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Old 08-23-2016, 07:07 PM   #1
STO54
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GMS engine quits


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Got a .60 GMS NIB for nothing. Get what you pay for. Made the mistake of putting in new plane. Was always a hassle to keep running properly. Starts no problem every time. But increase throttle and it's anyone's guess what happens. Sometimes it goes - like a bat out of RC Canada. Yeah I know but RCC is my personal hell at the moment. Anyway, last time out quit just as I was landing and ripped LG as it crashed to ground. really sucks. I know, get an OS.
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Old 08-23-2016, 08:16 PM   #2
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Re: GMS engine quits

I would bet a $20 its not a bad engine
need more info
What kind of plane?
Is engine inverted?
What kind of muffler?
What kind of fuel tank?
What size propeller?
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Old 08-23-2016, 08:44 PM   #3
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Re: GMS engine quits

Quote:
Originally Posted by cassat View Post
I would bet a $20 its not a bad engine
need more info
What kind of plane?
Is engine inverted?
What kind of muffler?
What kind of fuel tank?
What size propeller?
The plane is a Great Planes Evo Rocket. Engine is mounted inverted with a PItts muffler specific to the engine. Tank is 8oz plastic that came with the ARF. Prop is 11 x 6 APC. I have had very experienced guys look at the set up. Re set needle valve. Change glow plug. Their opinion is the engine is best used as a paper weight. They can't figure out why it runs erratically. Fuel by the way is 10%.

I never have a problem starting it. It is getting fuel into the carb. At low idle it is fine no sputtering. Increase throttle slowly and it hesitates a second then goes to WOT. Pull back and it's fine. Hit throttle again and 50/50 it will go or die. I have had numerous deadstick landings. Unfortunately last one was too close to the deck.

I know there could be hundreds of causes from dirt or debris in the carb, misalinged head....
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Old 08-23-2016, 08:57 PM   #4
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Re: GMS engine quits

I don't normally bash engines, but GMS and their Tower clones were known for a few issues. Along with quite a few over my own bench there were forums that covered a lot of them. If you check the GMS/Mecoa site you will see they discontinued the 47 through 76 completely, the ones that seemed to suffer the most.

First is that they will most often dead stick when new regardless of how rich set on the ground till at least through a few tanks. Once worn in a bit that seemed to settle down. Expect to kill or compromise at least one glow plug during this process.

Second problem were their carbs. 50/50 if you get one that works well. Often we swapped for OS as they were a direct fit. If you keep their legacy carb there is one particular problem that should be checked. When the receptacle, (holding the High Speed needle and distribution bar) was inserted the drilled hole that is supposed to line up with the fuel inlet fitting often turned to at least partially block the flow. You may be able to run a drill through the fitting with the HS needle removed to see if it goes all the way in. If not may be best to unscrew the fitting and physically check. Worst case you may need to clean up the obstruction or even drill a new hole close to the same diameter as the inside of the fuel inlet fitting.

Then there was the rather poor QC. Excessive gap between the crank and case bore allowed air to leak through the front shielded bearing and draw in enough fuel on the way to keep running even if the carb barrel was completely closed. The front bearing was rather narrow and did not hold up well, especially if the prop and/or spinner were out of balance. If worn it could again wear the case bore so this problem could show up later.

NOTE: If you had one that would run on with a closed throttle you could guarantee the front bearing would suffer from lack of lube relatively quickly since it would be drawn away from internal vacuum.

Easiest fix was to replace both bearings with good quality and the front with a rubber sealed type with the inner seal removed for better lube. Some went as far as filling the lube groove inside the bore between the carb inlet and front bearing, but that was not really solving anything.

Regardless of these fixes I found every one required such a rich mixture to run reliably you had horrendous fuel consumption and gobs of lube oil ejecting from the exhaust.

To be honest I have gone away from new 2-stroke installations and over to larger scale 4-stroke. Still work on a few, but nothing like a few years ago. Perhaps they finally got their act together.

Now to installation: first thing to check is tank height. Often with 2-strokes inverted then has the carb fuel inlet fitting quite low compared to the tank. That fitting should be even with the half fuel level in the tank.

Even then you may experience problems with siphoning once fuel is above that level. This will often lead to fuel leaking when engine is off and can have a big effect on mixture settings as it burns off.

When running 2-strokes inverted the standard #8 or even the hotter #3 did not seem to keep from quenching. I strongly suspect you are introducing more fuel than it can light with the throttle management you describe.

This fuel tends to pool in the head and overpower the glow element, cooling it off too much to keep working.

My solution was to run a 4-stroke plug, (or even the OS "R" type made for their Wankel. Some recommend an idle bar 2-stroke, but I found that did not work reliably.

NOTE: Inverted installations can suffer Hydrolock, where a slug of incompressible fuel/oil are trapped between the piston and head, (this is the reason you see ground crews walking the props through on old films of WWII radials. Doing it by hand allowed it to bypass or eject out the exhaust.

You should do the same, flip the prop through a few times with heat removed. If there is anything beyond normal resistance tilt the plane till the fuel can reach the exhaust port. If you try and force it through you will likely do internal damage.
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Last edited by Cougar429; 08-23-2016 at 09:15 PM.
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Old 08-23-2016, 09:30 PM   #5
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Re: GMS engine quits

Did the engine get broke in properly? I've seen engines give lots of trouble simply because they weren't broke in. The needle was inconsistent all the time. Before you mount an engine inverted you should have it running right. I've had several GMS engines that all ran just as good as my OS do. Cheers Floyd
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Old 08-23-2016, 09:38 PM   #6
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Re: GMS engine quits

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Originally Posted by floyd View Post
Did the engine get broke in properly? I've seen engines give lots of trouble simply because they weren't broke in. The needle was inconsistent all the time. Before you mount an engine inverted you should have it running right. I've had several GMS engines that all ran just as good as my OS do. Cheers Floyd
Same here Floyd...... I have a .47, two .61 and the sister company TH .75, a couple mounted horizontal and two vertical and all run great and never, ever dead stick. They don't shut down as easily as the OS but I've learnt to live with that as they run so well.....lol
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Old 08-24-2016, 07:53 AM   #7
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Re: GMS engine quits

Not enough back pressure from the pitts style muffler, block off one of the outlets to get more pressure in the tank.
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Old 08-24-2016, 10:52 AM   #8
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Re: GMS engine quits

I had a GMS .40 and it was a real dog!!! I tried a different carb, a shim under the head, nothing seemed to work. Every time I used the throttle it would go dead lean. After I did a real close inspection I found the cylinder to be too low in the crankcase. Wrong machining of the deck hight. I put a .050" aluminum shim under the cylinder and it ran a lot better and would maintain a good needle setting after using the throttle. One of our club members threw his GMS .47 it the bushes.
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Old 08-24-2016, 02:01 PM   #9
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I think that with the cheaper engines, you take a risk. It might work fine, or it might be horrible. For the price, they are probably pretty good.
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Old 08-24-2016, 02:11 PM   #10
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Re: GMS engine quits

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I think that with the cheaper engines, you take a risk. It might work fine, or it might be horrible. For the price, they are probably pretty good.
Agreed....maybe I just got damn lucky but they run as good as my OS engines other than a slight shutting down issue. They are not made anymore though so parts may not be available and will probably be just throw aways if something major breaks.
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