Hi,
I just bought an OS70 Surpass, which had obviously done a lot of running. I removed the head and cleaned out the inlet and exhaust passages, ground in both valves, re-set the tappets and started the engine. The compression is fine.
I runs fine, but when ticking over, it suddenly stops. This can happen even on a fast tickover. It makes a very slight pop sound, then slows down and stops, in about 2 or 3 seconds.
Any ideas as to the cause/remedy please?
Also what's the best way to adjust the slow-running needle on a 4-stroke. What are the symptoms of too lean/too rich?
Thank you.
Jack

G'day

I used to have a Surpass 70 but it has been gone for several years. When I bought it it, like yours, was well used. Its low speed needle was tuned too lean and as a result it would not start unless the throttle was wide open. I opened it about a full turn and it would then start. Then I tuned it by the method that I use on all my Saitos (most of my four strokes are Saitos though I also have some OS, Enya and Laser)

1. Start by opening the LSN buy a turn or so. This is not critical but it will make sure it is at least going to start. Too lean and it will simply not start.

2. Tune the HSN for maximum revs. Don't go lean though.

3. Slow the engine to the point where it will idle. This may be quite a fast idle.

4. Adjust the LSN for the best idle you can get but also test the transition. Good transition is more important than the absolute smoothest idle.

5. Repeat steps 2 to 4 until you get the idle you want.

If you can't get a good slow stable idle try some fresher fuel. Nothing mucks up idle better than stale or watery fuel. Also, a fresh glow plug may also help.

You can also try another trick. When the engine is idling, remove the glow plug. If the revs drop, it is probably tuned too rich. When it is tuned correctly, the drop will be minimal if the plug is good and the fuel is good.

You should use an OS F plug or other F type plug. The Enya #3 plug also works well in four strokes. Enya now also make an F type plug.

Hope this helps a bit.

My OS Surpass 70 was never great at idle and I eventually set it up with a single NiCd battery to keep the plug glowing all the time. I have since learned a great deal more about tuning and could probably tune it better these days. I use 10% nitro in all my engines apart from one. It is a Saito 90R3 radial engine and it likes to drop one cylinder on 10% fuel. On 20% fuel it is much happier so that is what I feed it.

Cheers

Mike in Oz

Hi Mike,
Thanks for your useful information. The engine is running a lot better now, starting is now very easy as well.
I think some new fuel will be the complete solution.
Happy New Year
Jack

You got good advice there, I would just add that you should not neglect backing off the HS needle a bit after achieving max rpm. I got sloppy about that and my usually dead-reliable OS 70 cut out on take-off and I had an awful crash. I think it was on the edge of lean, and then in a nose up attitude it got a bit leaner. My impression from playing with the engine afterward on the test stand is that there is a fine point in the needle setting where you go a little leaner and the engine just quits. It protects the engine from lean runs, I suppose, but it can be hell on airplanes!

But as long as it is adjusted properly it is a really sweet engine.

Jim

Hi!
Did you check the ball bearings!?

Thanks for that. I usually am fairly careful with engine settings. This engine was quite old, but new to me. I think it's sorted now.
The bearings are also very old, but there's no play.
Jack