RE: GMS Engine Tuning Problem
 

That's not substantiated yet.

They are taking new orders for GMS engines and parts.

Nor are the Tower Engines being shipped at the moment, they have exactly the same delays on them, the GMS and the SuperTigre engines.

All three have the same "expected" delivery dates on them, and all three have been pushed back by exactly the same amount.

Since all three come from exactly the same plant, which seems to be re-tooling after the aquisition of Super-Tigre and GMS, it is still possible that all three engines will be forthcoming... even though these ##$%) delays are annoying.

I did receive a part for the SuperTigre and GMS engines last month, that I had ordered in November...



Yup... but is that company going away? Or is it that they are retooling? Had a fire? etc.?

I written and talked to Tower reps several times, and they basically say that they are at the mercy of the deliveries from the factory.

When I press them about GMS engines no longer being produced, they tell me "no, we are still awaiting delivery of that product".

So there is no confirmation nor denial either way. AAARGH!

I've been tempted to cancel my orders for GMS engines, but at this point I want to see how this plays out.

RE: GMS Engine Tuning Problem
 

You may want to contact this Australian company who has GMS engines in stock [link=http://www.modelaeroproducts.com.au/shop/index.jsp]Model Aero Products[/link]

I've not had much contact with them but believe they can source parts for most GMS engines. The exchange rate is 1USD=0.93AUD so those in North America should be able to offset the postage costs with savings on the exchange rate.

RE: GMS Engine Tuning Problem
 

Can anybody help? I have a inverted 40GMS in a trainer that just cuts out after a while (after starting and walking to the take off point)- no running high, and after performing perfect on all attitude testing of the plane. I notice that the back pressure seems high when rotating the prop. Needle tuning does not fix the problem.

RE: GMS Engine Tuning Problem
 

Hi,

This has been stated before, but if you follow these procedures and did the mods identified in this thread, you should be OK.

Adjusting 2-cycle Glow Engine Carburetors:

First make sure there is no air leaks in the fuel lines, make sure the fuel lines are not kinked and the fuel tank is not too far from engine. Ideally, the center line of the fuel tank should be in line with the fuel pick up on the carburetor. The tank “klunk” pickup should not block fuel flow, that is, it should never touch the end of the fuel tank when the nose of the tank is pointed straight up (test when tank is out of aircraft). Also if a pressure tap from the muffler is used, make sure the tap is not clogged and the pressure feed to the tank does not have any air leaks, nor is blocked.

With the engine level and running at full throttle lean the mixture (turn the needle towards the closed position, usually to the right) until the engine wants to stop. Then quickly richen the mixture (open the needle valve, turn to the left) until maximum RPM is observed.

At maximum RPM, continue richening the mixture (slowly turn needle to left) until you note a slight drop in rpm. This should be will be the normal operating mixture for the high speed adjustment.

There are two styles of low speed carburetor adjustments, that is, a 2nd low speed needle valve or an air bleed adjusting screw. If it is a low speed needle valve, the needle valves would be in line, and opposite each other or in the center of the throttle control arm. If you have a low speed air bleed screw would be to one side of the carburetor and will open and close an air hole.

These both are low speed, idle, adjustments and such low speed adjusting should be done with them alone, the high speed setting should not be NOT TOUCHED once adjusted as described above.

Low speed idle adjust speed procedure:

Close the throttle to the slowest steady rpm. From that, quickly open the throttle.

TOO RICH (too much fuel in mixture):
Did the engine run rough, burp, want to stop? If it did, the mixture is too rich. If it slowly died, it is too lean – see next section.

If you have a low speed needle carburetor, this can be leaned by turning the low speed needle in (to the right) about 1/16 revolution at a time.
Repeat (after first opening the throttle to clean out any excess fuel in the crankcase) until the change from low to high is immediate and clean.
If a lower idle speed is desired, close throttle a little and repeat the procedure.

If the engine has an air bleed carburetor and the initial quickly opening the throttle indicates a rich mixture (as described above) the air bleed screw has to be opened a little. This will let more air in, try turning the air bleed screw to the left 1/16 turn at a time. Follow same test procedure as with the needle valve procedure when testing, that is, opening throttle quickly.

TOO LEAN (too much air in mixture):
If the engine ran smoothly during initial quick throttle opening the engine, then slowly quit. The mixture is to lean.

If you have a low speed adjustment needle, turn the low speed needle out (to the left to give more fuel) until the engine does not quit when the throttle is quickly opened,

If the engine has an air bleed carburetor, and the mixture is too lean (too much air) then turn the air bleed screw in, to the right, to limit the air in the fuel mixture.

When proper low speed setting is attained the engine will transverse from low to high smoothly time after idling with no tendency to stop. If not increase the low speed rpm very slightly until action is proper.

If when an engine has been set up with this routine and you still see the problem you describe the trouble is probably in the fuel supply system not the engine, see the notes at the beginning.

Hope this helps.

Fly4Fun,

Wayne Miller

RE: GMS Engine Tuning Problem
 

Thanks for the info, i'm going to work on mine soon, i have to ask how did you take the picture, it is super clear, what kind of camera did you use, thanks....

RE: GMS Engine Tuning Problem
 

Well i can now fix my GMS 120 to run Correctly i hope. Lots of great info..

RE: GMS Engine Tuning Problem
 

Good day guys just a comment on the gms .47 engine problems , i had a similar problem and solved it by fitting a os LC4/3 longreach glow plug as gms recomend using a long reach plug and i have noted some of you are using a os no8 plug wich i myself was using . I am running a gms .47 abc with a (silver centre) pipe and no baffles I also found that people some times over tighten the carb wich ovals the hole and affects tuning .
Hope this might help have a great day

RE: GMS Engine Tuning Problem
 

i have a very good running GMS 25 but the only problem i have is it will not shut down on closed idle unless i block off the exhaust . i have set the venturi to totally closed and this engine will idle for as long as possible at a very low idle rpm but will not stop. any ideas guys ?

RE: GMS Engine Tuning Problem
 

....most likely the carb is not down tight on the o-ring, and it has
an air leak there. This is common on the Tower engines as well.

Loosen the cinch bolt and press or tap the carb down tight until
the o-ring is compressed and sealing properly. Then snug the
cinch bolt back up.

FBD. [sm=teeth_smile.gif]

RE: GMS Engine Tuning Problem
 

In my tower 75 I heard a "click" when the carb was fully pressed!

Jorge

RE: GMS Engine Tuning Problem
 

I have a GMS 120 basically the same as the smaller engines. And the key to getting any little 2 stroke engine to run good is to make sure it is sealed. crankcase especially.. i bought mine on ebay for 50 bucks and when i got it it wouldn't hardly run at all. I took it apart and checked everything. and re sealed the crankcase and replaced the O ring for the carb. and put a gasket betwen the engine and muffler. And put a 4 stroke plug in it and now it screams. and will shut off upon throttle closing a great running engine once you get it sealed. the GMS and tower engine's are cheaply made and after they have been run and broke in they tend to develope leaks.. and wont tune properly Seal it real good and i'm sure there wont be any more problems this goes for the needles in the carb having a good seal as well.

RE: GMS Engine Tuning Problem
 

did you see my reply on the other post,,,,

people THINK ,, these,, or any model plane engine needs to rev un-Godly RPMs to run correctly,,, NOT true,, MANY tuning problems can be corrected by simply adding/mounting a larger diameter prop !!!


my reply,,,,

hey,,,,,,, my friend Doug had the same problem with his GMS, after 3 guys spent 2 hours trying to fix the problem I walked over and told him to put a larger prop on it,,,, problem went away !!!!!!!!!!!!!!!!!

true story !!,,,, his GMS kept stalling on take off,, I told him he needed to load the engine more, put a larger prop on it,,,, problem gone !!!!

Jim

RE: GMS Engine Tuning Problem
 

my GMS 25 runs very good but the only problem i have is, the darn thing don,t want to shut down. it will idle as low as possible and keep idling until i drop a rag on the prop.

RE: GMS Engine Tuning Problem
 

Hi

Typically when an engine does not shut off at idle, you have an air leak. This could be either at the carburettor base or the back plate.

If you can not seal the engine by tightening screws or replacing the carburettor "O" ring, you will have to smear a light amount of RTV (or other silicone high temperature sealant available from automotive stores) on the mating surfaces of the carburettor "O" ring, carburettor mounting base or the back plate, reinstall the components and let sit overnight.

Hope this helps.

Fly4Fun,

Wayne Miller

RE: GMS Engine Tuning Problem
 

Wayne,


You forgot to mention the most obvious leaks... Between the carburettor body and the carburettor barrel, which rotates within it.
If the carburettor parts are machined at the extremes of tolerance (the barrel is the smallest acceptable diameter and the carburettor body cavity is the largest acceptable diameter), a leak significant enough to prevent engine shutdown could result.

The other place is between the draw-bar/pinch-bolt and the engine's crankcase.

Both leaks can easily be overcome, with a small smear of silicon vacuum grease (that will virtually never wash off).

RE: GMS Engine Tuning Problem
 

Hi Dar,

I guess my description wasn't as good as it should have been. Thanks for the clarification.

Fly4Fun,

Wayne

RE: GMS Engine Tuning Problem
 

thanks both of you guys, i will try this.

RE: GMS Engine Tuning Problem
 

Wayne, FBD, (and company), Im only on page 8 and am reading very carefully...but had to skip to the end to see how the story ends:D and here you are, still harping on about how to fix this motor, almost 4 years later. My hats off, big time kudos to you bud....and my GMS will be screaming shortly;)

RE: GMS Engine Tuning Problem
 

Hi Dad_Roman,

I started this thread because of the problems I was having with the GMS adjustments.

I'm glad to say that now, after making sure the case and carburettor is sealed for air leaks, and the fuel intake hole drilled as described, all my engines work great. In fact, I've been able to pick up some at a very reasonable price because other's could not get them to work. The bigesst problem, besides the above mechanical modifications, seem to be the low speed needle being set too lean, or the motors not being fully broken in.

After your post, I looked at the number of people who have looked at this thread, it is hard to believe over 30,000 have looked - I wonder how many we were able to help?

There is no way I could have got the engines running without the help of everyone here. Thanks so much.

Fly4Fun,

Wayne Miller

RE: GMS Engine Tuning Problem
 

Yea Im glad I found this thread;)

My symptoms are slightly different, probably due to luck, from everyone elses, although I believe the solutions are the same.

My 47 idled ok, died in the pits ok, transitioned fair, screamed on the top end. If I did any aerobatics it would surge and act up a little. however, the weirdest symptom was if you turned the HSN it had little to no effect. It would kill the engine at appx. 1 turn out. Any thing more than that and the engine would scream, all the way to the needle litterally falling out in my hand! After reading all this it still stands to reason....fuel starvation. Just drilled my carb, fixin to run it in the yard to check it out. Will get back to ya:D

Thanks
Shane

RE: GMS Engine Tuning Problem
 

And the results are..........RUNS LIKE A TOP!

HSN is now sensitive within half a turn as it should be. I can go from slobby 4stroke to lean, as should be. Cant wait to fly it!

XJet said in another thread that he would shorten the LSN 1mm. If you look in your barrel at WOT (mine) the LSN is still recessed into the flow bar, this also didnt seem right to me, too much restriction. I shortened mine the 1mm. Now the LSN clears the end of the flowbar about that 1mm...I like it. I also drilled out the carb nipple and the misaligned hole in the flowbar below it. I used the nipple as a guide and believe I went 2 steps up. I ended up at #52 in the ole index at .065.

RE: GMS Engine Tuning Problem
 

Wayne, "Snoopy", all,


I also participated in this thread early on and even learned something new, about dynamic fluid flow properties...

Although I never owned any GMS engine, it seems strange to me that the fuel-nipple alignment 'bugaboo' affected such a large production volume of model engines, without anyone at the production plant bothering to notice... Without a positive step to rectify this production error...
...And from the sheer volume of onlookers in these pages, it does seem to be a very wide-spread occurrence.


If this engine is indeed made by the same company that makes Tower (and also Super Tigre) engines, I would also add the unseated carburettor that appears in the Tower web site photos of the Tower .61 and the Tower .75 engines...

Could it be a production error that does not allow the carburettor to be seated in its socket? Or is is just sloppy assembly!?

After all, this is a formal web site photo, Photoshopped and all... Not a sloppy two-bit novice job...


My belief is that such production errors must be rectified ASAP and 'on the fly', including backup fixes.

RE: GMS Engine Tuning Problem
 

I'm not familiar with Tower engines having never owned one, but after purchasing several new GMS engines, I know that they come un-assembled in the box. The carb is in it's own little plastic bag and it is up to the purchaser to properly seat and attach the carb and muffler.

You would think that the factory would have cottoned on to the problem, but it seems they haven't.

I recently purchased a new carb for my GMS .47 due to smashing the originel one off in a crash. I took the new carb home, set up the drill press with the right size drill and got it ready to drill out the fuel hole before even looking to see if it was misaligned. And sure enough, it was. Drilled it out, cleaned it and assembled it and ran the engine. It ran as hard and strong as it always did. I didn't bother to waste my time checking if it would run without drilling the carb. I knew it wouldn't before I even walked into the shop to buy it.

A post earlier in this thread says something about how drilling the carb is not solving the problem. It's only putting a bandage over the real problem. But for me, this bandage of drilling the carb is as good a fix as there will ever be as long as the factory keeps putting out misaligned fuel jets.

For those that have recently joined this thread - Don't be put off buying a GMS engine. They are good, strong reliable runners. I would buy one over an OS anyday. I know there have been problems with supply from Tower, but I think this is a Tower problem, not a GMS (or Sanye) problem. Our local importer has no problems getting them in.

RE: GMS Engine Tuning Problem
 

....yep, I must say my .47 is running absolutly perfectly. The restricted
fuel inlet being the key. Of course, like in the case of the Tower engines,
the carb must seal against the o-ring....or the engine will always be
problematic, with the air leak.

The problem with the Tower .75 was mainly failing to shut off, even with
the carb completely closed. I bought one of the the very first Tower 75's.
When I installed the carb, it was so tight in the case it would not go all the
way in with hand pressure. I looked carefully after I pressed it down as far
as I could by hand, and I could see that the o-ring wasn't seated. I made
sure the cinch bolt was loose, then I put a small block of hardwood over
the top of the carb and gave it a sharp little rap. This seated the carb, and
I could see that the o-ring was squashed and sealing properly.

I never had the shut down problem because of the carb air leak.

Just like with any engine, the owner should go over all the bolts and screws
to insure their proper torque. Too many times, and inexperienced enthusiast
will install and begin running an engine without insuring it's proper assembly.

Loose headbolts have caused their fair share of deadsticks.

All in all, I would recommend the .47 GMS to anyone. I think I paid $65.00
for mine. It turns the 9-7 MAS prop at 16,000 even, and really hauls the
sport Quickie 500 plane. ;)

FBD. :D

RE: GMS Engine Tuning Problem
 

FBD - Thats a nice looking quickie. I bet it hauls rear-end.


Many people don't realise it, but the GMS engines actually use two orings to seal the carb. there is the one on the carb spigot that you can see and squash down when seating the carb, but there is another one lurking around as well. If you look down the hole that the carb goes into (with the carb removed) you will see another oring sitting in a recess near the bottom of this hole just above the crankshaft. It is important you take note of this oring when seating the carb. I have pushed a carb down to seat it, and the had the bottom of the carb spigot grab this oring (instead of sliding past it) and dislodge it. A bit of oil on this oring will go a long way towards succesfully seating (and sealing) the carb.

Just another thing about GMS engines: they are supplied with 3 shims under the head. These 3 shims look like 1 thick shim ( I think they put 3 sheets of copper together then stamp out a shim from them) but if you look closely, you will see that they can be separated into 2 thick shims and one thin shim. This gives a lot of flexibility for adjusting the compression - the possibilities are:

-all 3 shims
-2 thick shims
-one thick and one thin shim
-one thick shim
-one thin shim
-maybe no shim at all, but I wouldn't recommend this

You should take this into account when tuning you GMS for different prop loads and nitro content. They run very well on low nitro fuel and high compression.
I run my 47 on one thick shim, 0% nitro FAI fuel and a hot A3 plug and around a 10x7 - 11x5-6 Bolly or MA or APC prop. It screams and idles and transitions as well as you could want.