I have an old, used FP .20 that will not peak out, but gets extremely hot and then starts to sag.

I am thinking that something is either bent, worn or warped. Do these symptoms suggest one or more of the following?

1. worn out bushing (the crankshaft has a lot of forward/backward play)
2. worn or bent connecting rod
3. warped liner or piston

It is not a fuel system problem; I'm running it on a test stand. The carb has been disassembled and cleaned and all engine bolts are tight.

Thanks, David

Those engines run cooler and better the more castor oil they injest. If you are using
synthetic oil they will get hot quick. Also a hot plug will not help, use a medium range
plug.

FBD.

Is that one of the older ones w/ the Iron piston? If so, and it has decent compression, it might be just as FBD sez...add some castor to your fuel and try (if you don't have already ) a #8 plug? Those older FP's and earlier MAX .20's etc. like a lot of oil, specifically castor...20% or even more.

I found an empty qt. can of Fox "Superfuel" and it's got the list of ingredients right on the front of the can:
29% (!) Castor Oil
64% Methanol
5% Nitro
1% Detergent
1% Ignitors

Thought that was interesting...

....it took that much oil to keep a Fox in one piece....

FBD.

please ignore this post

I am using 10% Power Master to which I added enough Benol castor to bring it up to about 22.5% oil. It does have an A3, so maybe I'll try again with a #8. It has decent compression and turns freely, that is, I can't feel any binding or roughness. Is a 9x4 too much prop for an FP .20?

I would expect it to turn about 12,000 revs. Over-propping would certainly add to
the overheating issues. The hot plug is not helping either.

So, what is the best prop for this engine? Thanks.

I have the 15 and the 25....the 20 is a "tweener"....have a look here:

....yours comes in right about here....it looks OK to me....

Thanks. I just ran it again with a #8, a bit more castor and an 8x4.

This is just a sick engine. The higher the rpms go the more it shakes and vibrates. At full throttle the rpms waver up and down and it sounds like it's getting ready to blow apart. The engine will shake violently for a second or two, then run smoother, then shake again, and so on. There is definitely a mechanical problem inside the engine. I think the larger prop was acting like a flywheel and damping some of the vibration. With the 8x4 it goes psycho.

Those engines have to be set several clicks richer sometimes, than what you think
would be the correct setting. They heat up after running for a minute, and need a
richer setting to keep them cool.....they will lean out and start to sag.

If you lean one up to maximum RPM then keep richening the engine slowly
click by click, you will be surprised how many clicks it takes to get the engine
actually richened up when fully hot. I counted 12 clicks open from full peak on a
15 size engine on time....meaning I had to richen it up about 8 clicks from where
the owner had it set....the engine was dying out in flight, time after time.

When you set an engine that rich, sometimes it takes the engine a couple laps
to heat up and get back to where it was on a previous flight....but that's what it
takes in richness to keep the engine from overheating.

FBD.

Hi!
Are you sure you haven't run it too lean..?
You should use 5-15% nitro, 20% oil and a OS 8 or Enya 3 glow plug and a APC, RAM or Graupner 8x6 or 9x4 prop.
The axial (back and forth) play in the axle is normal because it's a plain bushing engine (no ball bearings).
Check all screws and tighten if necessary. if things still doesn't work out it could be the connecting rod that is bent...or that the piston to liner fit is bad (worn ).
The cylinder liner can't be warped...just worn out.

Hi!
A 8x4 prop is way to small prop for a 3,5cc (.20) OS MAX engine.8x4 is best suited for a 2,5cc (.15) engine

I ran the rod out of my 25 engine after years of use....it was just worn out. Pull the
engine backplate, and see if the rod is loose as a goose.

I own a few other FPs ranging from the .15 up to the .35. None of my other FPs have ever behaved like this one. It is an engine line I am very familiar with having flown them for over 15yrs. This one is a complete anomaly.

FWIW, I always run all my engines on the rich side. I have never damaged an engine from running it lean. I still own and fly several engines that I bought in the early '90s and they all run as good or better than new. You could easily say that I baby my engines.

I think it's just worn out. I got it second-hand in a trade and never used it until recently. I have no idea how much abuse it saw from the previous owner(s).

Thanks.

Just to amuse myself, I ran one more tank. I tried every setting from stumbling rich to wavering/rattling/clanking lean and everything in between.

It will not maintain a constant rpm at any needle setting and pinching the fuel line sometimes speeds it up and sometimes slows it down- at the same needle setting, whether it's rich or lean. It is hardly producing any power regardless of the needle setting, and it shakes and rattles like it wants to explode. It makes the strangest noises of any 2-cycle engine I own - like it's banging itself to death.

Even though I can't feel any play in the rod, it's acting like the rod bushing are worn out. maybe the play gets worse when it heats up.

--------------


A 9x4 is the perfect prop for the .20FP, as is an 8x7-8 or a 10x4.

Is it possible that the engine hasn't been broken-in previously?

There were several versions of these engines sold. There was an ABC original and an then the later ABN version. There may have even been a lapped piston/liner version, but I'm not sure. If it is the latter, it will need a considerable amount of running in a fat, rich four-stroke before it will come in and stop overheating when peaked out.

As others have indicated, if the plug is too hot, it will advance the timing way too far and this will cause overheating and refusal to peak out.


Ed Cregger

Here's a old engine tuners trick to try. The 20fp is a brass bushing-ed motor,you mentioned alot of in and out crank play. Remove the prop thrust washer and place 1 or 2 {depending on thickness } teflon shim washers between thrust washer and crankcase replace the prop of your choice and try running it again. I'm willing to bet it cures your troubles. It's always cured mine on the fp's.

happy flying

Thanks, Ed. I've used an OS #8 for the last four attempts.

The engine is at least 15 years old, maybe older. I got it second hand. I have no idea what the previous owner subjected it to. I do know he was the type that went through engines a lot faster than anyone else at the field, so I guess he damaged it somehow. It may have even been crashed.

Here's the thing. It starts out sounding halfway normal for about the first 30 seconds or so. The longer it runs the more erratic and rough it becomes. Even at a very rich setting - and I mean rich - the head still gets hotter than any of my other OS engines.

All of my other FPs run flawlessly. I've never had a bit of trouble with any of them. Something is mechanically wrong with this one. If it was a needle setting or fuel system issue it would have been solved by now.

abacro, I will give that a try as a last resort. Do you know of a source for teflon washers? Do hardware stores carry them?