When your main needle is out 6 to 7 turns to get it to run....

....it isn't getting enough fuel !!!

FBD.

MJWS and Opjose, thanks for reporting back your progress. It's great to see your open minded approach to this problem has yielded results.

MJWS, let us know how your problem child GMS responds to further testing.

...but Dave...my OS is running perfectly which is what I expect from ANY engine (period). If I don't get reliable running from a normal installation (upright, tank within a 1/4" tolerance) then it's simply either going back for a refund or going on my shelf until I forget how irritating it was in the first place (at which time we go back to the beginning).

Granted, I am sure the OS with its smaller muffler has more fuel pressure...but I will try it this weekend with the GMS muffler for kicks. Anyone care to make a wager?

Incidentally my carb looked exactly like yours with the offset fuel inlet.

A friend of mine told me about this thread. Here's my story on GMS. I own 5of them. (2X) .46s and (1) .25 and (2) .32s. My friend owned several also. Mine ran flawlessly and would not die at idle. Never saw the not dying at idle to be a problem. Most of his had problems either at idle or would lean out in flight. One day he asked me to see how many turns in my idle screw was on a .46 that ran perfect. I checked and it was 1.5 turns out from bottom. Just curious I checked another. It was also 1.5 turns out. Than I had to check them all. What would you know!!!!!!!!! All my GMS engines are exactly 1.5 turns out from bottom on the idle screw. This was never by purpose. I had adjusted over the years just to make them run. Never had I counted the turns until this time. He set his at 1.5 turns on the idle and 3 out on the HS. Runs just like all of mine now. Flawless!

There are 2 different carbs on these jewels. Make sure the carb is pushed down tight on O-ring and tighten up. I have helped 2 other people with their GMS and their problem was low speed needle in too far and needed to be more flush with throttle arm. If tank position is good and 6 turns out tells me your LS needle in leaned way too much in and the HS has to be backed out to get any kind of Idle. Because at Idle the LS needle shuts off fuel flow.

Yep....and when the low speed needle is too close to the nozzle, the high speed
needle is non-operative that is to say it isn't working. They have their main
needles set at 6-7 turns out....they could turn turn them out even further and the engine
will not richen up.

They can turn it out until it falls out in their hand....it will never richen up because the
low speed needle is now a "regulator" and will not allow enough fuel through. I tried
to explain this to the fellas, but they don't get it.

I proved through my own testing (what I already knew) once you get the low speed needle
far enough away from the nozzle for the main needle adjustment to work properly, the
low speed needle can be backed all the way out, and have no effect on the high speed.

They claim that if you get the low speed needle up close to the nozzle, it will somehow
increase the flow....while if you get the low speed needle away from the nozzle, the high
speed doesn't work right, hence the 6-7 turns out is where they had the carb set.

Oh, well.

FBD. [sm=regular_smile.gif]

Speedster and Flyboy Dave.

I don't think you've read or comprehended the JDW thread at all. You couldn't have because if you had, you'd know that the lean condition experienced had nothing to do with the low speed being too far in and obstructing the spray bar. Forget that idea! In fact it was quite the opposite. Read the posts.

And by the way, it's not a claim - it's what really happened. Clearly, it's also what happened on the engines owned by MJWS and Opjose.

This will not necessarily happen to all engines, but it certainly applies to a significant number of them such that it's worth trying on a "problem child".

Enough of the denial - now get with the program.

That's better. You're on to it now...

This was the phenomenon that was demonstrated. This is what more and more problem child GMS .47 and Tower .46 owners are discovering. Come on in and join the revolution.....

Got a new one for my tuning a GMS engine. Just yesterday the same friend I had told to go 1.5 turns out on the idle needle has a Twin Star with two GMS .32s. Both start out fine, but 50% of the time on landing one dies. Always the same one. So, sure enough he checks the low speed on the one that dies and it's over 2 turns out. The one that never dies is 1.5 turns out. That makes 8 engines that run perfect with the idle set 1.5 turns out and the high speed at about 2.5 out. The eighth one is the one that he reset on the TWIN Star that runs fine now. I've rechecked myself and if the idle is set hard on bottom the turn out is closer to 1 3/4.

I guess I might as well let the cat out of the bag. I said earlier (twice) in this thread that I
was a "carb Guy from way back". There aren't too many "carb Guys" around these days, in
fact carburetion isn't even taught in modern automobile schools, because carbs went extinct
in 1988 in the U.S., almost 20 years ago. I have a friend that graduated from an well know
auto technical school....and he told me they don't even teach carburetion any more.

They teach electronic fuel injection, and the care and feeding of the computers that operate
them....as well as the troubleshooting equipment that is used to fix those systems.

Unfortunately, carburetion technicians are fewer and further in between. Fact is, if it wasn't
for motorcycle mechanics, off-road vehicles, and the old school marine mechanics, carburetion
would be totally obsolete due to environmental regulations, and the advancement of more
efficient (fuel injected) engines. These mechanics, when they reach Journeyman status can
work with complicated multi-carb systems. Not everyone has the skills and experience to be
able to tune and re-jet these carburetor system's.

The problems arise when the owners of motorcycles and off-road vehicles change the exhaust
and intake systems, and throw the factory settings so far off, the engine will no longer run
satisfactorily.

This is where ol' Flyboy Dave comes in....I was trained on these carb systems at the Yamaha
factory in 1970. I still have the diploma. I was a carb tuner Professionally for 15 years. Modifying,
tuning, and re-jetting the two, three, and four carb system was my specialty, and I became
Expert at it. I still do this work in my shop today, in fact I just re-jetted a couple machines lately
that had problems with accessory mufflers throwing the carbs out of whack.

This red bike has a set of constant velocity, altitude compensating, downdraft, racing style
carbs. Normally, this would be a challenge for any mechanic, but I still have all my tools
and my own shop to do these types of things. There are a lot of secrets and techniques
involved that I won't bother you with, but trust me....when you drill out the plugged and
sealed factory plugs to the jets that aren't supposed to be touched....

....you better know what you are doing.

See those tiny drills ? They vary as little as .001"....as a hint, sometimes you have make a jet
smaller, because they don't manufacture smaller jets. I have to solder them shut, and re-drill
the jet to a smaller size.

I've been doing this for 36 years....I'm an Expert at it. If you think you can fool me about
a simple carb as big as your thumb nail....think again.

FBD.

Here's another bike I did recently....another bike with a muffler change that would
barely run. Ever see a vacuum operated, constant velocity, twin carb set-up on a
V-twin engine ?....probably not. This one had a $85.00 "jet kit installed with the pipes.

I had to remove the jet kit and re-do it myself....I nailed it on the first try.

Like I said....I'm a "carb Guy".

FBD.

This might be interesting....I'm thinking if the big fat GMS muffler is leaking all over the place
the pressure to the tank could be almost zilch. Couple that with the mis-drilled fuel inlet and that
would account for a lot of the problems.

Low fuel pressure, and an upright engine leaning out, could lead to a lot of deadsticks.
The muffler swap could be beneficial with the process of elimination.

FBD. [sm=regular_smile.gif]

Yes, a leaky muffler has a very bad effect on the GMS47 -- but only because the carby is often defective in the first place.

I've seen otherwise well-behaved GMS47s go to hell in a handbasket as soon as they started leaking oil between the sections. I guess they were also leaking pressure and that meant they would start to sag in verticals and run erratically. Tightening and sealing the muffler fixed the problem and restored the engine to its previous reliable operation.

However, almost every other modern 2-stroke will run fine *without* muffler pressure so this is only one of many factors in the GMS saga.

I see they still offer the .47 with the stock mufler....

http://www2.towerhobbies.com/cgi-bin...p?&I=LXDXL5&P=

....this might be a better motor for general use, if you're not looking for the
extra RPM's afforded with the tuned muffler. It's 5 bucks cheaper too.

XJet,

JDW is one of the guys who has thrown his name into the hat to come down and pit his AT40 against Tokoroa's finest (I still owe you a PM with proposed dates once our guys come back to me), so he will be able to demonstrate to you this GMS carb phenomenon we are talking about that Flyboy Dave is in a state of denial over.

Does any weekend in December work for you guys or should we just shoot for January/February?

I've been down the muffler change road as well. That wasn't the problem either.

Well, I suck - I didn't try it with the GMS muffler and I didn't reinstall the GMS in the plane this weekend. I needed to get some flying in and simply didn't feel like fiddling.

Maybe I'll f'with it this weekend.

--------------


Or fill the high performance mufflers exhaust outlet with metal filled epoxy and then redrill it to a smaller diameter than the stock diameter. If JB Weld will hold up to exhaust flange duty, which I have witnessed, without melting, it should do fine plugging up the stock exhaust outlet.

Me, I'd rather work it the other way around and provide the carb with the fuel it needs via crankcase pressure and an Iron Bay or Cline regulator.

Ed Cregger

[quo

1) He found peak at WOT and backed off some (as you do when you first run up a new engine)
2) He started to lean his low speed while testing idle and transition (similar to what you and your helper were doing) until both were satisfactory
3) Flew the model but was frustrated with the sagging when on uplines or hovering.
4) To counter the sagging, he richened the main needle to over 6 turns out (he tuned it while vertical).
5) He then got frustrated with the very rich setting at anything other than vertical
6) In an act of desperation he turned the low speed 1/4 turn leaner and noticed that it richened the mixture at WOT, so he wound the main needle in one turn to compensate.
7) Repeat (6) a few times, such that the rpm peaked while vertical at about 2-3 turns of main needle rather than 6 turns,
8) THe engine is now stronger (500-600rpm more at peak), throttles perfectly and draws its fuel well (in other words; runs only slightly leaner when vertical than when horizontal).

He reproduced this sequence at will, just with the low speed setting. I would not have believed it myself had I not seen him do it. te][/quote]
Harry I read this several times. First the GMS47 is not the best choice for 3D (made for speed) This all boils down to the LS needle wasn't right to begin with and the HS was turned to compensate. The key is at vertical the pressure in the tank and length the fuel had to travel wasn't quite enough to force enough fuel to engine at it's needle settings , Hense the opening of the HS needle to get every drop of fuel forced into engine. Sounds like when you got LOW SPEED needle closer ,your HS setting got more normal ,plus you say you gained 500 RPM whitch tells me your needle settings were out of whack. Harry I thought you were an OS 50 GUY

This is correct, but not in a way you, or I (before I saw this demoed) would have expected. He had leaned the low speed to attain a good idle and an acceptable spoolup, using the pinch test and found that the main needle had to be 6 turns out to draw fuel properly when vertical at WOT. Leaning the low speed - yes, leaning the low speed - changed the main needle setting and fuel draw dynamics profoundly.

Yep, that's exactly what happened. "Closer" means leaner in this case. And that's the key point - the low speed was leaned further than where you'd normally end up if your criteria for a sufficiently leaned low speed were a good transition and a nice idle. Leaning it further richened the high speed - inconceivable in theory but it really did happen on this engine. He demonstrated it at will. It gained 500 rpm, drew fuel better, required three less turns of main needle and smoked less, which led us to believe that the fuel draw and atomisation was greatly improved by making the low speed even leaner than where we thought it was sufficiently lean.

Actually, I am not a single engine brand kind of guy. I fly YS, Saito, Jett and OS. In the last 6 years, I have only bought one OS engine: a Surpass .52. I get to help lots of newbies at the field and tune up a lot of engines for a broad cross section of flyers. I am a club instructor and train many newbies once I have dialed their models in for them. In the last couple of years, I have worked on perhaps 60 to 80 engines in active flying service. I have come to respect the consistency, reliability and ease of use of OS engines, based on this experience. This motivates me to call guys out who come on and proclaim OSs to be pieces of crap for various reasons both real and imagined. If this makes me and OS guy, so be it - I can live with that...