RE: GMS Engine Tuning Problem
 

As time wears on, the joints in the muffler leak more and more and are usually distinguishable by black, oily soot oozing from them. This has not caused my .47 any running problems:D.

RE: GMS Engine Tuning Problem
 

Hi Gary and Turbobeaver,

Gary,
Hope your field dries out shortly, we look forward to your input!

Turbobeaver,
The joints on my "silver" style muffler were very tight, infact it showed no signs of the black liquid seapage that usually occurs at bad muffler/engine joints, but it would be a good idea for others to check. I look forward to the day I can get to see the air museum in Ottawa - last time I was there I did some white water rafting. Glad your enjoying the post, I'm really enjoyed working with this group of modelers, they are great and its been very productive.

Fly4Fun,

Wayne

RE: GMS Engine Tuning Problem
 

Hi,
Just thought I would chime in and let everyone know I am selling a new in the box GMS2000 0.76 :D
I will sell for $75. :eek: paid $90+tax at my lhs I bought it for my 1/7 AT-6 Texan but never finished it ,
pm me or email me @ [email protected] I will send pics if requested
see GMS.com for stats on the 2000 0.76
comes with muffler& gasket and allen tools and i will throw in an new in pack OS glow-plug and a new carb cover for storage
thanks all.[8D]
Bill Ellis

RE: GMS Engine Tuning Problem
 

Hi Bill,

Thanks for the information, this forum is for the resolving of a GMS problem, and the wrong place for your ad. You probably would get a better response if you post in the "MARKETPLACE" section.

Good luck on selling your engine.

Fly4Fun,

Wayne

RE: GMS Engine Tuning Problem
 

O.K. Guy's!

We had our annual Frozen Finger Funfly today, and my friend finally turned his .47 over to me. The first thing I noticed was that his carburator has two O-rings sealing it, where it mounts to the crankcase, and my old .47, which runs perfectly, has only one. I would say that he should have a better seal around the base of the carb than mine does, but the O-ring is really stiff and hard. I can't help feel that changing to a silicone O-ring would help here. I will try to get another O-ring, as I am afraid I knicked the O-ring while removing it.

I next removed the inlet nipple. About 1/3rd of the hole in the brass insert was covered by the edge of the carb body. When I compared it to mine, the edge of the hole in the brass insert was lined up with the edge of the hole in the carb body. The newer carb appears to be consistant with the findings you have made with the bad runners. I took the rest of the parts off the boddy and laid the carb on its side and knocked the insert out with a rubber hammer. It came out with a couple firm taps. The hole in the insert was uniformly round on one side of the barrel, but the other side resembled the shape of a kidney. I can't be sure which side was facing the nipple. When I compared the holes in the brass insert from my good carb, the holes in mine were nice and uniform on both sides.

Everything else between the two carbs looked identical. At this point, I reassembled the newer carb, being careful to line up the holes. I used a nipple from an air tool to slide over the end of the brass insert. I then pounded the insert into place. I made sure I didn't pound it in too far. I wasn't sure if installing the high speed needle barrel would pull the holes out of alignment when tightened down, but it did not. The barrel is tight and everything is in proper alignment. All that remains now is to run the engine with a new base O-ring. I wish I didn't have to change the O-ring, as now I won't know if the engine runs well, if it was the alignment of the holes, the new O-ring or both.

I will see how quickly I can get a new O-ring, and report back. Engines don't run particularly well in the severe cold we get here, but I should be able to tell something. I may try putting the newer carb on my engine to see how it does. After all, my .47 doesn't have that O-ring inside the crankcase.

Till later!!!

Andy

RE: GMS Engine Tuning Problem
 

Hi Andy,

Thanks for the very detailed report.

I was wondering if the brass insert would come out without damaging it, and was afraid to try. I'm assuming you drove it out from the inside of the carb - if so, did you use a brass punch? If not, could you explain how you did it?

I also fly in cold weather, we usually put a couple of drops of lighter fluid in the carb immediately before starting, it works well. You may want to give it a try.

I look forward to your next report.

Thanks for taking the time to help us out, its really appreciated.

Fly4Fun,

Wayne Miller

RE: GMS Engine Tuning Problem
 

Leakage from the front bearing seems to be a common complaint on the GMS 47 and Tower 46. Excessive crankcase to crankshaft clearance in the area between the ball bearings certainly affects crankcase base compression and fuel draw ability. Has anyone taken the time to remove the ball bearings and check the actual running clearance between the crank and the crankcase? If the clearance is more than .0015", I would be very suspicious. While air and fuel leakage from the front bearing area may not bother an engine that is not used for aerobatics, it will always show up as a poor ability to draw fuel when the nose is pointed up.

The modifications to increase fuel pressure and remove the flow resriction in the carb have apparently helped the fuel draw ability of the engines involved. I do not feel that they have got to the root of the problem however. It would be interesting to see if these engines can be flown without muffler pressure.

Before the onset of cold weather I was just starting to help a new student learn to fly with a new GMS 47. While it ran good at times, I had it flame out a few times for no apparent reason. I will be looking at it in the near future and my first investigation will be to look at the above mentioned clearance as his engine also drips fuel from the front bearing. I will report my findings in due course.

RE: GMS Engine Tuning Problem
 

I have just read this post as I too have had simular problems with my GMS 47. I had a very frustrating time with mine this summer when I mounted it in my See Bee 40 size pattern plane. I had run the GMS in my trainer before and It seemed okay, but once I installed it on my See Bee I would always lean out in arobatic flight and dead stick. I also had a MDS 40 so I swapped it out with the GMS and it would do the same thing as the GMS so I naturally thought it was somthing with the installation. But I could not find any problems with the fuel system. At our annual KWFD scale rally Flite Craft had a great deal on Super Tigre engines so I bought a G-51 and put it in the SeeBee and it has performed great right from the start.
I did check under the inlet nipple on my carb and the hole does not line up on mine, but by blowing air through it I would say it doesn't seem to be affecting flow much but I will rill it out anyway, just incase I need to use this motor in the future.
PS. I ended up putting the MDS 40 with the GMS old style muffler on my trainer with floats
and it has crazy power! This plane is at my cottage and I did not have my tack but it absolutly screamed with that pipe.

RE: GMS Engine Tuning Problem
 

Hi Waco Driver,

I have attached a long piece of fuel tube to my carb input nipple, then blocked both the exhaust port and carb throat and blown into the fuel tubing to check for front bearing leaks. There was a little air escaping, but very little. Since I have both an older and newer GMS .47 (the newer one runs great) I compared the two and both leaked air about the same.

Since sealing the carb and enlarging the carb input and exhaust nipple, my "bad" engine runs fine now without the extra exhaust baffles.

I noticed we live fairly close together, and since we have 3 GMS's between us, perhaps we could do some comparisons. Send me a PM if your interested in getting together.

Thanks for your input, I look forward to your findings.

Fly4Fun,

Wayne Miller

RE: GMS Engine Tuning Problem
 

Hey Wayne!

I actually drove the insert out of two carbs. The first time, I heated up the carb body, and used a rubber mallet to hit the threaded end. The threads were not damaged at all. The body of my carb is worn out in the area of the jam bolt, so I wasn't worried about damaging the body. I didn't heat the body, I just hit it with the mallet a couple times. It came out with no damage. No punch is needed.

One possible fix for the alignment issue, would be to put a fiber washer on the inside of the carb, around the seat of the brass insert, as well as the one on the outside. It would move the brass insert out just enough, and it wouldn't allow the over tightening of the needle valve barrel to pull the brass insert in too far.

It is possible that www.smallparts.com may have these washers, I have not looked yet.

By the way, I use the lighter fluid too, works great!

Waco Driver:

I have heard the leaky bearing theory before. I wish I could quote some sources, but I'm afraid I don't remember the exact details or where I read this information[sm=surprised.gif]. It may have even been on RCU. Oil leaking through the front bearing offers lubrication. The way the two stroke engine is ported, some leakage is necessary for proper fuel flow. I heard of someone with, either a GMS .25 or .32 who didn't want the fuel leaking from the front, so they replaced it with a sealed bearing. The engine no longer had the power or RPM it once had. Surprisingly, Mecoa replaced the engine, in spite of the modification. I am counting on my memory here, and I am missing the details, but the leak is not as much of a factor as it may seem. I will say, that the fuel leaking from the front of mine is hardly noticeable. I remember seeing it when I had it on the break-in stand, but not since being in the nose of my SE. As I have mentioned before, my engine is on its 5th year and is still going strong.

Andy

RE: GMS Engine Tuning Problem
 

Hi Andy,

Thanks for the tip on getting the brass insert out - do you think that perhaps a "C" clamp may work as a "press" to remove the insert?

Just a thought, whenever I'm stuck for small fiber or rubber washers, I use "hole" punches to make them. I punch the small hole first, then punch around it with a little larger punch. This gives you a washer.

I have some gasket material, but have also used winshield washer bottles, margerine tub lids, inner tubes, leather, the inside of fuel container lids etc. when I felt they would work for a certain application. Such as: spacers, fuel filter gaskets, washers, seals etc.

You can usually buy these punches at places where they sell tools.

If your stuck for washers, you may want to try the above.

In your case, the washer would have to be fuel and heat proof.

Thanks for getting back to me.

Fly4Fun,

Wayne

RE: GMS Engine Tuning Problem
 

Hi Seagull Extra,

Glad to hear from you! I was at the KWFD as well.

If the engine was working OK on one plane, but not this one, then I would suspect a fuel delivery problem. Either tank location, fuel restriction, or air in the fuel.

If all these are OK then you may want to try the things discussed above, such as adding extra pressure to the fuel tank by drilling out the exhaust pressure nipple to the next size, or by adding an exhaust extension to create a little added pressure to the tank.

Hopefully, all the people working on this problem will get to meet someday. I hope GMS appreciates our efforts!

Keep us up to date on your progress with the engine.

Fly4Fun,

Wayne Miller

RE: GMS Engine Tuning Problem
 

Hi Wayne
One other thing I forgot to mention was that I also tried a OS muffler on the Sea Bee with the GMS motor and it would still dead stick when subjected to arobatic flight. I also lost about 1000RPM with the OS pipe but I too thought muffler preasure was a possible cause for the flame outs. One good thing is I now have a lot of practice with dead stick landings and somehow I managed not to lose the plane. I sure hope my GMS .76 that I am putting in my Seagull extra 300 does not have these problems.
By the way I don't live too far away from you. I am in Elmira.

RE: GMS Engine Tuning Problem
 

Hi Seagull Extra,

I also have a GMS .76 and fly it on 15% nitro with a Fox RC idlebar plug and it works great!

I'm sending you a PM. Please check your INBOX.

Fly4Fun,

Wayne

RE: GMS Engine Tuning Problem
 

I'm going to break out and mount up a GMS .47 tomorrow. I will
be checking out the fuel inlet thing, and see if I can help out. I like
the engine.... ;)

Dave. :D

RE: GMS Engine Tuning Problem
 

Well, it looks like there certainly is an alignment problem in the fuel
inlet spigot. I put a short piece of fuel line on the carb and blew into
it with the barrel wide open. Even at 3 full turns out on the high speed
screw....it seemed like it was restricted, and hard to blow air through.

This picture wasn't easy to get, but you can see the partial restriction.
It's not really as bad as it looks, due to the angle I had to use to get
light into that small hole....the restriction is more like 40% (eyeball)
it looks more like 50% in this pic.

Dave.

RE: GMS Engine Tuning Problem
 

Hi Flyboy Dave,

Thanks for the picture, I know it must have been hard to get, it cleared a lot of things up for me. My brass insert was rotated and was off center. Freq Flyer stated he was able to fix his by aligning it in and out, and I didn't really understand, now I know why.

So it appears it can be out of alignment either in/out or rotated.

I think the restriction is also worse when the brass inlet nipple is installed, I think the base of the nipple also covers part of the hole when it is fully seated.

Keep us informed of your progress.

Fly4Fun,

Wayne

Edited for typo

RE: GMS Engine Tuning Problem
 

Wayne:

The c-clamp would probably work fine, to press out the insert, as long as the carb/insert is placed between a couple pieces of wood, or something to protect the soft metals. I like your washer making idea. I really think that putting a washer on the inside, is the only way that the insert will stay in the right alignment, as vibration will likely cause things to shift.

Flyboy Dave's picture is a good duplicate of the carb I am working with, which leads me to believe that the specs are off at the factory. Has anyone made MECOA aware of this site, they should probably be made aware of all this information.

Andy

RE: GMS Engine Tuning Problem
 

Hi Flyer Freq,

It would seem that if the hole is rotated slightly, you could drill it out with no problems, however, if the hole is too far "in" or "out" then moving the hole back or forth would be the best option. The reason for this would be that when the hole is just rotated, the end of the brass insert would be in the center of the carb opening. When the brass insert is too far in or out, then the end of the insert would have to be repositioned to the middle of the carb opening.

I'm not sure if MECOA or GMS is following this thread, nor do I know how to get in touch with them, but they certainly should know - we are putting a lot of work into solving this common problem.

Again, thanks for your input.

Fly4Fun,

Wayne MIller

RE: GMS Engine Tuning Problem
 

I gave the brass insert a couple whacks with a wood block protecting
but it wouldn't budge. My brass insert wasn't twisted as well as out of
alignment, so I decided just to drill it. I used a # 44 drill, which is .085
or 2.2 mm....anything close to that will work. The drill did drift slightly
into the aluminum as I drilled the brass, and I did hit the threads a bit.

I figured this would happen, but it's OK....and I lock-tited the nipple back
in, and I don't anticipate any problem. I forgot to look at the inside of
the fuel nipple itself for any restrictions or problems. However, I gave it
the "blow test" with the fuel line....again with the barrel all the way open,
and manipulating the needle from fully closed to fully open (about 5-6 turns)
the flow seemed to be much improved, and linear.

The good, the bad, and the ugly:

1. Double o-rings on the needle...good
2. See the scroring in the barrel ? (not the throttle sleeve)....bad
3. In the fully closed position, the barrel guide slot is exposed....bad
(this could be one of those "engine won't shut off problems" ? )
4. See how the mounting flange on the carb is "knarled" ? ....bad
I wonder if the carb pinch bolt was out of alighnment os something ?
I don't recall if the carb was off the engine when I originally got it.

RE: GMS Engine Tuning Problem
 

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

RE: GMS Engine Tuning Problem
 

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

RE: GMS Engine Tuning Problem
 

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. [:o]

RE: GMS Engine Tuning Problem
 

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

Andy

RE: GMS Engine Tuning Problem
 

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.

RE: GMS Engine Tuning Problem
 

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.

RE: GMS Engine Tuning Problem
 

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

RE: GMS Engine Tuning Problem
 

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?

RE: GMS Engine Tuning 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

RE: GMS Engine Tuning Problem
 

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???

RE: GMS Engine Tuning Problem
 

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

RE: GMS Engine Tuning Problem
 

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

RE: GMS Engine Tuning Problem
 

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

RE: GMS Engine Tuning Problem
 

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

RE: GMS Engine Tuning Problem
 

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

RE: GMS Engine Tuning Problem
 

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

RE: GMS Engine Tuning Problem
 

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

RE: GMS Engine Tuning Problem
 

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....[:o]

Dave.

RE: GMS Engine Tuning Problem
 

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

RE: GMS Engine Tuning Problem
 

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