I have Evolution .46 engine mounted sidways with a pitts muffler. My problem is I can get the engine to run nice in idle and high speed so long as the plane is horizontal. As soon as I go to the vertical, the engine dies when in high speed. In idle it's good and I can get a good low speed idle and transition. I suspect I'm not geting enough pressure from the muffler to pressure the tank enough when holding the plane vertical. I've checke to make sure the carb is pretty much at the same level as the fuel tank and replaced all fuel lines and the fuel tank to make sure it wasn't any of those. Fuel system passes the pressure leak test no problem. Any ideas on what it is or how to further troubleshoot problem. My last resort is to mount engine normally and use a normal side mount muffler, but I'm trying to avoid haveing the muffler sticking out of the side of the cowl.[:@]

Have you tried to RICHEN the high end ?????

Yep.... no amount of hig speed needle adjust seems to help.

Miklos,


Pitts type mufflers have a tendency to reduce the fuel pressure to significantly lower levels, compared with the standard muffler, with an exhaust pressurised fuel-tank.

It makes itself known when the engine leans out, as you raise the nose.

It has been suggested in past threads, to crimp, or to plug-up one of the Pitts muffler's down-pipes.
This usually increases the pressure sufficiently.

The 'nose straight-up' test must only be passed at full power.
This, because under no possible flight condition will the engine need to overcome gravity, to siphon fuel straight-up, when at idle.

Miklos,
I've run into similar problems with the Slimline brand as well as the ones I've made myself. What I like to do is get some door screen material (the metal kind-stainless preferably) and mold a few layers (2-3) around the tip of my finger. Repeat process again molding the screen small enough to snugly fit into the exhaust tubes. I then take a pin drill (1/32" should be ok) and drill a hole clean through the exhaust tubes about 3/4" from the exit openings. Insert the molded screen "cups" into the exhaust tubes and pin into place with a pin from the wife's sewing kit. Push the pin through, bend over the excess, snip off point. Done.
This will increase the back pressure just enough and take the crackle out of the exhaust sound.
I'll try to get some pics posted.

I pulled out my Spit when I got home. Here are some pics showing a nice neat way to do it. This muffler works great!



I suppose you could 'cap' the outside of the tubes with the screen but that might look a bit hokey.

Thanks guys. This pretty much confirms my suspicions about low pressure. I will try both suggestions as soon as I get a chance. I'm hopeing not to have to mount the engine upright and avoid that nasty muffler sticking out the side. Will keep you posted.

Hi Jazzy,
That is the neatest idea I have seen for increasing back pressure with the Slimline pitts.
I'm hoping not to have to do it on my extra 300 (Goldberg BIY), the difference there is that I have the long extensions (9") on mine which will tend to increase back pressure. I've run about 6 tanks through the G91 ST and it seems fine, but now I know how I will increase the back pressur if needed.
Thanks,
Pete

Thanks Pete!

I've used SS scrubbers from the hardware store too. These look like crumpled up balls of tight steel coils.
I like this idea because the expansion chamber volume is extended to the screens (first point of any real resistance) which are near the outlet of the exhaust tubes.
I have the Slimline pitts muff. for an OS 1.08 on a Magnum 1.08. (I had to make an adapter/mounting plate.) The expansion chamber is decent sized but there is a noticeable back pressure problem there too. I put 3 to 3 1/2" neoprene extensions on the exhaust tubes but they don't do anything for back pressure. The neoprene does however help soften the sound of the 1.08 by dampening some of the vibration and 'ringing' in the muff.

You may need to double-pin each side on your .90.

Good Luck,
Jeff