OS .55 dying when trying to take off.

by Nathan Stanfield
(Anchorage, AK)

I am running a new OS .55 AX side mounted with a pitts muffler on a Great Planes Revolver.

After a good break in last week the engine is running fine on the stand, but as soon as I put it on the ground it starts to run a little slower.

Taxi out to the runway is usually good, but as soon as I start to give it throttle it dies.

I have it set just rich enough so that it does not lose gobbs of power when its pointed strait up, but I still feel like it is running rich.

Could it be that I am not getting enough back pressure with the pitts muffler?

I was thinking about pinching one of the two pipes off to give more pressure. The high speed needle seems to be out about 4 turns.

After about an hour I got it to run decent for about four minutes and then had a dead stick landing and that broke my prop. I almost kicked the airplane.

Any thoughts?

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OS .55 dying when trying to take off.

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Oct 19, 2009
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Pitts Muffler Problems
by: Simon

Hi Nathan,

Your problems are entirely due to the Pitts Muffler you are using and the fact that they do not generate the back pressure required, when compared to the stock muffler, in order to pressurise the fuel tank to the correct level. This is confirmed by the fact you are running with the needle valve turned out 4 turns, which is necessary to try and make up for the loss of fuel pressure.

I can fully sympathise with your frustrations as I have just spent months going through the same problems with an OS 91FX fitted with a Pitts muffler on my Ultimate Biplane. After consulting many forums on the subject, I eventually cured it by fitting lengths of neoprene exhaust tubing to the Pitts Muffler outlets and then inserting hardwood dowels into the ends of the tubing. Each dowel was drilled out in the centre to match the total outlet area of the standard muffler. This immediately restored full power and flexibility to the engine and bought the needle valve back to 2 turns open.

You can also block one of the outlets off altogether and just experiment with differing backpressures by varying the outlet size in the other. I have found this to be the easiest route to take; just increasing the outlet size by 0.5mm can result in a difference of several hundred rpm.

Hope this helps you in your endeavours, if not you may just have to bite the bullet and go back to the stock muffler.

Regards - Simon

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Oct 19, 2009
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Submitted by: Simon

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Hi Nathan, Your problems are entirely due to the Pitts Muffler you are using and the fact that they do not generate the back pressure required, when compared to the stock muffler, in order to pressurise the fuel tank to the correct level. This is confirmed by the fact you are running with the needle valve turned out 4 turns, which is necessary to try and make up for the loss of fuel pressure. I can fully sympathise with your frustrations as I have just spent months going through the same problems with an OS 91FX fitted with a Pitts muffler on my Ultimate Biplane. After consulting many forums on the subject, I eventually cured it by fitting lengths of neoprene exhaust tubing to the Pitts Muffler outlets and then inserting hardwood dowels into the ends of the tubing. Each dowel was drilled out in the centre to match the total outlet area of the standard muffler. This immediately restored full power and flexibility to the engine and bought the needle valve back to 2 turns open. You can also block one of the outlets off altogether and just experiment with differing backpressures by varying the outlet size in the other. I have found this to be the easiest route to take; just increasing the outlet size by 0.5mm can result in a difference of several hundred rpm. Hope this helps you in your endeavours, if not you may just have to bite the bullet and go back to the stock muffler. Regards - Simon

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Oct 13, 2009
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55 adjustment
by: Anonymous

It is easy to set the 55 way too rich, especially the low speed needle. You should check the low speed needle is less than 1 turn open, and the high speed about 2-3 turns.

Sep 12, 2009
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put stock muffler back on?
by: Matt

Is there any way you can put the stock muffler back on and see if it flies? Maybe leave the cowling off?

Or would that require modification?

That would be a good way to rule out the pitts muffler as the problem...

Just a suggestion.

Matt

Sep 11, 2009
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Posibly air
by: Conde

Hi i am tinking your engine is having troublems whit the bubles in the fuel tank or in the hose.

Try to check dis ting probabli for the bibration or check your carb i tink 4 turns open is to much.

I hope you can fix your problem becouse i know is terrible this caind of tings and be carfuly wen you fly make shure you have enough altitud for a safty landing if your engine dies.

If you can fix your problem please tel uss how did you fixed.