Low Speed/Idle Screw - Jams up throttle lever

[Chris Anthony]

Folks,

I wonder if anyone would be able to describe their process of adjusting the low speed/idle mixture adjustment screw.

My current method of tuning the idle mixture has been to screw the needle all the way in (with throttle set to 0% on sticks - idle) and then out a set amount, currently 1 turn (360*). From there, I will test, set idle (for 30 seconds) and ram on throttle, see what happens. If it stutters but picks up, too rich, if it cuts, too lean, and I adjust accordingly.

But after a few flights, I have found that the screw is moving itself, and the mixture is changing as a consequence. I checked the mixture screw position tonight after having set it originally at 1 full turn out from fully closed, and I then found it at 2 full turns. It was then that I realised that the mixture screw can actually impede the throttle movement, for example, if it is screwed in all the way - the throttle will jam and not turn when actuated by the control wire/servo. Is this normal, and how do you guys account for this when tuning?

The main worry I have is why the adjutsment needle has changed position.

The engine is an SC52 2s nitro btw, only 2 weeks old.

Thanks,

Chris

[David Davis]

You wouldn't be confusing the mixture screw with the throttle stop screw would you?

I can think of no reason why the throttle would jam if you were to screw the mixture screw in fully.

[Chris Anthony]

It is possible. I am screwing the small flat-headed screw that turns along the same axis that the throttle horn pivots around. This screw is seated basically inside the moving throttle part. The engine does seem to respond to changes in this screw's position; in line with the behaviour expected with normal idle screw adjustment that I have read about.

There is another external screw sticking out of the carb, with a load of threads protruding with the head, and a locknut on these threads. I have presumed that this is the throttle stop screw, and I haven't touched it. But it's very likely I am wrong.

[Peter Miller]

The slow running or idle adjusting screw is in the hole in the middle of the throttle arm. You are definitely playing with the throttle stop screw.

Go to the Parkins website, look at the SC 52A and open the Spar Parts list. THis is a PDF file and is an exploded view of the engine and carb. You will see the isdle screw in that draiwng

[Prop Nut]

That should be J Perkins website.

[Peter Miller]

Posted by Prop Nut on 16/05/2014 08:33:00:

That should be J Perkins website.

Yes of course. Well, it is early for me!!! My brain is only firing on two cylinders

[Chris Anthony]

Thanks chaps, I've checked out the J Perkins site, the diagram is great. Here is my engine:

**LINK**



I'm afraid my descriptions may not have been very good. The screw I am adjusting, assuming it to be the idle mix screw is 46844F, housed in the throttle arm as you describe Peter. What I have assumed is the throttle stop screw is 46825F. Is this correct?

Edited By Chris Anthony on 16/05/2014 10:34:13

[Peter Miller]

Yes, you have the right screw for idle adjustment. and the right one for throttle stop.

Right, now close the throttle to leave just a tiny opening and blow though the fuel line. There should be a slight hiss of air. If that is the casxe, then I cannot see how the idel screw could cause the throttle to jam.

Have you taken the screw out? There should be an "O" ring on it. If that is missing it will allow the screw to move.

[Willyuk]

I had the same on an irvine 46. When the idle screw was in too far, the throttle body jammed. The solution was to use the pin trick. I.e. Put a pin in the carb to stop the hole closing too far and blow through the line, open idle screw until air comes out. After that it was fine , by the way it was opened far more than the std 2 turns. No idea why, I have stripped and cleaned the carb but if the idle screw is in too far it jams. It works just fine now after setting the jet to allow the air through.