I am now at 5 months in my new to me Comanche. After abut 40 hours of flying (not bad during winter in the Canadian prairies) I have two questions for the seasoned owners on this board.
Not sure if it is my imagination but it seems like my engine runs rougher while drawing form the auxiliary tanks. Nothing shows up on instruments but I think I notice a difference.

Also any cruise above 7000' at 2400 RPM my MAP maxes out at 20" Is this normal?
I would like to comment that this is an incredible touring plane. I love it.

First, I don't know of any reason the aux would cause rough running other than is there were a fuel delivery issue or you were sucking air into the line by way of a degraded fuel line, fuel selector, or pump. Being that everything after the fuel selector is common to all tanks, that would lead to the wing, flexible fuel lines from the aux to the hard lines, as well as the connections to the same and to the selector valve. Also check that you are not drawing air through the valve itself. You may need to fit a clear line from the firewall to the edp to see this.

As for MP, at 7K, I would expect about 22-22.5" with typical intake drag and standard sea level barometric pressure. I would suspect a faulty MP gauge more than any other issue. On the ground with the engine shut down, it should read the current barometric sea level pressure, minus approx one inch per 1000 feet of field elevation. That is a good first check. They can also be non linear in their errors though so that is not definitive, but will show if the needle has sprung one way or the other.

Zach

Thank you for the input.
I have my AME coming out tomorrow night. to check these and a few other things.
Will let you know what he discovers.

Where are you located, Brad? I've owned the same 1959 "250" for almost 37 years and have made almost every mistake known to aviation.....
I can be reached at c-flhv@magtech.ca
Don Ostergard
Drumheller AB
(403) 823-0405

If speed is right then +1 on the faulty MP gauge. Just had the gauge in our 260 repaired for same reasons.

I had a slow and sluggish manifold pressure gauge that finally stopped and wouldn't move. Turned out the rubber hose that interconnected between the engine and the firewall lines (for movement) had degraded and plugged with a rubbery sludge. Cheap and quick to fix too.