Flyboy Dave

My Feedback: (21)

I can't remember If mine came with a rear needle....

....or it came like this.

Ernie Misner

Love your photos Dave. I'm going to get a Nikon D200 digital body for my older non autofocus nikon lenses soon. Then maybe I'll give you a little competetion.

If it's like the Tower 75, there is a cap that unscrews on the left side of the carb and then the needle valve ass'y. screws right into there!

Ernie

Turk1

Its obvious that those brand engines carb barrel slider groove geometry is wrong designed.Anybody who can design a better groove
geometry and produce replacement barrels will have a good earning.

JDW

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.)

Nutz4flying

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.

JDW

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!

Turk1

Totally agree.I see same weakness in industrial use too.

Flyboy Dave

My Feedback: (21)

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.

JDW

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!

rlbrobst

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.

macca68

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...)


Thank you all for your help! Macca68. CHEERS!

Flyboy Dave

My Feedback: (21)

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.

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.

Harry Lagman

I have to wonder if this poster has read any of the preceding posts in this thread at all.

As everyone else reading this thread knows, JDW is not dreaming up a theory - he (and now others) has done a practical demonstration that proves that changing the idle needle setting can dramatically affect the high speed setting at WOT. This is not a theory; it is easily demonstrable.

So this one's just for you Flyboy Dave: "The WOT high speed setting on the GMS is affected by the position of the idle needle relative to the spraybar. This is not theory, wildarse guessing or navel gazing; it's an easily demonstrated fact. Try it one day".

BTW, any contribution from Flyboy Dave on "venturi effect" should be tempered by this gem I found earlier in this very thread:

Go Dave!

Edited: Typos

JDW

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.

JDW

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.

Flyboy Dave

My Feedback: (21)

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.

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.

JDW

Flyboy Dave

It is you who should read the thread!!!

My carb also had about 40% restriction on the spraybar tube entry. Even with 70 % restriction the hole would still be plenty large enough to allow WOT fuel flow. This is simply NOT the problem.

The engines I have sorted could all be LEANED several turns on the main needle once the idle needle was correct. Yes, you read it right! - LEANING the idle mixture enabled the main needle to be LEANED heaps and all the problems that LOOKED like muffler pressure, fuel flow restrictions etc magically went away WITHOUT drilling, filing, sealing or otherwise modifying anything else at all.

Changing to a muffler with less back pressure (Tower) is also not a problem IF the idle needle is set correctly, so it's not a muffler pressure problem. Since there is VERY little flow from the muffler to the tank I'm sure a hole 25% of the standard nipple ID would be tons adequate. Remember that the pressure in the tank is NOT dependent on the nipple hole size UNLESS the flow rate is so high that there is a head loss through the nipple. With the flow rates we are talking about that is just NOT an issue. Remember that it is air we are talking about here. If fuel can get through a standard nipple without significant head loss at the flow rates we are talking about then you dont need much of a hole for air from the muffler to do the same!

The reason you have to get the idle needle correct is to have the geometry of the carb correct at WOT. Leaning the IDLE needle has no direct effect effect on WOT mixture BUT it does have a VERY dramatic effect on fuel draw - i.e the engine will only 'suck' correctly if the gap between the idle needle and the spray bar tube is correct.

As I said before, don't take my word for it, try it and see for yourself as others have.

Ed_Moorman

My Feedback: (1)

Have you guys noticed that there are very similar threads for the Magnum .52XLS, SK .50 and Evo .46 & 1.00? The solution given by someone who loves the specific engine is that the low end is too rich causing the top end and mid-range to be off. I am seeing a trend in the Chinese engines. I wonder if they use common carb parts or the same supplier and the same taper needle?

w8ye

My Feedback: (16)

Also the same type carb as on the Magnum engines is used on some of the RCV engines

Flyboy Dave

My Feedback: (21)

When you are using a rear needle set-up and have the main needle on the rear
needle set too rich....you are using the idle mixture screw as a fuel regulator.
You are basically using an idle setting from the idle screw, and the full throttle
wide open setting of the rear needle. The reason you had to lean the low speed
so much is simply because you had the main needle set too rich, and are using
the low speed needle more like one would with a front needle.

Once you drilled out all the problems areas, you were able to get enough fuel
to the engine, you had to lean it out. Once you got enough fuel to the engine
you were able to find the sweet spot settings, and the proper balance to get
all things good.

The theory about the distance of the low speed needle from the nozzle is
total hogwash. Once the low speed end is pulled clear of the nozzle in WOT
operation, it has no effect on the main needle setting what so ever in this type
of carb....the main needle is on it's own at full throttle.

Now there are different types of carbs, like the double needle "cat's eye type",
where the low speed setting has a direct effect on the high speed, but this is
not one of them. In that type of carb the low speed has a tapered needle like
the main needle.

On this type of carb, the low speed needle simply blocks off the outlet of the
nozzle, and only allows a certain amount of a predetermined amount of the fuel
in. Once the low speed needle clears the end of the nozzle it is no longer in play
....just that simple.

In this type of a carb, the low speed needle is really just a "plug", there is no
real idle circuit. This is incorrect as well....

You can remove the low speed needle entirely away from the nozzle, and it will
"suck" fuel just fine....a like c/l engine....without a low speed screw, just a needle and
a nozzle. The gap in between the needles has nothing what so ever to do with
"the geometry of the carb". in any way, shape, or form.

FBD.

XJet

It *is* possible that by having the low-speed needle closer to the main fuel jet, air travelling between the two (ie: over the end of the fuel jet) is accelerated faster than if the low-speed needle is further away.

Bernoulli's theorem says that the faster the air flows, the lower the pressure.

This means that *if* the air can be made to travel faster by bringing the low-speed needle closer to the fuel jet, then the engine will indeed run richer.

However, I'd be very surprised if this were the case (but I'm prepared to be proven wrong). The reason I say this is because the air will be far more inclined to travel *around* both the low speed needle and the fuel jet than between them - unless the gap is quite wide. This is because of the very low reynolds numbers involved in airflows around those particular components.

I spent considerable time trying to get the GMS carby on my GMS32 sorted -- and even though I got it dialed in well for operation with the stock muffler, there's no way *that* carby would flow enough fuel to work on a tuned pipe at higher RPMs. No matter where the low-speed needle was set, it ran so lean that it would just hunt up and down in RPMs.

I think the GMS carbs are probably "marginal" as shipped. This means some can be tuned to work okay (depending on your altitude, the fuel you're using, the prop you're running etc) but others simply can't be tuned for a reliable run. Whether you get the former or the latter seems to be pretty much the luck of the draw.

What I do know is that flowing the fuel-path and slightly shortening the fuel-jet makes the carbies *much* easier to tune and guarantees that they'll run reliably -- even if not tuned perfectly.

Performing these modifications does *no* harm to the engine's operation so I see no reason why not to do them.

Harry Lagman

Clearly, you haven't tried this on the GMS engine.

If you unscrew the low speed needle more than about .010-.015 from the spray bar, it will not draw fuel properly. Try it. We don't want to hear your theories, we want you to try it and report back.

JDW and others have discovered that in order for the GMS carb to draw and atomise fuel correctly, the low speed needle has to be within a few thousands of an inch of the end of the spraybar. Is this too hard for you to fathom?

Try it out. Unscrew your GMS's low speed needle all the way out and report back to us, if it's not too much trouble.

Are you up for it, or will you just delete this post too?

Harry Lagman

XJet, remember, Flyboy Dave doesn't deal in Bernoulli. Remember this?

Flyboy Dave

My Feedback: (21)

If you are setting the low speed needle this close to the spraybar at WOT, you
are using it to regulate the fuel flow, as you would with a forward mounted main
needle. Your high and low speed settings are out of balance, and you are blocking
the nozzle with the low speed "plug".

In other words, you are setting the "high speed"....with the "low speed'.

FBD.

Flyboy Dave

My Feedback: (21)

XJet....Bernoulli's experiments and theories were about water, not air.