Turbo arrow runs rough at altitude

dcs02d

I fly a 1980ish turbo arrow. It has an STC that includes an inter-cooler and a boost to 220HP for takeoff. It also has pressurized mags.

Beginning at about 11K feet the engine will stutter every few minutes. Similar to the feel of fouled plugs but it occurs every few minutes for a second or two. As altitude increases the problem becomes worse until the airplane is un-flyable above maybe 15K as the engine runs rough constantly.

There are no problems at lower altitudes. Fuel flows and cylinder temps seem normal at altitude. I generally am running about 2400RPM / 30 inches.

I did try each mag at altitude and the problem became significantly worse when running on either mag alone at altitude.

How should I trouble shoot this? Suggestions for a fix?

Thanks for the help.

Don

flyguydon

My guess would be the fuel servo. There is an altitude compensating valve in it that changes thing with changes in pressure. There is really nothing else that cares about altitude in the system. Most any mag, pressurized or not, should not really have any issues at that altitude... and for a symptom like that you'd need both to fail at the same time to create it.

Try moving the throttle to idle and back up to see if that makes it more stable... or move the mixture rich and lean to see if it has any effect.

dcs02d

I'll give it a try this evening. Thanks for the suggestion.