can rev it a few times, then its ok. Takes me a long time to start - any suggestions?

RE: GMS Engine Tuning Problem
 

....open the low end carb adjustment screw 1/8 turn. (ccw) ;)

....this will give it a bit more fuel, and should make it easier to start. :D

FBD.

RE: GMS Engine Tuning Problem
 

Actually since it starts and runs OK once it gets fuel... try this...

BEFORE you start the plane with no glow ignitor installed, open the throttle about 1/2 way.

Hold your finger over the muffler exhaust and turn the engine over by hand until you see the fuel hit the carb... watch the tubing...

Then turn the engine over the same way two more times.

Turn DOWN the throttle, and then start the engine as normal.

RE: GMS Engine Tuning Problem
 

I got a GSM .75. I would like to put a good heather or a higher performance muffler.

Any sugestions?

RE: GMS Engine Tuning Problem
 

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

RE: GMS Engine Tuning Problem
 

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.

RE: GMS Engine Tuning Problem
 

Make sure your fuel lines arten't reversed, and your vent tube is up at the top
of the tank where it needs to be. ;)

FBD. :D

RE: GMS Engine Tuning Problem
 

Yup that will do it too.

It's very easy to change the orientation of the vent tube or block it off inadvertantly causing the described problem.

RE: GMS Engine Tuning Problem
 

Thanks for the replies,
I checked the lines and the vent tube is at the top of the tank.
What are these cap screws you are talking about [sm=confused.gif]

Mark

RE: GMS Engine Tuning Problem
 

He's talking about snugging up all the screws on the engine

RE: GMS Engine Tuning Problem
 

Yes, especially those on the cylinder head which are usually "CAP" screws, requiring a hex key to tighten.

These WILL get fairly loose after your first runs.

RE: GMS Engine Tuning Problem
 

Ok thanks for the clarification.
I will look at these this morning

Mark

RE: GMS Engine Tuning Problem
 

I just went down ane checked all the capscrews on the engine. The ones on the back plate were fine but the screws on the head I could tighten not quite a quarter turn on each screw.

Mark

RE: GMS Engine Tuning Problem
 

Good job