I have an Irvine .46 with a Pitts muffler and 11x5 APC prop in a Rambler 45. Probably have over six gallons of 15% through the engine but it starts and runs okay. Recently I noticed that the mid-range is quite lean compared to WOT and idle. To peak the mid-range, I am 1200 rpm rich on the high end!!! If I set the high end properly, the engine definitely sounds lean at mid throttle in the air and sags when I point the nose up. I think the problem has become worse as the engine aged because I definitely would have noticed this before. Any ideas what could be causing this and how to fix it?

If it has 2 exit ports, plug one and see if the extra back-pressure helps.

If wide open throttle is ok you probably need to richen the low speed needle a little as it controls idle through mid range rpm's.

+1`

Quote:

ORIGINAL: ThumbSkull

If it has 2 exit ports, plug one and see if the extra back-pressure helps.

Quote:

ORIGINAL: Delman

If wide open throttle is OK you probably need to richen the low-speed needle a little, as it controls idle through mid range RPM.

RC-Fan,


I support both of the statements above...

Does anyone who changed over to an aftermarket Pitts style muffler, ever find it to be 'trouble free'?


They are designed to take up a limited space and be installed inside the cowl...
But with the exception of the [link=http://www.jettengineering.com/accy/turbo.html]TurboJett[/link]; none of them seem to actually be designed, to allow an engine to run normally...

Although some modifications; as ThumbSkull implied, could (in some cases) achieve reasonably good results.

EDIT: Language.

Thanks for the suggestions. I'd had this problem on another plane with a Pitts muffler and had to plug one of the outlets. However, this one was "trouble-free" until recently. I'm loathe to mess with the idle as the engine responds perfectly when I go from idle to full throttle quickly. I'll try plugging one outlet first and see how that goes. I'll let you know what works as, when I did a search, I could not find anything helpful.

Well, blocking off one tube on the Pitts muffler solved the problem. Mixture is right on at all speeds. Had to lean out the high needle a lot, but the idle has not changed. Lost about 200 rpm at WOT but that's not a real problem.

[sm=thumbup.gif] I've been there before. Glad it helped.