RE: GMS Engine Tuning Problem
 

I am a proud (I hope) new owner of 2 GMS 25's used on a twinstar. One engine runs like a raped ape, the other doesn't. Im guessing an air leak as I can close the throttle barrel all the way and the engine continues to run. :eek: Time to start reading.<div>
</div><div>Chad</div>

RE: GMS Engine Tuning Problem
 

If this thread doesn't sort it out then its beyond repair. Good luck.

RE: GMS Engine Tuning Problem
 

My solution was to rid my life of GMS engines.

RE: GMS Engine Tuning Problem
 

They (GMS) all idle with the throttle closed. Try pushing the carb down into the body before tightening the cinch bar. Next trick is to remove the fuel nipple from the carb and remove the swarf/casting flashing in the hole.

I am necrobumping this thread up due to a question. Hopefully some of the fellas that contributed to this thread are still around.

I recently acquired a near-new GMS .47 engine (non remote needle version) that I'm trying to get running good. I've read the whole thread with great interest and have a question in regard to the idle needle/spraybar interference issue. I already fixed the fuel inlet misalignment problem, but now I need to deal with the idle needle and spraybar interference problem. As has been noted, if the idle needle is set for good idle and transition, the high speed operation runs lean and no amount of needle adjustment on the high end will richen it up enough without having the idle needle set way too rich. Is it better to shorten the spraybar or the idle mixture needle and how much needs to be taken off to obtain good results? FWIW, I ran the engine with an Irvine carb and got good idle and transition, but the smaller throat carb robbed a lot of power from the engine - at least 2,000rpm. So I'm very motivated to fix the GMS carb. I do not have the GMS/Tower muffler, but I do have a red Jettstream muffler.

Thanks in advance.

I would advise you to FIRST verify the issue is that the engine is not getting enough fuel at high speed with the LS needle set properly.

Scribe or record your exact LS needle settings that you are using now.

Then richen the LS a bit and start the engine. Tune it for best HS performance.

Ideally you could then adjust the LS needle while the engine is running full bore, and see if there is any affect on the high speed performance.
e.g. if you richen the low speed and see a High Speed improvement, then you know the engine is starved of fuel when running all out.

I don't think shortening the spray bar or idle mixture needle is going to help, short of increasing the spray bar fuel hole size. Neither of the former will buy you more fuel flow at high speed as the idle mixture needle will be out as far as it can go already, and the modification will merely ruin the low end.

You may want to see if there is anything you can do to improve the fuel flow at high speed without involving the LS needle...

e.g.

Is there anything you can do to increase the fuel passages sizes?
What about pressurization? ( but don't use a one way valve ).
Are your lines SHORT.
Can you put a bigger nipple on the muffler?

If the idle mixture is sloppy rich, backed out 4 turns, the engine can be run rich to lean at WOT. Try setting the idle mixture from sloppy rich to a normal setting and the idle needle protrudes into the spraybar at least .5mm or more which negates the main needle setting grossly as the engine goes lean and no amount of richening the main needle cures the lean condition. It's very obvious the idle needle is obstructing fuel flow at WOT when set to the proper setting or even slightly rich of the proper setting. Clearly there is a problem that cannot be fixed with simple needle adjustments. That's a fact.

Sounds like you need a thicker needle or a needle with a steeper taper. Removing the very tip of the low-end needle might also work.

Other things to look into would be the fuel, compression ratio, and prop size. Changing any of these will affect the fuel needed and hence might solve your problem.

I think the needle needs to either be shortened a little bit or re-profiled to have a more gradual taper. I don't think it needs that much material removed, just enough to flow enough fuel at WOT and still idle and transition well. Pictures below show the GMS carb (brass spraybar) and a TT Pro carb (steel spraybar). I think I'd have good success if I chucked up the spraybar and shortened it a little bit and smoothed the end down some like the TT carb.

No, I don't think that there is anything wrong with the spray bar, it is the shape of the needle that you need to work on. A steeper taper for a larger difference in fuel delivery between two given throttle settings.

Fair enough. I read this whole thread, and some mentioned they shortened the spraybar, and some modified the needle (shortened it). Both ways netted satisfactory engine behavior. It was this reason why I wanted to go the route I asked about.

Am I better off to make the end of the needle more blunt with a slight taper or just feather the taper a little finer over more length of the needle? Probably the latter? (Essentially making the needle "sharper"). I'm sure parts are non existent for this engine since they've been discontinued for awhile, so I don't want to ruin it.

Do yourself a favor, ebay it.

I really dislike these kinds of replies. Some of us like to actually make these things work. I'm happy that you don't. That's how I've ended up with a lot of fine running engines. People couldn't get them right and gave them to me. I fiddle around a bit and make them run great.

So if anyone else has these same sentiments like "eBay it" or whatever... Please keep it to yourself.

Thanks.

I'm sorry you're so sensitive about this. I wasted at least 2 years tinkering on the two that I had. I'm just telling you the best suggestion I have. I got rid of mine after endless attempts to make them run right. I finally realized that I was wasting my time. I've been running glow engines since '67 so I'm no "newbie". Knock yourself out with the most frustrating cheap engine I ever bought. I apologize. Have fun.

Tinkering with engines is all part of the fun with the hobby, wouldn't do without it! ;)

In this case there shouldn't be anything wrong with the engine itself, it is just the carb that they messed up a little. One can always fit another carb, the ASP ones are not too bad for their price.

I haven't altered the needle on my engine but removing a bit from the very tip should be fine, I think. Try to check how much of the needle that is inside the spraybar at WOT and take it in steps...

It's not that I'm sensitive - it's no different than telling someone to convert their plane or car to electric because they can't tune the engine properly. I'm willing to give it a try, so why not offer some advice if you have any, or just sit and watch. This engine has a lot going for it even though it's cheap and has a fussy carb. I'll get it right, no doubt.

Amen brother!

I will see if I can get this carb to work. If I wreck it, I'll go to plan B at that time.

And no, there is nothing really wrong with the engine itself. The P/L fit is real nice (and REAL ABC unlike the OS stuff...), and the overall fit and finish isn't that bad other than the carb. Some have had to seal the base of the carb and backplate due to air leaks, but I haven't needed to do that yet. I'll see what happens when I get the needle issue sorted out.

Fixing motors is always time well wasted. I am not sure I would read all 640 replies though. An hour? I had read quite a few before when I saw the OP is Wayne, who was in my club.

Over the last week or two I read through the whole thing. A lot of points of view and a lot of quality information for dealing with the less than stellar carbs that came on the GMS/Tower engines. The engines themselves seem to be made pretty decent.

Before I went and reproduced the needle valve, I figured I'd run this GMS .47 again and try different fuel to see if it made any difference. First fuel was 80/20 methanol/castor fuel. Max rpm was 14,700 using a 10x6 APC prop and red Jett muffler. I could get a nice low idle of about 2,800rpm. Setting the idle needle for a good transition meant it would load up at idle and stumble a bit throttling up from a long idle (30sec or so). Setting it leaner so it wouldn't load at idle meant it was lean in the midrange and would cut out. Opening the main needle didn't help. I tried some hotter glow plugs and that helped some plus gave a little better top end power, but it would still load up at idle if set for good transition. Next I went to the opposite extreme and put some 20/20 fuel in the tank. Top end power went up to 15,180rpm and throttling improved, but still would load up some. On both fuels, it took over a half turn of the needle richer to get a drop in rpm. For most of my testing today I set the needle right at peak just to ensure the top end would be clean and fast. I think re-profiling the needle may still be necessary, but I don't think it would be the very tip that it should be changed but further back a little to richen the midrange so the needle can be set leaner for idle. Thoughts?

Yeah, that sounds like what mine used to do. That "loading up" really bothered me. Geez I got good at deadstick landings.

Yeah, it's been said way back in this thread by a lot of guys saying that the idle needle is super sensitive - lean/rich threshold is super close. In my case, the most minute adjustments couldn't find the sweet spot. I swear I will find a way to fix it.

I have a GMS .47 that I got a few years ago in a Tower Kaos ARF. After I dumb thumbed the Kaos I put the engine away, full of ATF. Got it out a few months ago & put it in a Escapade ARF. When I maidened the Escapade, I found that I couldn't make the engine 4-cycle by richening the HS needle. I figured there was a restriction to fuel flow somewhere but it ran OK with the plane's pointed up. I went ahead and flew it; it didn't quit in the air. Haven't flown it since that day. I had an APC 11x6 on it, running 10% Powermaster with castor/synthetic blend oil.

Reading this thread got me wondering about the LS needle position so I looked in the carb. The idle bar in my engine goes all the way across the carb throat! I have no Idea how old this engine is. When did they change the carbs? Anyone?

CR

Charley, god only knows. I have had a love hate relationship with this engine. Drove me crazy.

My God this thread is still alive. . . . It seems everyone has since gone electric. I was very frustrated with this engine. Bought it cheap. Found this thread and drilled out the fuel inlet. The engine has run fine ever since. It now pulls my very old Avistar for training missions.