[Florin]

I have an old OS 46 SF that has some noise in the bearings and I decided to fly it like that because it did not affect the performance, but last week I noticed a big drop in power after flying for 1 minute.

When I take off the power is OK, but after a couple of circuits the power drops a lot.

I replaced the glow plug, but nothing changed.

Can the bearings be the culprit (found some cheap replacements on eBay) ?

[bbbair]

An OLD OS 46... how old?

Bearings making noise and you didn't replace them - asking for trouble.

Loosing power after One Minute - once the engine warms up the compression is going - probably a bad ring in the cylinder.

Bottom Line - It's time to buy a new engine!

If you have a buddy with lots of spare parts - you MIGHT be able to resurect this old workhorse - but if it truely is OLD, perhaps a funeral is in order and get a replacement.

[Cougar429]

If the compression is still good once the engine is warmed up the likely culprit is a leaky front bearing. This leans out the air/fuel mixture.

I would second bbbair in that you should deal with the bad bearings before running the engine. Along with the leak I described, contamination from bearing bits can damage other internals in the motor and slop in the crank can wipe out a nice tight fit between the crank and nose housing, (required for good fuel/air delivery). If that happens replacing the bearings will not return the motor to full serviceability and if bad enough mean you've wiped the motor.

There are lots of other reasons for a performance drop in flight. Any air leak elsewhere in the motor housing can worsen with temp changes. One common point is the O-ring sealing the base of the carb.

Another place to check is if there are signs of burnt fuel/oil at the head/cylinder junction. This could indicate a leak between the two and may take nothing more than retightening the head bolts once warm.

However, without knowing the environment, condition and setup all this is a guess.

[DO335]

Originally Posted by Cougar429

If the compression is still good once the engine is warmed up the likely culprit is a leaky front bearing. This leans out the air/fuel mixture.

That's news to me.

The seal at the front of the crankshaft is created between the crankshaft and the front part of the crankcase. The bearing has nothing to do with it.

When you open the engine, you will find that both the front and the rear bearings have lots of open space in them between the balls which are held in a cage. Some front bearings have a seal on the outside surface. Some, including my HP 40 Gold Cup and Webra Blackhead 60 do not have any seal.

So by the logic presented to you in the quote, they would both run lean, which they do not do.

If the bearing happens to have a seal on the outside, that's to keep out foreign material. It will not seal against blow by or crankcase pressure.

You will also find a very small diameter hole or channel which goes from the intake to the rear of the front bearing. The purpose of this is to draw back fuel which may leak excessively out of the front housing.

In any case, the first place to start if you know that you have faulty bearings is to replace them or has been suggested, if the mill is just over the hill, replace. It is not cost effective to start buying pistons and cylinders if it's an ABC version. It might work out if it's ringed though but not usually.

If it's ABC, there is also the possibility that the liner plating is peeling off and that is contributing to your problem.

One last point - before all the experts start jumping all over me, the OS 46 SF came in both ABC and ringed versions.

[Cougar429]

I agree the seal is there to primarily prevent foreign material from entering the bearing, but what I meant in the original post is if the bearing is pooped the seal can no longer work effectively. The excessive crank play from a bad bearing, (or both) that slop consequently taking out that sealing surface between the crank and housing would be worsened by a leaky rubber style bearing seal. The steel guard type would really not be affected.

Perhaps I could have worded that better.

Some engines have the oil return channel behind the front bearing and in the case of the GMS and the Tower clones is the likely culprit for the motor run on problem. One solution suggested is to fill the groove with JB weld as there is still sufficient lube.

That's why I posted with no first hand knowledge of the motor, how it is set up or other factors such as fuel or external fuel system component condition, all this is just guessing.

[Florin]

"Bottom Line - It's time to buy a new engine!" - Hey I bought this engine in "used, good condition" in May 2011, and took it out to the field 2 or 3 times. First time, some really bad black "stuff" came out of the exhaust. Next two trips to the field it got clean but lost the power... doesn't seem like "good condition" to me.

As I don't have a friend with lots of spare parts, it's not worth investing money in brand new parts - I might spend too much and I rather just replace the engine, as advised by you guys.

Thanks for your replies.

[Flyinbullmax]

I think you should try changing the bearings, you could be surprised how good this little thing is still worth. Go to the nearest local bearing shop or at Rc Bearings and buy a nice fresh set of bearings and follow the instructions at this link: http://www.rcmodelreviews.com/bearings.shtml its pretty easy. Then after you did this, bring all ajustments back to base settings + a new glow plug and I'm sure this engine will prove my sayings.
Keep us up to date with it.
JP

[Cougar429]

Florin, if you are heading to the London Swap Meet and are willing to wait, bring the motor with you and I can at least give it a boo.

[Florin]

Thanks for all the answers, guys!

Cougar, I will be there and I'll bring it with me. Please PM me the info about how to meet you there.