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-   -   GMS Engine Tuning Problem (https://www.rcuniverse.com/forum/glow-engines-114/2290854-gms-engine-tuning-problem.html)

JDW 08-19-2006 04:10 AM

RE: GMS Engine Tuning Problem
 
Flyboy Dave

It's simply not worth discussing this with you - you simply don't read what is written and you obviously have no idea at all about fluid flows and pressure effects with accelerated flows. As I said before - try it - you might like it!

Xjet is on to it - I guess you'll bag him as well!

In the meantime I'll wait until someone else has something intelligent to say on the subject.

Blade47 08-19-2006 09:30 AM

RE: GMS Engine Tuning Problem
 
To add to the mix ... I have a .47 and it has all the mods from this thread and with the idle needle set to .003 ...IT seems to run fine but the low end is still a bit boggy for transition ...top end rpm seems a bit slow ( with out a tac I am not sure ) .....BUT.... when I use a one way pressure valve I have been able to tweak out the top end for a bunch more RPM and I sure can hear it from the sound.

I had to tweak low end but not much.

So in this case pressure did make a difference.

just my $0.02

Motorbrain 08-19-2006 10:13 AM

RE: GMS Engine Tuning Problem
 


ORIGINAL: JDW

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.
Within 1/2 turn is ok on other carbs????



2/ The idle needle must be AS LEAN AS POSSIBLE. Remember that the idle needle WILL effect the position of the main needle for full throttle performance.
Closing the idle needle even a few degrees should make it too lean -i.e. it should just die when the throttle is opened.
If you are doing this your engine will quit running quite often in flight
when you throttle back to idle after running full blast and the engine
is hot. Adjusting the low speed lean is inviting deadsticks.



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!!!
Really? An incorrectly set idle will idle nicely?

Put a front needle carb on the engine and it will act differently. The problem
with these engines is quitting in flight even when the seem to be running ok
on the ground. When an engine quits in flight it is from leanness and overheating.

MB

Motorbrain 08-19-2006 12:03 PM

RE: GMS Engine Tuning Problem
 
Blade,

open the idle screw a quarter turn and see what happens.

MB.

XJet 08-19-2006 02:32 PM

RE: GMS Engine Tuning Problem
 

ORIGINAL: Flyboy Dave

XJet....Bernoulli's experiments and theories were about water, not air. ;)
I suspect you're confusing the use of the word "fluid" in Bernoulli's work.

For the purposes of most CFD (look it up), a gas (such as air) is simply a compressible fluid. Mr Bernoulli's work applies to gasflows. Remember, it's often used to explain how a lifting-section airfoil creates lift and it's the driving principle behind the venturis used in most carbies.

JDW 08-19-2006 03:04 PM

RE: GMS Engine Tuning Problem
 
Motorbrain,

1/ Yes, I have experienced many carbs where you can 'get away with' an idle needle set a bit too rich. I'm not saying that they don't benefit from being right, but they don't misbehave like a GMS (particularly at WOT) with an incorrect setting. On most other brands the idle setting has little or no effect on WOT performance in my experience.

2/ Since I got my engine sorted it has not quit in flight. I don't keep an accurate count but I would guess that it has probably been something approaching 100 flights - It hasn't even coughed or hesitated in that time and it still has the same OS # 8 plug. It is now the best starting, most reliable and consistent of my 8 engines (notice I didn't say the best) and it will idle for ages at under 1500rpm and then accelerate perfectly to WOT! All this is a far cry from where it was before I got the idle needle setting absolutely correct!

Remember ALL I did to solve the same problems others have been experiencing is set the idle needle as I have described.

Flyboy Dave 08-19-2006 10:50 PM

RE: GMS Engine Tuning Problem
 
1 Attachment(s)

ORIGINAL: JDW

Flyboy Dave

It's simply not worth discussing this with you - you simply don't read what is written and you obviously have no idea at all about fluid flows and pressure effects with accelerated flows. As I said before - try it - you might like it!

Xjet is on to it - I guess you'll bag him as well!

In the meantime I'll wait until someone else has something intelligent to say on the subject.
Well...you know, JDW....I took you up on your challenge....I figured I might
as well check out your Magical carburetor geometry and tuning theory.

I must admit, I am a little Old Fashioned and set in my ways. I thought I might
even learn something new. I have real bad news....your theory was just that, a
theory. :eek:

It didn't pan out. In fact, everything you stated in your theory proved to be wrong.

At least I gave it a shot. I took my GMS .47 out today and did the idle screw
test. You said the idle screw had everything to do with the high speed mixture.

I said it had nothing to do with the high speed mixture in the WOT position.

Your position was that the leaner you set the low speed, the better the engine
would run....due to a bunch a magical theories that you alone know about. You
claim that leaning out the low end not only cures all the ills of the problematic
GMS .47, but is the sole source of the fix....and all the other problems
are not in themslves problematic....rather, a waste oF time.

Your magic tuning theory will overcome all these slight problems !

Well here we go....

1. shows the position of my idle screw, after I had done all the mods, and had
the engine running perfectly....with about .008 distance from the spray bar. It
looks more like .010, but I eyeballed it at .008, so let's leave it a that.

2. shows the actual gap. For those keeping score at home, a matchbook
cover is .013".

I started the engine, and let it warm up for a minute....then I shut it off and
opened the low speed needle 1/2 of a turn. I restarted the engine with an
electric starter each time. I figured that as I enrichened the low end....
moving the low speed needle away from the nozzle would kill any chance
of an idle. I was right.

I opened the low speed needle 4 times....1/2 turn at a time, two full turns out.

Pic 3 shows where the screw ended up. See how much further out it was
when I started, and trust me....it was set perfectly.

Now for the fun part ! See the gap in Picture 4 ? A whopping .060 :eek:.

Guess what happened to the high speed ?

Nothing. The high speed rpm stayed at about 15,500 or so at every stage.

PIc #5 shows the combination of feeler guages that were needed to measure
the gap inside the small carb.....060"....

....acording to your theory, this is impossible. Acording to my theory, in which

"once the low speed needle clears the nozzle, it has no effect on the high speed
setting....in any way, shape, or form".

This turned out to be true, just as I expected....sorry. [:o] The engine ran the
same on the high end, fron .008" gap to .060" gap....just like I said it would.

PIc #6 shows how I left the main needle the last time I ran it. Trust me on this,
it is a richened position....I don't shut my engines off at a full peak setting. Let's
call this a 10:30 setting, shall we ? This measured two turns, plus two clicks.

Pic #7 shows the position of the main needle at full peak with the gap at .060".
Guess what ? It is set at 1/4 turn leaner than the .008" setting !!

As any tuner knows....when you enrichen the low end (opening the gap) you
will probably have to lean out the high speed.

Pic #8 shows the way I left the needle...richened, like I did before. The setting is
1 and 7/8 turns out....with the .060" gap.

This setting is 1/8 of a turn away from the initial setting (richened) of two
turns, plus two cliclks.

Sorry, my friend.

FBD. :D


Motorbrain 08-19-2006 11:06 PM

RE: GMS Engine Tuning Problem
 

ORIGINAL: 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.

I think this is where the Lad went wrong with the theory. The low speed
does have an influence on the high speed, but only to a certain point. This
will vary on different carbs. The gap is really quite meaningless. The way
the carb operates is the important thing.

MB.

Flyboy Dave 08-19-2006 11:32 PM

RE: GMS Engine Tuning Problem
 
Well, Motor Brain....

....the thing is, the Guy seems to be wanting to help. He posted his theory
many times in the thread, and I applaude him for the effort. Where he rubbed
me wrong is when he stated that everything we did (curing the obstructions)
was not needed, and simply "leaning out the low end" would solve everything.

The fact of the matter is....the engines are quitting in flight due to a lean condition.
They will run OK on the ground, but the extra richness needed to sustain flight
is not available due to the few restrictions in the fuel system. We have gone
through this problem with a fine tooth comb. We have been successful in resolving
the problems. ;)

Our new friend JDW comes into the act and tries to tell us that our efforts are
in vain....he has the tuning secret....and we are wasting out time fixing simple
fuel and air restrictions.

Would you agree that an engine suffering from a lean condition will not be
helped by leaning it out further ??? :eek:

FBD. :D

Motorbrain 08-19-2006 11:57 PM

RE: GMS Engine Tuning Problem
 
This is what caught my attention about this


ORIGINAL: Flyboy Dave

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. :D

You are right.

MB.

Flyboy Dave 08-20-2006 12:11 AM

RE: GMS Engine Tuning Problem
 
It is interesting, though....some of the stuff these Guys come up with !! :)


FBD.


rlbrobst 08-20-2006 01:01 PM

RE: GMS Engine Tuning Problem
 
I found the best solution yet for these motors! Put them on Ebay as I have done mine and buy a REAL motor.

XJet 08-20-2006 02:35 PM

RE: GMS Engine Tuning Problem
 
Yeah, I just keep buying Thunder Tiger 46Pros instead. They start first-flip, are a breeze to tune, run forever and produce excellent power.

Consider the difference in price between a GMS47 ($65) and a TT46Pro ($74) and I'm more than happy to pay an extra $9 for that peace of mind and extra quality/consistency.

You can burn more than $9 worth of fuel just trying to get the GMS's needles set properly and if you factor in the time/effort required to modify the GMS for *reliable* operation, then the TT is *much* cheaper in the end.

rlbrobst 08-20-2006 03:43 PM

RE: GMS Engine Tuning Problem
 
You got it dude. I buy the SK 50's for 59.99 from kangkeusa.com and they start first flip and require only setting the high speed needle after breaking in. There are soooo many far superior engines to use than to spend so much time tinkering with these GMS engines. I like flying not crashing because they have to be tinkered with so much! I do have a Thunder Tiger 61 but have not built a plane for it yet. I like ASP and Magnum engines as well as I have never had any problems with them.



ORIGINAL: XJet

Yeah, I just keep buying Thunder Tiger 46Pros instead. They start first-flip, are a breeze to tune, run forever and produce excellent power.

Consider the difference in price between a GMS47 ($65) and a TT46Pro ($74) and I'm more than happy to pay an extra $9 for that peace of mind and extra quality/consistency.

You can burn more than $9 worth of fuel just trying to get the GMS's needles set properly and if you factor in the time/effort required to modify the GMS for *reliable* operation, then the TT is *much* cheaper in the end.

Motorbrain 08-21-2006 05:23 PM

RE: GMS Engine Tuning Problem
 
Its more fun to fix a problem and get the satisfaction
than it is to sell the problem. Anything made on an assembly
line can have a problem, the fun of the hobby is being
smarter than the engine.

MB.

rlbrobst 08-21-2006 05:37 PM

RE: GMS Engine Tuning Problem
 
Motorbrain,

This is true but there comes a point where it is not fun anymore and it becomes a job to just keep it running and in the air. Time to ditch it then. If you like them so much and like tinkering all the time with them I have a 1 flight GMS listed on Ebay. Cheap!

ORIGINAL: Motorbrain

Its more fun to fix a problem and get the satisfaction
than it is to sell the problem. Anything made on an assembly
line can have a problem, the fun of the hobby is being
smarter than the engine.

MB.

Blade47 08-22-2006 08:50 AM

RE: GMS Engine Tuning Problem
 
I have a Pair of GMS .76 ..can some one tell me what a good start setting for the low end needle is ( number of turns out )

Thanks

Cheers

Ed_Moorman 08-22-2006 12:08 PM

RE: GMS Engine Tuning Problem
 
1 flight on a GMS .47! Mine didn't tune real well until I had about 1 gallon of fuel through them.

2dawgs28 08-23-2006 11:24 AM

RE: GMS Engine Tuning Problem
 
Hey I need some guidance, I have a GMS .47 ABC. The motor has about four or five tanks of fuel run through it. It idles perfect with the low speed slightly off of rich. It will transition from low to high with no stuttering. The only problem is when I hold the plane straight up the engine slowly begins to lose RPM's and finally dies off. I have checke the fuel lines, replaced the glow plug and pulled the tank and replace the fuel line and tightened the plug on the front of the tank with no luck. Am I looking in the right place with the fuel tank or could my problem be elswhere. The plane is a Hangar 9 Twist and the tank is installed with 1/2 inch foam arond the tank. (foaming in tank???)

Chuck

Flyboy Dave 08-23-2006 12:01 PM

RE: GMS Engine Tuning Problem
 
1 Attachment(s)
Remove the fuel inlet nipple from the carb. Check the hole where
the fuel passes into the needle assembly and see if your's is
mis-alighned like mine was. Also check to make sure the nipple on the
muffler that pressurizes the fuel tank is drilled correctly, and also look
at the nipple you removed on the carb inlet for a nice clean inlet hole.

You are not getting enough fuel into the carb, and it is leaning out. This
is not good, because it causes overheating, and could damage the engine. [:o]

It is easy to fix. ;)

2dawgs28 08-23-2006 03:58 PM

RE: GMS Engine Tuning Problem
 
Well I believe it may have been the fuel tank. I swapped tanks and so far it seems to run like a champ. I shall see once it's in the air. I did check the fuel inlet nipple and it was about a 1/4 blocked so I drilled it just to eliminate that problem . Thanks for the help.

chuck

JDW 08-23-2006 04:00 PM

RE: GMS Engine Tuning Problem
 
2 dawgs28

See my post on page 13 0f this topic. Your problem is EXACTLY what I experienced. I found a fix which worked on my own .47 and which I have now also used to cure other misbehaving .47s. It's worth trying this before you start any drilling of holes etc.

2dawgs28 08-23-2006 04:51 PM

RE: GMS Engine Tuning Problem
 
Ok JDW I will check it out thanks.

Chuck

dickj 09-10-2006 08:53 PM

RE: GMS Engine Tuning Problem
 
Thanks to Dave, I found my fuel inlet was not sufficiently sized and mis aligned. I got the motor started immediately after it was bored out. I am still confused on the proper needle settings however.
dickj

buckshot-II 09-13-2006 02:14 PM

RE: GMS Engine Tuning Problem
 
Good reading about the GMS .47 I have had nothing but trouble with it. Overheating is (was) the major problem. I tried drilling the fuel inlet nipple, muffler nipple, adjusting the low needle ... etc, to no avail. UNTIL, I found the real cause of it. As it turned out, I had several GMS .47 carbs to try and they all resulted in the same thing, the dreaded overheating. What finally woke me up and clued me into the REAL problem was that the high speed needle valve did not make any significant changes. Starting with 2 turns all the way to 12 turns open, the engine would run the same and then overheat. So, I took a closer look at the needle itself and found a small line/grove part way on the tapered needle. How did that get there you ask? ... well, I found that when I got a new carb out of the box and looked at it, it did not have the very small grove or line. How did that get there then. Manufacturing difference.. no, it was ME! When I began the break in process of the engine, I tightened the needle valve very tight just to find the zero position, so that I could get the correct number of turns to start ... approx 2 turns open. Well that did it, when you OVER tighten the new needle valve in the brass spray bar, which seems to be tapered inside, it puts that grove on the needle and from then on extra air enters that little space within the spray bar... in other words, the needle valve is damaged. I tried this with several carbs, and purposely destroyed a $9 needle valve to prove my theory.. and yep that did it. The good carb worked great, fabulous idle, quick transition, 14,400 rpms , temperature at 225 degrees and ran like the dickens and never overheated. Then to really mess it up, I tightned the needle valve past the stop point to crank it in real tight and then took it out and noticed a grove/line on the needle valve. Ran the engine again and alas, it overheated in a few minutes and the needle valve did not make much of any difference. Also, a note/warning, when you screw in the high speed needle valve to the stopping position make SURE that the carb barrel is open, otherwise the low speed tube/bar touches the end of the needle and may cause damage to either component. I learned that the hard way, only after the needle is open a couple of turns, should you close the carb barrel. Voila, it runs like a top and cost me a few needle valves and spray bars... what a nightmare. Thanks


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