Brian,

Please read what I just posted for Dave! I think you just answered the
question with your observation ... the mechanic has rebuilt the carb
several times!!!! I bet he is trying to adjust it to standard
specifications with a throttle shaft which bores are egg-shaped! This
causes leakage. It also creates proper adjustment problems. If you
set it to specifications, a stumble can result. If you set the idle
higher, then you are operating a little bit off the main feed system,
and then, the problem is covered up! But, then it is not as efficient,
and REALLY in fact, out of adjustment! Ideally, the carbs should
probably be replaced. but, as always, it comes down to money! YES!
Upping the idle is okay in most instances since there really is no down
point other than using a bit more fuel at idle. Old engines do not
function like new ones YES!

But, I think my response to Dave will also give you some good insight
in your inspections also. Dirt, buildups, wear, and leakage create
many problems and they come in many forms creating many weird gremlins!
ANY negligible movement of the throttle shaft affect the idle quality
and the functioning of the this fine adjusted system!

take Care!

Brian wrote:
I have about 60 hours in a pa28-151 (160hp) that is a rental locally.
It does the same thing. The Mechanic as rebuilt the carb a couple times
and still can't figure out what is causing it. Only does it
occasionally and doesn't seem to require a fast reduction in power,
just a reduction in power. In fact it is more noticable when doing
smaller power reductions. It has been doing it for a couple hundred
hours now.

Would love to know what causes it as would the Mechanic.

Brian
CFIIG/ASEL