Additionally, I find that at WOT the fuel distribution, especially on the small Continentals, tends to be quite uneven as evidenced by uneven EGTs and uneven CHTs. ?I find that even a 1/8" to 1/4" reduction in Manifold Pressure makes a big difference in evening out the fuel distribution between the cylinders as the throttle plate gets slightly in the way of the air flow and causes better mixing of the fuel by disturbing the air flow.
FWIW, the MA-3 and MA-4 series carbs have a backsuction "economizer" jet that leans the mixture at less than full throttle by using the difference between the pressure at the venturi vs the ambient pressure. The MA-4-5 and larger Marvel Schebler carbs use an economizer valve that is pushed off it's seat at full throttle operations to enrich the mixture for cooling purposes at full throttle, and conversely leans the mixture at less than full throttle. For those with an interest, minutia level details on the operation can be found at <http://www.insightavionics.com/pdf%20files/MA-4%20Carb%20Manual.pdf> -Jeff Scott Los Alamos, NM > ----- Original Message ----- > From: brian.kraut at eamanufacturing.com > Sent: 01/27/14 01:17 PM > To: KRnet > Subject: Re: KR> now WOT > > I used to always fly wide open also on cross country. WOT at 8,000' is > about 75% power and I am always 8-13 thousand feet. Since I got the M2 > with the fuel flow meter I have changed this. I have found that if I > pull back to 19" manifold pressure (fixed pitch prop) which gives me > 50-60% power depending on what my altitude is that I get much better > range and economy. I only decrease my speed from 191 MPH to about 180, > but my fuel burn goes from somewhere over 8 GPH down to 6.25. This also > many times gives me one less fuel stop which makes for even more fuel > and time savings than going up and down one more time. > > > One thing that I have found on my plane with an MA4-PA carb is that the > last inch of throttle gives me a much higher fuel burn even with the > mixture pulled back at just LOP. The experts tell me that there is an > enrichment valve in the carb that kicks in for the last bit of throttle > to cool the engine on climbs and the extra fuel does not atomize all the > way. But aside from this there is still a big difference by decreasing > power and it is due to the much higher drag at the higher speeds.
