scratchbuilder46

Men, I am having some trouble with my O&S .61FX burning out glow plugs. Some history- It is mounted inverted in a Royal Spitfire. I have changed the tank lines, put a new head gasket on it, checked all the bolts on the head and the rear gasket. It will turn up to 11000 rpm with a master airscrew 12x6 glass prop. At full throttle I can open the high end needle valve to 4 turns and it will not sag or load up. The fuel remains in the lines from the tank to the needle valve and from the needle valve to the carb. I have been in the hobby for 50 years and this one has me stumped. I have had some pretty good engine men in the club take a look and they are at a loss too. I have a home made exhaust system on it like a wrap around muffler. Plenty of space for expansion and the pipes are large enough for no back pressure. I will get about 1/2 a flight out of it and it goes lean and of course looses power and eventually dead sticks. It runs great on the ground, runs just fine when I hold it up, transitions great from low to high power. What am I missing here guys?

jeffie8696

My first suspicion would be no back pressure, no fuel tank pressure.

1QwkSport2.5r

Burning glow plugs happens from too much nitro for the compression ratio or too lean of fuel mixture. I'll almost guarantee it's the latter. What fuel and what glow plugs are you using?

scratchbuilder46

More info. Using Cool Power 10%. Fox long plug with idle bar. If I can open the high speed needle 4 turns I am pretty sure it is not running lean. If I don't have back pressure how is the engine getting fuel? It is syphoning down to 1/2 tank then going lean?

jeffie8696

If it is happening at 1/2 thank then maybe you have a tank line issue? Check for bubbles at half tank full throttle.

earlwb

If it isn't running lean at half a tank without muffler pressure, then that may not be the problem. Usually we all use muffler pressure as the carburetors are expecting muffler pressure to aid with good fuel draw with the engine. Too much compression could do it, especially if the engine was made for the European market and was setup for zero nitro glow fuel like they use in Europe. Some people have bought engines in Europe and brought them home, etc. So sometimes the engine is setup for European zero nitro fuel then like that.

But you say it is inverted. Ok, then what I think may be happening...is... When a engine is inverted and you stop the engine and let it sit like that for a while, then the fuel residue and oil left inside tends to pool on top of the glow plug which happens to be the lowest point in the engine. Later you go to start the engine again and the glow plug element burns out from the oil residue on top of it. So what you do then is with the engine still running, clamp off the fuel line and let the engine lean out and die, and then their is less oil residue inside and it takes longer for it to drain down and clog up the glow plug. Some people put their planes upside down when not using them too.

Also looking at the glow plug element can give you clues about what may be happening as well.

scratchbuilder46

I think at this point I may just start over with the tank and work forward. I thought I was pretty careful putting it together, but may have missed something. All your comments are most welcome guys. I will post results after some experimentation to see if I can get this engine to run right.......

the pope

Running lean or bearings going south . Cheers the pope

scratchbuilder46

Bearings are good. I think the front seal has gone bad. I swapped out another .61 I have and will check to see if the same thing happens. If so then I know it is the fuel system as well as the muffler I made for it. Will do a pressure test on the bad engine and see if I have any air leaks. I will just bet the front seal is bad. We shall see, will keep update.

1QwkSport2.5r

The bearing seal isn't what seals the crankcase, it's the oil film created between the crankshaft and bearing race. If you have a leak at the bearing beyond what's normal, the engine likely wouldn't idle well at all. I would put the engine on a test stand with a known good fuel system and run the homemade muffler as well as the stock factory muffler and see if the engine behaves the same. No pressure test is really necessary.

scratchbuilder46

That's just the problem. It idles great, transitions to full throttle with no burble, turns 11000 rpm, but goes lean and hits over 200 degrees pretty fast. I know it has enough back pressure from the muffler to run. I will keep at it until I figure this out. Take all the suggestions a work on this one. Just hate to give up, like to know why something is happening, when all seems to indicate this engine should run as it is.........

1QwkSport2.5r

Being able to open the needle valve 4 turns and it doesn't go rich tells me it's not getting enough fuel. On most carbs, 4 turns out would barely run at WOT. I suggest if your are insistent on using the homemade muffler, try running it up to WOT and as close to the carb as possible pinch the fuel line momentarily. If the rpm doesn't change or sags it's too lean. If it speeds up then its rich. Easy way to tell if your muffler is providing enough tank pressure or not. It could be the piston fit might be suffering wear too if the engine has a lot of time on it.

flycatch

My Feedback: (26)

You have an air leak cause by a warped head or bad bearings. Check the output of the crankshaft for excessive fuel blow by if none found than take the head off. Once the head is remove check the top cylinder head with the gasket removed. If you see any brown streaks than you know the cylinder head is the problem.

Prop_Washer2

My Feedback: (26)

An OS Type "F" should keep it from burning out, then you can work on your Fuel Flow, Muffler Pressure, Tank Height, Air Leak, or other problem...

I run OS "F" Types in all of my 2-strokes....especially the bigger ones.

CafeenMan

Have you tried to blow through the lines with the tank empty? Maybe there's some kind of obstruction. If your needles are that far backed out and it's running properly then it's not getting enough fuel. There are a ton of reasons that could be happening.

If it always happens at 1/2 a tank of fuel there could be a cut in the feed line in the tank at about the half level where it start sucking air. It could be there's some dirt in your carburetor. I would put the engine on a stand and see how it works there and completely rebuild the tank with new tubing (metal and silicone).

the pope

I just rebuilt a tank the other day and just put a bit more of a bend on one of them and it snapped in 1/2 . So I would do as cafeenman suggests and completely redo the tank . Clean out the carb which is just a 5min. job and try a new plug . Your homemade muff. might be a prob. too. Maybe it causes things to go lean mid flight so definitely try the os one as well . Had a ST 34 that the muff. nipple was just barely hanging on . So I thought id silicon it on and it blocked the hole and caused a bit of trouble and head scratching until I figured out what was going on . Cheers the pope

aspeed

Post # 6 pic says pretty much what the troubles are. Less compression will help, an added shim. I found Fox plugs to blow more easily than some others. Running inverted , droplets of fuel can dislodge the element as well as post #6 says, as well as running lean of course. If the motor can run rich enough, and reliably enough, then muffler pressure isn't even needed really. It make things more even, when pointing the nose up, or when the venturi is fairly large.

davidhand

I had an OS 46 that started burning out plugs (OS #8) it turned out bearings starting to fail. Small pieces of metal going round inside the engine. Changed the bearings and everything was fine.

bjbellino

My Feedback: (176)

4 Turns out on the needle valve sounds really rich setting? Would try just a regular non idle bar O.S. glow plug and see what happens. Might also leave plane on flight stand and let it run and see if it goes lean and dies at half tank just anchored on the stand. Master Airscrews have always been hard to balance for me. Could be causing some air in the fuel, GOOD LUCK.

Sport_Pilot

I suspect it just nedds more muffler pressure, or pehaps a regulator. I had a similar issue with an old Red Head ASP .91 that would not run right unless using its stock muffler, or with a Perry Pump. It would run fine on the ground and burn about a quarter tank, more if not doing aerobatics, then lean out and die. Blew a plug or two but not too bad. I solved the problem temporarly by running it rich enough to run right on a half a tank, but too rich on a full tank. Replaced the muffler but was a large power loss. I eventually added a Perry Pump.

earlwb

You know, the OP didn't mention what muffler he was using. Some of the in cowl or Pitts type of mufflers have little muffler pressure and the oversize carburetor is then starving for fuel as it cannot draw fuel very well. He may simply need to restrict the exhaust outlets. Some people used to block off one exhaust outlet on some of the Pitts mufflers too.

Sport_Pilot

He said that he had a home made wrap aroung muffler.

JohnnyS

If you can open the main needle 4 turns and it doesn't die rich then you may have a restriction in the fuel system somewhere that causes the problem. It also sounds like it could be overheating or have too much backpressure.

FWIW: Hope this helps!

Hemikiller

My Feedback: (125)

Not enough tank pressure due to the exhaust system not producing enough back pressure.

Had the exact same problem on an ST90 in a Super Skybolt. Ran great on the ground, died at a half tank like clockwork. Did all the same fuel system checks as mentioned, no joy. It had a Tatone Pitts style muffler with dual outlets. Blocked one of the outlets and it ran perfectly throughout the flight and never dead sticked on me again. I recommend you try restricting the outlet of the exhaust to increase pressure in the tank and try again. Do your tests at half tank, as that is where your problem occurs.

scratchbuilder46

Thanks men for all the advice. Problem solved. Hemikiller nailed the problem. Not enough back pressure on my home made muffler. I blocked off one of the tubes, and voila runs like a clock. Just starved for fuel. No wonder it was turning 11000 rpm! What kind of tipped me off also, was the Pitts for my .91. Taking a look at them together, I could tell my home built had a lot more volume then the Pitts, and larger exit tubes. Blocked one tube off and good to go. Thanks all...