Dave:

It is that "knarled" area where mine is worn out, after 5 years of running. However, mine is knarled higher up. From the positioning of the marks on yours, I would guess that your carb was not fully seated. This will definately cause the engine to suck air. If your engine has the o-ring at the bottom of the throat, where the carb slides in, you may not get a good seal. Don't worry if this is the case though, as the carb body is only about $9.00.

Andy

Hey Guy's,

I just sent the following e-mail to MECOA. I hope we get some input from them.

Andy

Dear Mecoa:

I have been running a GMS .47 for about 5 years now, with great success. When a friend bought one like mine, and we couldn't get it to run right, I went in search of answers on RC Universe. I found a thread where people were trying to solve the same problems that my friends engine has. This is apparently a very common problem. Please go to the site address where we are working on the problem of high speed flameouts. You will see that what we are dealing with is not on your Q & A page. We think we just about have this common problem solved. My user name is Flyer Freq. I believe the issue can be solved easily in manufacturing. Here is the site address:

http://www.rcuniverse.com/forum/m_22..._1/key_/tm.htm

Any input from you would go a long way in establishing your dedication to quality. Thanks in advance for your help.

Andy

Good going, Andy....we need some replacement carbs from the GMS
factory. Even though I don't mind having to make a small modification
my carb is not the way it should be, and I haven't mounted it yet, or
ran it !!!

Dave. []

Thanks Dave! I hope they respond appropriately. An appropriate response would likely be the best advertising they could do. I have high hopes.

Andy

I just fixed a GMS32 for a friend this weekend. This was the second engine he'd been sent and had the same problem as the first -- the carby would not go into the crankcase.

Turns out that the little pin hadn't been ground deep enough and was (no matter how much you fiddled with its orientation or position) fouling the section of carb that inserts into the crankcase. I ground the hollow out a fraction and it went together just fine.

What we must all tell ourselves is that these are engines built to a price. If you want *consistent* quality then you often have to pay a little more.

I recently replaced 2 x GMS47 engines in my planes with 2 x OS47AX's for similar reasons as above.

The GMS's were near new, run in, but were causing increasingly slow take-offs and flight. They started suffering from underpower, dead stick landings with burnt plugs and coming in hot and smelly. No they were not set lean - this is where the plot thickens.

As I richened the needle from peak and the rpm's dropped, I found the more I richened the needle, it didn't appear to go into 4 stroke mode as most engines do. This made me suspicious that perhaps somehow it was leaning back out again although I was richening it. Now, I wasn't setting it too rich either for flight like slobering rich as this is not good for an engine either.

I also found the GMS's noisy and as noise is an issue at our field, it was easier just to junk them. The new OS46AX's run smoothly, quietly, with plenty of power, no dead sitcks, not too hot after landing and can go into 4 stroke mode if desired.

I originally went away from OS when I had a problem with a dodgy 46FX, but now I am back.

Hi,

Today I was lucky enough to spend some time with Don Hancock from the Brant Flyers in Ontario. Don is a very experienced modeler and very knowledgable with engines. Don had a problem GMS .47 engine from one of his student pilots and invited me to be there when he took it apart.

The symptom is: the engine seems OK, airplane takes off, then the engine quits. The pilot has also noticed air bubbles in the intake fuel line on the ground, which may be caused by fuel tank vibration. This engine also has an exhaust extension/deflector and a rear (remote) high speed needle valve.

The one thing we all should remember is that we are working with three different styles of GMS engines. The first is an older style front needle valve engine with the "shiny" muffler, the second is the new style front needle valve with the "finned" muffler and third is the new style rear needle valve with the "finned" muffler.

Before dissassembly, Don noted there was some air leakage through the front bearing, but not where the carb mounts into the engine.

The motor was completely dissassembled, and except for some minor rust on the rear crankshaft plate, the engine seemed OK.

It was noted that when the brass insert for the front fuel intake was removed, that the brass insert is under cut (grove around the insert) where the fuel comes in. The insert has fuel intake holes on both sides of the insert in the under cut.

This means that it really doesn't matter if the holes under the intake valve nipple line up or not. The fuel will flow through the intake nipple and around the brass insert (using the under cut) and can find its way in to the carburettor through either of the two holes. Basically this means drilling the hole under the nipple probably does not help. However, it would not hurt anyone has already done this.

This leads us to believe that the problem is probably one of two things with this particular engine. One could be fuel foaming due to an air leak or tank vibration. The second would be low exhaust pressure (we know this carburettor has poor fuel draw).

In previous posts, we know modelers have got the GMS .47 to run by increasing the hole size in the exhaust pressure nipple, by drilling one drill size larger (we may even need more) and adding an exhaust deflector/extension causing a little more pressure.

Don will keep us up to date on how the engine works, and any other modifications he may have to make.

If anyone still has problems with their GMS, they may want try and analyse the problem and let us know the results by:
1. Check for air leaks at the carb, fix if necessary.
2. Check to see if the fuel line air bubbles at "start up" clear quickly, if not, chances are the tank pressure is not enough.
3. Then drill the exhaust pressure nipple larger for more tank pressure, again check to see if the "start up" air bubbles clear quickly. If this fixes the problem, great!
4. If not, try an exhaust deflector/extension to help with fuel pressure.
Note: I did not have to add an exhaust deflector/extension on my engine to make it work OK.

If anyone does the above, please let us know your findings. Thanks for your help.

Fly4Fun,

Wayne Miller

Wayne:

While I was skeptical at first about the alignment issue with the hole in the brass insert, I am still leaning that way. I don't see how the fuel can enter the carb, except for that hole. Maybe I need to look at it a little closer,... I hope to be able to get my friends engine to the field on saturday to see if my realignment job solves the problem on this engine. When you say that the brass insert is under cut, just so I am positive I understand what you mean, are you saying it is hollowed out around it??? If this is what you are saying, is it a large area, or if the hole is pulled in too far, could the carb body still interfere with the fuel flow?

I have a Perry carb that fits the diameter of the carb throat very well, but it is too long. I wish that I could find someone to machine it down about 1/8 of an inch, to see if the engine run's well on it. I wish my GMS carb wasn't so knarled up, so I could run it on the new engine. I just want one more piece of positive proof that the problem is in the carburator.

By the way, if you go to www.mecoa.com, you will see where they deal with the leaking bearing issue. As you will see, their explanation backs up my statement, that I noticed the bearing leak when I was running the engine in, but not since.

I'll keep you guy's posted!

Andy

P.S. Is there any way you can run the carb from the engine that always did run well on the engine that had the problem?

Hi Andy,

I had a hard time trying to describe the "undercut" and your "hollowed around" is a probably better description. The holes are opposite each other. I'll put a picture at the end of this post, hopefully it will help. Again, this was a rear needle valve engine. My drawings are not the best, but I hope it helps. If you want to check, the brass insert is removed by:
1. Remove carb from engine
2. Remove carb barrel
3. Remove fuel nipple and needle valve or nut - what ever applies to your carb.
4. Put carb on a piece of hard wood, then place a piece of hardwood on threaded end of brass insert and GENTLY tap it out.
5. To replace brass insert. Line up the fuel intake hole and start the insertion of the brass insert into its seat with finger pressure. Take a dowel with a drilled out center (or a hollow brass punch) and GENTLY tap the insert back into place.

I never did swap carbs on my engines, but both engines are working well now, so it wouldn't be much help. I did three things at once when I fixed the problem, that is, I drilled the intake nipple, drilled the exhaust nipple and drilled the brass insert - I wish I had done one at a time to isolate the problem.

I'll take a look at the mecoa site, thanks for pointing this out to me.

Thanks for keeping us up to date.

Fly4Fun,

Wayne

Thanks Wayne! Great drawing. I have my carb apart on the bench. I'll have to study it a little closer tonight. I wonder if the nipple threads down into the hollowed out area. If it does, with the hole in the insert being right at the edge of the threads in the carb body, the hole still could be all/partially blocked off by the edge of the nipple. Even, if it is not totally blocked, it could cause turbulance in the line, which would cause problems. In my field of work, if a fluid or a gas is forced to take sharp turns close to what it is feeding,(In my case, a boiler burner), the turbulance generated can cause valves to fail, or the burner to be blown out. The faster the fuel is forced to travel, the greater the turbulance. It is rare, in my situation, that a boiler fails when it is idling, it is when it is at high fire. I think this theory can easily be transferred to our engines. The faster the engine runs, the greater the turbulance in the fuel entering the carb, the less fuel being mixed with air, the leaner the engine runs. Poof,...flame out! When we richen the carb up at full power, so it doesn't lean out, and then we cut back on the throttle, now we have a fat midrange, and the plug gets put out by raw fuel. I think this could cause greater turbulance in the tank also, since the flow starts and stops so rapidly, thus foaming. Not so sure about the last part, but have a good feeling about the first two scenarioes. Any thoughts???

Hi,

One of the things discussed when we were looking at the engine, was whether the fuel nipple threads went in far enough to block the the hole partially. Don measured the length of the threads and the depth of the hole with calipers - it appears to have lots of clearance below the threads.

As far as the turbulance of the fuel flow, I took another look at post 95 and noticed the hole in the brass insert was away from the carb throat. This means the fuel would have to go into the fuel nipple then be drawn away from the carb throat before entering the hole and going to the carb - I don't know much about fluid dynamics, could this create a problem? If so, the hole I drilled into the brass insert would have taken care of this and allowed the fuel to go straight from the fuel input nipple into the brass insert drilled hole.

I looked at the macoa site and in particular the fuel leaking through the front bearing. Thanks for the link.

Look forward to your next step.

Fly4Fun,

Wayne

In spite of the new five inches of snow on the ground, I am going to try to fly the .47 tomorrow. I did have one other thought, though. When I was comparing my engine, which runs well, to my friends engine, which doesn't, I did notice that my engine only has one head shim in it, and my friends has three. I only run 5% nitro, and I get all kinds of power with one shim. The more shims, theoritically, the higher nitro you want to run. My friend was running 20% nitro in his, so, in theory, it probably should have been O.K.. But, perhaps some of the engines that aren't running right, aren't all related to the same problem, maybe some of the issues are due to the wrong nitro content vs. the compression ratio. I know, it shouldn't be that touchy, but I thought I would throw it out there. I guess I wonder if the higher compression ratio in my engine might cause it to draw fuel better.

I am basing what I said on the following information on MECOA's website. Here is the link: http://www.mecoa.com/faq/compression/compression.htm

Andy

Hi Andy,

Thanks for the info, that is something I never thought of it would be interesting to see what difference it would make. I don't think compression has anything to do with fuel draw, but you never know until we give it a try.

I'm leaning towards making the exhaust nipple bigger by one, and perhaps two drill sizes if possible, to cure the problem. Also, if its fuel turbulance as you suspect, drilling the brass insert hole to be directly in line with the fuel intake nipple may help as well.

I'm curious as to what you will find. Hope you get a good flying day in!

Fly4Fun,

Wayne

ONE HUNDRED AND TWELVE POSTS 112 Postings to get a carb problem fixed!!!! Get a Perry punp or cline regulator pump and see if that helps. Forget the stock carb. Any carb that fits the carb bore should work. One of these 2 ideas will work...I bet. Capt,n

Hi Captinjohn,

I personally think its great that modelers from all over the world have pulled together to solve a common problem, and may have the problem now fixed.

You must be working on the same problem, otherwise you wouldn't have commented. Let us know if the perry pump or cline regulator pump fixes the problem.

Thanks for your help.

Fly4Fun

Wayne Miller

I think it is great the rc engine people all over the world do come together to solve a problem. For the RC people that want a quick ...maybe even better fix. Try a differant carb. I was given a old Fox engine long ago. The carb was junk, compared to other carbs. The engine itself had very good compression and the bearings felt good. I took a Perry carb off a engine about the same displacement, and put it on the old FOX. Boy did that engine run after the carb transplant. If you need a bushing adpter.....turn some round stock to the dia of the old carb and then drill it with the tailstock to slip fit of the smaller carb. Works great. Also if you think you got a leak around any carb...clean real good and use some clear silicone sealant to seal it. If that takes care of the leak...your in business. There are all kinds of sealants you can use. Basically...if a engine has good compression and seals and bearings are good....it should run good with the right carb. Or in some cases a good carb. After-all you only got 3 moving parts in a 2 stroke engine. Carbs...on the other hand can have many more parts. Also most Supertiger carbrs are quite adjustable. The brass piece can be turned to effect the mid range. Good carb if you take the time to read about how to set them. Well I hope the GMS problem is solved. May buy one some day. I have talked with guys that love them. Best to all....Capt,n

Hi Captinjohn,

Thanks for your comments. I have also had to swap carbs in the "old days" as well. I used to prefer Kavan Carbs and have put them on Fox's and Super Tigers to get reliable operation.

The problem here is that some GMS .47 engines are just fine, others are a pain to get running reliably. This is what we are trying to isolate/fix. I think we are down to a fuel feed problem, and we have a fix by drilling the fuel intake nipple, brass insert and exhaust nipple - we just need to find which one is the fix and want some more testing to confirm.

Wish you had one, it would be great to have another troubleshooter. Thanks for your input.

Fly4Fun,

Wayne

I am not going to drill out the exhaust pressure nipple, or the fuel
inlet nipple just yet....I'm 99 44/100 % sure the drilling of the inlet
that was restricted will be the fix. Regulators and pumps shouldn't
be needed on any simple installation.

I'm not a fan of those Cline regulators....have you seen the price ?
SIXTY DOLLARS for two cents worth of plastic....I'm sorry, to me
that's an insult....[]

Dave.

Hi Dave,

That's a pretty high percentage , hope that's it! One thing for sure, your going about it the right way - one thing at a time.

Let us know how you make out. Got my fingers crossed!

Fly4Fun,

Wayne

Hey Guy's!

Just got back from the field. I forgot how much fun flying with ski's was! I must have had about 30 or 40 take off's and landings. The mid range felt sloppy, no matter what I did. For regular flying, the engine was fine. I was actually impressed. Rolls, loops, vertical climb's, out of site. I mean litterally, straight up, out of site. No more full throttle flameouts! I was thinking the slightly ritch midrange may have been due to the cold. I decided to do some hovering before calling it quits. The engine could not transition in a hover. It would try to die. If I throttled back, and got back on the low needle, the engine would stay running. I couldn't make sense out of it, so I went up to the hobby shop to talk to the owner. He said it sounded like it was transferring from the low speed needle to the high speed needle too early. As we studied what I did to the carb, to align the hole, we realized that not seating the spray bar(brass insert), changes the transition point. It does cause it to go on the high speed needle early. So, in short, it appears that I did prove the turbulance theory. Yes, I did change the o-ring at the base of the carb, but I don't think that was a factor. We looked at a GMS .47 that he had in the case, and the hole in the spray bar was properly aligned in that one. To truly make our engines run properly, I recommend spending the $8.00 for the new carb body from Tower or your LHS. I am still hoping MECOA surprises and impresses everyone, by contacting me back, or answering us in this forum, and somehow making this right. If they even just made new spray bars available to us,... Well, that is my report. I feel confident this is right. I guess I can prove it further, by putting my spray bar in my friends carb. I'll have to see if time allows it.

By the way, Perry doesn't make a carb that fits the GMS properly. I have one that is close, but a little tight going in. The biggest problem is the length of the throat. It is too long, and won't allow the carb body to compress the top o-ring. If you had a machine shop, and could turn it down, it would probably work.

Andy

Andy

Hi Andy,

I am not quite sure what is meant by "not seating the spray bar". I'm guessing that it was not in all the way in order to keep the brass insert hole aligned to the fuel intake.

If this is the case, if the brass insert was fully seated, then the hole drilled in it directly below the fuel intake nipple, do you think this would cure the brass insert positioning problem?

Sounds like you have certainly made progress in getting this engine running and are enjoying your skis.

Thanks for the report.

Fly4Fun,

Wayne Miller

Hey Wayne!

Yes, I think if the hole were centered under the nipple, and the insert was seated, or pushed in all the way to the shoulder of the insert, the engine would run properly. My friend just wants to buy a new carb body, but curiosity has the best of me. I want to put my spray bar in his carb and see what the alignment is like. If it is good, then I will run the engine. The midrange problem is evident enough on the ground, that I probably won't need to fly it to know if there is a difference; although, I would like another crack at those ski's! For some reason, the ground doesn't scare me when there is snow on it . I'll let you know what I find.

Andy

Hi Andy,

From what I understand the carbs are cheap, but I'm not sure if you can order it as an assembly or parts.

I looked up your town on the map and your just a little farther north than me. I'm in a town called Kitchener and about half way between London and Toronto.

I hope your friend will let you have more time with the engine.

Fly4Fun,

Wayne

The Tower .46 carb may be the way to go for a replacement....

....you know Tower will stand behind their parts 100%. [sm=thumbup.gif]

I have 2 Magnum 40-46 carbs NIB in stock, but I'm pretty sure this one
will work....If it doesn't I'll try for a replacement....they need to improve
these things.... The Magnum is .5 mm smaller than the GMS carb, and
being as we are using this as a "Club Racing engine", I'd be at a slight
disadvantage if I changed the carb....

....and if you knew the Guys I fly with, that wouldn't be a good idea. [sm=stupid.gif]

Dave.

Hi Wayne and Dave!

The Tower .46 Carb, while it looks like it would work, it won't; I've tried it. I don't remember the exact measurements, but the throat is smaller by a few thousandths than the GMS. It sucked air like water through a screen door. I tried to put the GMS carb on the Tower engine also, and it didn't want to go. You can get the individual parts for the GMS though. Here is the link for the carb body and spray bar: http://www2.towerhobbies.com/cgi-bin...?&I=LXEGM2&P=7.

I took the carb back apart, and measured the distance from the shoulder of the spray bar to the edge of the hole. I didn't have anything that would measure that fine, but there was a difference. In fact the bad spray bar was drilled at an angle. The good spray bar was roughly .051 inches. The bad bar is roughly .039 inches on one side, and .035 on the other. These are real rough measurements, as I guessed at the distance between marks on a mm scale, then used a conversion calculator to inches. I put my spray bar in my friends carb. Hopefully I can get a chance to try it.

Wayne, I think you might have looked up someone elses profile by mistake, I am probably 4 to 6 hours south of the Canada/U.S. border, in Appleton, Wisconsin. I wish we were a little closer, as I would love to fly with you some time.