The stumble you describe around 1200 is pretty normal - it's where the transition takes place from the idle circuit.

The stumble at altitude is a little different. From the message, I can't tell if you throttled back then up through the 1200 range or if you had a stumble at a higher RPM. The fuel pressure you're getting on the mechanical pump is probably sufficient. (The TCDS for an O-320 is a minimum of .5 PSI.) So, if you're getting a stumble at higher RPM, it may very well be the two observations are not related.

As you describe the power issues - first thing I would probably do is to check the Tach at low and high RPM.

You could call Penn Yan - I've had pretty good luck when I've worked with them.

Dan

hesitation issues
 

1st let me say that in spite of it being a common squawk that lots of folks have had to learn to live with hesitation, stumble, bogging ect is not normal. The reason a lot of people live with it is that it can be very tough(read expensive) to isolate and fix. The previously posted link to Sac.sky-ranch covers the possabilties quite well. In my experiance the no.1 most likely cause is a fuel issue. This is were I would start. Make sure you have proper pressure and flow to the carb, make sure the part number (not model number)carb installed is correct for the engine, make sure the finger screen in fuel inlet is clean and in good condition, check for any leakage as indicated by stains, check idle mixture setting, at 800ish RPM a slow (3 sec.)pull to idle cut-off should produce a 50-75 RPM increase folowed by shut-down, and finally check for correct carb float and float setting. If all checks out ok and you still have a problem check and see if the one piece venturi AD has been performed on that carb. If so borrow a carb that it has not(still two piece venturi or manufactured new), install and test. If this fixes your problem either have your carb rebuilt with a new tempest venturi and main jet installed, or buy a new tempest carb. I have found this squawk is commonly caused by a low quality one piece venturi and/or mismatched venturi/main jet in rebuilt carbs. Some engines will require a richer main jet when using the one piece venturi. Lots of good info relating to carbs @ www.msacarbs.com Good luck, Russ

My O-360 carbed engine and fixed pitch prop 6A will have the fuel pressure perhaps below 1 often. Noticed that today at 11,500' in the mountains. I don't worry about it anymore

Add wheel pants and fairings

But Russel, the Lycoming manual shows multiple carbs for several engine models.

As an example, the O-360-A1A shows both a 10-3878 and a 10-4164-1 carb. as acceptable.

The MSA carb link you show adds a 10-5193 and doesn't mention the 10-4164-1. Lycoming says this is OK on an -A4M, but my catalog may not be up to date.

I's sure the major differences are in the jet sizes and they have been optimized for different airframes. How do we really know which one is best for RVs, remembering that the carb. part number should match Lycomings documents if you want the 25 hr Phase I time?

The same engine is in the Cherokee I rent. It's placarded against opening the throttle too quickly -- no quicker than 2 seconds between idle and WOT. Believe me, you will learn the need for this if you have to do a go around. :D

carb
 

For the 0 320 carbs the higher the part number the later the model of the carb. The single piece venturi is a known problem, so instead of tearing the fuel system apart, why not start with a known problem. The single piece venturis have manufacturing flaws that cause many of the problems mentioned above. Instead of spending $100 or more on a new venturi, the roughness, etc in the bore of the venturi can be polished out. If you don't want to do this yourself contact TerriLee Bell at Aircraft Fuel Specialists in Hagerstown MD. You can also find a lot of information on the yahoo Lycoming forum