RE: GMS Engine Tuning Problem
 

I didn't get to fly this past weekend because we got dumped on by mother nature. Instead, I took an old glow plug, drilled it out, and silver soldered a Dubro nipple onto it. I hooked an air hose up to it, plugged the carb intake and nipple, and the muffler nipple and exhaust, and applied 5-10 lbs of pressure to it. I then submerged the engine in water and watched for bubbles. I was really amazed at how tight the engine was, without adding rtv anywhere. as I rotated the prop, the air would pass from the cylinder to the crankcase. When this would happen, air would come out the front bearing, but nowhere else. As I rotated the carb barrel from fully open to fully closed, I got a little bit of a leak around the end of the barrel when fully closed. Hardly enough of a leak to mention. After this test, I pulled the engine apart, including pulling the front bearing, and oiled everything up real well. Once I saw the machined channel for the oil return at the front bearing, the air loss there made perfect sense. I am no longer concerned about that at all. I am all the more convinced that the problem with this engine was fixed when I changed the spray bar. The poor transition in a hover, in the frozen climate, I am convinced is related to the engine being so new, and the fuel thickening from the cold. A higher nitro content would likely help in this area.

I honestly didn't expect to find this engine so air tight. I didn't even see the leak people talk about around the carb jamb nut. You may recall though, that I had changed that o-ring when I damaged it taking it out of the housing throat. The one supplied by GMS seemed rather hard and stiff.

Andy

RE: GMS Engine Tuning Problem
 

I have to say that this thread has been very informative but I don't see whatI'm looking for. Can anybody tell me why my .47 will scream like a banshee with an idle bar pllug but won't run for crap with an OS8? The idle bar plug lets this thing just tear up the sky. When I put the OS8 into the engine it won't go past half throttle. I'm really perturbed to the point of getting rid of it. Problem is, I don' have the cash flow right now and don't really want to buy another motor yet.

RE: GMS Engine Tuning Problem
 

You've already run this problem through the mill in your own thread. You recieved responses from some very knowledgable people at that time.

It might be time to try some of those suggestions?

You could also take a couple hours and read through this thread?

Here's your own thread....

http://www.rcuniverse.com/forum/GMS_...2611432/tm.htm

Enjoy,

Jim

RE: GMS Engine Tuning Problem
 

If you look at the OS plug guide, the OS 8 was designed for .60 size engines and up. Of course, we all know that engines have their own temperment, and we will solve problems, sometimes, by going against recommendations. In this case, however, the OS 8 is the wrong plug. The A3, as mentioned by w8ye in the thread you started, is the way to go. My engine runs great on it, and it might be on its original plug, after 5 years,...not sure though. The OS 8 is definately the wrong heat range.

Andy

RE: GMS Engine Tuning Problem
 

Here are my findings as promised in my post # 86

As Wayne has indicated in his post # 106, after completely disassembling the engine, we did not find anything mechanical that might cause sudden flameouts. After seeing the annular reservoir formed by the .030" deep by .250" long undercut on the outside diameter of the main jet, and the 2 main jet inlet holes, I discounted the theory that fuel flow restriction caused by misalignment of the main jet inlet holes and the fuel inlet nipple, was the cause of sudden flameouts on this particular engine.

The carb venturi diameter of .312" impressed me as being very large for a sport .47 displacement engine. This is the same size as the venturi used on .60 size engines of a few years back. While this size venturi would give great power at the high rpms turned on a 9" prop, it could certainly affect fuel draw when a 11" or 12" prop was used for aerobatic flying. I therefore made up a venturi restrictor with a .281" inside diameter, the approx diameter of the venturi on my old OS 45 FSR.. I also opened up the muffler pressure tap from .062" to .070".

After thinking about the possible reduction of muffler supplied fuel pressure caused by resriction of the rear mounted main needle valve, I decided to mount the needle on the carb replacing the acorn nut and plastic plug in the end of the main jet. With these mods in place I mounted the engine on my test model which normally has the OS 45 FSR engine installed. The fuel tank is particularly well isolated from vibration and fuel feed has never been a problem in this model.

Last weekend's weather provided a great opportunity to check out the engine. Details are as follows; temp. 28 F, 8" powdered snow; Master Airscrew 11x6 prop; home brew fuel with 5% nitro, 20% oil, 5% acetone; Fox long idlebar 1.5 volt plug; mixture set backed off approx 300 rpm rich from peak .

First takeoff- engine quit abruptly at about 50'. The exact same problem as encountered several times last fall with the engine in the owner's trainer.

I changed the gloplug to a new OS F four stroke plug, reset the mixture as above and took off. Engine ran beautifully during many take offs and landings and aerobatics.

Changed to a new Fox Miracle plug, reset mixture, another engine failure at 50' altitude on T/O.

Changed to an old used Fox idlebar long plug, reset mixture, took off and the engine ran beautifully, equally well as with the OS F plug.

Changed back to original Fox idlebar plug, engine again quit at about 50' altitude.

I removed the GMS and installed my OS 45 FSR and installed the owner's Fox idlebar plug which was giving the engine cuts. I noticed that when removing the igniter current that there was a greater than normal rpm drop but the engine tuned up ok. I did many circuits and aerobatics during 4 flights and suffered no engine cuts. I used the same fuel and prop and the weather was identical to the first day's except that the temperature was 30 F.

My conclusion- Gloplug selection even amongst plugs of the same brand, is very critical with this particular engine, and may well be with many GMS engines . What is particularly disconcerting is the fact that the symptoms exactly match those of an interrupted fuel flow.

RE: GMS Engine Tuning Problem
 

Hi Waco Driver,

Thanks for the very detailed analysis. Your thinking regarding the venturi is the same as Don H out of Brantford, in fact you may both belong to the smae club - you may want to give him a call.

I personally stick with the Fox Idle Bar plug, the price is right, its been around for many years and seems to work in all my engines. The only time I've seen them let me down is if I use them during engine break in - they fail prematurely after that. Typically I'll use an older plug for breakin.

I live in Kitchener, perhaps we'll get a chance to meet and compare notes.

Fly4Fun,

Wayne Miller

RE: GMS Engine Tuning Problem
 

Wayne ...I am Don H out of Paris. Cheers

RE: GMS Engine Tuning Problem
 

Hi Don,

Sorry, I failed to connect your RCU name to you. Again, thanks for your input and for all the things you've shown me, and the help you've given to others in getting started in the hobby.

Fly4Fun,

Wayne

RE: GMS Engine Tuning Problem
 

Hey guy's, go back to entry 109. My theory on the hole missalignment appears to be correct. We had temperatures today of 47 degree's F, so off to the field I went. With the warmer temperatures, the engine ran like a top. Transitions were snappy, in fact, that may be an understatement. Due to ground clearance, I could only run my APC 11X3. It turned it consistantly at 15,700 and at times saw as high as 16,200 rpm. In fact, on my second flight, I blew the element on the A3 glow plug. I put another plug in, and was back off to the races. The regular summer crowd was at the field, except for the engines owner. No one could believe it was the same engine, as before I replaced the spray bar, you were lucky to get the engine off the ground. Fortunately my witnesses will vouch for its incredible performance. I am confident that if everyone gets a new carb body with the hole in the spray bar, their problems will go away. I must add, that the engine was very easy to set. No putzing was required!

I am dissappointed, however, that MECOA has neglected to return my e-mail as to our issues.:(

Andy

RE: GMS Engine Tuning Problem
 

Hi Andy,

I went back and read 109, and I understand your analysis led you to believe fuel turbulance at the brass intake hole may have been the problem.

Your last post said to order a new brass insert to solve the problem. I'm not quite sure why the new brass insert will fix the problem.

Originally I stated I drilled both the intake and exhaust nipple and the brass insert to get mine working. I suspect it was the drilling of the brass insert hole larger was the fix for yours, but I'm not quite sure. Can you explain what you think the fix is?

I'm glad to hear the engine is working well, congratulations, your hard work paid off!

Fly4Fun,

Wayne

RE: GMS Engine Tuning Problem
 

Hi Wayne:

Unfortunately, to get the brass insert, you have to get the whole carb body. Fortunately that is only 8 or $9.00. I put the brass insert (spray bar) from my good engine into the carb of the engine that was not running right. When I first tested it like this, it would not transition well in vertical flight. I had wondered if it was due to the below freezing temperatures, as a friend with a new O.S. .46 AX was having the same problem in the cold. Since in full throttle, vertical flight, the engine ran well, the only way I would know if the difficulty in a hover was a real problem, was to fly in temperatures above freezing. The 47 degree weather was perfect for that, and it proved the spray bar to be the correct fix.

No drilling was done on this spray bar at all. The hole was just spaced further away from the needle valve. I believe I listed the rough distance from the hole to the shoulder of the insert in another entry, but in simplest terms, it was more centered on the nipple. Looking at the port where the nipple screws in, the entire whole in the insert can be seen, rather than just the edge. The undercut is irrelevant. While it is helpful at lower rpm, at higher RPM, the flow is so fast, it can't make that sharp turn, from the end of the nipple to the hole in the insert, without turbulance impeading the flow. I believe that the fuel may not be entering the spray bar on the taper of the needle either, though I haven't tried to measure that. If the fuel is entering on the flat of the needle, or even on the transition from the flat of the needle to the taper, I believe the adjustments would be very touchy, if not impossible.

I hope I cleared that up, if not, let me know and I will try to write it another way.

Andy

RE: GMS Engine Tuning Problem
 

This thread is starting to get old!! Jeaze, It's not a Capiche Thread. Seroiusly over one engine, the solution would be to make it as a fishing weight, or paper weight. I hope the mods shut this thread down, get a OS or Saito for cryin outloud! Or Webra, or Rossi, or Tower, or Thunder Tiger, or Super Tiger, or MVVS, or some Magnums, or K&B, or Mecoa, or Irvine, or Norvel, or Cox, or Abitar LOOL, or Conley. OK did I miss any engines? Oh yeah; Tigershark!

RE: GMS Engine Tuning Problem
 

Hi Andy,

You did a great job of explaining it, thanks. We may have done the same thing when we drilled out the brass insert, and this centering the hole.

Thanks for all your help.

Fly4Fun,

Wayne

RE: GMS Engine Tuning Problem
 

Well, After drilling my muffler nipple and the remote nipples and the carb nippleon my new GMS .47, I did not see any difference in the way it ran or died I should say. BUT, I took the end cap out of the carb and looked thru the spray bar hole and saw some trash in the spraybar from where I had drilled it I guess. cleaned out the trash, reassembled, re-tuned and now it runs better than it has yet! I have a run little over a gallon of 15% tower hobbie fuel in it, and it runs better with every flight. I am still tweaking it as it wants to flood out if run at a lower speed too long, and it has to have the fuel blown out after a taxi and just before take off or it will die at 10 feet. But I am much happier now. I am just learning how to fly and a first timer doesn't need this, he needs stick time!

RE: GMS Engine Tuning Problem
 

Hi Yoyo27983


I'm glad to hear you got yours running, that's another one "off the shelf" and in the air.

If the plug is being flooded at low RPM, you may want to tweak the low end needle just a very small amount, like 1/16 of a turn from where it currently is (it may go either way, in or out). Another fix is to try a Fox Idle bar plug. It may also need just a little more break in - these engines are very tight.

Thanks for keeping us up to date.

Fly4Fun,

Wayne Miller

RE: GMS Engine Tuning Problem
 

The restricted fuel-flow argument seems to be gaining following around here.

The two GMS 47s in our club both suffer from a tendancy to flame-out when the planes they're in are placed in a dive steep enough to cause the revs to rise by 20% or so. It's as if they just can't suck enough fuel to sustain those revs even though they're tuned slightly rich of optimium at full-throttle.

(no, it's not a clunk or fuel-line problem, you can hold the planes at the same or even steeper nose-down angle on the ground and the engine runs just fine).

We drilled out the exhaust pressure-tap nipple and that helped a little (the engine on one plane now picks up after appearing to die during a bit of a dive).

One can't help but get the impression that these engines might be just a little over-carbureted too.

RE: GMS Engine Tuning Problem
 

Hi XJet,

Good to hear from you.

Has anyone there been bold enough to drill out the intake nipple and brass insert yet?

Thanks for keeping us up to date!

Fly4Fun,

Wayne Miller

RE: GMS Engine Tuning Problem
 

I drilled the bass insert due to misalignment on mine in post #96-100.

Dave. :D

RE: GMS Engine Tuning Problem
 

Hi Dave,

When I saw the happy face I knew you were pleased and, just to confirm, I went back and reviewed your post.

In my mind, I was really asking if any of XJet's buddies had drilled out the brass insert and nipple so the hole was directly below the intake nipple.

On another subject, our local hobby shop charters a buss to Toledo for the Weak Signals event very year, I plan to attend this year. If anyone else is going, I would like to get a chance to meet them. Let me know if your going to be there.

I'm really pleased on how well we have worked well together in trying to solve this problem.

Fly4Fun,

Wayne Miller

RE: GMS Engine Tuning Problem
 

Here's another data-point.

We fitted a Thunder Tiger 46Pro muffler to one of the GMS 47s in our club and it significantly improved the reliability.

Power was down but the damned thing didn't miss a beat no matter how much we tried to make it die.

Perhaps the extra restriction of the TT muffler is creating more muffler-pressure which would support my theory that these engines are over-carb'd.

I've also noticed that the GMS mufflers have their pressure tap right smack in the middle, not necessarily the best place -- ideally it should be in the place of maximum stagnation because the slower the gases are moving the greater their pressure. I'm going to try moving the pressure tap on the GMS muffler and see what happens.

RE: GMS Engine Tuning Problem
 

Hi XJet,

Moving the pressure exhaust pressure tap is a new angle and I don't think it has been tried yet. It will be interesting to know your results, I wonder if there is anyway of measuring the pressure at different locations along the muffler?

Thanks for your help.

Fly4Fun,

Wayne Miller

RE: GMS Engine Tuning Problem
 

I have a Tower 46 instead of a GMS 47. The tuned type muffler fell apart and Tower was out of stock. So I used a MACS One Piece muffler. The engine top end power was down a few hundred rpm but all the other sins were forgiven due to the increased muffler pressure to the tank.

Enjoy,

Jim

RE: GMS Engine Tuning Problem
 

After switching to a tuned pipe on my GMS .47, I began to experience problems with the engine shutting off in any high-G maneuver, especially tight loops. After reading this thread, I removed the fuel nipple to discover the brass insert misaligned with the carb body. I was able to remove the insert by heating the carb body with a torch. I drilled the insert larger to match the body, and now it is like a different engine. I have it on a tower kaos and it will easily do unlimited vertical, turning an apc 11-6 at 13500 and doesn't flinch during any maneuver the plane will do. Thanks to those that discovered the problem!

RE: GMS Engine Tuning Problem
 

Hi Obxwes,

Thanks for letting us know, we really appreciate the feedback. I expect we will be getting more feedback as the season progresses.

Once we isolated to a fuel feed, we were having a hard time identifying if the muffler pressure was the problem, or adding extra muffler pressure just forced more fuel and just hid the problem. From the feedback we are getting, it appears the carburettor has a fuel draw problem and it can be fixed by drilling out the fuel intake nipple and drilling a hole in line with the brass insert to line up with the intake nipple.

I purchased a new GMS.47 last week and again the brass insert hole is not aligned, it is off center towards the direction of the needle valve, away from the carburettor throat.

I'm going to drill both he intake nipple and the brass insert before breaking in.

Just as a reference, I also noted the following while the engine was new:
1. The gap between the low speed needle and the carb brass insert in the throat of the carburettor is 23 thousands when wide open.
2. Hi speed needle adjustment was: 3 3/4 backed out (from tightening the needle in until it just stops)
3. The lows speed needle was backed out 1 7/8 turns when the throttle was completely closed.

in the future, this may be a good starting point when working with a carb that is way out of adjustment.

After I break the engine in, and adjusting the carb, it will be interesting to see how much of a change there is and if it will be closer to the final adjustments I posted earlier in this thread.

Again, thanks for the feedback, I'm looking forward to more results from others.

Fly4Fun,

Wayne Miller

RE: GMS Engine Tuning Problem
 

Hi,

I haven't seen any responses for a while so I'm guessing we've narrowed the problem down to fuel draw at the carburettor input. I'll try and summarize what we have found in this post.

The problem seems to be fuel turbulance 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 in worst cases, a fuel air lock. This makes carburettor adustment 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 modler, Andy, in an earlier post and you should read it.

We have tried many things to fix this, such as, more muffler pressure, enlargening fuel/exhaust nipples and drilling the carburettor 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.

I'm going to try and upload a picture to help those who have not done the modification yet. 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 as close as possible to the "shoulder" of the brass insert and directly under the fuel intake nipple.

If you chose to try this modification, please make sure you do not have a carburettor air leak (where carburettor 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 auto motive 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 carburettor:
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 carburettor.

If you wish to remove the brass insert to drill (prefered 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 carburettor first.
5. Using a hardwood dowel or brass puch, 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 carburettor 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 carburettor nipple. Put the carburettor over the hole in the wooden back up block and gently tap the brass insert in place using the hollow punch.
9. Reassemble carburettor.

After the above, you should adjust your engine as described in earlier posts.

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

If anyone else has any input, please let us know.

Thanks for everyone's help in solving this problem. If anyone is going to Toledo on April 2nd, let me know by PM and perhaps we can meet.

Fly4Fun,

Wayne Miller