Dave,

Your problem rings some bells for me since I have experienced such
problems with engines before. I have a pretty decent background. So,
I offer this.

The float level is probably okay. I normally do not see any
significant changes onse set. in fact, I have modified such settings
for efficiency. Setting the float lean, results in a lean situation
(bog) when accelerating or being placed under a heavy load. A rich
setting probably would not affect such a situation which you are
experiencing. A real real rich setting would be evident by observing
wet fuel within the venturi etc. This situation would also be noticed
by potent exhaust and would be evident at all times and RPM levels.
Thus, I do NOT think it is a float problem.

A vent hole plug could be a possibility but idle quality should show
this. Based on my experience, I would check for two probable
conditions. First, DIRT!!!!! I have found that dirt from fule tanks,
corrosion, etc. can develop into such hard to identify gremlins! It
has happened too many times! Ultimately, if let go, shall cause a
failure. Based on this, I would suggest several things. The carb,
really needs to be removed, inspected, and CLEANED! Then the float can
be verified (yes). All orificies need to be checked with light and /or
inserting a pin or fine wire. The light is good to see fine things
like strands of misc. stuck deep in passageways, etc. Then, once
cleaned, and checked, it needs to be assembled and adusted properly to
spcifications. While performing this function, INSURE that all housing
surfaces are FLAT and seal PROPERLY. Once this is complete, the fuel
filters should be replaced. You should also then, cut the old filters
open, and carefully look for an accumulation. Then, the air inlet and
filter assembly should be inspected and replaced if dirty. Once a new
filter or existing filter is re-installed, make DARN sure that the
intake inlet housing is totally sealed. If dirty air is being drawn
past the filter, it shall clog the air bleeds in the idle tube
passages. This could then, POSSIBLY cause your condition. If the carb
is in good condition, it would probably be more noticeable. Why you
ask. Well, as carbs age, performance at idle is usually compensated
with just incrasing the idle screw. That's okay, I guess. But on an
older carb with some wear, this puts some of the idling onto the main
circuit and covers up the problem on the idle circuit! Since the main
circuit can feed plenty of fuel, no noticeable problems can be felt!
But, if everything is in decent order, and the idle is truly adjusted
properly, a problem becomes more evident. I am not talking big
extremes here just small changes. Anyway, when everything is adjusted
properly, dirt, or some leakage, shows up much more easily ... like
idle air bleeds being clogged! If the carb is operating partly off the
mains, they are also operating of the main air bleeds also. These
bleeds would provide enough air, since they are designed for higher
RPM's. And, since the idle air bleeds are so much smaller, they are
easily plugged before the main bleeds would be.

The second are which needs addressing is a vaccuum leak. During rapid
throttle closing, the vaccuum spikes quickly which can expose and pull
outside air into the carb which results in a lean stumble! So, all
intake gaskets should be visually checked and even sprayed with say
WD-40 or carb spray and notice if RPM changes are evident. If they
are, then you found the problem!

Based on what you have described, you shall have to me quite meticulous
in every item which I mentioned here. I think that if you are, you
shall find the problem. Additionally, knowing the possibilities, when
you find the problem, it shall stick out like a sore thumb!

Finally, if the idle air bleeds are obstructed, a can of carb cleaner
would clear this up immediately. From there, it would be wise to pull
the top of the carb, and check for sediments and dirt. Based on what
you find, would determine if the carb had to be pulled. Normally, if
you catch a problem early enough (like you described), not too much
follow-up is needed other than removing the current problem.

Sometimes the simple things create the biggest hardships because they
are not always evident. But, you are doing the right thing! You noted
something wrong, and are investigating it, and you shall solve it, I'm
sure. Most people, would probably just dismiss it or worry about it
when it was convenient for them. But, left too long, can create a bad
situation!

Take Care, and I hope that this information helps!

Dave wrote:
Hi All!

OK, since the collective talent in this group greatly exceeds
mine and those around me...

Today the engine in our 151 (O 320-150hp)Warrior stumbled
during power reduction, and we were able to repeat the symptom.....

Here are details

OAT -5 C, 2350 rpm, kinda rough air, pulled back throttle to
decend after passing traffic..

I think we hit a bump which caused the throttle to be
retarded quicker than normal, engine stumbles for 1 second and
resumed operation at the new (lower ) throttle setting.

We were able to repeat this several times, carb heat on/off,
fuel pump on/off, leaned rich of peak/ full rich, left tank/right
tank, rt mag/left mag/both. - as long as we waited a couple of minutes
to try again. No evidence of carb icing, dew point was +10C

Moving the throttle immediately back to cruise, then sudden
retard, no problem. The stumble could be repeated by waiting 2 -3
minutes .

Recent maintenance, oil & filter change 1.5 hrs ago.

The stumble was like the mixture was momentairly too lean,
similar to the lean test during runup...

At 3500 ft, lean setting was at 60% of mix lever travel to
full rich..

Any thoughts from you engine talents out there?

Cheers!

Dave