os .91 4 stroke
 

Ok here we go guys I picked up a older plane with a os 91 4 stroke. It runs good but I have to turn the high speed needle until it stops to get a nice high rpm. What does that mean is wrong?I can run it if I back it of a 1/4 turn from there but don't have the top end it should have? What does this mean about this motor?

Some of the older engines(pre Surpass) liked about 4 turns on the high speed needle. Too lean and they would/will backfire and spit the prop off!

It ran great at the setting i had.. just seemed like it would be to warm but it ran nice. Just never had a engine run with with the needle that far in.

Fairly common problem. The seat for the high speed needle wears. If the idle is good I'd fly it. At some point you'll be needing a carb or parts to make it the way it should be. I've run a couple for a long time that way.

I had the same problem with a 70 Surpass. It's not too hard to fix it.

When you wind the needle right in the knurled part bottoms out on the needle carrier rather than the valve seat. Remove the needle then the carrier from the carb. Carefully file down the carrier where the knurled part of the needle touches it so that the needle can be adjusted further in. Use a fine tooth file and make sure you clean the carrier thoroughly before re-assembling. File it enough that the needle bottoms out on the seat with a small gap between the knurled section and the carrier.

Re-assemble and you're done.

Dave H

Thanks guys that helped. I did fly It 2 times it fly great just a little less power because it had to be a little rich.

I pulled the high speed needle holder off my pre Surpass 90. I took a few thousands off the end to shorten it on my belt sander. Now the needle doesn't bottom against the housing. Worked great! Thanks, Gerry.

It's Dave, but no problem, glad I could help. Belt sander was a good idea.

I wonder if rcworld fixed his or if he's still running rich.

Dave H

I will try that cl

I could guess Bill for William or Dick from Richard but Dave from gerryndennis is a new one for me. Hehehe. Glad to make your aquantence.

Likewise, I can't claim that's the best screen name out there but Geraldine and Dennis were my two much loved cats. They were both fascinated with model aeroplanes and would insist on sitting in the middle of any plan I had out, or jumping on my lap just as I had both hands full with glued balsa, or walking over freshly layed down wing panels, chewing on servo leads etc.

I miss them both but at least I have lovely reminders every time I open a plan and see muddy paw prints and holes from their claws.

Cheers,

Dave H