Ever had an engine that wouldn't lean out?
 

Thought I'd post here since you guys know a thing or two about tuning engines. :)

Engine is a basic OS 40 FP running 10 x 6 prop. Today I literally couldn't get it to lean out and scream, it stayed slightly rich no matter what. I turned the needle valve in about a half turn more then it started to die which is when I knew I had gone too far. It simply never would develop that high end scream like it should. It blew a little smoke indicating a slightly rich mixture as well.

I will say I noticed a few air bubbles in the fuel line, maybe an air leak somewhere? I also am running some old gas, would that make it less likely to lean out?

Thank you!

RE: Ever had an engine that wouldn't lean out?
 

Check the fuel lines, make sure the tank is padded, get some new fuel and check your glow plug. Probably one or more of those things.

RE: Ever had an engine that wouldn't lean out?
 

Bubbles in the fuel line will LEAN the engine.....air leaks anywhere will also LEAN the engine......foaming fuel LEANS the engine as well....you sound like you have a RICH problem....Just a suggestion but I have a Perry Carb that the High Speed needle backs itself out and richens the mixture. If you screw a needle in to tight on some carbs you can damage the spray bar/needle and will never be able to lean the engine, because you can't shut down the fuel supply.

RE: Ever had an engine that wouldn't lean out?
 

Thanks guys, I know the needle valve isn't bottomed out, there was plenty left to turn in. The plane in question is an Avistar ARF and the fuel tank simply sits in plywood formers, no padding. I thought this was unusual as I've built planes for years and always surrounded the tank with foam. Anyways, we've had the plane for several years and never had a foaming problem so I'm thinking I have a fuel tubing problem or old gas contributing to the problem. I'll try a few things and report back, thanks again.

RE: Ever had an engine that wouldn't lean out?
 

The inability to get the engine to rev up can be many things. Most likely bad fuel or plug, but could also be binding bearings/bushings. Did you try pinching the fuel line to see if it would rev up? If pinching the line doesn't work, it's not from being too rich.

RE: Ever had an engine that wouldn't lean out?
 

Irecently had an obstruction in the exhaust system which caused the symptoms you describe. The needle would not lean the engine out and it did not make any where near full rpm. When the obstruction was removed, all was well. For those interested, the problem was a piece of the exhaust gasket that had broken and lodged in the opening at the rear of the mousse can covering most of the 1/4 inch opening. The fuel pressure was so high that the needle could not reduce the fuel flow to make the engine run correctly.

Wiggy

RE: Ever had an engine that wouldn't lean out?
 

Good detective work, Wiggy. That's a case of "if something can go wrong, it will".
I had a Royal .50 that would never run a full scream, as well as a Cox .049 car engine and a ST .35 stunt engine. These engines were set this way on purpose, I guess. The trick could have been very low compression, I'm not sure. The purpose I figure was to eliminate the chance of "run away" screaming runs that the user could not control.
In the case of the .40 FP, if it is either worn out or varnished up, it will refuse to scream, but will tolerate a lower temperature setting.

RE: Ever had an engine that wouldn't lean out?
 

I believe Articflyer's point is not that you are now running with a bottomed out needle - but perhaps the needle may have previously been closed a bit to hard and wollered out the seat, such that now it can not be properly leaned.  As a kid I did this to more than a couple of unsuspecting cox engines.  Always sneak up on the seated needle slowly and gently - if you must.

RE: Ever had an engine that wouldn't lean out?
 

Yes, this is the right approach. First thing to check if your exhaust is obstructed. Too much backpressure will cause the engine to run "fat"no matter what.

Plug is an easy thing to check too. If you think the fuel is suspect, it probably is.

Nothing on the intake side of the engine will force a rich run regardless of needle position.

If none of this fixes things, then it is time to look inside of the engine.

Bob

RE: Ever had an engine that wouldn't lean out?
 

Thanks everyone. Disassembled the engine and pulled the tank, there were no obstructions in the muffler/exhaust port. No leaky fuel tubing (blew in one and plugged other, tank is sealed) High speed needle valve was 1 1/4 from bottomed out. Fuel residue inside engine did seem slightly gummy, maybe this is normal? (ashamed to ask since I've been in the hobby so long)

Also ashamed to ask, this engine has a small needle valve in the very front of the carb. Is this a midrange mixture screw? I know all the car engines have this but back in the day no airplane engines had a low speed mixture that I can remember. I thought it may the idle screw that limits how far the barrel rotates but this wasn't the case. I'm thinking about sealing the backplate and carb for airleaks also. We do this for RC car engines all the time, is vital there's no air leaks in car engines.

Sorry for the long winded post, thanks again guys.

RE: Ever had an engine that wouldn't lean out?
 

Oh also, the old gas that was given to me is 10%. I have a bunch of 25%, is that recommended in a sport engine such as the FP?

RE: Ever had an engine that wouldn't lean out?
 

Bijan, you haven't made it clear if this is an engine that you've had that normally does scream, or if it is a used engine that you are trying to get running for the first time?
How long does the cylinder stay pumped up after you've brought the piston to TDC?

RE: Ever had an engine that wouldn't lean out?
 

Bijan, the screw in the front of the carb is going to be an air bleed adjustment. If that is off, it can foul the way your engine runs all the way thru the range. But if you haven't messed with this setting, it's probably not that far out, and probably not the culprit.</p>

I agree with others who've suggested new fuel. Old fuel can absorb moisture from the air and will cause problems just like you are describing. I have had fuel go bad once before in as little as 6 months. The 25% will not hurt your .40FP, so long as it has at least 17-18% oil in it. Ideally, you'd run 10-15% nitro in this engine, but 25% shouldn' t hurt it so long as you are running a prop in the 10x6 or smaller range.</p>

RE: Ever had an engine that wouldn't lean out?
 

The engine is several years old, maybe 7-8 years old but I will say we've only put maybe 2 gallons on it in that time frame. We bought everything used so I have no idea how much run time was on the engine before we acquired it. There was a 4 year period we didn't fly. (it's my father's airplane btw) Up till last week it had run flawless, idled perfect and yes would lean out and scream.

I'm not sure how to perform the test to check for compression? I put the piston at TDC and let it rest there. After a couple minutes I rotated the prop and it kicked over, still retaining compression. What is the best way to perform this test and how long should it stay pumped up? Great idea by the way, thanks.

RE: Ever had an engine that wouldn't lean out?
 

Lomcevak,

Thank you and sorry, I have a habit of messing with things :) Yes, I screwed it all the way out, any idea where I should start with this adjustment? Oops.
I'll definitely try the new gas and report back. 20 years ago the engines I ran never had these complicated air bleed adjustments. Flew K&B's with Perry carbs and pumps, now that was technology. Also had an OS 46 SF that was incredibly reliable, even after being plowed into the dirt at 100 mph :)

RE: Ever had an engine that wouldn't lean out?
 

The air-bleed screw will lean the low end as it is backed out and richen as it is screwed in. If the engine will idle then it is probably close. On egood thing about air bleed carbs is that they have less affect on the high end. It still sounds like bad fuel or plug to me. Did you try pinching the fuel line while it is running at full throttle as I suggested earlier? If it won't rev when you do that, the problem is not that it is too rich.

RE: Ever had an engine that wouldn't lean out?
 

Bijan, all you do to test the compression seal is to rotate the engine by hand until the piston is at TDC and then wiggle it a little to feel for that "compressed air feel". A real good engine will stay pumped up longer than I care to stand there holding the engine, a servicable engine will stay pumped for at least 5 seconds. The definitive test is to seal off the exhaust port and rig up a sealed off tube going into the intake. Dunk the engine in water, blow into the air tube with moderate pressure and look for any air bubbles. Rotate the engine if your hands aren't too full. This is how a local small engine repair guy quickly diagnoses weedeater engines to see if they are worth repairing. The same principle works with our 2 strokes. If you have ruled out the easy stuff to look for, the "dunk test" can eliminate a lot of "poking and hoping".