Newsgroups: rec.models.rc
Subject: Re: Responses from Engine troubles
From: kaiser@elec.canterbury.ac.nz (Chris Kaiser)
Date: 19 Aug 92 10:46:51 +1200
Distribution: world
Organization: Electrical Engineering, University of Canterbury, New Zealand

In article <1992Aug18.030343.20950@c3177208.ssr.hp.com>, bill@c3177208.ssr.hp.com (Bill Chidester) writes:
> jam@ee.uwm.edu (Jeff McKelvey) writes:
> : 
> : I recently posted a question about a problem I had with an OS SF .25.
> : It would run fine on the ground but after 3-5 mins in the air it would
> : suffer a severe loss of power and RPM.  Here is a brief list of the
> : possible solutions I recieved:
> : 
>   (stuff deleted...)
> 
> : noticed that the engine reaches peak RPM when the needle valve is about
> : 6 turns out, from the responses I've been getting and from the manual
> : this seems like a lot, is it?  Should it be closer to 2 or 3?
> : It's been raining here all week so I don't know how the engine works in
> : the air yet but, as usual, runs great in my parking lot.
> : 
> : Thanks again,
> : Jeff
> : 
> : 
> 
>    You still have a whopper of an air leak and I'll bet I know where it is.
>    Every O/S engine I have seen (well, every two-cycle engine) leaks around 
>    the needle valve.

Yeah? If this is true it can't be a serious problem, since OS's are quite popular over
here but I've never come across one that wouldn't run properly with the standard needle
value set up. However, the OS 46 SF in particular has an o-ring seal around the needle
value, which seems to indicate that at least on their more expensive engines they consider
it worthwhile to seal it.

>    Our standard fix is to put a short (1/4 to 3/8") section 
>    of medium or large fuel tubing over the needle valve, where the needle 
>    valve screws into the needle valve housing.

I might try this on my Magnum 25 (which runs quite well as is) to see if it makes any
improvement.

>    Most O/S engines will go from rich to lean between 2.5 to 1.5 turns and 
>    should go from lean to rich in one turn.

I concur with this, 6 turns sounds like heaps too much - the needle value must be almost
falling off the end of the thread! There might be some sort of fuel blockage, perhaps a
blocked fuel filter or some such thing. I used to use fuel filters on all my engines,
but after some problems getting consistant runs I removed them. I now just filter the
fuel going into the tank, and haven't had any problems since.

>    One of the most perplexing engine problems we've seen recently (our flying
>    club is 'we', I doubt they'll ever elect me Pope) was an OS .40FP that
>    behaved exactly as you described.  Ran on the ground, but no more that 3
>    minutes in the air.  It was noted that the needle valve was very "touchy"
>    and sensitive to sideways pressure.  A sure indication of a bent needle. 
>    The valve looked good, but was replaced anyway, to no avail.  Finally, it
>    was found that the needle valve was fine, but the casing had been bent
>    during an unexpected density transition (from air to Earth).  A new needle
>    valve casing solved the problem.

I had this happen to my Magnum 25, only in my case it was obvious that the whole needle
valve assembly was bent! However the motor still ran enough to fly (as long as I didn't
go inverted!) so I ignored it for a while. When I eventually got around to replacing it
the motor ran great.

Chris Kaiser

Postgrad, Elec. Eng. Dept.
University of Canterbury
Christchurch, NEW ZEALAND

Newsgroups: rec.models.rc