GMS motors - Page 5 - RCCanada - Canada Radio Controlled Hobby Forum
RCCanada - Canada's Radio Control Hobby Forum
Engines Discuss engines

Closed Thread
Thread Tools Display Modes
Old 04-07-2010, 02:00 PM   #41
Wayne MIller
RCC Supreme Contributor
I am: Wayne MIller
Join Date: Aug 2004
Location: Kitchener/Drumbo, On
Radio of choice:
Hitec A9 JR 9303
# of RCs: 19

Feedback: 11 / 100%
Posts: 2,168
Total Props: 8
Re: GMS motors

       Remove this ad - become a site supporter!

This has been posted before, but I can’t seem to find the post. This may help some who are having troubles with their GMS engine – this is related to the 46 specifically, but may be helpful for other models of the GMS as well.

Before doing any modifications with a GMS engine, you should first remove the engine back plate and smear a little red high temp RTV (It stands for Room Temperature Vulcanization and is reference to “RTV Silicon Adhesive Sealant”) around the mating surfaces to seal any air leaks. The RTV is available at automotive stores – CTC or Princess Auto. Also remove the carb and smear a little RTV around the “O” ring to seal any air leaks there – push down on the carb while tightening (not too tight) to make sure the “O” ring seals properly. Let the RTV dry overnight. This may be all you need to do.

NOTE: the GMS engines have a very hard cylinder sleeve and piston ring and will take a long time to break in, about ten tanks running rich – but it also will last a long time and will take a very long time to wear out because of this.

I have a thread over at R C U and we think we have isolated the problems with the fuel delivery and draw in the GMS carb. Drilling out the carb intake nipple and brass insert seems to have fixed the problems - if sealing the air leaks does not cure the problem.

If you want to see what the learning process was, this is the thread:

See post number 95 for a picture of the problem – the hole in the brass tube at the bottom of the fuel intake nipple is not lined up and needs to be drilled out.

The problem seems to be fuel turbulence when fuel leaves the fuel intake nipple and takes a sharp turn when it enters the needle valve area (see drawing attached). This can create foaming, and/or leaning out at high speed and in worst cases, a fuel air lock. This makes carburetor adjustment difficult and may even cause the engine to quit while in flight. To give credit where credit is due, this problem was identified by another modeler, Andy.

We have tried many things to fix this, such as, more muffler pressure, enlarging fuel/exhaust nipples and drilling the carburetor brass insert. Drilling the hole larger in the brass insert seems to have done the trick; I have just drilled three and all work well after the modification. Drilling the nipples does not seem to affect the performance, but doesn't hurt either.

The modification can be done with the brass insert in place (although you have to be very careful not to damage the nipple threads) or by taking out the brass insert and drilling (preferred).

The hole in the brass insert should be directly under the fuel intake nipple.

If you chose to try this modification, please make sure you do not have a carburetor air leak (where carburetor enters engine body) or an air leak at the engine back plate. If you do, you must seal first. Seal with Ultra Copper Hi-Temp RTV by Permatex (available from automotive stores) or replace "O" rings as necessary. Testing for air leaks is described earlier in this thread.

Please be aware that any modification will void your warrantee.

See instructions below and refer to the attached picture for part location. I've included two sets of instructions, one for drilling with brass insert in place and the second to remove brass insert for drilling.

If you chose to drill the brass insert in the carburetor:
1. First remove the needle valve retaining nut, needle valve and the intake fuel nipple to avoid damage to the needle valve. Drill the fuel intake nipple one (or two) size larger.
2. Reinsert the fuel intake nipple and then use it as a drill guide, this will protect the nipple threads.
3. Very carefully drill through the fuel intake nipple and into the brass insert.
3. Remove all brass filings from inside the brass insert and re assemble the carburetor.

If you wish to remove the brass insert to drill (preferred method):
1. Remove the intake fuel nipple
2. Remove the needle valve assembly by taking off the needle valve retaining nut retaining nut to avoid damage to the needle valve.
3. Remove the retaining screw
4. Remove throttle barrel, be careful, there is a spring behind it. Note: small end of spring goes into carburetor first.
5. Using a hardwood dowel or brass punch, carefully tap the threaded end of the brass insert until the brass insert comes out. Note: be careful not to damage the threads.
6. Drill the hole into the brass insert at a slight angle towards the carburetor throat (same angle as fuel intake nipple).
7. Clean out all brass filing out of brass insert and remove any drill burrs.
8. VERY IMPORTANT! Reinstall brass insert so the enlarged drill hole is directly underneath fuel intake nipple. To reinstall brass insert, you will need a hollow punch and a backup block with a hole (preferably made out of a hardwood block). Set the brass insert in place so the newly drilled hole will line up under the carburetor nipple. Put the carburetor over the hole in the wooden back up block and gently tap the brass insert in place using the hollow punch.
9. Reassemble carburetor.

At first I had a difficult time adjusting the engine high speed and low speed settings - these seemed to interact 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 (low speed needle adjustment affects high speed needle 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) for the idling speed you want (you may want to keep the idle a little fast while breaking in).

5. Once you find the best idle speed for your prop, then "poke" full throttle after idling for a coupe of seconds. Turn the low speed adjustment no more that 1/4 turn either way until the acceleration from low speed to high speed is smooth and does not hesitate. My low speed needle adjustment needed to have 1/8 turn leaner (tightened "in" ) .

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

As a starting point for you, the final adjustments for me ended up as:
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 those who are having problems - let us know if it does.

I have just done this with three engines and they all work well.

Let me know how you make out.


Wayne Miller

Last edited by Wayne MIller; 04-09-2010 at 02:28 PM.
Wayne MIller is offline   Quick reply to this message.

Sponsored Links - Subscribe to remove this ad.
Old 04-08-2010, 11:41 PM   #42
RCC Apprentice
filtr's Avatar
I am: Ron F
Join Date: Dec 2002
Location: Webb, Sask
Radio of choice:
# of RCs: 20

Feedback: 1 / 100%
Posts: 30
Total Props: 0
Smile Re: GMS motors

I have 6 GMS motors and only had to turn the High end needle and the low end needle to set them. They have been flying great. I have purchased them off people who could not make the work. Then I take them to the flying site and show the old owners how great they work.

I think alot of people try to many things all at once and then make the problem worse. I have had OS Junk engines also. They had real problems with the liners peeling. They also said that it was rusty bearings after a two gallons of gas.
filtr is offline   Quick reply to this message.
Old 04-09-2010, 07:16 PM   #43
RCC Supreme Contributor
AJCoholic's Avatar
I am: Andrew Coholic
Join Date: Sep 2002
Location: Timmins, Northern Ontario
Radio of choice:
JR 12X

Feedback: 5 / 100%
Posts: 9,873
Total Props: 1
Re: GMS motors

Originally Posted by g-manz35 View Post
Buddy of mine quit some time back and started again just lately. He has some HB's and Webras. He said that OS was the "cheap knock off" brand in those days. I think he's still trying to get his head around the fact that they are considered quality engines by many nowdays.

Thats funny, since O.S. is one of the "originals" having been in the model engine business for close to 60 years!

Andrew Coholic -MAAC #26287L

1/2A to giant scale, IMAC, SAM, R/C sport, turbine jets, Heli's...
if its got a wing or two and an engine - I like it!
AJCoholic is offline   Quick reply to this message.
Old 04-09-2010, 07:22 PM   #44
RCC Supreme Contributor
AJCoholic's Avatar
I am: Andrew Coholic
Join Date: Sep 2002
Location: Timmins, Northern Ontario
Radio of choice:
JR 12X

Feedback: 5 / 100%
Posts: 9,873
Total Props: 1
Re: GMS motors

if you want to buy a good out of the box made in China engine, I would take a look at the JEN series (sold by Just Engines in the UK).

I have run a few .56's (now they are sold as .57's) and a .37, and a .91 and two .47's.

The ones I have run (a few of them quite a bit, and for 4 or so years) run really, really well.

With our strong dollar VS the British pound, they should be pretty "cheap" relatively speaking.

I wouldnt recommend them if I didnt think they were good engines.

Just an option - although the .37 is the smallest they sell. No .25's...

Andrew Coholic -MAAC #26287L

1/2A to giant scale, IMAC, SAM, R/C sport, turbine jets, Heli's...
if its got a wing or two and an engine - I like it!
AJCoholic is offline   Quick reply to this message.
Closed Thread

Quick Reply

Register Now

In order to be able to post messages on the RCCanada - Canada Radio Controlled Hobby Forum forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.

Member names may only be composed of alpha-numeric characters. (A-Z and 0-9)

!!ATTENTION ADVERTISERS!! If you intend on advertising anything on this forum, whatsoever, you are required to first contact us here . Additionally, we do NOT allow BUSINESS NAMES unless you are an Authorized Vendor. If you own a business, and want to do sales on this site via posting or private message, you will need to follow the rules. Shops, Stores, Distributors, Group Buys without being authorized will see your account terminated.
User Name:
Please enter a password for your user account. Note that passwords are case-sensitive.
Confirm Password:
Email Address
Please enter a valid email address for yourself.
Email Address:
Your Name
Your first name and last initial is required if you plan on using the forum.
Which city & province you live in. This is mandatory for classified listings.
Radio of choice?
Which radio is your current favorite to use?
Number of RC Vehicles?
How many boats, cars, planes do you own?


Human Verification

In order to verify that you are a human and not a spam bot, please enter the answer into the following box below based on the instructions contained in the graphic.

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
vBulletin Message

Thread Tools
Display Modes

Forum Jump

All times are GMT -5. The time now is 08:41 AM.

vBulletin Security provided by vBSecurity v2.2.2 (Pro) - vBulletin Mods & Addons Copyright © 2016 DragonByte Technologies Ltd.