Let me preface this by saying it was recently in a crash. Here's what its running on. 10% and 20% pro pattern fuel....same results. APC 12-7 prop. I used to get around 10,000 RPM's with this set up. I have completely broken the engine down and mic'd the crank which appears to be straight and true. Replaced EVERY rubber O-Ring, replaced the head gasket. added sealant to all mating surfaces. Checked the timing 3 times, Checked the valve lash multiple times.
It will start cold fine but has a lot of smoke regardless of where I set the mixture needle. It used to not do this. I can set the low end to get a good transitions but it stil seems very smokey compared to what it used to be. When I run it up to max power and lean it out i get about 9800 RPM which is great but then something instantly changes and what little smoke was there goes away and it drop to 8000 RPM and stays here now matter what I change on the needle. From then on as long is its warm the engine won't go past 8. If I let it cool down and start over it does the same thing. Initially peaking at 10 then dropping back down. If I let it idle for 15 seconds or so and then goose it, it will initially peak to 10 and drop right back down. The high speed need doesn't seem to do anything much in the way of adjustment of the RPM's until I hit so lean that it just dies.

Help Please

So frustrated.

Just an opinion here: Too much prop and that is spoofing you into pushing the needle further.

I used to run the second series 70 many years back on a cross country airplane (a tanked up Kaos) and any 12 inch prop unless it was quite flat was a pretty unrealistic load for the engine. I used mostly 11x7 and 6's and it was good for several hours and 45 miles. There was an even older seventy that was a bit of a problem and it was replaced in the market very quickly and I am sorry I do not know how to tell them apart.

If you want to run a twelve inch prop I believe you need to unload the engine somewhat and use no more that a 12x5.

John

Well, I would agree to some degree but that prop falls right in the middle of the recommended stunt plane range. Plus its the same prop i have been using with zero problems. I know I was always getting around 10k but now I struggle to get 8. So incorrect prop aside im still not making the same numbers that was making before

I had a similar problem with one of mine after I flew through a tree. Nothing I tried worked so I broke down and bought the complete needle valve assembly, problem solved. The needle valve got a slight bend in it as the plane smashed it's way to freedom before it crashed into the rocks!! I could have probably just replaced the valve but for the low price I wanted to make sure I fixed the problem.
At that time the engine was in a Decathlon and using a 12X8 APC and when it went into my Tiger Moth it had a 13X6 APC. I only mention the prop at all because it's a combo thing and depends on the plane the engine is used in, I didn't see what plane you were using.

I have it on a great planes mustang kit that I have built. Actually its the third time I have built that kit. Man, that Surpass II with the Spring Air retracts on it is such sweet rig. One of the best flying planes I have ever owned.

OK....soooo.....Update. Just for the fun of it....I threw a 60P carb on there from a 91 surpass. It did the same exact freaking thing. ***. So...I guess that eliminates the idea of the carb being bad. Even threw a brand new needle in....yeah...Same crap different day.

Now it's time to get a dial indicator and check the crank and rod. The simple is now eliminated.

Yup....already did that. That was the first thing I did after the crash. Its straight as an arrow. Valves are adjusted to listed specs as well. Put a run out on the valve rods as well.

Have you tried a new glow plug? Eliminate all the simple things first even if you don't think they could be the problem.

Yup, tried like 5 of them. still nothing.

Hi!
Are you sure you have the OS F glow plug!?

Subbed because I own one of these engines.

When the engine was reassembled, was the camshaft installed properly? Here's the way it's supposed to go:

I suggest setting it up richer from the start. Peak it out then richen the mixture 3 to 400 hundred RPM. A lean mixture will cause detonation which slows the engine down, richening the mixture may not pull it out of detonation, and so it just runs at the lower RPM.

Yup, Positive....Tried numerous of them.


Yes, made very sure that the timing was correct.

Done that many times. I have richened it so far back that the needle can almost come out. It just stays stuck where its at till the mixture gets so rich, that it chokes and dies.

If it has really bad detonation then that is how it would act. I was suggesting that you make sure that it starts up rich.

SO, I have no idea what it was but I gave the engine to a friend. They took it to their work and ran it through the ultrasonic cleaner. I have no idea what it cleaned out but its running like a top now. I took that stupid thing apart all the way like 5 times and cleaned it. Must have been something in there that I couldn't get to somewhere.

Crankcase vent plugged?

I dunno maybe. I ran some small guage wire down the nipples several times. Even took them out and checked them. Didn't see anything. I dunno but what ever it was the ultra sonic cleaner got it.

OK, so, update. I gave the engine to a friend that repairs full scale aircraft and he happens to have an ultra sonic cleaner. I don't know what kind of magic that thing is capable of but it fixed what ever was wrong with the dumb engine. 10300 RPM rock solid now. Put 3 flights back to back on it and not so much as a cough. I would love to know what cleaned out of there that I didn't get, cuz I took that stupid thing apart at least 5 times and cleaned every part with kid gloves. Oh well, it runs great now.