This is why we do runups

I noticed that the runup is pretty bad on my new plane. I started
leaning during taxi and it helped a little, but not as much as i thought it
would. Then I tried leaning for the taxi back, and flew again the next
morning and leaned again on taxi, big improvement. The one other partner
told me always to use two pumps of primer, even if it was just running five
minutes ago. I think they are drowning the bottom plugs, and then running
rich on taxi. The plane requires about two inches of leaning on the mixture
control to get it clean. I have yet to check the RPM rise before lean cutoff
to see if the idle mixture itself is too rich. We adjusted the mixture on
the IO-360 (on the bi-plane) and wow, what a difference in all around
performance! This one is an O-360 though.

Oh yeah, the roughness is about the same on either mag but goes totally
away when on both.
Suggestions?

Wayne


"Roger Long" om wrote in
message .. .
I think the most significant point is that the engine got steadily better
over an hour of subsequent cruise flight. Four hours later, it's back to
normal. If there had been an underlying cause, it should have gotten
worse.

The weather here in the east has been terrible. While those in LA and
similar environs might chuckle at what we call hot and humid, we still
adapt
to the conditions. Perhaps some other club members aren't as aggressive
about leaning as they should be and it hasn't been a problem until this
long
muggy stretch.
--
Roger Long

The only other things I can think of are dirty fuel, leaking valves, or
perhaps the wrong type of spark plugs was installed.