I have a GMS .46 in a Sky Raider II that I broke in well and have flown about 5 times. It's been a great engine right from the start. It's run very strong and has always started with no ore than a single bump of the starter. Just lately, it has begun to just stop running. It starts and runs well then just stops taxing to the runway or halfway into my takeoff run. Sometimes it will run for a minute or two in the pits and die abruptly. It always restarts right away but the problem repeats. I've tried to run it lean to rich and everywhere in between, same result. I've flushed out the fuel tank and all other tubing. I've cleaned the high speed needle valve and flushed the carb. The tank is isolated well in foam. I'm using 15% Omega and an OS #8 plug. Any advice?

Could the fuel filter be dirty?

When in doubt try a new OS 8 and fresh fuel.

This engine doesn't have a ton of time on it or it didn't eat a lot of dirt or dust did it? Engines get fussy as you describe when they are worn out.

Bill

Were you adding castor oil before? Is it running hotter?

Bruce, start from the begining again. Set the needles to the manual settings and then start tuning from there. Tune the high speed first, then tune in the idle. Go back and do both at least two more times to peak them out.

If you try to just sdjust the high speed, or just the idle, you can get itnto some strange reactions from the engnine.. The adjustments are interactive.. When you focus on the high end, you are screwing up the low end. If you start "fixing the low end, you are screwing up the high end.. So, start with the factory settings and work the top end in first, go to the idle end and peak it out, then back to the top, back to idle and after three trips through, you have an engine that is balanced out OK.

Don

check for any air leaks in the fuel lines (these can develope) and a new plug its probably stuffed after the breakin, what prop are youu using? i wouldnt use bigger than an 11x6 Greg.

There is a long thread on getting GMS engines running properly in the glow engine forum.
To save you a lot of reading, the bottom line is that with GMS engines, you need to drill through the fuel inlet. Take the carb off the motor, and using a 1.5 mm drill (1/16" is close enough) drill through the fuel nipple into the brass bit where the fuel gets metered by the needle valve - but try not to go through the other side. When the GMS engines are assembled, the holes for the fuel don't always line up. Drilling through the inlet nipple takes care of this mis-match. The problem usually shows up when running at WOT and the carb is not able to supply enough fuel.
I have several GMS engines (including the .47 like yours) and I have done this on each one and never had a problem. My .47 used to always cut out on me until I drilled it. Never had a dead stick since.
But, in your case, it may be something different, as it is cutting out while taxiing, not at WOT. I was going to say replace the GMS plug, as they are rubbish, but I see you are using a #8, which is good.
Another thing you can do is to take out one of the head shims. GMS engines have quite low compression as supplied. There are 3 shims under the head - 1 thick and 2 thin. Try taking out one of the thin ones and see if it improves things.
I now run my GMS .47 on 0% nitro with only the thick shim in place and it screams, and idles just as well as when I ran it on 15% nitro.

Have you changed the nitro percentatge? Sometime having nitro to the point that it is about to detonate can cause strange problems. Also try a fresh plug, if that doesn't work try a cooler or hotter plug and note the differance in RPM and performance.

Bruce,


Please define 'well' (in respect to the way you broke your engine in).

If you did it [link=http://www.rcuniverse.com/forum/Tapered%2DBore_Engine_Break%2Din_%2D_Upgraded/m_1850473/tm.htm]this way[/link] then you did do it well.

If you did it very rich, well...

Also, has the engine run a bit on the too lean side, on occasion?

The glow-plug in many engines will not run very well, after the break-in, due to FOD (break-in produces particles from the parts wearing in...), if it had not burnt out, that is.

Also, have there been adjustments, or weather changes? If the weather became warmer, or more humid, an adjustment change is necessary (which you did not write about doing).

Another thing; I believe your engine is a [link=http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXDXL5&P=ML]GMS .47[/link], not a GMS .46...


5 flights is too short for any change to happen, unless something wrong happened...

Wow, I didn't expect so much help. Looks like I've got a lot to check out. Thanks!

No problem at all, Bruce.

That what us 'geezers' are here for (even this 45 year old one... ).

I'm no expert but does anyone think a screw has vibrated loose? Like the one holding the carb to the case or any of the backplate screws?

I'll say it again... and it requires almost NO effort.... Put a new plug in and bum a tank of fresh fuel. It the problem doesn't go away, then look for problems elsewhere. Others have given outstanding suggestions but until you've tried a new plug and new fuel, you shouldn't try trouble shoot further. I've "learned" this lesson about 30 times over the last 40 years.

The problems you describe sounds like fuel that has gone bad.

Let us know what you find.

Bill

It may very well turn out you are completely correct but these types of problems don't usually lead to the problems described. A loose backplate screw and carb screw usually result in a lean condition at higher speed. Abruptly quitting at idle (or near idle) is usually bad fuel. Quitting when you remove the igniter is usually a bad plug.

Bill

hey,,,,,,, my friend Doug had the same problem with his GMS, after 3 guys spent 2 hours trying to fix the problem I walked over and told him to put a larger prop on it,,,, problem went away !!!!!!!!!!!!!!!!!

true story !!,,,, his GMS kept stalling on take off,, I told him he needed to load the engine more, put a larger prop on it,,,, problem gone !!!!

Jim

Jim : I would guess his engine was simply too lean...

you would think,, but richening it did not help !!

Jim

Jim : Then I would look at the vibrations induced by higher rpms (smaller prop). Was the fuel tank and fuel lines well insulated from vibrations ???