Posts: 19
Joined: 3/30/2005 From: Auckland, NEW ZEALAND Status: offline
Turk1 No, I don't think there is any problem with the slider groove geometry. Once they are set up correctly they seem to maintain a very consistant fuel air mix. My own engine now idles at 1500rpm, transitions perfectly every time, is powerful at full throttle and is happy and sounds 'on song' at all throttle settings. I think the issue is with the basic design and perhaps with the course thread on the idle needle. The problem is not that they can't be set up correctly, it's that they are very very sensitive to the idle needle setting and give misleading signs (behaving lean when they are rich etc.)
Posts: 8
Joined: 8/25/2003 From: Cary,
IL, USA Status: offline
I agree with Turk1. Any slop in the carb barrel, no matter how slight, causes irratic behavior for the GMS 47. The set screw design does not allow a close enough fit to prevent the carb barrel from vibrating or otherwise not returning to the set position consistently, despite the barrel being held by a spring. If you can push the barrel in with slight pressure with your finger and wiggle it, even slightly, there is no way to get consistent performance from this carb. The head of the set screw that is provided is designed to be tightened against the carb body. The better design is a longer screw that is adjusted to hold the barrel snuggly but still allow barrel rotation and then the screw is held in place by a tightened nut against the carb body.
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Posts: 19
Joined: 3/30/2005 From: Auckland, NEW ZEALAND Status: offline
Planetome, I still have to disagree! 1/ It doesn't matter which method is used - the end play of the rotor is determined by the diameter of the machined end of the screw compared with the width of the scrolled slot in the rotor. The length of the screw has nothing to do with it as it's not tapered.
2/ Have a look at any brand of engine with a scrolled rotor. They ALL have end float and all use a spring to 'load' the rotor out. The end float I have observed on all the GMS carbs I have seen is similar to that on other brands. (they are copies after all!)
3/In my experience the spring is totally adequate to hold the rotor 'out'. I have yet to see an engine running with the carb rotor vibrating in and out. Only an extremely stiff and misaligned throttle linkage would overcome the spring and this would be a problem with any brand.
4/ You CAN set these engines up to perform well. You DO NOT have to drill, file or redesign anything. You DO however, need to spend some time and get the idle setting absolutely correct. I'm not saying the carb is as easy to set up or as forgiving as most others, but once they are right the engines perform really well and have excellent mixture stability throughout the RPM range. We have a number of these engines in our club performing very reliably indeed.
5/ These engines are what they are. They are obviously not a patch on say OS and they are not as well made as some other Chinese engines but they can be set up to run well without redesigning and rebuilding them. The items that DO need changing in these (and other Chinese engines) are the bearings. In my experience the Chinese bearings have a very short useful life!
The items that DO need changing in these (and other Chinese engines) are the bearings. In my experience the Chinese bearings have a very short useful life!
Totally agree.I see same weakness in industrial use too.
Posts: 13468
Joined: 3/20/2002 From: San Bernardino County,
CA, USA Status: online
The high speed needle screws into the nozzle, which is integral to the carb body, that is to say even if the carb barrel moves back and forth, it won't effect the high speed running of the engine. It will however, effect the idle adjustment.
Have you ever adjusted the idle screw with the engine running ? When you push on the idle screw with the screwdriver, it pushes the barrel "in" towards the main needle....as far as the "slop" in the guide (screw) will allow. The low speed screw is afixed to the carb barrel, so when it moves "in" it leans out the mixture while you are pushing on the screw.
When the engine is running wide open, the space between the idle screw and the nozzle is quite large, so even if the barrel were to move in and out slightly, it would not effect the running of the engine because the low speed is no longer effecting the high speed setting, and the ngine is running solely off the high speed needle....on this type of carb.
FBD.
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Posts: 19
Joined: 3/30/2005 From: Auckland, NEW ZEALAND Status: offline
Wrong Flyboy Dave! The MOST critical aspect of these carbs is the position of the LOW SPEED NEEDLE relative to the end of the spray bar tube (or nozzle)at FULL THROTTLE.
It is NOT a large gap as you suggest but is about .002in and is very critical. It is NOT critical because it changes the low speed mixture, but because it changes the venturi effect and therefore the fuel draw at full throttle.
There is no doubt that moving the rotor in and out say .015in (typical end float) WOULD dramatically change the carburation at full throttle but THIS DOES NOT HAPPEN IN REAL LIFE so is not a problem and quite frankly this discussion is not helping anyone with a GMS tuning problem!
Posts: 30
Joined: 3/14/2005 From: Muncie,
IN, USA Status: offline
I was actually able to set mine up at about 1/4 throttle and just set the idle mixture screw until I got a nice smooth idle and then transitioned to full throttle and adjusted the main needle. It took some tinkering but I finally got it! I made ABSOLUETLY NO modifications to my motor, sealed NOTHING and my motor runs just as good as my thunder tigers. My final settings were 1 3/4 turns out on the idle screw (factory setting was 2 turns out) and 3 1/4 turns out on the main needle(factory setting was 3 turns out). So as you can see it did not take too much adjustment to make a dramatic effect.
Also, For these motors to be truly broken in they require an extensive amount of running and fuel. I have run mine with almost a gallon of fuel and it just now seems broken in.
I have noticed the movement on the idle mixture as well while tuning but when you remove your screw driver or release side pressure on the needle then it does return back to the proper setting.
Now as far as the chinese bearings..... YOU ARE QUITE CORRECT!!! They are not near as good as any of their counterparts. I see this at work every day since I am a maint. man at a local box factory. What seems to kill them the quickest is HEAT.
I would recommend not using cool power fuel on these engines as well. The 100% sythetic lube in cool power seems to break down much faster than castor does. I switched to Omega fuel since it is a mix of synthetic and castor and it seemed to help out alot in these GMS engines. I think that is how I got this motor to finally break in.
Posts: 2
Joined: 6/17/2006 From: murray bridge, AUSTRALIA Status: offline
G'Day All!
I'll keep this short, I have had the same experiance as other moddlers on this thread. I was given a GMS 47 by a mate that thought I "might be able to use it". It came with what I would call a 'very well' home made muffler. (aluminium tubing).I am yet to fly with this engine but this is what I found.
1. It would idle smooth as.
2. It wouldn't run WOT for any period of time (high end wound out 6-7 turns).
After reading this very informative thread these are the modifacations I have done.
1. Copper insert: hole out of alignment, re drilled with the fuel nipple still in place. Ran the engine and it ran WOT but I was unable to get it to reach max rpm. What I did notice was that there was air bubbles in the fuel line. This pointed towards lack of tank pressure.
2. While engine was running at WOT I squashed the end of muffler (aluminium pipe) with a pair of plyers reducing the size of the elongated opening. This gave more pressure to fuel tank, which then gave more fuel to the engine. It actually flooded the engine and made it stall!
3. I re-set high and low end needles and re started the engine. This time I was able to fine tune the engine to a few hundred rpm rich of max rpm. Transition from idle to WOT is excellent!
I wont be able to 'try it in the sky' for a few days yet but I am confident that it will be fine.
Sooo. I reckon the problem is fuel tank pressure. I wish I would have taken more notice of fuel lines before I had taken to the brass insert with a drill bit!(My two cents worth...)
Posts: 13468
Joined: 3/20/2002 From: San Bernardino County,
CA, USA Status: online
The clearance on my spraybar is about .008 or so at WOT, so it is quite clear....the low speed setting cannot effect the amount of fuel coming out of the nozzle in any way shape or form. You can argue that it it does, but clearly it doesn't.
quote:
It is NOT a large gap as you suggest but is about .002in and is very critical. It is NOT critical because it changes the low speed mixture, but because it changes the venturi effect and therefore the fuel draw at full throttle.
I don't know where you dreamed this up, but this isn't the case. You cannot change the "venturi effect" of the carb....no matter where you set the low speed needle. The piston can only draw a certain amout of air, wether it be at idle, mid range, or full throttle....that amount is constant....only more times per minute as the speed increases.
If you removed the low speed screw from the engine entitrely, it would run perfectly at high speed, just like a control line engine. The low speed setting will not, and cannot effect the high speed setting while it is quite clear of the nozzle.
Your theory about changing the "venturi effect" of the carb, due to a slight adjustment of the low speed needle is totally incorrect....sorry.
FBD.
FBD.
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An engineer says.... "That won''t work". A mechanic says..."Oh yeah, watch this". "Old Age, and Treachery will overcome youth and skill". Revver Bro #4.
Posts: 19
Joined: 3/30/2005 From: Auckland, NEW ZEALAND Status: offline
Flyboy Dave,
The venturi effect is not only dependent (as you seem to think) on the airflow in the carb throat. It is also a function of the geometry of the fuel outlet. This is changed if the idle needle is in the wrong position relative to the spray bar tube.
Dave, why dont you just try it instead of relying on (incorrect) theory. Get a GMS 47 and tune it as I have described. Now run it at full throttle and then open the idle needle half a turn. You will find that the main needle will have to be richened by SEVERAL turns to keep the engine running AND the power will drop off as well.
If you look at MACCA 68's post you will see he had the same issue - main needle 6 - 7 turns open. I dont agree with the way he 'solved' the problem by closing off the muffler outlet - this is unnecessary. Assuming there is nothing unusual with his fuel plumbing and tank position I believe he still hasn't got the idle needle lean enough. THIS IS THE KEY to setting up these carbs.
Posts: 19
Joined: 3/30/2005 From: Auckland, NEW ZEALAND Status: offline
Flyboy Dave
I just noticed something else in your post. You say that the clearance between your idle needle and spraybar tube at WOT is about .008in.
If this is the case then your carb is NOT set up for optimum performance. I've no doubt that the engine can be made to idle and transition OK at this setting but in my experience it WONT deliver full power (paticularly when vetical) and wont be as reliable as it should be.
Try setting the idle needle - spray bar gap at .002in and start from there. ALL the good performing 47s I have looked at or set up have a gap of between .001 and .004.
Posts: 13468
Joined: 3/20/2002 From: San Bernardino County,
CA, USA Status: online
OK....I'm going to make this as simple as I can, my pc time is limited, and I don't have the time to teach basic mechanics and engine tuning over the Internet.
quote:
If you look at MACCA 68's post you will see he had the same issue - main needle 6 - 7 turns open. I dont agree with the way he 'solved' the problem by closing off the muffler outlet - this is unnecessary. Assuming there is nothing unusual with his fuel plumbing and tank position I believe he still hasn't got the idle needle lean enough. THIS IS THE KEY to setting up these carbs.
If a carb has to be set at 6-7 turns out it isn't getting enough fuel.
If an engine is not getting enough fuel (too lean)....leaning the carb out further will not remedy the situation !!! You are heading the wrong way. If the engine is not getting enough fuel (too lean) you must richen the engine.
In the case of this engine, they suffer from a large exhaust muffler with no baffle which results in slightly lower pressure to the tank. My engine suffered from the exhaust nipple needing to be drilled to clear out the "flash" because it was restricted causing a further loss of pressure to the tank. The nipple feeding the carb inlet wasn't perfect either, and the fuel inlet to the spray bar was missalighned, and had about a 40% restriction. I had to drill that out too. Have you read this thread ?
Having corrected all these problem areas allows enough fuel to the needle, so that a proper adjustment can be made.
And I do, BTW, have one of these engines, and I have tuned it to perfection. My engine, FWIW, was set too lean on the low end and was bogging in transition. I just checked my engine, I ran it a couple weeks ago...it has a bent wire on the main needle making it very easy to count the turns....
....two turns, plus two clicks. My engine is obviously correct.
Please, rather than trying to tell everyone that they don't know what they are talking about....won't you read up a bit on carb's ? Trust me, we are so very happy that you didn't have an engine with all the problems we, in this thread have had.
By simply tuning your engine correctly, you were able to get it running correctly....that is wonderful.
Thank you for your contribution.
Flyboy Dave.
< Message edited by Flyboy Dave -- 8/18/2006 11:28:17 PM >
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An engineer says.... "That won''t work". A mechanic says..."Oh yeah, watch this". "Old Age, and Treachery will overcome youth and skill". Revver Bro #4.
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RE: GMS Engine Tuning Problem - 
