I get a kick out of folks that remove muffler mutes.

The carburetor throat size/fuel draw ability is matched to the amount of pressure that the muffler can provide the fuel tank. Change any factor in that equation and it spells trouble.

Okay, you just ran your engine on the test bench with the muffler mute in place. Let's just say for kicks that you tached the engine running at 11,750 peak. Now you remove the mute from the muffler, put it all back together, start the engine, readjust the carb (yep, you changed the amount of muffler pressure going to the fuel tank - so you have to readjust the carb). This time, when you tach the engine at peak rpm, you get a 150 rpm increase. Wow! A 150 rpm increase in rpm for nothing!!!

Not really.

What? Not free! Really?

You take your model to the field, range check it, fuel it up and fire up the engine. Then you go through the nose up running check. Uh, oh. Something is wrong. The old needle setting of two or three clicks richer than peak isn't cutting it today. Your engine is too lean when you hold the nose up. Readjust the high speed needle another two or three turns richer. There! That did it.

You get permission to takeoff and your buddy places the model on the runway. Now you are ready for takeoff. You begin your roll, perform a little rudder correction for the slight cross wind and you're up and off the ground. But something is wrong. Your engine is way too rich. But that doesn't make sense. You just set it perfectly. You and your buddy look at each other with a what's up shrug of your shoulders.

You land the model successfull, walk over to it and lean the mixture one or two turns then takeoff again. Better, but now the engine is leaning out too much in the climbs. ***?

Well, there is the story in a nutshell. Some folks never connect the dots and realize that their engines carbs were coordinated with their mufflers, keeping the metering and fuel feed in the "choice" range with all of the components intact. Change one little thing and you are off into dangerous territory. I have seen fellows leaning their engines out, once de-muted, in order to get back those two or three hundred rpm they lost when they adjusted their needle valve to compensate for the lack of muffler pressure. These folks are the ones screaming about "junk" engines that are either burned up immediately after put into service, or are engines that are constantly dead-sticking. These folks were sold a bill of goods when their equally ignorant buddy mentioned the idea of getting that extra bit of rpm by de-muting their muffler.


Ed Cregger