w.pasman

Last friday I replaced the bearings and since then my engine runs very hot, it even idles hot.

Just before I replaced the bearings, the engine ran pretty cool and never noticed that it gets hot while idling.
Also I had to crank up the idle throttle quite a lot, to keep it idling.
This is on a Raptor 50V2: OS50SXH.

I did not change anything with the needles during replacement of the bearings. Sure enough the carb was taken off but I am very careful to align it as it was before using the 'marks' on carb and engine. The full throttle point and throttle cut point (related to the ATV on the Tx) were spot on after the replacement. Needles were not turned, in fact the idle mixture screw is VERY tight.

I can't find anything out of order. everything looks smooth:
the bearings in the engine are both new, look right and feel right
the starter shaft feels smooth. I can stop it easy with my finger while the engine is running. I can rotate it by hand and feel little if any drag.
Idles smooth, no rattling, can idle at very low rpm
clutch is not dragging

I checked my (uniflow) exhaust system.
To be sure I replaced the main klunk line, although it seemed OK.
I can pressurize it and keep pressure.
I tried to run without uniflow pressure, by keeping the vent open. I also took out my inline fuel filter. This did not make any difference.

I checked for play on the crankshaft. There is none.
To be sure I took off again and replaced the carb. Made no difference. Carb Orings look fine.

I cleaned the carb, there was nothing inside


Now that I run out of options to 'fix', I took the engine apart again and noticed this scoring on the liner set.

Could this cause my problem?



The piston also has scoring, but this was there already for a long time




{PS this post was also posted on runryder but here I hope to get manufacturer advice}

w.pasman

BTW compression (turning the cold engine by hand) feels OK: it's very hard to turn quick but you can turn it slow with some force.

Bax

My Feedback: (11)

It sounds like something's not aligned, or it's bent. If an engine is difficult to turn over when the glow plug's out, you have a sure sign of a bent or misaligned part. A bent rod or misaligned crank or bearing will cause added friction, which makes for hot running. When the engine gets too hot, the piston will expand enough to squeeze out any oil and scuff against the liner.

The only way to find out is to completely disassemble the engine and carefully check everything.

w.pasman

Okay I will disassemble the bearings again and reassemble them.
I found one place with a slight play: the joint between the crankshaft and the connecting rod has a very slight play. Anyways I heard a slight 'clicking' when the piston changes direction from up to down and it seems that this joint is causing it. It looks very small, maybe 0.1mm or less play, I can see a little oil moving only.

w.pasman

BTW don't get me wrong, the engine turns over smooth if I remove the plug. When I talk about 'hard to turn' it's when I check the compression, and that is done with the plug in place.

And also, now the oil is out I have a lot less compression. But I suppose that's normal.

w.pasman

Bax you don't comment on the liner or piston. Do I get it right that means that they look OK to you

and of course, thanks for your help!!

Bax

My Feedback: (11)

Slight scoring on any piston and liner will result over time as an engine is used. This indicates that some foreign material got in there, and/or you've had a short-duration overlean situation a time or two....nothing serious.

If your engine has good compress when the plug's in, but turns over smoothly when the plug's out, it doesn't sound like anything's seriously wrong. If you can run the engine rich of peak RPM "all day", don't worry about its temperature. It WILL get hot. A model engine that's running normally will become too hot to touch from just below the exhaust port to the top. After it's shut down, it would be normal for that heat to distribute itself throughout the engine.

We don't worry too much about the actual emperature of any model engine if it's running well.

As far as the clearances go. Look very closely. If you can only see oil squeeze from between the crankpin and the connecting rod, you're fine. You can have between .006" and .008" of clearance before you should consider replacement. The engine will run with as much as 0.010", but you're on borrowed time as far as when the rod may actually fail. Most properly-run airplane engines will have the piston/cylinder fit wear out long before the rod/crank fit will wear out.

w.pasman

Thanks for your more detailed info!

Well it did get very hot at the *back plate* and that worried me. Especially because it already did while idling. My plug didn't like it either, it was definitely hot.

OK I am pretty sure that I dont have 0.2mm (0.008" ). That would give an awful lot of play.

I took everything apart of this engine, removed the brown residu from the piston and re-assembled. Hope it's OK now.

w.pasman

OK I just did a test flight with the fully rebuilt engine.
The only thing I did was polishing the gum off the piston.

AND NOW IT RUNS FINE!! [X(]

That means, no overheat during idle and hover.
I still have to put in a lot of throttle for idling and hover throttle is rather high too
My throttle curve is now 59% 80% 100% for the upper 3 points (0deg, 5deg and 10deg pitch) in *normal mode*.
This is not normal, it was 61-67-100 in *idle-up*. But it seems that there is some more space to tweak with the needles

I could not yet test idle up in my backyard.


Conclusion: I am confused at what caused this problem. Can it be the gum on the piston?? Or is the problem actually fully solved?

Bax

My Feedback: (11)

If your fuel had much castor oil in it, then over time, it will build up some varnish that will remove the clearance between the ring and piston, and between the piston and cylinder wall. Cleaning it will bring those clearances back. However, if your engine's had significant running time, then cleaning the varnish off can reveal that the piston and ring are worn, and compression is lower as a result. You'll wind up having to open the throttle a bit more to get back to the same power levels as before. Usually, if the compression is getting bad enough to seriously-affect running, the idle and acceleration will also start go go bad.

w.pasman

Bax

The engine was running cooler after some time, and I could tweak the needles further. I'm back on my old power now.

On runryder someone suggested that the standard SKF bearings that I used have too tight clearance, and that I need wider clearance bearings. It seems to me that that could indeed explain this weird behaviour and the 'running in' effect.