I for one, wouldn't hesitate to buy another GMS....I like them.

FBD.

mine (GMS .47)has impressed everyone who has seen it fly,new out of the box it started as soon as it got fuel. never a moments trouble and would buy them again. can get 30 min. flights on daughters trainer, it really sips gas.

I don't know what is going on with RCU, but I still have not been getting notified to replies to the forums.

Anyway, I currently am fighting with an O.S. .40 SF that is hard to prime, and the fuel rapidly flows back to the tank. Very frustrating! It runs great in the air for a few minutes, then dead stick! I have installed a check valve in the line to the carb, which makes priming easier, but it still doesn't run right. All I can think, is that for some reason, there is an excesive backflow of air through the carb to the tank. Apparently, this isn't an uncommon problem. I have a K&B 1.00 doing this right now also. I experimented with the K&B by adding the uniflow tank set up, and it didn't work well. If anyone has an answer to this one, many people with many types of engines will be greatful. I ordered a Perry carb yesterday for the K&B. If that solves the backflow problem on it, I would be inclined to believe the problem is in the carb on all these engines. I will keep you posted!

Andy

The Perry Carb solved the backflow problem on the K&B, although the carb started to leak above 3/4 throttle. That is a hole different story though; Conely is helping me figure this one out. At the recommendation of a friend, I took the O.S. .40 apart, and soaked it in carb cleaner. He said that dirt or buildup of castor will cause the passage ways to stay open, when the carb is closed. This will allow the fuel to be drawn back into the tank at idle. I was very skeptacle, but did as I was told. I did not place any parts with o-rings in the carb cleaner, as this would swell the o-rings. I just wiped these parts off with a rag soaked in Marvel Mystery oil. Before re-assembling, I blew all passageways out with compressed air. I was a skeptic, but it worked. We put over an hour of flying time on that engine, last night, and it performed great! The kid I fixed it for was really happy! I also broke in a new GMS .47 for him. It was a very, very nice performing engine. I mounted it on my SE, by the way, with the uniflo tank. Very sweet combo!

Andy

Hi Andy,

If fuel is flowing back into the tank, the problem could be the low speed mixture needle adjustment. One of the symptoms is that the engine to quit in flight when throttle is reduced, or increased, to mid throttle. The following is what works well for me when a carb has been fiddled with and you need a good starting point. Hope this helps, it was referred to me as the "blow method" for carb adjustment:

Try resetting your low end needle. Open carb to about 1/4 throttle setting. Turn high end needle about 5 turns from closed. Open carb to full throttle and use a clean piece of fuel tubing attached to the carb and blow into it with your mouth. This ensures your high end needle is allowing fuel (in this case air) to flow into the carb. If it doesn't, than try another turn out or finding the obstruction.

Once you've verified the high end needle valve is working properly and allows air to be blown into the carb at full throttle, now (keep the same high end needle setting 5-6 turns out) and close throttle to about 1/4 throttle setting. Turn low end needle valve so it is completely seated (meaning closed byturning it clockwise, do this gently and don't force anything). Now with the carb opened at 1/4 throttle setting, SLOWLY rotate the low end needle valve counterclockwise (to richen the low end mixture) while simultaneously trying to blow air into the carb through the clean fuel tubing. Stop richening the low end needle valve when you first "feel" (more accurate) or hear air leaking into the carb.

Now, return to high end needle valve and close it completely and open it about 2 1/2 turns from closed. Recheck to make sure this 2 1/2 turn setting allows air to flow into carb and double check the low end needle setting to see if you can blow air into it with carb at 1/4 throttle setting. If all checks out, then attached fuel tank line to engine, prime engine manually with engine at full throttle, return to slightly less than 1/4 throttle, attached glow driver and start with starter.

As soon as it fires up, don't touch anything. Leave glow driver on and let engine warm up for about 30 seconds. Now with glow driver still on, slowly advance throttle setting. You should be able to get it to full throttle. If it dies from going from 1/4 to 1/2 throttle and above (with glow attached) it probably means you are too lean on the low end needle valve. Just open it (low end needle valve) up a little bit (about an 1/8 of a turn or less). Repeat as necessary until engine can advance above 1/2 throttle without dying. Once it can run at 1/2 and above, remove glow driver and go to full throttle. At full throttle, lean out high end needle valve to max lean rpm and then richen the high end needle mixture by 400 rpm or so when you pinch the fuel line briefly at full throttle the engine speeds up a little and then returns to its setting. The pinch test verifies if you are too rich or too lean. If you are too lean, when you pinch and release the fuel tubing the engine will die or not speed up at all. If you pinch and release and engine speeds up a lot, you are too rich, etc..etc.. The right setting is when your engine speeds up a little, about 300-400 rpm (a tach would help with getting your ears tuned in this area)

Have fun and just use a methodical method to tune your needles. Remember, high end needle normally works at 1/2 throttle and above and low end needle affects the idle and transition to full throttle. Also, if you adjust low end, you must check and adjust high end. The two needles can interact with each other. If you forget which direction to turn the needle, just use a clean piece of fuel tubing attached to the carb and blow into it to determine if your adjustment reduced the flow or increased the flow for whatever needle you were adjusting. Its really simple once you get the hang of it.

Hope this helps!

Fly4Fun,

Wayne Miller

Wayne:

Good refesher to tuning an engine[sm=thumbup.gif], but I was referring to the last major discussion, regarding fuel rushing back to the tank. In an earlier thread, I had remarked how GMS engines are not the only engines that can be accused of having poor fuel draw, and that a couple other engines(non GMS) that I was working on were having fuel draw issues, and that after you primed them, the fuel was rushing back to the tank. My last entry was describing what I had found to solve the problem "in general", and not specific to the GMS. I was pinpointing the condition of the carb, and not the engine. The K&B carb was a total loss, and required the installation of a Perry to stop the backflow of fuel. The O.S. carb required a good cleaning. Hard to say if it was dirt or dried castor build up, but a good cleaning solved the backflow issue, and the flameout issue at high throttle. The two issues seemed to be related. I guess my hole point was that there are some problems that are related to engines in general, that some people may be trying to stick on the GMS line alone. I don't see any fuel draw issues with my GMS's, and wonder if a good carb cleaning wouldn't solve some of these problems. I am sure some problems were solved by replacing the carb body, due the bad location of the spray bar hole. I am sure Wayne and others would agree that you can get a new engine from any manufacturer that could have a blockage in the carb, that would cause a fuel draw issue. If the engine ran fine and all of the sudden doesn't, look for the obvious first. I would consider the obvious to be:
1) a clunk that has fallen off
2) leaks around fuel fittings
3) holes in fuel lines
4) bad plug
5) kinked fuel line
6) clunk that has become trapped at the front of the tank
7) throttle servo that hunts, instead of consistantly returning to its programmed position
8) fuel line that is too large for the application(can help generate bubbles)
9) Leaking tank or tank stopper
10) Carb not fully seated
11) Cracked or torn O-ring at base of carb
12) Help me out guy's,... there is bound to be another dozen EZ solutions. Anyone care to add any?

After checking the EX fixes, then tear the carb apart and clean. If that doesn't fix it, then it is time to get more radical.

Hi Andy,

Sorry, I misunderstood. Thanks for the information and update.

Fly4Fun,

Wayne

Very interesting thread. I have been considering a GMS .47 for a scratch warbird I am building. It will be a completely cowled engine and I would like to use a Pitts muffler on it. Does anyone have any experiences or suggestions for running this engine on a Pitts muffler?

Thanks
Jim

Is anyone still watching this thread?


Jim

Yes....I am....

Dave,

Do you have an opinion of the GMS motor with a Pitts muffler?


Thanks
Jim

Hi,

I'm still watching the thread as well.

I've never tried the GMS with a Pitts muffler so I'm not much help. Since the standard muffler does not have any baffles, and niether does the Pitts, I'm guessing the Pitts Muffler should work OK.

Sorry, I'm not much help.

Fly4Fun,

Wayne Miller

Sorry....I can't help much either. I don't use Pitts mufflers.

Dave.

Jim:

I have used the GMS .76 with a Pitts Muffler with great success. I would suspect the .47 would do well also.

Andy

Thanks for the replies. Which Pitts muffler would you guys recommend? Tower has a recommendation for one. Should I follow their recommendation?


Thanks
Jim

I just bought a bird with the GMS .47 and tuned silver pipe.

Runs fine then, in flight loses power.. needle out 3 1/2 turns....lands motor a littel hotter thnan ormal but not smoking. High end needle unresponsive....sounds like everyones probelm.


Great thead, I will be doing the carb inlet inspection and tank position inspection.

This thing screams ....but it needs help to maintain its flow. Wayne, I will post my results.

This thread is a good source on this engine, I hope everyone who search's on GMS will find it.

Hi Bosshossv8,

I'm glad you found the thread useful. You should also make sure you chek for air leaks around the carb and engine back plate as well, especially since yours seems to be running hot.

Please keep us informed and let us know how you make out.

Thanks for the kind words.

Fly4Fun,

Wayne Miller

I just bought a new Tower 46 from a friend in our club. The engine has never been run. Since it is made by the same people that make the GMS engine, is there anything I should check before trying to break it in? Also, how would you guys recommend I break this engine in?

Thanks
Jim

Jim....

....the Tower engines come with outstanding instructions for break-in. I would
start with a 10-6 prop, and a fuel containing castor. I add some extra castor
to a fresh can of fuel when I am running-in a new engine....the engines love it.

Just start it up....1/4 throttle....let it warm up for about 45 seconds. Go to full
throttle, then close the main needle until the engine peaks out at full RPM. Don't
worry....it won't hurt it. Then open the needle until you hear the engine slow down
....if you have a tach...slow it down about 6-700 RPM's with richness.

Follow the instructions and you will have a sweet engine. Just don't baby-it, or
lean it out and get it too hot.

FBD.

Hi,

I've noticed there has not been much activity with this thread for a while.

We've just gone through another flying season, and I was just wondering, now that people have had the chance to give the modification a try, if anyone has anymore feedback or information on how this modification has worked for them. Thanks.

Fly4Fun,

Wayne Miller

I hope to have a NIB GMS .47 this weekend and after reading this long and informative thread had a look at a couple engine manufactures sites and found this

http://www.jettengineering.com/tech/tech.html

then look for RC Carb setup ... after reading this information and knowing full well that I will be taking the Carb apart to check that all is OK .. I will use Jett suggested way of setting up the engine for fine tune THEN put it into my plane ( new built Cutlass45) after being set up on a test stand first.

I will add my findings to this thread once the test has been done.

Has any one tried to use one of these? http://www2.towerhobbies.com/cgi-bin...&I=LXG850&P=ML to keep the pressure in the tank when you throttle down then up? for 5.69 it may be a cheap fix of keeping the pressure you get with the muffler in the tank and reduce back flow.....

That might just be the third best thing on this Planet....

....next to sliced bread and canned beer.

FBD.

I modified a GMS47 carby this week and tested it on the weekend. The engine ran perfectly, no daadsticks and the fact that it could be richened right up (not possible before the mod) shows that the standard carby can have an excessive amount of flow restriction in the fuel circuit.

Hi Blade47,

The device looks good and let us know how it works.

The way I think muffler pressure should work, is that it should supply pressure when running, then back off when idling. The concern I would have is that the device may allow pressure build up during flight, but would not bleed off the pressure at low throttle and cause flooding, and stalling. However, we won't know until its tried.

Let us know the results of your tests.

My engines all worked well after drilling out the intake to the carb and following the setp procedure described earlier.

Fly4Fun,

Wayne Miller