Bisson OS .91 pitts muffler issues

[JBjr]

Hello folks.

I'm posting this to poll you all and see if some of you are having issues with our pitts muffler for the OS .91. We've been making the pitts muffler the same way for many years now with no major issues. We recently had a customer send us information that they were having issues getting the motor to run properly with our muffler. If any of you are having any issues getting your motor to run with our pitts muffler on it, please contact us at sales@bissonmufflers.com or reply here so we can address it for you.

It seems since the new AX line came out there have been different/more problems running the motors with our mufflers. Any input or feedback you can provide us will help us in resolving any of these issues.

Thank you,

James Bisson Jr.

[Sevans16]

I have had great luck with the Bisson on 2 of my OS 120 AX's. I had tried several mufflers and thought the 120AXengine was a bum till I got a Bisson for it.

[JBjr]

	Quote: Originally Posted by Sevans16
	I have had great luck with the Bisson on 2 of my OS 120 AX's. I had tried several mufflers and thought the 120AXengine was a bum till I got a Bisson for it.

Thanks for the feedback Sevans. We had to address issues with the 1.60 over a year ago and started putting tailpipes with smaller openings in them. This has since resolved those issues and we haven't had any more problems.

[sdawes]

Hi James,

There is indeed an issue with running the pitts muffler on the OS. 91 FX engine. I have recently fitted the BCMPOS91/70 muffler to my 91 FX which is installed in a Black Horse Ultimate Biplane. The engine, along with the stock OS muffler, has been previously installed and flown in another airframe and performed flawlessly. The Ultimate/Pitts muffler combination would not allow peak power to be obtained, maximum of 9400 rpm on a 13.5 x 8 Bolly Clubman prop, or 13 x 9 APC, instead of the 10500+ I would normally expect. The engine would not respond positively to the needle settings, needing 3 turns open minimum to get anything like a decent run. In flight the engine hunted all the time and generally lacked any sort of performance.

Thinking that lack of back pressure to the tank was the problem I tried plugging up one of the exhaust outlets. Immediately the engine sounded on song and gave full power, 10700rpm recorded now, is very responsive to tuning and pick up is instant. Apparently the OS 91 FX needs a high level of tank pressure to deliver the high performance it is rated at, and this exceeds what the pitts muffler can deliver in its current form. My suggestion would be to offer some after fit restrictors with the muffler that can be used to tune the back pressure as required. In the case of the OS 91 FX the standard muffler has a single 8mm internal diameter outlet. By my calculations this would require two restrictors with an internal diameter of 5.7mm to give a similar backpressure. I haven't tried this out as yet and if I had access to a suitable pressure gauge I could be more precise in my findings, until then it will be a case of trial and error until I get the backpressure in tune with the engine requirements.

Hope my findings are of use.

Simon Dawes

[jamesrxx951]

i have a .91fx in a sundowner 50 and started off with a bisson pitts muffler. i could never get it to tune for high rpm either. it just never ran right. a fellow club member was helping me with the issue and he said that pitts never work real well with these 2 st glows. i put the stock muffler on and it ran fine. i wanted to put the pitts back on so was going to buy a fuel pump for it so i could use it because we felt we were not getting the full tank pressure i needed to work. i havn't even flown the plane yet this year because i have been to lazy to order a perry pump to stick on it