RE: GMS Engine Tuning Problem
 

Wayne & friends: After several hours of reading this thread, I'm encouraged and can't wait to try these mods on my GMS 47. My engine was new last year and was broken in on an SE last summer. This Spring I installed the GMS on a 4 Star 40 I had built as one of my winter projects. I have not had a single successful flight with the 4 Star. The engine tunes up great in the pits: good idle, smooth transition and great WOT, but it won't fly. The GMS quits in the air every time. The engine has all the symptoms mentioned in this thread. I'll keep you posted and I hope to be able to add another success story soon.

RE: GMS Engine Tuning Problem
 

Hi Planetomb,

Welcome aboard!

Please keep us up to date - look forward to following your progress.

Fly4Fun,

Wayne Miller

RE: GMS Engine Tuning Problem
 

What all mods are you going to try? Best of luck,

Ernie

RE: GMS Engine Tuning Problem
 

Does anyone want to purchae a couple of these in the .40 size? One has been mounted, broken in and never flown, The other has the problem where it has been in the air three times and it just dies. I spent too much time and money on the first plane building it to have it do what the second motor is doing and crash it. Corsairs are tricky enough as it is to fly.

RE: GMS Engine Tuning Problem
 

The modifications I've made so far: I had previously sealed the rear plate, the exhaust and the cylinder head. I had also completely checked out the fuel supply system to insure that there are no leaks, crimps, bends, or anything that would inhibit fuel flow. Using suggestions found in this thread, I removed the carburetor body from the engine, removed the fuel nipple from the carb body and observed the partially blocked hole. I first enlarged the fuel nipple, then reinserted the fuel nipple and enlarged the hole in the carb with a drill bit through the fuel nipple so that the threads of the carb body are not damaged. This is a blind operation because you can't see what you're doing, but by going slow and checking several times by removing the fuel nipple and taking a look, it takes only a few minutes. I gave the carb a bath in alcohol to remove any filings from the drilling operation. The only issue I had was when reinstalling the fuel nipple to get the engine back together, I twisted the fuel nipple off leaving the threaded base stuck in the carb body. I should have anticipated doing this because the enlargement of the fuel nipple weakens the walls of the threaded base. I was able to back out the threaded base and install a new nipple in the carb body. I haven't enlarged the new nipple. I sealed the carb body when I reinstalled it on the engine. I don't think I'll need to shorten my spray bar. I have a gap between the end of the LS needle and bar when at full throttle - the gap is small but visible. I will run the engine tonight and report results soon. (I have not enlarged the exhaust nipple for increased back pressure nor have I tried a baffle yet - these steps will follow if it still quits in the air.)

RE: GMS Engine Tuning Problem
 

I Have a Gms61 same problem, flies for abour 3-4 mins then dies, tried all the fixes mentions on this forum-no help. replaced carb. with one from super tiger 61 runs good no more problems. super-tirger carb fits, no mod's.
Les

RE: GMS Engine Tuning Problem
 

I have both a GMS 47 and a Tower 46. The GMS has a Bisson Pitts muffler on it. Neither engine has ever died in flight on me. Wont even die when the throttle is closed. Only problem is that they want to sputter on the take off roll when get past midrange. Both are on warbirds so this is a pretty big problem. I am still a fairly new guy to IC having come up in electrics but I was wondering if this might be because the high speed needle is too rich coming off a good low end setting? Plan to lean them out a little tommorrow to see. What do you guys think?

Jim

RE: GMS Engine Tuning Problem
 

I am surprised at the number of people who are drilling fuel nipples, carb orifices etc in an attempt to solve carburation problems on GMS 47's.
Before doing this I suggest you try a simple test. Start the engine and run it at full throttle. After tuning it for max revs, OPEN the needle. If the engine slows it is TOO RICH. This means that you are getting more fuel than you could ever need and therfore there is absolutely nothing to be gained by drilling out nipples or the carb!
Similarly I see nothing to be gained by filing the end of the spray bar tube. Opening the idle needle will have Exactly the same effect without the disadvantage of moving the end of the tube away from where ot should be (in the centre of the carb throat)
In my experience (having sorted quite a few of these engines) the problems are almost entirely due to the idle needle setting. While the engine may well tolerate an incorrectly set idle needle AT IDLE, it will invariably lead to problems transitioning and in the mid and high throttle ranges.
In my earlier post on this subject I suggested that the gap between the end of the idle needle and the spray bar tube should be about .002 to .004in at full throttle. This is a good starting point and the needle may need adjusting slightly from there. Another good way of establishing a starting point for the idle needle is to attach a long piece of fuel tube to the carb so that you can blow through it. Close the throttle rotor on to a modelling pin and adjust the idle screw until you can just start to hear air escaping (with your ear close to the carb) when you blow. This should give a full throttle idle needle position just clear of the end of the spray bar tube.

My 47 has many many hours under it's belt and runs perfectly. It pulls a 12.25 x 3.75 APC at about 12500rpm on a Tower muffler, idles reliably at under 1600rpm and hasn't even coughed let alone quit in flight for a year. And all this from an engine that was misbehaving badly, had no power at all and I thought was beyond saving UNTIL I GOT THE IDLE SCREW ADJUSTED RIGHT. Remember that ALL I did was adjust the idle screw and it solved all the mid and full throttle problems!

RE: GMS Engine Tuning Problem
 

In my earlier post today I should have added that :-

1/ The idle needle on these engines is very very sensitve. It MUST be set correctly and 10 degees rotation can have a dramatic effect. By comparison OS (and some other) engines are quite tolerant of slightly rich idle settings and generally their idle srews are not unduly sensitive - within 1/2 turn is OK.

2/ The idle needle must be AS LEAN AS POSSIBLE. Starting from the .002 to .004 gap mentioned in my earlier post, run the engine and tune the main needle for full power. Allow the engine to idle at under 2000rpm for at least 30 seconds then open the throttle rapidly (at about servo speed). If the engine dies with no smoke it requires richening. If it coughs, hesitates and smokes it needs leaning.
Keep leaning the idle needle untill you have gone too far i.e. it will not accelerate away after a 30 second idle. Now richen the idle needle a few degrees at a time untill the engine accelerates quickly when the throttle is opened. Now adjust the main needle again for full throttle performance and CHECK THE IDLE NEEDLE AGAIN. Remember that the idle needle WILL effect the position of the main needle for full throttle performance.
Keep repeating this untill you arrive at the leanest possible idle needle position that will allow instant acceleration to the best full throttle performance you can get.
When it is dead right, the engine should accelerate cleanly after prolonged idleing every time. Closing the idle needle even a few degrees should make it too lean -i.e. it should just die when the throttle is opened.

The really confusing and undesirabe characteristic of the design of these carbs is that an incorrectly set IDLE MIXTURE will effect mid range and full throttle performance dramatically and yet the engine may well idle really nicely!!!

RE: GMS Engine Tuning Problem
 

JDW: I've found that the idle setting is, as you say, very sensitive. But for my engine, getting the settings right was not possible. Prior to drilling out the fuel intake in the carb body, I had very little response to the high needle setting (as many others in this string have also stated). Without any modifications, I could get my 47 to run and transition on the ground perfectly. But it would die in the air EVERY [&o] time, sometimes on takeoff, sometimes on a climb - it just quits. After modifications, I can set the high speed needle (remote needle valve) with noticable change to engine performance. This allows me to get even closer on the low speed needle than I've been previously able. I'm also running the stock tuned muffler rather than the tower muffler as on your engine. Reaming out the nipple on the muffler creates just a little more tank pressure so that the engine can maintain fuel flow when the plane is vertical.

RE: GMS Engine Tuning Problem
 

Planetome,
My engine (and the others I have sorted) all have remote needles. My engine also has a partly obscured spraybar fuel entry hole.
Even so the hole is plenty large enough to pass 5 times the quantity of fuel necessary to run the engine at full throttle!

The approach I outlined has worked for me with either the standard muffler or the Tower muffler. In fact the standard muffler gives slightly more fuel pressure than the Tower muffler (the Tower muffler requires a slightly richer main needle setting).

Remember that if it possible to make your engine too rich at full throttle by opening the main needle then you CANNOT possibly have a fuel flow problem and drilling out fittings CANNOT fix the problem. It MUST be something else!

I also simply cannot accept that drilling out the muffler nipple will increase tank pressure AT ALL. Fuel is obviously a far more viscous fluid than air. Since the fuel nipple and muffler nipples are the same bore, it is inconceivable that the exhaust cannot enter the tank fast enough to maintain pressure! We are dealing with an almost static situation here - the fuel flow is relatively low (probably less than 10cc per minute at full throttle) and there is no doubt that whatever the flow, if the fuel can get through the fuel nipples the exhaust can sure as hell get through the muffler nipple to maintain the tank pressure. A bigger hole will NOT increase the pressure.

I actually drilled out my muffler nipple (when I was desperate and before I really thought about it). It made no difference at all to the poor performance I was then experiencing. When I thought about it a bit I realised why!
Why did I drill it out? Simple, I had a totally unresponsive main needle that needed to be about 5 turns open to allow the engine to run at all when vertical. It seemed like a fuel pressure problem. Once the idle needle was set as I have described I was able to close the main needle by MORE THAN 3 TURNS and gain over 1000rpm!!! I then replaced the drilled muffler nipple with a stock one - no change.
I can demonstrate this anytime I want by simply richening the idle needle. The main needle becomes unresposive and has to be opened a number of turns, the full throttle performance falls away (particularly when vertical) and the engine is a dog. It still idles OK though!

The engines I have sorted perform identically horizontal or vertical. My engine is in a fun fly model that spends a lot of it's life vertical. It has great power either horizontal or vertical (no change in revs) whereas it used to drop 1000 rpm when vertical.
The idle needle hasn't been touched for a year and the main needle has only been adjusted about 3 or 4 clicks depending on the weather.

RE: GMS Engine Tuning Problem
 

I have found another problem with these engines. On the lock down ring for the throttle arm if it is not fully agains the butterfly then you will notice that the distance from the spray bar will always change. They have over machined out the butterfly so that there is way too much slop. If you think this may be your problem just open it at full throttle. Grasp the throttle arm and give it a slight pull away from the carbeurator and you will notice the distance changes. The way I fixed mine was to stick a wooden dowel down in the throat at full throttle and loosen the throttle adjuster plate and then slide it inward towards the carb. Then lock it down snugly. This is a pretty cheesy carb design if you ask me. But you get what you pay for. I am getting rid of my GMS engines and switching over to SK engines. Cheap, reliable and better design and built!!

RE: GMS Engine Tuning Problem
 

If I have interpreted your post correctly, your engine must have a different carb to any that I have seen on a GMS. On all the ones I have seen the throttle arm ring (which is attached to the throttle rotor with a set screw) is the same diameter as the rotor and does not bear against anything on the carb body so cannot control end float in the rotor. In any event it moves away from the carb body as the throttle is opened. Even removing the throttle arm ring completely wont change the end float.
The end float of the rotor is determined ONLY by the clearance between the stepped end of the screw in the top of the carb and the scrolled slot machined in the rotor.
A bit of clearance here should not present a problem as there is, of course, a spring behind the rotor which will load the screw against one side (the inside) of the slot at all times.
The critical setting is the position of the idle needle relative to the end of the spray bar tube at full throttle with the spring loading the rotor 'out' of the carb body.
I agree with you comments re the design and getting what you pay for!!

RE: GMS Engine Tuning Problem
 

JDW, mine may be different as I do not have the spring to keep it loaded inward. I have two of these motors and checked them both out and neither have the spring. These are the .40 size. You were right though the throttle ring bears against the rotor so it does have no effect. My bad. I was playing with them again last night and noticed the problem again and checked it out to find that the throttle ring does just butt up on the rotor and does not fix the problem.

There just seems to be too much slop on the machining of the rotor where the screw holds it in. This seems to be a problem on both of my motors. I checked my Super Tiger, SK, K&B, and Irvine motors and none of them have that type of slop. The K&B has more slop than the rest but not over .001 of movement. The GMS has between .0015 and .0025 of movement when checked with feeler guage. Not sure how much of a problem this is since it is so minute.

I am going to go back this afternoon and try to re tune like you stated with the idle mixture. I did this the day before yesterday and thought I had it but tried to get it a little better and went too far and my glow starter and starter's batteries were dying so I could not get it to re start. I was setting at approx 2 turns out on the idle setting and 3 turns on the main high speed needle. I believe the 3 turns out is the factory spec. I will have to find the instructions this afternoon before re trying.

RE: GMS Engine Tuning Problem
 

RLBROBST
The spring I'm talking about is located inside the carb behind the rotor and loads the rotor OUTWARD not inward.
This is a very common set up and is used by most manufactures. My guess is that the .40 carb is identical to the .47 and if you remove the screw in the top of the carb and slide the rotor out you will find the spring hiding in there.

I have seen other brands with similar end float, but as I stated before it shouldn't be a problem because of the spring. The only way I can see the end float causing concern would be if the throttle linkage in the model was badly misaligned and was pushing the rotor in against the spring. This could possibly, I suppose, create a condition where the rotor might oscillate in and out. If the movement is only .0015 to .0025 I doubt that it would be an issue.

RE: GMS Engine Tuning Problem
 

JDW
You are absolutely correct. I adjusted the idle screw to a more lean position and am able to idle for well over a minute and then throttle to full throttle with no problems. I believe I have the needle out appox. 1 1/2 turns out and the main needle is out 3 1/4 turns. It now throttles up great and does not spit and sputter and die! Everyone should try leaning out the idle needle instead of all this filing and drilling out stuff. It is VERY SENSITIVE! You move it 1/8 of a turn and it could kill the engine in either a too lean or too rich setting. I have now ran 30 oz of fuel this evening without incident! Before it always would die in transition and you would have to dead stick the engine. Thanks everyone for all your information and inputs but I will just stick with the adjustments and not modify anything!! They build these this way for a reason. Why modify if you can adjust the problems away?!?!?!? I also confirmed all this information with a Tower Hobbies, where I purchased my engines, Technical Rep. who DOES NOT RECOMMEND filing or drilling ANYTHING. The only problems they have been unable to solve is if you use a Pitts style muffler on anything less than the .61 on the GMS engines.

RE: GMS Engine Tuning Problem
 

I tried the same thing last Sunday with my motors. Both the Tower 46 and the GMS 47. Although I did not fly the GMS, it seemed to fix my problems. It will idle forever and seems to throttle up now with no problems. Have to kill it by grabbing the spinner as it will not even die with the throttle closed. The Tower is the same. But I did fly it and did not have anymore problems. However, the heat was so bad, in excess of 100 degrees that lift on our airplanes was much diminished. Since the 47 was in my pride and joy I did not attempt a flight.

All in all I must agree that leaning out the low end needle seemed to be my answer.

I will see if I have a pic of my 47.

Jim

RE: GMS Engine Tuning Problem
 

Scratch FW 190 A with GMS 47. Flies fantastic.

Jim

RE: GMS Engine Tuning Problem
 

PLBROBST
I'm glad you were able to sort your engine. If it's like mine you wont have to touch the idle needle again.

I know of 2 guys running pitts style mufflers successfully on GMS .47s. Both had major initial problems and couldn't get the engines to run reliably. Eventually they both (independently) came up with the same 'fix'. The fix involved plugging one of the two outlet holes from the muffler. This obviously increases muffler pressure, so it seems that the standard pitts muffler doesn't produce as much pressure as the standard (or Tower) mufflers.
While I haven't seen either of these engines checked with a tach, I don't think the performance was as good as on the standard (or Tower) mufflers.

RE: GMS Engine Tuning Problem
 

Just wanted to say thanks to everyone who has posted here. This has been very informative to me because I am new to this hobby. I also own a Tower .46 and just received my GMS .47 today. Brand new in the box for less than $50 on ebay. I had the same probs with my Tower engine and got so frustrated that I sent it back. I installed my GMS on my Hanger9 trainer and ran three full tanks through it. I then filled it up again and adjusted the high speed a little and leaned the LS about 1/8 of a turn and put her in the air. It ran so good I couldn't believe it! Thanks again everybody!

RE: GMS Engine Tuning Problem
 

JDW, That was exactly what I did on my corsair to get the pitts muffler to work! Guess I am not the only one who has done this. I posted that suggestion last year on this thread.

RE: GMS Engine Tuning Problem
 

I have had no problems running th Pitts muffler on my GMS 47. Gobs of power.

Jim

RE: GMS Engine Tuning Problem
 

....I beg to differ....an engine running slightly rich on the low end will not
exhibit the problems that were associated in the original post's in this thread.

As a matter of fact, my engine was set too lean from the factory on the low
end....I had to open the low speed to get the engine to transition....without the
slight bog.

The problem I have seen, and what has been described in this thread, is with
the engine dying out at full throttle, for no apparent reason. This situation
would normally result from an engine leaning out in flight (normal) with
a lean condition....not a slightly rich condition. We all know that an
engine has to be set slightly rich, because as the engine takes flight, it tends to
lean out for a few reasons.....leaness will cause an engine to die in flight.

A slightly rich condition, caused by a rich setting on the low end....will not cause
an engine to quit at full throtle....in my experience. :eek:

FBD. ;)

RE: GMS Engine Tuning Problem
 

Flyboy Dave,
I agree with you that with most carbs, a SLIGHTLY rich idle setting will not cause in flight problems at FULL THROTTLE. The most common problem with a rich idle setting, in my experience, is at part throttle when the throttle is advanced after a period of low throttle operation. In this situation MOST engines with a rich idle setting will be prone to gag and possibly quit.

In the case of the GMS carb however, the idle setting is extremely sensitive and definately does effect full throttle performance if set too rich. If your engine was delivered with the idle setting too lean and you have richened it just enough to allow it to transition properly, then yours is not too rich and so wont exhibit the problem we are talking about.

There is no doubt that these engines will idle and even transition OK with a very rich idle setting. The downside however is that in this configuration, the idle needle will withdraw too far from the spray bar tube at full throttle causing (I believe) poor fuel draw and atomization. This in turn requires a VERY VERY rich main needle at full throttle. In my case richening the idle needle by 45 degrees requires the main needle to be opened by over a turn!!
The problem is NOT the fact that the IDLE mixture is rich, it is that this rich setting means that air/fuel flow of the carb is screwed up at full throttle due to the relative positions of the idle needle and spray bar tube. With an engine set up like this there is no doubt that it is very likely to quit unexpectedly in flight - mine certainly did.

Once the idle is set correctly, the engine behaves normally - the main needle can be leaned - it is progressive and predictable - the engine delivers more power and becomes reliable.

I have set up engines of all brands over many years and have never before experienced a carb where the idle setting has such a dramatic effect on the physics of the carb at full throttle. I stress again that it is NOT the idle mixture that is the problem it is the resultant change in the geometry of the carb in the full throttle position.
Most engines can be tuned perfectly for full throttle operation whatever the position of the idle setting. The GMS certainly falls down in this respect.

It seems that a number of others have now tried setting their carbs as I have suggested and found that it has worked for them as well.

You are correct of course in saying that engines tend to lean in flight and the MAIN needle needs to be set slightly rich to compensate for this. The IDLE needle however certainly DOES NOT need to be set rich for any reason. I can see no advantage with any engine under any circumstances for having a rich idle setting.

RE: GMS Engine Tuning Problem
 

Just curious, can the remote needle valve on these engines be moved to the front position, like on the Tower 75 engines?

Thanks,

Ernie