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Wayne Miller 10-26-2004 07:05 PM

GMS Engine Tuning Problem

I have two 2000 GMS .47 engines. One is older, that I just got, and I also have a new one. The new one has a muffler with fins and works fine. The older one has a muffler with a polished center section and is giving me problems.

I put the older one on a SE and took it to the field today. I'm using 15% fuel with synthetic/castor and the engine has about 1 gallon of fuel through it.

I had a very difficult time adjusting wide open throttle, however the idle adjusted fine and would shut down at low throttle with no problems. Transition from low to high throttle is good. The problem occurred if I left it at wide open throttle - it would all of a sudden stop (no warning) as if I had a fuel blockage, or fuel pressure problem. I noticed it was not over heating (not too lean). I checked tank, fuel lines, clunk, filter and checked needle valve and seat for contamination - all were clear. Also checked for leaks in fuel line, and pressure from muffler - all is OK. I also changed glow plug with same result, but RPM was slightly better.

I found the wide open throttle adjustment was very broad, like one turn either way, and never did seem to really peak. On my new engine the adjustment is very touchy - like two clicks either way.

From reading the posts here at RCU I remember that GMS had an air leak problem. So I returned home, put a new "O" ring on the carburettor then mixed up some soap and water. I plugged all openings on the engine (exhaust, pressure feed, carburettor intake, and fuel intake).

I then sprayed a little soapy water on the engine. Hooked up fuel line to the carburettor intake and blew into it with all the other openings sealed so there was no place for the air to escape.

I could see soapy bubbles (air leaks) at the carburettor retaining bolt as well as a very small amount at the front bearing.

I sealed the carburettor mounting bolt with copper RTV, and also the carb "O" ring for good measure.

I still have a small leak at the front bearing.

I'll see how it runs tomorrow, but I thought I'd check here to see if anyone has any other suggestions, or experince with the same problem.

Thanks for your help,

Wayne Miller

w8ye 10-26-2004 07:43 PM

RE: GMS Engine Tuning Problem
Your experience is typical of the GMS 47/ Tower 46/ Hobbico 46 engines.

On a side note, the shiny side muffler doesn't make as much tank pressure as the finned muffler.



Wayne Miller 10-26-2004 08:13 PM

RE: GMS Engine Tuning Problem
Hi w8ye,

Thanks for the information, my assumption is that they must be all made by the same people?

I understand they have a good warrantee, has anyone had any luck sending them back?

Do you know if what I have done should fix the problem?


Wayne MIller

w8ye 10-26-2004 08:20 PM

RE: GMS Engine Tuning Problem
Try the engine now that you have sealed the carb leak.

If your needle is still insensitive, try switching the mufflers. If it's still not sensitive, try opening up the LS mixture srew a little.

Typically with the shiny sided muffler, the engine will most usually have a rich midrange.



Wayne Miller 10-26-2004 09:00 PM

RE: GMS Engine Tuning Problem
Hi Jim,

I should be able to try it tomorrow if there isn't to much rain. I'll let you know what I find.



Flyboy Dave 10-26-2004 09:08 PM

RE: GMS Engine Tuning Problem
If it won't peak up, you have it over-propped. Some engines will complain
about over-propping, by just quitting....with out severe signs of over-heating.

FBD. ;)

Wayne Miller 10-26-2004 09:38 PM

RE: GMS Engine Tuning Problem
Hi Flyboy Dave,

Good thought!

I'm using a 12.25 X 3.75 which was recommended to me for this engine/plane combination.

I questioned if this is correct, and if blocking the carb airleaks does not work, I'll try a 10 X 6 or 11 X 4 and see what happens.

Thanks for your help.


Ed_Moorman 10-26-2004 10:09 PM

RE: GMS Engine Tuning Problem
A 12.25-3.75 should be just fine. I have run that size on mine with no problem. It will not over load or over heat your engine unless there is some problem with your specific engine. I have a GMS, Tower, OS, Magnum, Evo, Thunder Tiger, all in the .46-.52 size and they all run fine on that prop. I use that prop, an APC 12-4, APC 11.5-4 or an 11-6 on all these engines with either 10% or 15% Omega fuel.

The fatter muffler with the shiny center is the "tuned" muffler, usually known as the Tower muffler. They will normally add 700-800 rpm to OS and other engines. The muffler with the fins is the "Standard" muffler. I run one of those because I did not see much difference in rpm on that engine with the tuned muffler, maybe 200 rpm, while the OS & Magnum gained 700-800 over their standard mufflers. The prop I used for the test was a 12.25-3.75 APC.

The tuned muffler has a larger volume than a standard muffler and let the exhaust flow more freely. It does produce less pressure for the tank so you have to tune a little richer for acro planes or expect them to lean out in tight maneuvers. As I recall, the tuning was different from the tuned muffler to the standard when I made my rpm tests, but that is to be expected with tuned exhausts.

Wayne Miller 10-26-2004 10:18 PM

RE: GMS Engine Tuning Problem
Hi Ed,

Thanks for your input, I'll try a little richer settings as well and see what happens.

I have lots of stuff to try - they are calling for 30% chance of rain, but I still hope to get out.

I'll let you know what happens.



Stiks 10-26-2004 10:35 PM

RE: GMS Engine Tuning Problem
Some of the GMS engines take longer to break in than others. Seen one that would die at full throttle for some time but eventually it went away with a little patience and more time on the motor. Mine runs on 5% nitro and it has the 'tuned' muffler, loves to run on either a bolly 11.5 x 6 or and apc 12x4. The only tuning problem that I have had with mine was using a MA10x6 after it had broken in, the thing just revved it's head off and could not get any decent sort of carb setting from it, many hours of frustration latter we tossed the MA and put the bolly on and not a problem since.

Flyboy Dave 10-26-2004 11:22 PM

RE: GMS Engine Tuning Problem

ORIGINAL: Wayne Miller

Hi Flyboy Dave,

Good thought!
Wayne....:D....if you get 100 responces, you will get 100 prop/plug/fuel ideas.

Put a 10-8 Master Airscrew on it, or a 10-7 Zinger....and watch what happens. ;)
Forget those 12" props....that engine is a cranker. ;)


XJet 10-27-2004 03:15 AM

RE: GMS Engine Tuning Problem
I just spent the afternoon trouble-shooting a GMS76 (ringed) engine that was behaving very strangely.

After about 2 minutes in the air it would behave as if the throttle were cut back to about 1/2-2/3.

My first thoughts were that it was overheating, but it had done over an hour's running and richening up the needle until it was burbling wouldn't stop it from losing almost 50% of its power. What's more, it didn't have that usual "strained" sound that an overheating engine tends to produce -- but it did have an erratic metalic knocking noise.

We checked the fuel-lines, played around with the mixture, propellor sizes, tried two new plugs, and just about everything else we could think of -- all to no effect.

The guy who owns the engine was using straight 80/20 fuel with Klotz Super Techniplate oil and he tried mixing up a new batch with methanol from a freshly opened brand-new drum -- no different.

Then, when we tried some of *my* fuel, the thing ran like a dream with no signs of any power loss or other problems.

I'm using CoolPower blue (synthetic) that I've blended in a 2:1 mixture with castor oil (making 66% synth, 33% castor) and 10% nitro.

Since he didn't want to toss out the gallon of Klotz-oil fuel he'd just made up, I suggested that he add a few ounces of straight castor and a squirt of nitromethane.

This did the trick. The little bit of additional castor (about 5%) and the small amount of nitro (about 5%) completely changed that engine's performance.

I know that the Klotz SuperTechniplate has a small amount of castor in it -- but obviously it's not enough (for this GMS engine anyway).

The original poster doesn't say how much castor he's running in his fuel but if the engine is relatively new and seems to be sagging, try throwing a few more ounces of straight castor in -- you might be surprised -- I was!

I don't bother buying CoolPower Pink or Klotz SuperTechniplate because I think the levels of castor in them is too low. It's better (IMHO) to buy straight synthetic and add your own castor. This way you can vary the ratio to suit the engine. On my TT42GP I use half castor, half synth, on my TT46GP I use 2/3 synth, 1/3 castor, and on my little norvel I use all castor.

DarZeelon 10-27-2004 03:29 AM

RE: GMS Engine Tuning Problem

Klotz Super Techniplate (KL-200) contains 20% BeNOL Castor oil and 80% KL-100 Techniplate.

You can also get straight BeNOL from Klotz; just degummed Castor (bean) oil.

Ed_Moorman 10-27-2004 02:38 PM

RE: GMS Engine Tuning Problem
I would say that ringed engines won't break in very easily or quickly on an all-synthetic fuel. The ring doesn't seem to want to seat without some castor in the fuel. After a gallon or so of castor fuel, then the all-synthetic is OK, but I still prefer some castor. I have been known to run high nitro heli fuel in Saitos.

As for the GMS .47 ABC, mine did take longer to break-in than my Mag .52XLS or OS .46AX. it seemed like a gallon before it ran really well. I am running an APC 11.5-4 on it in a light Heckler 3D plane.

Wayne Miller 10-27-2004 07:44 PM

RE: GMS Engine Tuning Problem
1 Attachment(s)

I have isolated the problem and I would like to thank everyone for their help. I'll first answer some of the questions asked, and then explain what I did to troubleshoot the problem.

The fuel I am using contains castor and synthetic oil, but I don't know what percentage. It is the Omega Fuel by Morgan and contains 15% nitro. Again, I have 2 GMS .47's with two different mufflers. The engine with the shiny center section muffler with no fins is the problem engine.

Last night I had sealed the carburettor throat leak with copper RTV. I tried the engine today at the flying field with the different suggested props, but I still was not satisfied with the engine performance. As Ed identified in a previous post, the muffler pressure was very low and you could not see any "bubbles" moving to the fuel tank, this means the tank was not being pressurized.

I suspect this is why the "nose up" test before flight caused the engine to go lean, even with the needle set rich.

I was lucky to have lots of interest at the field and one modeler, Merrick, came up with the idea to insert a baffle in the muffler to create a little back pressure. This was made by cutting a 1.25 (1 1/4) inch disk out of the bottom of a soda pop can, and then putting a small hole in the center for the long muffler screw to go through. The inside of the muffler diameter is 1.5 (1 1/4) inch and the exit hole for the exhaust is .25 (1/4) inch. We felt that the .125 (1/8) inch clearance around the baffle would be sufficient. We only used one baffle, and if you reference the picture below, it was the baffle closest to the front of the motor. (Note: In the final version, I chose to use two baffles).

The engine worked very well after the addition of the baffle, and you could see the "bubbles" in the exhaust pressure line moving towards the tank. All the props that were suggested, including my original prop worked well.

Another modeler, Al, suggested we go to his shop and make more professional aluminium baffles.

The suggestion was to make the baffels with 1/16th aluminum, 1.25 in in diameter, then use wheel collars, compressing "O" rings, on the long screw to hold the baffles in place.

Instead I opted to put the screw in the front of the muffler, slide on a brass tube spacer, a washer, an "O" ring, the first baffle, the next "O" ring, washer, the next brass tube space, washer, "O" ring, baffle, "O" ring, washer, and next brass spacer. The reasoning, I was concerned about the collars coming loose, and thought the brass tube spacers may work better.

A picture of the set up, with center section left off for clarity, is shown below.

Hope this helps others in the future with this enginr/muffler set up.

Again, thanks everyone for your help.


Wayne Miller

Jerry Sigur 10-27-2004 07:56 PM

RE: GMS Engine Tuning Problem
Wayne, I have a GMS.47 with same muffler as you.
Before running the engine for the first time I sealed the
backplate and carb with red RTV. (I do this with almost
all my engine.)
I've never had any probs
with this engine. It runs as well as any engine in it's class.
Oh, I use Omege 15% in it.


Wayne Miller 10-27-2004 08:06 PM

RE: GMS Engine Tuning Problem
Hi Jerry,

Thanks for the input. I also sealed the carb and back plate as well as tightening the head last night. I still had the problem today before inserting the baffle.

It was obvious when I looked at the exhaust pressure line that it was not pressurizing the tank.

When I dissassembled the muffler, there was absolutely nothing inside except the long retaining screw. Once I inserted the baffle, the engine worked great at all orientations, and with lots of power.

Can you tell me the diameter of the exhaust hole in your muffler, and is there any baffles etc. inside your muffler?

I'm curious to find out why some of these engines work fine and others don't.

Thanks for your help.


Wayne Miller

Jerry Sigur 10-27-2004 08:13 PM

RE: GMS Engine Tuning Problem
I'll take the muffler apart tomorrow morning, but I believe it to be
empty of any baffles. I'll let you know.

Flyboy Dave 10-27-2004 08:21 PM

RE: GMS Engine Tuning Problem
I am curious as well. I thought maybe yours was missing a baffle from
the factory....I took the muffler apart on my new GMS .47 to have a
peek....no baffle. Now that I think about it, none of the Tower mufflers
have them either. That is the "tuned muffler" as opposed to the "stock
muffler" with the fins on it. The stock muffler may have a baffle in it, I
would think that it should.

I'm starting to think possibly something is amiss with the pressure
nipple (hole to small) vent line in the tank kinked, os something along
those lines. We ran a half dozen of those engines up this summer....

....no problems. The problem is not lack of baffling to increase the pressure
to the tank. You have worked "around" the real problem. :D

FBD. ;)

Wayne Miller 10-27-2004 10:02 PM

RE: GMS Engine Tuning Problem
Hi FlyboyDave,

Do you have an exhaust extension on your muffler? One person at our field does, and it this improves exhaust pressure for the tank - we think it may be the bend in the extension that gives a slight restriction to cause more back pressure - just a thought.

Another comment at the field was that the pressure nipple may be close enough to the front of the muffler to create a venturi effect, thus it may actually be causing a vacuum at the pressure nipple. I can't comment on this, but the baffles would help if this is the case.

I took a look and measured the "new" muffler with fins that works OK with the GMS .47. The body of the new muffler is smaller in diameter, but there is a slightly larger exhaust opening (.3 in compared to .25 in).

I wonder if the smaller diameter of the muffler body gives enough back pressure to cause more pressure to the tank?

When first starting up the engine, and without modification, you can see the "bubbles" on the pressure line of the "new" muffler moving toward the fuel tank. However, with the "old" muffler the "bubbles" were either stationary, or moving exceptionally slow.

It was mentioned that perhaps there could be a blockage or kinked pressure line, this was one of the first things I checked. It was visually clear, and fuel flowed freely out of it when refueling. Last night I also removed the tank, took off the stopper, and checked the lines and "clunk" just to make sure there was no air leaks and made sure the lines were routed without restriction.

All I can say is that with my new makeshift baffles, all is OK and the tank seems to be pressurizing OK, and the engine works very well.

I've heard a lot of people complain about the older problem GMS's, and other than sealing the air leaks (which may or may not have worked) there has been no definite fix.

I'm hoping that the baffles "fix" may help someone else who has previously run out of options. It is a great engine when it is running properly.

Thanks for your input.



Flyboy Dave 10-27-2004 10:27 PM

RE: GMS Engine Tuning Problem
1 Attachment(s)
I don't know, Wayne....maybe the nipple is in a bad spot, but if it were
you think the Manufacturer would have been made aware of it long
before now. I just looked at at Rossi .45, and a Tower .75 with tuned
mufflers, and the nipple is in the exact same place....way up front. :eek:

The thing that gets me is....we started up a half dozen of those engines,
and in every case, they ran perfectly right out of the box....even to the
point that the low speed needles didn't have to be touched....they idled
perfectly....and that in itself is quite extraordinary.

The bottom line is....you gotta do what you gotta do to make it right,
and that's just the way it is. I would however, like to find out exactly
what the particular quirk is in that engine, for future reference. It must
be something unusual. [sm=stupid.gif]

Good luck with your motor, Wayne. Mines going on a Sonic Q-500 ARF.
We have a group, and a "Club racing" thing going on.....lots of fun racing
those things....and we agreed on the GMS .47 over all the other engines
on the market. :D

Edit:....no, I don't have the exhaust extension.


DarZeelon 10-27-2004 11:52 PM

RE: GMS Engine Tuning Problem

This GMS muffler is the same as the Tower/Hobbico muffler.
It is used in stock form by many, as a low grade tuned pipe, since it adds significantly to the power of OS.40 and .46FX engines, Thunder Tiger Pro .46, OS clones of all types, like your GMS and some other engines with the same exhaust configuration.

There have not been many complaints of low fuel pressure with it.

Your solution will probably raise the fuel pressure, but will nullify the whole point of the muffler boost.
The 'open' construction is there for a point (pressure waves, exhaust supercharging) and no baffle exists, intentionally.
It was not inadvertantly left out...

I believe you have another problem with your fuel system.

Your solution is power destructive and you will have less fun flying your model, with less power.

It is like driving with your parking break on and stepping more heavily on the gas pedal to maintain your speed...

Jerry Sigur 10-28-2004 09:32 AM

RE: GMS Engine Tuning Problem
As stated above, no baffles in my muffler, the exhaust outlet is
roughly 7mm (as best I can measure).
Good luck with it, hope you find the prob.

AirGar 10-28-2004 12:53 PM

RE: GMS Engine Tuning Problem
Wow, I'm actually in agreement with Dar! :D

I don't know if ol' FlyBoy remembers, but my Sonic with the GMS 47/tuned muffler had the same problem....cutting out at full throttle for no apparent reason. Unfortunately, that was the last time I flew it so I'll be expecting that next time I ring it out (Never had the chance to work on it). My other 47/tuned muffler runs like a top, as do all the others where we fly.

I'll keep Wayne's problems/solutions in mind....fer sure

I don't think I'd go to the extreme of adding baffles and countering the reason for having the "tuned muffler" in the first place (as Dar stated), as there are too many of the same that run perfect. Has something simply been overlooked? It doesn't look like it as far as Wayne is concerned, so I'll be anxious to see what I find out with mine.

BTW, was running PM 15%.


DarZeelon 10-28-2004 01:17 PM

RE: GMS Engine Tuning Problem

If you take a closer look into it, you will see we agree on a lot more...

It is just each of us, from his own experience and perspective, is barricading behind his own position, so it looks like we are arguing.

If everybody agreed on everything, it would be a boring world. Don't you think so?

But since both you and Wayne are encountering a similar problem, with GMS .47 engines, exchanging ideas may bring you closer to a mutual solution.

...The baffles surely ain't it...

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