Got about two hours on a new OS 35 AX engine on a Tiger 2 and really like the engine. It has a stagger though after idle and going to mid-range of throttle, as if it is loading up with too much fuel. Tried leaning the idle mixture a lot but it doesn't affect the stagger. The engine normally flies fine, but after a long taxi it might stagger and die just when you are ready to take-off. I have been running it on the rich side to break it in well, but have never had this problem with dozens of other OS engines. Any ideas on resolving the stagger?

By the way, I am using the pressure tap off the muffler to pressurize the tank. No fuel filters. Have backed out the main needle valve to clear any possible dirt. All ideas appreciated.

It really does sound like the low speed needle. Keep leaning the low speed needle in 1/8 turn increments.

You've tuned and flown other O.S. Max engines, and you say you've done your best to dial in the low speed needle. Let's assume it's not the low speed needle setting itself.

If you're not using an A3 plug now, try a hotter glow plug like the O.S. Max A3 or the F-type 4-stroke plug.

If you are using an A3 plug now, try an idle bar glow plug like the Fox RC Long plug.

If a change in glow plugs doesn't improve things, you might try a different fuel mixture with slightly lower oil content. You could add a little bit of straight methanol to your existing fuel, or try a different commercial blend with slightly lower oil content. Too much oil content can cause the transition problem you're describing, too.

It sounds like it is loading up with fuel in the crankcase. Leaning the low end would help. My goal is always use the 25 second period of idle before giving it throttle to check acceleration. I don't do pinch tests. If you can get it to idle 25-30 seconds and transition to full throttle you should be good to go. This takes fiddling with the low speed needle settings until you get it right where it should be. Some people are happy with just getting in the ball park.

I'm not that familiar with this engine, but tuning it the best you can and trying out an A3 plug wouldn't hurt. I don't know if it is too hot for that engine.

Guys,


If new, there no longer is an A3 plug from OS...

Although it is virtually the same, as far as its heat-range is concerned; the nomenclature is now #6.


As far as OS is concerned, the #6 is designed for slightly smaller engines, up to .32 cid (plus the .40LA).
Running the US norm 15% nitro further stresses this (but it could be more tolerable, if you're using 5% in Thailand).

Yours seems a problem that should be solved by adjustment; rather than by component replacement.


If 'the 25-30 second idle -> full test' Barry suggested shows it is loading up, keep leaning the low-end until it does not.
If off-idle perfection causes mid-throttle fuel starvation, the fuel metering-valve might need to be modified to provide a slightly 'fatter' mid-range.


In any case, ThaiLazer, running your engine any richer than it needs to be (you are way past break-in), is not insurance and will not extend its service-life expectancy.
But it could cut short the life of your whole model, by causing the engine to conk-out when it must not...

The fuel needles are your engine's immediate ignition-timing controls, with all other criteria unchanged.
Allowing the engine to load-up at low-throttle, is just a 'fringe benefit'...

Thanks everyone for the suggestions! I'll try some of those ideas provided and report later on what ends up working.

After 30 or so more flights, I've got the engine running great. Most posters hit it on the head to adjust the low speed idle mixture, and I did indeed have to make small 1/8 turn adjustments. Now I have a very very slow idle with no loading up. This is turning out to me my favorite OS engine so far. Thanks guys for all the suggestions.

Quote:

ORIGINAL: thailazer

...Most posters hit it on the head to adjust the low speed idle mixture.

Thailazer,


It is important that you understand this.

The low-speed (not just 'idle') needle is your engine important and primary fuel control, since it determines the nature of your engine's behaviour, over ~80% of its entire working range.

The idle mixture is just a very small part of this needle's realm... the salt on the food, if you will.
So just call them "low-speed mixture" and low-speed needle.