I was flying my Spectre the other day OS 91 VR DF when the engine went lean in the air and quit. I normally run the aircraft rich so I tracked down the problem. It appears some dirt
had choked up the fuel filter. I guess it got in through the pitot tube of the fan as I fly of grass.
The engine now lacks compression. Is this one short lean run enough to damage the piston /liner ? I have tried to take the head off but can't loosen all the bolts. Now I have one of the bolts with a rounded head from the allen key . HELP ! How can I get this and the other bolts loose?
If I need to change the piston liner should I change the bearings as well? The engine has done about 40 flights. What would normal wear be for pistons and bearings?

Thanks

Hawkflyer.

Try heating it up and then use a standard hex driver or next size metric driver to loosen the enlarged bolt. The OS is known to take a good amount of lean run before seizing. You may have had little oil in your fuel. I used to use Wildcat in my BVM 91/96 which has 22% oil.

If the piston and liner are damaged, then change the bearing if they appear rusty or trun rough. The brg cost about $20 US. Towerhobbies has these engines for $280 or so, may be better to buy a new one!

A well treated OS 91 DF should last at least 100 flights. Run them as rich as you can for take off. I would also use pipe pressure if you can...Good luck!

I had one round out when the wrench slipped. I took a square drive bit from a multipurpose screwdriver and drove it into the bolt head. OS head bolts are sooo soft the bit drives into the bolt head easily and you can then take the bolt out with little trouble. Do youself a favor and get some hardened bolts to replace them with. Heat the bolt will never hurt either. Also another trick for getting steel bolts from aluminum threads is to take a punch the fits on top of the bolt head an smack it very sharply with a hammer. What happens is that the aluminum threads and steel bolts kinda weld themselves together via corrosion. The sharp rap with the hammer breaks this corrosion loose.

"Is this one short lean run enough to damage the piston /liner ?"

This is what one quick lean run did to an O.S. 91
It didn't have any compression, either..
JA

[X(]

You mean, eeh, some dirt is liable to destroy a whole piston / liner set ????

Ouch... I think I'll put one or two more fuel filters on my next setup ...

Thanks guys

I got it apart, the piston/ liner fit is very loose at the bottom end and the piston looks better
than the one in the photo but it has defintley been hot.
I run 22 % castor 5 % nitro fuel and run it as rich as I can on the ground so that it doesn,t
sag, go rich to much when the nose is held up.
I think I will invest in an inflight mixture control if I can fit it in somewhere.
I have got new piston/liner and bearings on the way.

Hawkflyer

Hi Guys,

Whilst we are on the subject of OS-91's, I read in another post to chuck the remote needle supplied with the engine, and invest is a BVM mixture control. I don't have any spare channels on my TX, but I bought this anyway, for a time when I can afford a bigger, better radio. Is the mixture control placed in the same position as the remote needle would be, and as I don't have the spare channel, is the mixture control just redundant, or is it going to be easier to use manually than the remote needle?

Tim.

timrob,

if you do not have a spare channel I would just use the remote needle supplied from os, it is just fine for that application. as far as using the os as an inflight, I would agree the bv would be the better choice.

Hi,

Hawkflyer, in relation to your particular problem an inflight mixture control would not have been any help. There are very few DF flyers in Australia running inflight mixture controls. They were popular a few years back when everyone wanted to go 200mph+ and wanted to screw every last bit of speed out of the aircraft. My suggestion would be to get rid of the fuel filter as any slight obstruction will cause a lean run with consequential damage. I filter my fuel through the pickup in the fuel container, again just befor it enters the pump and a third time after the pump. I have always run pitot pressure in a Ramtec fan and have flown from all different surfaces and have never had a problem.
Tim, personaly I would stick with the standard OS remote needle. I have seen guys have far more trouble tuning with the BVM needle than most with the OS.

regards,

Russell.