This morning at the field, my brother's Calmato 60 plane with an OS 61 engine was misbehaving.  The symptoms were as follows.

1.  It runs fine above about 40% throttle.

2.  It starts fine with the ignitor on, with a very slight delay (couple of seconds of starter use).

3.  It stutters and stops on idle after a little while (10 seconds or so).

4.  It stutters and stops immediately when throttle is suddenly advanced from idle.

5.  It stutters and stops immediately in idle if the glow ignitor is removed.

6.  If we ran it high throttle for some time (10s of seconds), and then brought it to idle and left it there, the time for it to quit was longer (as compared to [3] above).

7.  The engine was on another plane, and had just been moved to this one.

Me, Mike and my brother (Jamal) were trying various things, and finally we figured it out.  But as an exersize / education in nitro engine diagnostics, I request RC India nitro fliers to apply their diagnostic skills to this problem (with reasoning as to why such diagnostics is offered). 

PS: Should we have a prize for the right answer ?

I guess you had a bad glow plug.

I think you could check you Idle needle . Due to weather changes , you might have to lean it a bit .

Tried 2 other *good* plugs. 

No amount of low needle tuning helped.

Sounds like mixture is rich at idle and engine is loading up. I'd look for reasons for more fuel getting in or less air.

How can more fuel get in, other than what is controlled by the low speed needle (LSN) ? So if we take the tuning done to the LSN (4 turns down all the way to almost closed), we should have hit a balance point somewhere.

Once the air intake hole on the carb is ensured to be completely devoid of any thing blocking it, the issue of less air also seems improbable.

Or are there cases where this (too much fuel or too little air) can happen ?

Was the tank higher compared to the carb ?

Not having seen your setup, I can only ask generic questions. Is there any chance of higher fuel pressure ?

if all else is ok, then the plumbing has an airleak. (the fuel tubing or the fuel tank clunk issue etc)

Of course... I am stimulating discussion

We checked for any fuel/air leaks, did not find any.  The tank was mounted correctly.

I am trying to see the logic behind the suggestions too.  So if there is an air leak or higher fuel pressure (what would cause high fuel pressure normally, like if the carb inlet/needle jets are clogged ?), which of the symptoms *above* would it cause ?

a fuel tank leak or a tubing leak would cause the engine to cut off after the throttle is increased due to low pressure to pump fuel. however, if all this was checked off then the next step would be old fuel. that is would be my next suspect. let me know if that was the cause. if not, will scratch further, else i rest !

Has the engine been used recently or are you using it after a while ?
If its after a while , then often castor tend to form a gunk and clog the carb . When you run the engine , it runs a bit lean and after a while when the heat is supplied , it melts and excessive castor is given with the fuel . If you run it continously for a while and let all of it melt and exit the engine as excessive oil .
Maybe this is your situation. It happened to me also a while ago.
Happy flying ,
Sahevaan

Is it possible that the needles have gone bust? Or the O ring on it worn out which doesn't allow the needle to seat properly?

6 is inconsistent with other symptoms. What exactly did you mean by stutter ? Rich idle will have a longer stutter while a lean condition could also be called a stutter, but it will be short usually. What is the result of pinch test at idle ?

Is the carb barrel secure in its place ?

Almost all symptoms show an excessively rich mixture. But like RotorZone said 6 is confusing to me. 3, 4, 5 are definite signs of cylinder loading with fuel.

On the pinch test after bringing the engine to running temp, it should rev up for a few seconds and die. If it revs for several seconds, its too rich. If it doesn't rev at all and just dies, its lean. This is how I check my low end tune.

@second_chance/chand - Fuel was new.

@Sahevaan - While your point is valid, that would only prevent the engine from starting properly, or struggle for a short time. As more combustion happens, all the gunk should get thrown away. This engine was working flawlessly on higher throttle.  The engine was mounted on another plane inverted, and was being used all along.

@ajo - Nope, that was not it.

@RotorZone - Stutter means there is breaks in combustion. There was variable delays between successive combustion, and finally the engine would quit. At least once the engine started in reverse.  We did not try the pinch test at all, since we realized early on that any amount of LSN tweaking either way was not fixing it.  

For the record, what is the diagnosis when an engine quits when the glow igniter is removed OTHER THAN the glow plug itself is weak ?

increase the idling and lean a bit of high speed

I guess if its not your glow plug , maybe it isn't tightened enough of MAYBE you have forgotten to but the brass washer . If you have put the washer , maybe the plug isn't suitable for that use . Try another OS hot plug (Or if you are using the same , try changing it ) . The Glow Plug sounds most suspicious to me

Why dont you upload a video so all of get a better idea as to whats the problem.

Not sure what the video can show you other than how exactly it stutters and stops, which seems consistent with less air getting into the carb (as RotorZone suggested).  In any case, since it is already fixed, we will have to reverse the fix and capture it on video


So if the plug is good but the engine quits when you remove it in idle, it is an indication of fuel mixture being too rich ?

PS: A major clue is in this post : http://www.rcindia.org/fuel-and-engines/interesting-nitro-engine-problem-and-diagnosis/msg23174/#msg23174

Maybe your fuel is contaminated .

We were unanimous about it initially, but we were proven dead wrong ! All 3 glow plugs were working fine, and were fitted properly. Improper fitting of the glow plug would result is air escaping from the combustion chamber and would show up as other symptoms.

How would it affect the idle alone ?

This is a wild guess - Maybe once you throttle up , there is so much fuel coming in , small amounts of water content or sediments wont really matter as there is continuous (fast) combustion .
I might be completely wrong but thats what i think . Its like having a dent of a size on a large wing and the same size dent on a smaller wing . The smaller wing would have more drag (Again i might be completely wrong , please bear with me as im no expert )

The clue seems to be related to the "inverted" mounting on your earlier plane. But I have NEVER encountered a problem with ANY of my 4C engines, which don't seem to care any which way they are mounted!

BTW was it just the engine or the tank also, that was changed?

You are close Sushil bhai   But it was neither the engine nor the tank.  The 3rd piece of the puzzle was the issue, and that is why we all got stumped earlier on.