ORIGINAL: JDW

Hi,

I recently found a totally unexpected fix to problems I was having with a GMS .47 fitted with a Tower muffler and running a 12.25 x 3.75 APC prop in a 3D model.

The problem was that, even though the engine started easily, idled reliably, transitioned OK and had reasonable power (12,000+ rpm) in horizontal flight, it would sag as if lean when vertical.
The only way to achieve sustained vertical flight was to tune it with the model vetical. This involved opening the main needle an ADDITIONAL 3 turns (i.e. 6+ turns open!!). The engine still started, idled and transitioned OK but was way down on power at full throttle (only 11,000 rpm). It also smoked like crazy when vertical but would sag and die if the needle was closed even a couple of clicks.

This looked like a fuel supply or muffler pressure problem so I did all the appropriate things (many previously mentioned in this thread).
I removed and pressure tested the tank, replaced all the fuel tubing, removed the fuel filter, stripped and cleaned the carb, drilled out the pressure nipple, sealed the carb to the crankcase, replaced the glow plug etc etc.
NONE OF THIS MADE ANY DIFFERENCE WHATSOEVER TO THE PROBLEM!

In desperation and because there was nothing else left to try, I leaned the IDLE needle 1/4 turn (despite the fact that it had already been set apparantly correctly by the pinch test method at break in and the engine idle was already slow and very reliable)

The effect was immediate and dramatic. I was able to LEAN the main needle one full turn. I leaned the idle needle another 1/4 turn and, again, was able to lean the main needle another turn. After a few more cycles of this and with some final fine-tuning I had the engine pulling 12,600 rpm horizontal and vertical, transitioning beautifully with a reliable idle at just under 1,600 rpm.

I refitted the fuel filter and even an 'undrilled' pressure nipple and went flying. In the last couple of months I logged many flights with no problems at all - I haven't had to touch either needle and the engine has never hesitated or even coughed. It is now transformed into a very powerful, reliable, user friendly, 'happy' little 2 stroke.

Remember that all I did to totally transform the FULL THROTTLE performance was to lean the IDLE needle!!!

"That's only one engine - it probably had a bit of dirt in the carb that finally moved --" I hear you say.

NOT SO - Not only is the problem reproducible on my engine if I open the idle needle a turn or so, I have also resurrected another GMS 47 (also fitted with a Tower muffler) that was discarded by anothewr club member as being too unreliable - it kept 'leaning' in flight and stopping. This engine is also now running like a clock. A third near new 47 was also improved considerably by getting the idle needle correct instead of APPARENTLY correct.

So why does the idle needle position dramatically effect the full throttle performance?

A friend (and fellow engine 'nut') and I believe we may have the answer. We think it has to do with venturi effect and fuel atomization.

If you look down the throat of the carb (in the full throttle position) you will see that the idle needle is retracted just clear of the end of the spray bar tube. In the three engines we have 'sorted' recently, the gap between the end of the needle and the end of the tube is about .002" to .003" when correctly tuned. Remember that it is poissible to get a good idle and transition with the idle needle much richer than this - i.e. a much larger gap, but this seems to result in poor and unreliable full throttle performance.

We think that this is probably because the venturi effect at the end of the spray bar tube is compromised if the gap is greater than a few thou. This, in turn, results in less fuel 'draw' which, of course, "looks' like a lack of of tank pressure. We believe it also reduces the atomization of the fuel so that the engine 'acts' lean even while it's smoking.

So, forget about drilling out pressure nipples, changing tank positions, sealing carbs etc at least for a while - It's all too hard and, on my engine at least achieved nothing. Try checking that idle needle to spray bar tube end clearance and working from there to see if that solves the problems.

Good luck,

John.