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-   -   GMS Engine Tuning Problem (https://www.rcuniverse.com/forum/glow-engines-114/2290854-gms-engine-tuning-problem.html)

Wayne Miller 06-20-2005 04:24 PM

RE: GMS Engine Tuning Problem

Good point FBD! I get caught with this every year, someone always brings out old fuel, and your right, it has the same symptoms as a bad plug!




Dean in Milwaukee 06-20-2005 10:53 PM

RE: GMS Engine Tuning Problem
Ok, I'll give it one more round using the wd40 leak finding method. Maybe it'll find the problem finally. I am tempted just to sell it off and buy a magnum 52 like my buddy as in his 4d. That one has run fine right from the beginning, and has lots of power, he's turning a 13x4 at almost 13k rpm.

For the record, this is whats been done so far:

all new lines, twice.
new tank
uniflow setup
new glow plug
fresh fuel, both 5% and 10%
new carb base o-ring combined with rtv compound
drilled mismatched carb inlet
drilled muffler nipple oversize

The motor is improved over where I started, but still is'nt right, certainly not trustworthy for 3d aerobatics as it could lose power or die any time the nose is pointed up.

Hopefully I'll have a chance to try again tommorrow before work.

Dean in Milwaukee

XJet 06-21-2005 04:02 AM

RE: GMS Engine Tuning Problem
The simplest fix is a restrictor in the muffler tailpipe. I've made and fitted these two a couple of GMS 47 engines now and it makes all the difference in the world. One is being used very successfully in a 3D ship and the mod has produced a very nice engine with reliability and throttle response every bit as good as an OS or TT engine.

NM2K 06-21-2005 01:46 PM

RE: GMS Engine Tuning Problem
Many of the Asian engines are sensitive to nitro content above 5%. Especially before they are fully run in.

I wouldn't run anything larger than a 10x6 or 11x5 for the first gallon of fuel. The last thing that you want to do is to stretch the liner larger than it needs to be for optimum power.

Ed Cregger

RC-Captain 06-21-2005 06:57 PM

RE: GMS Engine Tuning Problem
After 251 replies I was wondering if you ever got the engine to run properly. What I found on my MAGNUM .28 xl after dead sticking to death , was 15% was to darn much nitro. When I changed to 5% it ran and transitioned like a charm. I think it was over heating and then when I let it cool and tried to start the engine again it was just like starting the break in stage all over again .

Wayne Miller 06-21-2005 07:15 PM

RE: GMS Engine Tuning Problem

Just an update regarding wheter people got the engines running properly, I've had many PM's and emails from people with thanks for this thread and were able to get their engines working as they should. I think once they get the engine running, they just move on to other things and forget about updating the thread.

However, you bring up a good point that I never remembered until now. Perhaps someone else can verify if the following is true or not.

I had an older modeler tell me never to break an engine in on fuel that only has pure synthetic oil, it should have some castrol. If it is pure synthetic, the lube is so slippery that the engine will never break in properly. It will run "tight" until it over heats - expands and just stops. Apparently the synthetic fuel loses its lubricating qualities at a lower temperature than castrol and breaks down when it gets hot. I was told to properly break an engine in, you should have at least some castrol in the fuel. Can anyone confirm?

I always use fuel that contains at least some castrol, I'm currently using Omega - you have to look all over the label to find where it says castrol, but it also states it on their web page.

Hope this helps.


Wayne Miller

RC-Captain 06-21-2005 07:30 PM

RE: GMS Engine Tuning Problem
AS far as my four engines all of the instructions say use 5%-10% nitro with at least 18% castor. What you explained sounds true but of course me being a myth buster , on my OS .46 fx I used 15% to break it in and I am not sure what the other percentages where but it was an OMEGA synthetic blend. This weekend I used 5% in the OS .46 and it ran like crap. Guess I have to start carrying to jugs of fuel.

newglowhead 06-21-2005 10:44 PM

RE: GMS Engine Tuning Problem
I've got 2 GMS .40s, and have had my share of problems with them. Let me tell you what I found/discovered.
The first engine I got was just a screamer. It ran easily like my son's os .46fx. And it ran great all the time. So, I bought another one for my son for Christmas. We couldn't get it to run good no matter what we tried. Drilled out the nipples, mod the carb, replaced the carb, nothing. The only problem with my good running engine was it wouldn't shut off unless you put your finger over the exhaust. Air leak. So when my engine started running bad, I didn't know what to do. It would run it's usual screaming self, but after about 5 min in the air, it would just quit. Riched it up, tried again. Same. This went on for about 5 weeks. I finally started looking around on the web for some resolution. In the FAQ on OS web, several questions caught my eye. First, air leaks. According to OS, an air leak is no big deal, as it only affects an engine at idle. The amount of air sucked in once throttle is opened is so much more than the air leak, it makes no difference. OK, what else can I find? A question about an engine running great, then suddenly dying after a few minutes of flight. Again, caught my attention. It attributed this to overheating, and leaning out. It said it's either too much backpressure, or not enough. Since my GMS mufflers are totally unbaffled, I made some baffles for the muffler. I first tried just a 3/8" hole in the center, but this seemed to make no difference. I then tried another with a 1/4" hole. Engine ran just fine. Of course I lost some power, but the dying problem went away. Put the same sized baffle in the other engine. It started right up, and ran just fine. Transitions nice, idles nice, and opens right up to full throttle. I need more testing, but think my engine problems are solved. One other note: Before the baffles, the engine was very insensitive to changes on the high speed needle. Now, I can tell when the needle changes.

Dean in Milwaukee 06-22-2005 12:18 AM

RE: GMS Engine Tuning Problem
Well, I tried again today with the backplate and carb rtv'ed in place, all leaks are now history, and: not much better. Still has to be set rich to go nose up, so I bought and installed a magnum 46a tonight, hopefully get a chance to start breaking it in tommorrow. The gms will just have to wait to go into another plane that has the gas tank closer than this plane has its tank.

Thanks for all the help guys.

Dean in Milwaukee

XJet 06-22-2005 12:44 AM

RE: GMS Engine Tuning Problem
If anyone wants to try one of the exhaust restrictors I've made and report back, drop me a PM.

Wayne Miller 06-22-2005 07:31 AM

RE: GMS Engine Tuning Problem

Thanks for the information NEWGLOWHEAD, at the beginning of the thread we started out with putting restrictions in the muffler and that was our first clue that the fuel draw was probably the problem. Creating more exhaust pressure pushes more full to the carb. Thanks for the detailed idescription. Can you send us a picture of how you installed the restrictor? I wonder why one engine worked fine, then started having problems?

Hi XJet, I wonder if you could publish a description or pictures of the exhaust restrictor that cured your problem?

Hi Dean, sorry we weren't able to get the engine going. Perhaps in the future we can get back at it. Thanks for all your feedback, we'll miss ya!

Thanks for helping out.


Wayne Miller

newglowhead 06-22-2005 10:24 PM

RE: GMS Engine Tuning Problem
Here are photos of the restricter I put in the muffler. You can see it is the diameter of the muffler, and is held captive by the two halves fitting together.


newglowhead 06-22-2005 10:29 PM

RE: GMS Engine Tuning Problem
I guess I don't know how to add a photo to the post, but click on gallery and you'll see them.

Wayne Miller 06-22-2005 11:05 PM

RE: GMS Engine Tuning Problem
Hi NewGlowHead,

The pictures seem similar to the one in Post 15 at the beginning of this thread, if someone has a hard time viewing this on, try going to Post #15.

Hope this helps.


Wayne Miller

Flyboy Dave 06-22-2005 11:37 PM

RE: GMS Engine Tuning Problem

....I'm surprised the muffler needed that much additional restriction. That is the
"sport" muffler....as different from the "tuned" muffler. I would think that muffler
would have plenty of restriction to provide the exhaust pressure to the tank

I see also you have a rear needle carb set-up. I have a gut hunch that needle
is causing a restriction, that the additional fuel pressure is masking.

It was the same thing with the .47 GMS. The additional fuel pressure helped
pass fuel through the restricted inlet hole. The problem wasn't the muffler pressure.


newglowhead 06-23-2005 07:36 AM

RE: GMS Engine Tuning Problem
So why would the high speed needle be causing the problem? Are these known to be bad? I did check and clean the entire fuel path from the tank to the carb. I found no clogs anywhere. I suppose I could throw on some other needle to see what happens. On the engine my son has, the needle was very insensitive, and needed to be open more than I thought it should be.

Flyboy Dave 06-23-2005 09:53 AM

RE: GMS Engine Tuning Problem
It seems to me that something is restrictive in the fuel system. The high
speed needle being insensitive is the key. Possibly the clunk needs drilled
out, the clunk up against the rear of the tank, the vent (pressure) line
from the muffler kinked, feed line to the needle valve kinked inside the
plane, ect.

Dave. ;)

Dean in Milwaukee 06-24-2005 09:44 AM

RE: GMS Engine Tuning Problem
Just an update for those you think their gms .47 fuel supply might be due to their fuel system and not the motor itself:

Installed and ran through 2 tankfulls on the ground of my gms .47 replacement, the magnum .46a, then leaned it out properly, richened it to lose about 300 rpm and went nose up, and the leanout/quit problem was...........gone completely. It was indeed the poor fuel draw of the gms .47 that was causing the problem, not the fuel system.

As a side note also proving the poor fuel draw problem, when you prime the magnum .46 by opening the throttle, covering the carb air inlet with your finger, and slowly turning the prop till you see fuel in the line make it to the carb, the magnum .46 takes about 2 motor revolutions to get fuel to the carb where the gms can take 10 or more, and once you stop turning it over, on the gms you can see the fuel immedietely start flowing back to the tank where on the magnum it stays full with no backflow.
I don't quite get why it backflows, but it does.

Dean in Milwaukee

newglowhead 06-24-2005 04:14 PM

RE: GMS Engine Tuning Problem
Very interesting. I also notice slow fuel movement in my GMS engines, although I've tried everything to free it up. Is it as simple as lousy needle valves?
I just threw a new OS .46AX on my plane and priming is very easy.
By the way, what field do you fly at in Milwaukee? I'm out in Menomonee Falls by the trailer place.

Dean in Milwaukee 06-24-2005 08:56 PM

RE: GMS Engine Tuning Problem
I fly either at the county field on oakwood in franklin or on sundays at Vincent high school.


skylane62 06-27-2005 10:54 PM

RE: GMS Engine Tuning Problem
A very interesting thread indeed. I also have two GMS 47's. Both have run without adjustment right out of the box, including my latest with a rear mount needle valve. I tend to run them on about 7.5% nitro. (I mix my own buying 5 and 10%). I find that these engines need considerably less nitro. The power is superb and the enhancement to the models performance is very rewarding. One engine is in a Falcon Ready on floats, the other is in the 40 sized Ultra Stick.

Good flying to all.

Ed_Moorman 06-28-2005 08:07 AM

RE: GMS Engine Tuning Problem
1 Attachment(s)
I'm with you, Skylane. I keep wondering, "What are they doing to these engines?"

I have 2 GMS .47's on a Cedar Hobbies Twin Stick because they run reliably and have excellent power. I do have the front needle versions. I have not had any trouble under acceleration, any fuel feed problems or having them quit in flight. I am running Omega 10% fuel and APC 11-5 props. I choke mine with 3 flips and they hand start, usually in one back snap.

FOD MAN 06-28-2005 08:14 AM

RE: GMS Engine Tuning Problem
i have not had any problems either, the one i have has run perfect since new. hand starts on first flip most of the time.

Wayne Miller 06-28-2005 08:18 AM

RE: GMS Engine Tuning Problem

I'm glad to see the recent responses regarding the GMS's .47 working well without any modifications. I like what I'm hearing. I'm wondering if GMS has corrected the problems?

How old are your engines?

Thanks for keeping us up to date.


Wayne Miller

Rupurt 06-28-2005 08:36 AM

RE: GMS Engine Tuning Problem

ORIGINAL: Ed_Moorman

"What are they doing to these engines?"

Considering the length of this thread its fairly obvious there are some genuine problems. Thats not to say this is always the case.

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