IO-540: Which Plugs? URHM38E or URHM40E

[9GT]

I noticed a slight lack of full power while I was on my last flight so I returned to the hanger for some trouble shooting. I got a lot of stumble on one of my mag checks. One mag has less than 50 hours TTSN and the other about 12 hours since a 500 hour inspection and repair so I de-cowled the plane and removed all the plugs. All are Champion's: 6 are 38E's and the other 6 are 40E's with a little over 500 hours on them. That's what came on the engine when I purchased it used. I have been rotating them upper to lower whenever I clean them. One is completely dead on the ohm test so I have decided to just go ahead and replace all of them with the new Tempest plugs. I have no idea why the previous owner had the mix of plugs and the application guide from Tempest has both the 38E's and 40E's as applicable plugs for my IO-540V4A5 engine. Any guidance help for me which to order would be appreciated. Thanks! Dave

[BobTurner]

My IO-540D4A5 came from Lycoming with the 38's. Perhaps the former owner was using the hotter plugs on bottom to try to burn off oil?
BTW, besides top-bottom, you should rotate plugs in the firing order (or 1 to 2, 2 to 3, etc will work) to even out electrode wear and maximize lifetime.

[9GT]

That makes a lot of sense Bob. Thank you!

[rleffler]

Mine came with 38s as well.

bob

[F1R]

When you get your Tempest plugs please report the resistance on all 12.

I am in the process and 6 of the first 7 plugs were 1350 to 1700 Ohms.

However one of them reads 600 for an instant then jumps to 1990for about 1 second then open circuit . Has anybody else found a new plug to behave like this?

[9GT]

Thanks Bob. I ordered the 38E's. F1R, I'll measure the resistance when they arrive and post the results here.