Strange beahvior on Mag Check

I have one mag and one EI. I set the EI to produce 25* BTDC at 1800 to get consistent RPM drops on mag checks. I usually have about 10-20* difference from side to side. Today I was getting 40-50, so something has changed. I did some testing and noticed that when run at 1000 RPM on the mag alone, the #1 EGT rises quite a bit more, relative to the others. The other 3 cyl's move in relative unison. All 4 move together on just the EI.

I was thinking plug or wire. Rising EGT could indicate a change in timing, but not sure what would cause that on one cylinder. Plug looks good and #1 measures 950 ohms; within 100 of the other three. Plugs were just cleaned and gapped 10 hours ago. No lead deposits.

Any ideas or guidance on what is causing this? I will pull the mag and put in the spare as well. It has 300 hours, so I'll tear it down and IRAN.

Thanks,

Larry

Hi Larry
If you have not already done this, move the suspect plug to different cylinder and repeat test.
At that point, you can either eliminate or isolate the plug as the problem.

Thought I would post my results. Replaced the mag with the spare and swapped a couple of plugs with spares with no change. I noticed a bit of oil weeping near the mag carcass that I use as a pickup for my EI. I suspected it had loosened and moved. I pulled it, replaced the gasket and reset the base timing. Problem solved and mag drops back to 10-20 RPM.

I think that I was observing some results the other day, such as inflight, while leaned. The EI deals with lean conditions much more gracefully and it creates a more pronounced difference. The variability in RPM drops during run-up was due to having too much timing, relative to the mag.

As typical, paying attention to little changes in behavior can help avoid larger problems.

Larry