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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...

Wayne Miller 10-28-2004 04:10 PM

RE: GMS Engine Tuning Problem

I agree that in theory the baffles should not be needed, however I just got back from the flying field and have tried several things. The results are below.

First I'll re-state that this on the older style GMS, and all air leaks have been sealed, except for a very small amount of air escaping through the front bearing. Some state this is normal - I'm not sure. I should also let you know my newer GMS works great and so I have experince with both styles.

Today I tested without the baffles, and also with different number of baffles. In all cases I ran with a 12.25 X 3.75 prop.

I should let you know the older GMS works fine with the newer GMS muffler. So I have concluded the problem is with the muffler and the pressure feed. If you have any other suggestions to increase the pressure feed, I would appreciate them.

I reinstalled the older muffler and worked with it.

I first tested with the two baffles as shown in the previous picture, and all worked fine, adjustments were easy, idle was good and high speed was constant.

Second I tried smaller baffles, that is, 1 inch instead of 1.25 inch. This gave me a broader adjustment range at high speed, but I did get more RPM. The engine would quit after a while at full throttle for no particular reason.

Third I removed the baffles completely and again I got more RPM but the engine would quit running at full throttle after a while.

Fourth, I installed just the original first baffle (1.25 inch) at the front of the muffler. The muffler was a little louder, but all adjustments were easy to make. The difference in RPM was 200-400 less than without any baffles installed in the muffler.

I settled on just using one baffle, the one closest to the front of the muffler and flew the GMS with a Somethin' Extra for 6 flights with no problems.

I'm convinced the problem is with either the fuel draw, or low exhaust pressure to the tank. Whatever the reason, the baffle does the trick for me.

In this case I have a working, flying engine instead of sticking it on the shelf.

I appreciate everyone's input, however, if you haven't tried the above, and are commenting, I should let you know I would have agreed with everyone BEFORE I got this engine. If you have an old GMS .47 that isn't working, give it a try, I think you'll be surprised at the improvement. In one of the previous posts someone stated "you gotta do what you gotta do to make it right" - good advice.

If you have any other suggestions I can try, I will give them a shot, just let me know.

If you have one of the older GMS .47 with the same problems, I hope you will give the above a try and give us your feedback.

Thanks for your help,



AirGar 10-28-2004 06:39 PM

RE: GMS Engine Tuning Problem

The 2 47's I have are the newer ones, both with the "tuned muffler". What gets me is, one has been flawless outta da box... :D , while the other is cutting out at full throttle. I haven't tried anything yet, but I will let you know what I find out with mine.

Both running the same fuel, same plugs, same prop (APC 9x8) and both mounted upright. Doesn't make much sense does it? ;)

Glad you got yours running. If it came down to it, and I couldn't get it to run right....I'd probably take them both apart and check/compare everything before I installed a baffle. Ya just never know....I may end up doing what you did! lol

I DO know they will run as they come from the factory, so I'm sure my stubbernness won't allow me to go your route unless it's a last resort.

BTW, did you try relocating the pressure nipple and/or enlarging it? Perhaps there a burr/flashing in it or the muffler causing restriction? I'm sure you checked this, but thought I'd ask anyway. Maybe that's what I shoud do first....compare mufflers.


Razor-RCU 10-28-2004 06:52 PM

RE: GMS Engine Tuning Problem
I need to get a GMS and the Sonic so I can play on the cracked dirt with Dave and gary----

I would like to argue with someone but A. I don't have this engine 2. I am not smart enough to know my own point of view and c. Why would any group decide on GMS for club racing? :)

AirGar 10-28-2004 07:00 PM

RE: GMS Engine Tuning Problem


I need to get a GMS and the Sonic so I can play on the cracked dirt with Dave and gary----

I would like to argue with someone but A. I don't have this engine 2. I am not smart enough to know my own point of view and c. Why would any group decide on GMS for club racing? :)

We agreed on that combo, as not everyone has the moola like you and I. :D

I have another (yes, #3 NIB) that I'd be more than willing to part with.....didn't you say you had an extra Rossi 45, or Jett 50 lying around gathering dust? ;) :D


Wayne Miller 10-28-2004 08:08 PM

RE: GMS Engine Tuning Problem
Hi AirGar,

I feel bad that you have the same problem, but I'm glad I got someone to troubleshoot with.

I did check the exhaust pressure nipple, and it is clear - however, I'm not sure if making it a little larger will help. I'm reluctant to make "destructive" fixes (drilling) except as a last resort, I don't like to order new parts, so I always try "fixes" that can be easily undone. You suggested relocating the pressure nipple - any suggestions where it should go?

I do know the local SuperTiger fans always drill the fuel nipple (into the carb) one size larger and this fixes their fuel delivery problems.

If you suspect the problem is the muffler pressure, see if the fuel in the line (residue from filling) migrates towards the tank when the engine first starts. Note, at first starting the tank begins to pressurize, after it starts, the tank is pressurized and the fuel moves slowly, so you have to see if pressurizes it when the engine first starts.

If lack of pressure seems to be the problem, I would first try the "good" engine muffler on the "bad" engine, if the problem clears up, then we know it is the muffler - if not, then it is something else. If the problem is in the mufflers, then very carefully dissassemble the second muffler and examine the exhaust nipple, both ends, before removal and see if there is any differences. Look at the openings of the nipple to see if there is any metal flashing or if there is special orientation compared to the other. This is the only thing I can think of that may be different between two similar mufflers.

The other thing you may want to try is to remove the carb from the "good" motor to the "bad" motor, if the problem is fixed, then the carb settings will be the problem. If the problem is not fixed, then it is an air leak, fuel delivery problem, muffler.

My motor is working fine now, but I'm sure there is a better fix than the baffle, I'll post anything new I find, please let me know what you find.

Thanks for your help.



Wayne Miller 10-29-2004 06:37 PM

RE: GMS Engine Tuning Problem
Hi AirGar,

Just an update.

I was flying today and all went well with the baffle in place.

However, I put a lot of thought into the problem and when I got home, I decided to dissassemble and take a look at the differences between my "old" and "new" GMS engines fuel delivery systems.

One of the things that did not look right was the fuel opening into the carb under the fuel nipple (the one the fuel line attaches to). It did not seem to quite line up with the hole from the fuel nipple.

To explain, if you take the fuel nipple off of the carb body, you can see a brass insert under it in the carb. The brass insert has a hole in it. The hole in brass insert did not quite line up with the hole in the fuel nipple. It seemed to pass air OK, but I'm not sure about how it delivers fuel. This could be causing the fuel restriction at full throttle - I'm not sure yet, I still have to test. But this would explain why more muffler pressure helps at full throttle.

I then reassemble, attach a fuel tube to the carb nipple, and blew with the throttle wide open, and needle valve backed out a little. I listen to the amount of escaping air.

I then removed the nipple again and carefully hand drilled (not electric - I didn't want to damage the threads for the nipple) through the brass so the hole to the carb was slightly larger. I inserted a pipe cleaner into the needle valve opening to catch the brass particles.

I expected that if this slight restriction, due to misaligned holes, was causing the problem, this would allow the fuel to flow freely into the carb from the nipple, since the openings now line up.

I also drilled out the fuel nipple and exhaust nipple to the next size larger - I figured this wouldn't hurt anything, and may help. Now when when I reassembled, put the needle valve back to the last setting, and did the "blow" test at full throttle, more air escaped with less resistance.

I don't think I'm going to be flying this week end, but if I do, I'll let you know what the results are. I expect the engine to perform better, and I may be able to remove the baffle.

If possible, would you mind carefully removing the fuel nipple from your "bad" engine and see if the hole in the brass insert in the carburettor lines up with the fuel nipple?

Hopefully we are getting closer to solving the problem, I let you know my testing results as soon as I know.

Thanks for your help.



AirGar 10-29-2004 10:28 PM

RE: GMS Engine Tuning Problem
Ahhhhh Wayne....I think your onto something Buddy!

I'll be gone all day tomorrow, so I'll try to take a look at that on Sunday.

Maybe you should try the first flight without the baffle.....

Look forward to hearing your result!


Flyboy Dave 10-30-2004 12:24 AM

RE: GMS Engine Tuning Problem
You know....I went in and looked at my engine....I backed the main out
about 3 turns, and blew into the carb/fuel inlet line....

....it seemed like it was pretty restricted. :eek: Not a real scientific test, I
know....I'll have to look further. ;)


Wayne Miller 10-30-2004 08:29 AM

RE: GMS Engine Tuning Problem

Thanks for checking, I will be interesting to see what we come up with.

Just a hint. I was really concerned about damaging the threads where the nipple goes in and was very careful when I did the drilling. If I was to do it again, I would first remove the needle valve, then remove the fuel nipple, drill the fuel nipple to the next size larger (be careful to keep the drill in the center of the nipple!), then put the nipple back in and use it as a guide for the same drill to carefully drill through the brass insert. Again, I would put in a pipe cleaner or piece of rag in the needle valve hole to catch the brass filings when drilling.

Its raining here today, and the Yahoo Weather site is forcasting rain for the next few days, so I may not be able to test for a while - if there is a break in the weather, I'll let you know the results as soon as I can.



Wayne Miller 10-31-2004 04:52 PM

RE: GMS Engine Tuning Problem

Just an update.

After I drilled out the carb input, I ran it today in the back yard first with the baffle left in - it worked great except it would tend to lose power with the nose pointed up.

I removed the baffle ran the engine again, had to readjust the high speed settings, but the engine ran very well.

It looks like tomorrow will have a cloudy, rainy day with some sunny periods. If I can, I'll test it at the field tomorrow, only "real" flying will tell us how it works.

I suspect the problem was fuel starvation at high throttle.



flyoz 11-01-2004 02:18 AM

RE: GMS Engine Tuning Problem
all very interestin stuff guys +
my 2c's worth; I read with the TT motors for example, you should always check the fuel pressure nipple that it doesnt have any mold flash/residue/blocks or isnt completely blocked.
That will cause hassles Maybe your baffle overrode this?

Wayne Miller 11-01-2004 09:01 AM

RE: GMS Engine Tuning Problem
Hi Flyoz,

Thanks for your input, we are heavily overcast this morning with a few sunny breaks expected, I'm going out to the flying field and will let you know how the motor works with the exhaust pressure nipple, input nipple and carb input hole drilled out.

I'm hoping all will go well.



Wayne Miller 11-01-2004 07:35 PM

RE: GMS Engine Tuning Problem

It was very cloudy with a few sunny breaks today and I had 5 flights with the GMS .47 today without the home made baffle(s).

The first flights two were used to fine tune and adjust the engine (with plenty of landings to tweek the needles) and the last 3 flights were great without adjusting the carb.

I'll go over what needs to be done. The problem seemed to be the fuel hole that is below the fuel input nipple in the carb, not lining up with the fuel nipple hole. I think this causes fuel starvation at full throttle.

What I did was to first carefully drill out the exhaust nipple and carb input nipples to the next size larger (to 5/64 of an inch). I then "free handed" carefully drilled the brass insert below the carb nipple to 5/64 inch (in the future I would drill the nipple, then install it, use it as a drill guide to carefully drill into the brass insert to avoid any damage to the carb's nipple threads). The new, larger holes now lined up and gave me better fuel draw and better "exhaust to tank" pressure. Be sure to remove any brass filings when you drill the carb. Note: Drilling the holes will modify the engine and therefore void your warrantee, so if you chose to do this, be very careful since its at your own risk.

At first I had a difficult time adjusting the engine high speed and low speed settings - these seemed to intereact with one another. Example, leaning the idle, also leaned the top end - I would readjust the top end, and it would affect the low end, and I ended up with a never ending cycle.

What worked for me was to:

1. First go to wide open throttle (WOT), adjust the high speed needle for max RPM then, while while still at WOT, tweek the low speed adjustment to fine tune the engine at WOT.

2. Then go back and adjust the high speed needle and then back again to tweek the low speed adjustment. Keep this up until you have the best WOT performance.

3. This will be very close to the proper adjustment at both WOT and idle.

4. After you get the best WOT with both high and low speed needles, carefully idle down and adjust your low speed RPM (by programming the transmitter or adjusting linkage).

5. Once you find the best idle speed for your prop, then "poke" full throttle. Turn the low speed adjustment no more that 1/4 turn either way until the acceleration from low speed to high speed is smooth. My low speed needle adjustment needed to 1/8 turn leaner (tightened "in" ) .

6. Test at WOT with plane level and nose up. Mine worked OK at this point.

The final adjustments for me were:
Turn in high speed needle in until it just stops, then back out 2 1/2 turns.
Turn the low speed needle in until it just stops, then back out 1 3/4 turns.
This may not be dead on for you, but will give you a good starting point.

I hope the above will help someone else make their GMS .47 the power house and reliable engine that it can be.

I hope someone else will give the above a try and let us know if it worked for them - this one test is not enough to verify if this is the "fix".

If you have any questions, or I can assist with anything, please let me know.


Wayne Miller

flyoz 11-02-2004 12:05 AM

RE: GMS Engine Tuning Problem
Outstanding Wayne- boy you sure earn your wings for dedication.

Im glad the "check the holes for blockages" idea was it :) ( n0w I can tell my wife I can prove Im a legend lololol)

Well done mate and happy flyin.
PS the whole string has been a real learning experience again well done mate and 10 points for effort :D

Wayne Miller 11-02-2004 08:17 AM

RE: GMS Engine Tuning Problem
Hi FlyOz,

Thanks, however, we still have to get some other people's tests and results in before we are sure this is it.

Nice country you live in, I've only worked in Sydney and Melbourne and liked the people and really enjoyed the zoo and aquarioum in Sysney.



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