Engine Misfire at Altitude.

[basloane]

A few years ago, I purchased a complete Mag Pressurization kit from Jeff Shapiro at TurboPlus. I believe they still exist in Washington.
If you are experiencing misfiring, might want to check the resistance on your spark plugs. If the resistances are much greater than 1,000 ohms ( >50,000 ) they start misfiring in the outside harness connection at high altitude.
I was experiencing misfiring above 15,000, and spent 4 years and upwards of $3,000 chasing, cleaning, testing and changing pressurization configurations until ( by chance ) we found an article about the problems ( about 6 years ago ) folks were having with AC plugs. We tested all the plugs and most of the resistances were off the scale. We could even see arcing outside the plug.
Sympathetic to my experience, the local rep very kindly provided a new complimentary set of plugs which we installed in the front engine. As an experiment, we switched out the rear plugs for Tempests and then filed for FL220. Both engines purred for about a year, then I started getting "misfires". Found a plug on the FE testing over 1,000,000 ohms ( yes, it was an AC ).
I would be sure that AC has addressed the problem by now, but I would like to see some more hard data......( just the skeptic in me ).....
Or,.... only use the AC plugs in non-pressurized/non-turbocharged aircraft......

[edasmus]

This thread does not apply to my naturally aspirated C337G and I do not claim to understand the dynamics that impact mags at high altitude. I can however second the previous post on spark plugs. I finally made the switch 2 months ago from Champion plugs to Tempest plugs and the improvement has been remarkable. For 18 years always purchasing Champion plugs, (for no other reason than habit) I finally made the change. Poplar Grove recently overhauling my front engine, convinced me to do it.

I have always been battling fouled plugs and I have read many articles on the resistance issues in Champion plugs that apparently Tempest plugs do not suffer. After 30 plus hours now of operating with the Tempest plugs, my freshly overhauled front engine, and my 32 year old SFRM and 300+ hour past TBO rear engine have been purring perfectly with not one single fouled plug or plug misfire at anytime. I find the improvement amazing.

Good luck with the resolution to your issue.

Ed

[JAG]

Ed - thanks for that info on plugs...I have always been a "Champion" guy myself given experience on other aircraft types (and have stayed away from AC plugs). I will give Tempest consideration going forward!
Jeff