SCOTT42

My Feedback: (1)

Hello all well I think I trashed my os .91FX, I run to small of a prop on it and over done it. I had mounted a 13-4 on it hoping to get more tork out of it and I did for the short flight that it lasted, my fault all the way. So my question is what can I do to salvage it? new sleve? new piston? or could have the block warped aswell? Ive never done this so its a tough pill to swallow. I putt a larger prop on it and run it in the test tsand but it wont run but just a minute then if I try to advance the trottle it just knocks and dies plus it gets hotter than hell in that short run time. All most looks like a parts engine at this point.

jester_s1

It sounds like it's just lean. I may be asking a dumb question here, but did you try tweaking the needles to see if it was out of tune? And did you try a new glow plug to see if the hot run ruined it?

Assuming you did, then you either trashed the bearings or the piston and liner. I doubt anything else is ruined though. If there is still some compression and the liner isn't peeled, put some new bearings in. You'll want to change them anyway if it does turn out to be the liner, so there's no harm in doing only that first.

Luchnia

My Feedback: (21)

Don't write it off yet. Take the exhaust off and take a look at the cylinder to see how it looks. If you have not done much damage to the cylinder you may be able to clean it up. I have used light emory and good to go. I had a 91FX that I ran hot and all I did was put it on the test stand, add a small amount of Klotz (cannot for the life of me think of what the oil is called now) oil for cooling and standard prop and ran a few tanks through it at different RPM levels.

I then changed my cowl setup on the plane, put the engine back on, and that engine is one of the best running 91FX engines I have ever had. Starts and runs like a top.

Let us know how it turns out. Even if you have bad bearings they are a piece of cake to replace and very cheap from RC-Bearings.

JohnBuckner

My Feedback: (1)

If any actual damage did occur and I agree with Jester it simply sounds still lean, then suspect valve springs that may have lost their temper.

I have no idea if this applies because I do not know if your installation is cowled but many, many folks have more problems simply because of the refusal to let enoungh air in especially even more air out. This of course requires cutting of the cowls and so many refuse to do this.

Just a few thoughts

John

jetmech05

Sounds like you are running lean, what could have changed to make you lean. A pin hole in your fuel line? Or perhaps you bumped the high end needle? I'd re-tune and see what happens

SeamusG

Very easy to determine if piston / line damage has occurred. With the OS91FX it is a simple task to remove the head - 6 hex head bolts. Do it while the engine is still in the air frame with the prop mounted so that you can turn the crank using the prop as a handle.

Rv7garage

My Feedback: (10)

Valve springs on a 91FX? I think not...

jester_s1

Since we live close to each other Scott I'd be happy to walk you through a bearing replacement if you want, as well as try to help you get it running as is. I know you're pretty mechanically savvy too so maybe you've already figured it out yourself.

JohnBuckner

My Feedback: (1)

Thanks RV I did indeed miss the FX part

SCOTT42

My Feedback: (1)

LOL, john you had me worreid there for a moment, Paul ive changed many sets of bearings and thats not a problem to do I just havent figured out why its running so hot? I do have some klotz oil , havent tried none of that yet its still mounted in the run up stand so I will give it another go first chance I get, as of now Ive got a 14-6 prop on it thinking it would not wind it up to a high rpm when its running.

pimmnz

RPM isn't the problem, it only runs hot if there is either no air over the fins, or no oil in the fuel, ie too lean. Consider that about half the cooling is done by the air, the other by the oil in the fuel...
Evan, WB #12.

Luchnia

My Feedback: (21)

I remember now it was Klotz castor. When mine first ran hot a guy at the field walked up and told me, "put a little Klotz in it until you get it broken in, it will run cooler and don't let it get too hot in the cowl" and I did that also changing the mixture.

Made the difference in the long run and like I stated it is the best 91FX I have had now...very smooth runner. If you have not damaged the engine you will be fine once you get her broken in. I ran a huge amount of fuel through mine before it got broke in. I bet it was two or more gallons - about a gallon or so on the bench and then at least a gallon or more in the air. She has that OS "pop" now [8D]

speedracerntrixie

My Feedback: (29)

I beg to differ. If you run too small a prop it wil overheat. A 13X4 must have had it running 14,000 to 15,000 RPM Way too fast for an FX 91

Gray Beard

My Feedback: (-1)

And when you do figure out your engine problem do yourself a favor and read up on prop basics and rethink the pitch for a speed prop.

Luchnia

My Feedback: (21)

Agreed. I remember trying to recommended prop for my 91FX and it was a mistake. I had to run a smaller prop initially and once the engine was broken in I was able to run the right prop. I think it was a 14x6 (not positive though - just grabbing from my memory).

From my limited experience some engines simply don't perform well on the high end recommended props, sometimes even after they are broken in.

aspeed

It is bigger than what I am used to, but if it is ABC it is likely OK. I would take the backplate off and see why is is rattling. Maybe just the rod rattling. If it is ringed or lapped steel, it may have got a bit too hot. More oil added may help to seal it better if it is loose or warped. Maybe 25% oil like the old worn out Foxes like. I'll be watching this thread because I have a .28 Thunder Tiger Prothat overheats like that too. I always thought overpropping a motor made it overheat. Either way Iwould run it on the rich side for a while.

da Rock

I'm running a handful of OS91s in my warbirds. Every one has a MA14x7 three blade on it now and it's a match made in heaven.

However, every one also has the airflow through the cowl controlled with baffles and the position of the hot air exhaust holes. Block the entering air except in front of the engine. Make sure most of the exit area behind what you want to cool. Consider whether the exit is in a positive or negative pressure area. If you can see the exit hole from the front, it's probably in a positive pressure area and just might not work as well as you expect.

Gray Beard

My Feedback: (-1)

Great looking Spit!!!!!!!!!!!!!!!!!!!
I have kept my mouth shut about over heating engines. I have never had one over heat except one but it was caused by a crash. I have pulled the engine apart and measured the crank and rod to see if I had bent them. According to the service they are fine but when I fire it up it cooks itself so fast it dies in about 15 or 20 seconds? I have reversed the piston and rod giving it a try but it reacted the same way. This was caused by a crash and not an over heating lean problem, I have never done that to an engine.
This is an SK .91 engine, I like them a bunch but at $100.00 I'm not going to over think it or keep working on it. The OS is worth the time and money. I would measure the rod and crank then do a visual of all parts. I think I would ask Bill Baxter what the correct size of the piston is too and measure it and then measure the sleeve too. Often when you over heat an engine the piston seizes and will leave aluminum on the sleeve.
You really can't tell the whole story looking into the port, you have to strip the engine down and look and measure things!
There are a lot of things that can happen to an engine if you over rev them and none of them are good.

jetmech05

Saw a helio guy once really overheat an engine....Very little smoke.....all of a sudden there was a pop...and he auto rotated down....Pulled the engine apart, there was an almost perfect hole in the middle of the piston the size of the glow plug.....
I'd like to add to my orginal post about being lean....look everything over after you change the prop
Good Luck

Ed_Moorman

My Feedback: (1)

I've owned a .91FX and it will turn some rpm. I've run 12" props on it with an Ultra Thrust tuned muffler.

If your engine knocked and quit very hot, you are running it way too lean. An engine will lean out in the air due to ram air in the intake and g forces, so you always need to set it slightly rich on the ground. I like mine to leave a little smoke trail in the air at the first of the flight. After 3 or 4 minutes, the engine leans out enough so the smoke trail is gone.

I recommend an OS #8 plug and some Omega 10% castor blend fuel.

If you really want to go, try a Jett tuned muffler and a 12-6 APC. Just don't run it over lean.

SCOTT42

My Feedback: (1)

First of all I just want to say thank you for all the input and help and secondly I want to say is that my engine, well its dead! I just went out to pull the head off of it and when I went to remove the head the sleave had welded its self to the head as it pulled out when I lifted the head off of the block, inside the sleave is deap pitted scars and the piston and ring are scared up really bad to. At this point Im asuming its just a parts engine. Costly mistake on my part, but wont happen twice!

jester_s1

You could replace all of that and have it running again. It's just a matter of if you think it's worth the expense and trouble.

aspeed

Sorry about your loss.  Cool though. I usually break a crankpin or crank when I rev. too much.  An ASP or Magnum is about the cheapest replacement.  About  $80, likely cheaper than a piston / liner and who knows what else.

SCOTT42

My Feedback: (1)

I just looked at tower hobbie for the parts to fix it, all I can say is its just gonna be a paper weight for a while. I cant get over how exspensive these parts are.

piston 39.99
ring 21.49
sleave 71.99
total 133.47 before shipping

yes it has turned out to be a costly mistake for sure!