On Oct 12, 8:03 pm, wrote:

We've put new points and condensers in both, including internally (and
of course externally) timing them. No change. I've done full-power mag checks
and misfires would be apparent in that case. New plugs make no change, either.

One other possibility: A bad plug lead or two. They can be open
(arcing internally) or shorted (arcing to the coax). They often won't
show up on mag checks but will, like a bad resistor in a plug, cause
some roughness if trhe mags aren't timed pretty close together. The
firing of the early mag can create enough pressure rise that the
second has trouble firing its plugs. I would doubt that yours are that
far apart.


: If the carb is a Marvel Schebler/Precision Airmotive unit, have
: the ADs been done with regard to the two-piece venturi and fuel
: nozzle?

Venturi AD's checked and CW. Restriction or mixture would show up as low
MP or abnormal EGTs anyway, no? Neither is the present.

The two-piece venturi had the risk of losing its secondary
venturi into the engine. The AD-mandated one-piece has squared-off
support arms that disrupt airflow around the nozzle and a second AD
required a new nozzle to try to fix this. The new nozzle made things
worse on some engines, so a third AD allowed the reinstallation of the
old venturis and nozzles, to be checked every 100 hours for looseness.
Except that Precision had recalled all the old parts and crunched
them. I know of one rebuilder that dresses the secondary support arms
and achieves better running. EGTs won't show anything, since it's
random turbulence around the venturi that screws up the atomization.
My 60-year-old Bendix/Stromberg is a better carb than any of
these MA-3s or -4s. What does all the money get us? Maybe Tempest will
fix these problems. They've purchased the carb line from Precision.


Dan