asp 91 stroke throwing the prop
 

hi.
I bought new my asp 91, installed it with a 14x8 prop. ran ok. second time ran for about a min and threw the prop. this is with a lock nut, the woodroffe key in place, exhaust ports ok, and a suitable spinner. this now happened three times. running on 10 per cent nitro, always been ok before. prop drive in good shape, we are all out of ideas here, any thoughts please.

mike

This is probably a mixture problem causing the engine to backfire and throwing the prop. What does your smoke trail look like?

Most probably running it to lean. Could also be that you have to high a nitro content, try some 5 or 10 % nitro with at least 18% oil content.

Could be you're not getting the prop tight enough. You should leave a mark in the hand that holds the prop.
are you using a wood prop? They like to loosen

thanks all. will try running it a lot richer and see how that goes

Many people do not tighten the props enough. I have several four stroke 91s. One of them fires backwards often, but never throws the prop.

ASP is a great company, I had a .91 I bought used and had many good flights on it.
One day I was running her up on the taxi way when at full throttle she locked up braking off the spinner.
I boxed it up in a OS box since I didn't have the ASP box, included a note with explanation including type of fuel and prop I used and sent it off.
Engine came back repaired like new, invoice listed all parts used and the charge was $0.00, engine ran great.
Thanx ASP.

I don't know your specific procedures but here are some points of a long time model flier.

1.Any time a wood prop sets up for a couple days all tightened up, and you fail to tighten the shaft nuts, you have a very good chance of throwing a prop.
2. Non-wood props usually make it a bit longer before the prop comes undone. :confused:
3. Just yesterday, I went out and sent a Magnum .60 into the blue after said Magnum had been on the shelf a month+. Now I well know better than doing such without tightening the prop. Yep, prop, spinner and hardware are all somewhere on a 50 acre flying site. Whose fault? Saw him just today in the mirror! :p DUMMY!
4. Any spinner plate definitely needs a sandpaper disc between the thrust plate and any spinner plate. Without such, short flight and/or start-up.
5. Lots of things are not in the owner's manual.
6. Lock nuts will never replace the emery cloth (sandpaper) between the prop shaft driver and the spinner-back plate, along with keeping mind on what you need to do and/or setting up while B-Sing with the other fliers around you. Don't ask me why I say That!!!! :mad:

The absolute best thing for adding friction to keep the prop on is open mesh plumber's sandcloth or the open mesh drywall sanding sheets. Both are very course and are equally rough on both sides. I don't bother with it if the prop is going directly against the metal drive washer of the engine or against an aluminum backplate that is textured to grip the prop, but it is mandatory on a plastic backplate.

Hi Mike, If you are still experiencing the problem and if that suitable spinner included a plastic back plate then perhaps its time to upgrade to at least a spinner with an aluminum back plate. Plastic spinners especially on engines that large and up tend to give the fellows all sorts of fits and spitting props off is one of them.

John

I've never had the problem your describing with any of my 4-stks, not my Saito 180, YS 91, ASP 91, OS 120 or others.
Could the problem be caused by too large a gap on the intake valve rocker arm ?

It's generally caused by the engine being too lean. There is a point in the tune you can't even hear it doing it but it will loosen up the prop nut, even two nuts. I have a number of the older YS 120s that require you to balance the tune with just the high speed needle then the brass screw in the center of the regulator. It's a true balancing act. I have had them running what I would have thought perfectly on the ground and then on take off had them back fire. Just a little pop. That little pop once bent the aluminum spinner at the prop grooves and loosened up the spinner, spinner adapter and prop nut.
There are a lot of reasons for the prop to loosen up but it us usually just a little lean and you may never hear the back fire.
I saw the wood prop mentioned and they really do compress and loosen up. On my gas engines that I use wood props on I loosen the prop nut after a day of flying so I have to tighten them back up every time I use those planes. Carlos, you have been very lucky with your engines. I have had one really close call with a prop coming off an engine I was working on. Scared me enough to give me a lasting respect for what they can do.