Full throttle fuel flow is determined by only two things - the diameter of the main metering jet and the pressure drop across it. It is impractical to mess with the pressure drop, so the only way to change the mixture is by changing (enlarging) the metering orifice. Removing the brass orifice is not very difficult - just remove the cap on the opposite end of the mixture control. It just slides out, but make sure it gets put back in with the same orientation. The orifice is off center and the mating port gets rotated by the mixture control, closing off the orifice. Pretty simple, really.
But first you have to know how much you would want it enlarged. The mixture at takeoff should be noticeably richer than best power - if you lean at full throttle on the ground you
should see a slight rpm increase. And yes, the EGT should be at least 150 ROP. But what is peak EGT on the ground? Do you want to run it in that condition long enough to find out? You might want to richen in increments of 5 percent or less, and therein is the rub. To richen the mixture 5 percent the diameter must be increased by only 2.5 percent. If you are really lucky you might have a drill just that size - I did - but most likely you will have to take it to a shop that has reamers that can open it up. The other thing to try if you like is to put a radius at the leading edge of the orifice, which has a chamfer already. A very slight "break" of the sharp edge will increase the fuel flow a couple of percent. Naturally, you have to keep everything perfectly clean. The good thing is that you can test it without flying. After an attempt or two you will either have what you want or will be
ready to send it off to an "expert" for the adjustment. You've "scrapped" the metering orifice, but they're not very expensive.
The point is that for minor increases in fuel flow you can make slight changes without much trouble. If you overshoot you are back to plan B. The good news is that once you have it right it never has to be adjusted again, unlike on the Continental system.
Gary Casey
I believe my fuel servo is not delivering sufficient fuel to the engine in the takeoff position. Why I say this is because the difference between the peak EGT and the takeoff EGT is 70 degrees F on the leanest cylinder and 120 on the richest. The book says this spread should be anywhere between 150 and 200 degrees F. Why do I care you may ask.. Well my
CHTs are too high at full throttle takeoff. I have to work to keep them below 430. I have done everything possible to maximize cooling airflow with little improvement in the CHT situation.
Anyhow, I would be interested in information from the group on the following
Like to compare fuel flows at full throttle at close to sea level (mine is between 15 and 16 gals/hr)
Any info concerning experience with enriching the takeoff flow on the bendix style RSA-5AD1 servo (this setting is internal, I have found no reference material on the procedure)
Thanks
Tom