Just got my engine back after having the piston and sleeve replaced and went to the field to break it in. Ran a tank through it adjusting needles and letting it cool every couple minutes. Runs great but I noticed that as it heated up it lost compression and eventually died unless I played with the throttle a bit (up and down). Am I correct to think this is normal in that the piston and sleeve are still seating themselves and the compression will improve as it breaks in further? It is fine when cool.

Thanks!

Compression never goes up with engine wear, either something is wrong with it or your doing something wrong to it.

I have no idea what could be wrong with it then. Should I just run it some more and see if it improves?

as it heats up it may lose some pinch at the top of the stroke.  Is this normal.  Is this what you are seeing?

Jack

When the engine is hot it is very easy to turn the prop all the way around with just a little resistance going through compression. When it is cool it takes more force to push though compression. After running for a few minutes it becomes unstable, in that the rpms go up and down without touching the throttle (mostly down). At WOT you can hear the rpms slowly drop and if you let it go it will die.

Sounds like it running very lean.

Sounds like you may have run it lean and already used up your new piston and sleeve. Have you used an engine before or is this your first time?

P.S. You do have a prop on it right?

ringed engines start off with less compression then get better.my new magnum .91 4 banger has the most compression i have ever felt
adjusted the valves on it and wow talk about compression.i was sure it was hitting the valve or something but it wasnt.
im not sure if ill be able to start it with my dinky starter and old lead acid battery.

check the compression when its cold. it should feel the same as when you first got it, or a little less.
check to make sure you arnt to lean. if you are not then i would keep running it.
i have a old magnum .40 that has some,but not a lot (compared toa semi-newgms .47) ofcompression. but it runs really well. not very powerful by design (bushing engine)but it wont quit in the air and could care less what plug/fuel i ran through it.
the only problem is it spits out loads and loads of castor. my gms.47 and .32do leave oil on the wing, but this engine just covers everthing in oil (the plane is aspad so i could care less)

Science adn JimmyJames,</p>

I have to laugh, as I have an old TT PRO-46 on a SPA3DT. The sucker has almost no pinch cold, but it will pull that 4.25 pound spad with authority and it covers everything in sight in Castor oil.</p>

I still don't understand why yours is dying.  Is it very hot, finger sizzling hot, when it dies?</p>

Jack</p>

Yeah, I guess I could have specified non ringed, but I didn't cause his engine isn't ringed. Ringed engines get better because the rings actually seat and seal better after they do.

It is pretty hot when it stops running. I put some water on my finger and it will sizzle on the top of the motor.

Too hot! Richen it up, and get some help from an experienced flyer.</p>

I had some help out there, they told me to fire it up and go fly. That didn't sound right so I bench ran it for almost 2 tanks first. I adjusted the high speed needle until I got a good transition from low rpm to high rpm. Then set the low needle for a smooth idle. </p>

Do you think I really damaged the engine that much? If I run it rich for a few more tanks will it be ok?</p>

If you're using fuel with castor oil in the lubrication mix, the engine is probably still fine.  If you're using 100% synthetic, then it's tough to say.</p>

As long as the engine will still run, it isn't "ruined" or anything, so don't sweat that too much.</p>

I saw a similar situation at our field a week or two ago where a pilot was having a heck of a time breaking in a new O.S. .46 LA on his Kadet.  His top end would rise and fall and he couldn't keep the engine running reliably.</p>

Like 90% of all "engine" problems, it turned out to be problems with his plumbing (fuel lines and tank setup).  The tank was actually set up pretty good, but the lines to the carburetor were too long.  He had too big of a loop going from the clunk line on the tank stopper to the rear needle valve intake, and the upside down "U" portion wasn't drawing enough fuel reliably.  He also had too much line going from the rear needle valve outlet to the carburetor intake, and it was bulging out a bit in a sideway "U" shape.</p>

We had him trim about 3/4" from the tank to the rear needle valve and about 1/4" between the rear needle valve and the carburetor.  We fired it up, retuned a bit, and his engine is running great now.</p>

My advise is to go through your fuel lines and tank setup to make sure there isn't anything restricting the flow of fuel.</p>

I'm close to the minimum length of line for intake and output but I'll check the lines when I get home. I guess I'll richen the mix a bit and see if the situation improves.

I had a problem with compression on my previous engine.</p>

When I installed it into a new plane, I went too go and test run it while taxing the plane up and down. I firstly made the mistake by doing it in very hot conditions with now wind too cool it off and the engine running inside a cowl. Secondly I didn't richen the engine too help with the cooling. After a min or two taxing it up and down and giving it a few "sprint runs". I decided I want too take her up and I went too refuel. Refueled and tried to fire the engine up - but nothing - no compression - nothing!</p>

Now the engine will start easily then start dying unless you play with the throttle, because of loosing compression. Once it dies, you will NOT get it running after about an hour.</p>

I think you are def running too lean. If the engine dies and you are capable of starting it with ease only a min or two afterwards, I think your engine may still be fine. If not - I think you will have to replace that sleeve and piston again.</p>

Even when it was hot, it would start easily and run. Just had to play with the throttle to get it to keep running after a while. </p>

The low speed needle is used for low to high rpm transition adjustment, by doing it backwards you made it very lean. If you get it tuned right you will get lots of time out of it, but prolly not as much as you would if you did it right the first time.

The compression is normal.
The rpm description sounds like it is running lean.
When you turn the high speed needle counter clockwise the engine should slow down.... does it ?
If it slows down will it run at wide open throttle for the whole tank of fuel ?

I'll fire it up when I get home tonight and report back on how it performs.

Richen it up. It should NOT sizzle on the ends or top of the fins. That is too hot and indicative of running lean on the high speed needle.

While you are <u>learning how to tune</u> an engine, always make sure you have fuel with at least 1/3 castor. If you want to switch to all synthetic later on, go for it. Personally, I still use some Castor in all of my engines. It stains, but it's cheap engine insurance.

jack

Just got in from testing it out. Opened the HSN about half a turn or so and it ran a little bit better but still lost some rpm on WOT. Still got very hot after each run and the compression was lost, but returned as it cooled. I noticed that as you turn it past compression it feels like there is a second compression or resistance as the piston goes back down past the exhaust outlet. I took the glow plug out and there was no feeling of the pinch at the top of the stroke. Not sure if TT engines are supposed to have that or not.

So I let it cool and took the top of the cylinder off to see what I could. The top came off rather easily, by which I mean it turned and slid right out. The piston and liner were clean but one "quadrant" of the piston and associated liner were black, almost like they were burned. There was a matching mark on the cylinder top, almost like something leaked out.

Question: is there supposed to be some sort of gasket between the cylinder and the top? Right now it's just metal on metal.

Thanks!

There should be a thin copper shim in there usually. I haven't had my TT's head off, but my OS has one.

If the piston or liner has a blueish color anywhere, that is a sure sign of overheating.

There was no gasket present. I checked on the TT website and one is supposed to be there. One more thing to note, while running there is a bead of fuel that builds on the fuel line as it passes the cylinder head. I couldn't see a leak but this kind of makes sense now. Couldthis be causing the problem?

my gms .32 has that same 2 stop compression thing along with the magnum .40. the magnum is a lot worse than the gms if i kill the engine the prop almost always stops at that 2nd compression point. </p>

</p>