Moki 180, now muffler pressure

[Bob Richards]

John,
 
I came to the same conclusion years ago. I was helping a guy with his engine, it would not hold a reliable needle setting. Hold the nose up and it would sag, even when set stinking rich when level. While I was holding the nose up one time, just to try something I stuck my thumb over one of the two exhaust tubes and the engine richened back up. So, I plugged one of the tubes, tuned the engine, and it ran fine in any attitude. No loss of rpm, in fact it gained about 100rpm. Slightly quieter as well. Go figure.
 
In our quest to make as much HP as possible, we always assume that opening the exhaust will always increase HP. This is NOT the case with most of our two strokes. I have had success with sport .40 sized engines by adding a simple after-muffler I designed. At least 6dB noise loss, no loss of power, improved needle valve consistency, and longer fuel run. I'm convinced that PROPER application of backpressure, NOT LESS backpressure, is very important with our two stroke engines. After all, a tuned pipe will have more AVERAGE backpressure than most mufflers.  A lot of this has to do with the way the cylinder ports are timed, however.
 
The real problem with noise limits is trying to convince club members that planes can be quiet and still perform. I had to use my tach to convince people that the engine was still running the same RPM after I added the muffler.
 
When it comes to noise, we are our own worst enemy.
 
Well, I did not mean to get so far off topic. I'll climb off my soapbox now. :-)
 
Bob R.


John Ferrell <johnferrell at earthlink.net> wrote:
I have proven to my satisfaction that Dick Hanson's advice that no Pitt's style muffler provides adequate fuel pressure. 
 
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