I have a GMS 47 with a APC 11X5 prop mounted in a Sig SE . It has lots of power, will go vertical forever. Mine idles great, transititions good, and even shuts off. Now for my problem; Unless I set it very rich on the ground (guessing 1500 below peak with lots of smoke) it wants to quit at the top of a loop. I am thinking it must be leaning out as it goes over the top, but it will go vertical as long as I want. I have checked the tank, fuel lines and clunk. It seems if it was a pressure problem it would lean out when vertical also. Anyone else have this problem? Does this engine just really unload in the air? I also have a OS 46FX and a Magnum XLS46, they are both good but the GMS puts out more power.
UPDATE
Read my last post in this thread for things I found out.

Is your engine mounted upright ?

Yes, engine is upright. I might add, I have tried the needle valve in the remote position and in the carb, no difference.

Loosen the carb pinch-bolt (not all the way loose) and put a piece of
wood over the carb. Tap it down tight against the o-ring, and retighten
the carb. It might have a small air leak....that may be why you have
to set it so rich.

Try to set the low end a tad rich as well.

FBD.

Dave, been there done that with carb. I even put a little high temp RTV on the O ring and pinch bolt to seal them.

Here's one way to see if the upright mounting is acting silly....

....get the plane up high enough, and push it into an outside loop. Push
a lot of down into the control, and listen to the plane. If it richens way
up, and goes into a four stroke....it's your upright system doing the bad
things. If that is the case, you can either live with it....or mount the engine
sideways to eliminate those pesky fuel problems.

FBD.

Jerry,

It sounds like your fuel tank is mounted too low.

It should be mounted so at level flight attitude, the fuel tank centerline is at the same height as the carburettor spray-bar.
If you cannot raise your tank high enough within the model's fuel compartment, remount the engine at 90 degrees to the right, setting the spray-bar at crankshaft level. Given the fuel tank is directly behind the engine mount, you will have the height problem solved. Make sure the tank as as far toward the engine as possible.

When mounted at 90 degrees to the right (of the model), the exhaust stack is pointed down and the weight imbalance is couteracted by engine torque.

An engine in an R/C plane should not change its speed from Gs in a maneuver (like a control-line plane), but only from pitch change (nose up, Etc.). It should also sound the same; whether upright, or inverted.

I have discovered that if I go over the top of a loop at full power I do not have the problem. I normally fly at 1/2 to 3/4 power and that when it happens. I think the problem is caused by leaning out when the plane goes vertical at partial throttle. The reasoning behind this is the fuel has to be pushed more upward when vertical and at partial throttle the muffler pressure is not quite enough. The problem is less severe if I set the engine rich so it shows a good smoke trail in the air. Also set the low speed needle on the rich side so it dosent interfere with the mixture at partial throttle. My low speed needle is set so that when the carb is fully open the low speed needle is just starting to enter the spray bar (no gap). This is a little rich at idle and gives a little hesitation when quickly opening the carb but seems to run better at mid range. By the way I have a Tower 46 that does the same thing. I think that GMS makes tower engines. I think the large mufflers on these engines are tuned for max power and do not make a lot of pressure at lower RPM's. The only other problem with these engine's is sometimes not wanting to shut off due to air leak through front bearing. But they sure make a lot of power when you open them up.

Jerry,

The so-called idle needle controls the fuel curve at all but 80-100% open throttle.
It is imperative that you fully understand this.

Since you did not mention the tank level issue, I understand that you have not yet checked it yet.
If this issue is not checked and rectified, there is no base to make any mixture setting corrections.

You must make these measurements and the tank center-line must be at the same level if the fuel jet, plus or minus 3/8".
No higher and no lower. Many kits and ARFs are not built this way and the engine is too high to allow this.
It may necessitate changing the engine mounting to 90 degrees to the right (of the model; muffler pointing down).

Before making these changes, should they prove necessary, no adjustments can overcome the in-flight mixture changes you are encountering.

If at 1/2 throttle, you raise the nose of your plane straight up, does the engine lean out?
It should become somewhat leaner (speed up).

This is not a natural phenomenon, since when doing it in flight, the plane is slowing down, so less effort is needed for bring fuel into the engine, than in a static 1G condition.

If the fuel tank center line is lower than I suggested, your mixture will become leaner in high G maneuvers and it will get worse as the fuel level goes down, below half tank.

Also try sealing the rear cover with Hi Temp RTV. That always seems to leak also.

Jerryj39,

Sorry to hear about your problems; I've had a bunch with my GMS 47, but I think I might have most of them licked.

Since it's running so well at other times, other than the tank/carb level issue, I would take your tank, and REPLACE every bit of tubing,
and try a sintered clunk. I won't bore you with all the problems I had with mine but the fix for many of them was to do just what I suggested. Even though it LOOKS good, replace the tubing, and try the sintered clunk. A difference in GMS full-throttle loops and half-throttle loops should be nil, unless there is a pressure, foaming, or clunk problem, and all are corrected easily.

I had mine out today on a Debonair with a 12x4 APC and it did fairly well until the throttle arm retaining hex nut came loose and I lost throttle control and had to use the "Terra firma kill switch". Replaced the broken APC with a wooden Zinger 12x8 (just because I had one!) and what a dog that flight was. Replaced that with a wooden MAS 11x6 and had a totally enjoyable flight with gobs of power and fun.

Let us know what you come up with!

Blue skies,

DL

Went flying today and think I found the problem. First off I changed the engine in the SE to a Magnum 46 XLS that I had and it ran flawless at all throttle settings and attitudes. That pretty much eliminated fuel delivery problems. So then I put the GMS back in, adjusted high speed needle to 14K RPM then richened to obtain 13.4K Rpm at full throttle, pinch test was good. Now the problem, still on the ground I reduced carb to about 1/2 and 2/3 throttle and when I pointed the nose vertical engine would slow down; LEAN! At full throttle this would not happen. Knowing that the low speed needle is not just for idle I started turning the low speed needle out until RPM did not change when I pointed nose up at partial throttle, had to readjust high speed a few times during process. Flew it with this setting and engine ran great without dying out as it had before. Now the problem, with the low speed needle set so the mid range is good it is very rich at idle and does not transistion good. Everything is sealed good except for the well known front bearing leak, maybe the idle problem. I have been around quite a few engines and it seems like the problem with these might be resolved if the carb barrel cam slot was cut to give more travel to the low speed needle. That is let it close more to get a good idle but move out of the way quicker for enough fuel at mid and high speed. I have one thought I may try, file the bottom of the spray bar to about a 45 degree angle pointing down. My thinking is this would allow more fuel at part and full throttle and I could still screw the low needle in to get a good transition. Any thoughts on this?
Now for the ugly! I was having fun doing rudder stall turns with down spirals. Did one rotation to many, SPLAT! No more SE.
UPDATE after examining the carb more carefully I see that filing thr spray bar would let more fuel in at idle also, so I think I will taper the last 1/16 of the low speed needle a little more. This should allow the idle to be set leaner but allow more fuel as the carb is opened. I am also going to seal most of the slot between the front bearing and the intake port on the crank, I think this is where most of the air leak comes from.