Cutting out mid-flight

[Ben B]

My engine is regularly cutting out and I'm out of ideas about a possible cure (and so are the club wise elders). I'm getting plenty of dead stick landing practice but it doesn't add up to a relaxing flight. Any advice gratefully received!

 

The engine is a Magnum 40GP (OS 40FP clone). It's an old engine but had been sat unused in someone's loft for a good decade or so before I got hold of it. It starts okay and idles nicely but intermitently it just cuts (sometimes even at full throttle). And the tricks bit is it only cuts in the air! It's more likely to happen when the fuel tank is running low but has happened on take-off with a full tank.

 

So far I've tried

 

1) replacing the fuel tank clunk line and fuel lines

2) replacing the whole fuel tank

3) replacing the glow plug

4) replacing the needle valve o-ring (in case it was leaking)

5) cutting down the silencer fueltank pressure tube (in case it was folding over in the airflow and causing a lean run)

6) wondered if it was the engine beginning to run lean as the tank ran down but tried running it increasingly rich and it still did it.

7) packing the fuel tank in foam (in case it was fuel foaming due to vibrations)

 

but still no joy. It still cuts.

Any ideas? I'm using 10% Irvine formula (tried new fuel). I'm using the recommended prop size.

 

It's a bit frustrating! This evening I'm going to strip down the carbs and check there's not any grot in the needle valve. Anything else to look for at the same time (apart from a new engine ).

 

Thanks in advance for any pointers.....

[sharpy1071]

Hi there,

I had a similar problem with an OS 4stroke engine when I went from castor to synthetic fuel and was told that it was the new fuel getting rid of the gum formed by castor and fouling the glow plug! The other thing I thought of is is your clunk a standard one or the type used in petrol engines that actually stop air getting in the fuel line?

[Josip Vrandecic -Mes]

Hi Ben , I do not know what fuel you use ,but try full sintetic with 10% nitro.Might help

Jo.

[Braddock, VC]

Check the pressure nipple on the silencer is clear, take it out to do this job as it may well be some teflon tape or similar used to seal it getting over the open end. Reseal it with epoxy.

[Ralph Yeates]

Cleaning of the carb/needle valve is a good step as debris can enter and have some weird effect especially what you are describing.

Do you run the engine with an inline filter?

do you fill up via a filter on the filling pipe ?

Also check the screws that hold the carb on to the body as they can leak air also the head bolts.

[Ben B]

I'm using a standard clunk and did wonder if it was the clunk line being too long (allowing the clunk to suck itself against the back wall of the tank) but shortening made no difference. I'm using Irvine Formula fuel with 10% nitro. I think the Irvine formula is essentially fully synthetic.

 

Will have a look at the silencer nipple to make sure it's okay.

 

I do wonder about grot in the fuel line. I never used a filter in the fuel line to the carb but used to fill up with one in the line from the pump to the tank (though I must admit the other day I took it apart to clean the element and found it didn't have an element- I must have decided it was once too dirty to clean and just screwed the filter back together again). Whoops.

 

Off to the model shop to get some more fuel and some filters and try again.

 

Thanks for the tip Ralph re the screws that hold on the carb in place. I'll put some sealing goo on them to make sure it's not an air leak there.

 

the only other possible suspect that I didn't mention was that I fitted a remote needle valve assembly for an OS 40FP engine, I can't see why that would cause the problem but if all else fails I'm going to try reverting back to a normal finger-nail chopping location and try again.

[Paul Adams]

I had a similar problem with an OS 55. After trying all sorts I changed the fuel to Castor based, and it's no longer a problem.

 

The other thing is how old is the fuel you are using / how is it stored. Fuel is hydroscopic so absorbs water, so if your fuel container is half empty, the fuel will absorb all the water from the air above it. If you empty your tank by reversing the direction of flow in your filling equipment (hand / 12v) it normally finish up bubbling air through the filling container, which results in more moisture being absorbed by the fuel.

 

So I would suggest scrounge a tank of fuel from someone else and see if that cures it.

[Ben B]

I did try getting some new fuel but it still did it- interesting that your OS seemed to need Castor, I can understand why that might happen with an older engine. The castor is thicker than the synthetic and might compensate for more relaxed tolerances than on more modern engines. Might give that a try (shucks- just bought a whole load of synthetic). Then again I believe Castor protects against lean runs better than some synthetics and that (a lean mixture setting) might be the culprit.

[Ralph Yeates]

Ben the remote needle should not be an issue our club trainer is a remote needle and I see no issues on that front. You should always use a filter at least to fill the plane and to be safe in the fuel intake as the exhaust pumps unknowns into the tank. Don't forget to post the answer once you get it cracked it may help others pulling their hair out.

 

That's why there are so many bald people about they keep scratching their head wondering about things ;o)

 

[Ben B]

Well my vote goes for crud in the needle valve. I took it all apart and certainly when I first tried to peer through the needle holder I couldn't see much, after blowing through it, it seemed to be much more "open" [IE I could see down the tube much better]. Okay it could just be a meniscus of fuel stuck which I blew out rather than a foreign object (certainly I didn't see crud coming out the other end) but after doing that (and installing a filter on the inlet line) it seemed to work much better.

 

In fact I got through a nice quarter gallon of fuel over the weekend with about ten flights and only one dead stick the entire weekend when I was still fine tuning the needle after having removed it.

 

Then again it could just be that it was a new old stock engine and it was still running in. Who knows- could have even been carb icing (it was nice and toasty this weekend and previously when I'd run it, it was cold).

 

 

Edited By Ben B on 12/04/2011 12:59:25

Edited By Ben B on 12/04/2011 13:00:07

[Shaun K]

Hi Ben, I've just spotted your thread late, but your problems are certainly well in line with a similar problem I had with an OS 32 2 stroke.

 

After several harrowing weekends, new fuel tubing, glow plug, testing tank seals etc etc, I found a small piece of grass stuck in the fuel inlet nipple on the carby. It was just enough to cause a lean run every so often and kill the engine.

 

With the grass gone, so were all my problems.

[Martin Harris - Moderator]

I have a bicycle pump with an adapter for my air retracts which I use, with a piece of fuel tube to the inlet nipple, for clearing carbs in the field - first blow through sharply needle out then needle in loosely - works extremely effectively!

 

Keep all eyes (yours and any assistants') well away from the blast - methanol and eyes do NOT mix!

[Peter Hogstrom]

Maybe you have solved the problem allready. But I had similar problem with my

OS-46 FX. It was the o-ring under the carburator that was damage so the engine was sucking air under the carb. Might be worth to check that if you still have cut outs.

Hope you found the problem, not fun with thease dead stick landings...

 

BR

Peter H

[Ed Darter]

Is the engine inside a cowl ? Could be overheating.....

[Craig Spence]

Hi Ben B,

I know a lot of people have mentioned a lot of these problems but I have from my experience found the following problems on sc engines.

The carb bolts came loose, letting air in, in turn cutting the engine at higher rev's due to the vibration letting the bolts loose. I only found out after the carb actually came out, first year flying.
Used epoxy instead of thread lock as the thread locks were ineffective really.

Another problem was that the gasket between the exhaust and manifold had become damaged, this also cut the engine once airborne. Easy fix with epoxy.

The high speed needle is probably the top tip and test, I have had this twice, took the needle out flushed the hole and jobs a good un.

One rarer one of late was the the carb barrel retaining screw/ bolt had came loose, then when throttling up the carb barrel actually came out when the servo pushed the arm forward. Took me a while to figure that one out lol, again easy fix, used thread lock this time.

Sometimes the tanks metal tubing cuts the silicon tube, hard to see a lot of the time so just replace it all if in doubt, main thing is to eliminate the possibilities until you nail the problem.

Good luck, oh and all these problems have been on SC engines never had a problem on any other engine, I still buy Sc though funnily enough lol.