My pistons looked like this about 1 month ago:



Think I should decarb before my dyno tune? My motor was running a little rich but I suspect that most of that nastiness has to do with oil going back into the intake. I did not have a catch can or atmosphere vented PCV system on my old blower setup, and that would explain why I would periodically get some mid range KR (3K to 4K RPM) but mostly be clean throughout the run. Nothing like nasty oil gas to lower your effective octane, eh?

Anything special I should consider when doing the decarb?

 

Na, you should be fine... I've seen alot worse where it really became necessary.


Bill

 

Methanol would clean that up nicely.

 

I am surprised you didn't clean that up when you had the heads off. When I pulled my heads last, I used a scotch brite pad on the end of a pnuematic drill to clean up the pistons. They looked like brand new when I put the heads back on. I tried some straight Carb. Cleaner and some Brake cleaner on a rag. The solvents melted the stuff, but was more messy and didn't really get the baked on stuff off. The carbonized oil/fuel would actually chip off the top of the pistons better than when using solvent. I don't think a decarb will completely clean up your pistons, it might melt some of the stuff off, but not all of it.

I will give you an example of how a decarb (GM fluid) may have helped me recently. I say, "may have helped" because I am sure the new vavle spring had some influence too. I had a broken valve spring when my car got to Xtreme. It compression checked 190, but a leak down showed 11% on that cylinder with the broken spring. The car had been started numerous times, so there was most likely a bunch of carbon on the exhaust valve. It doesn't take much too create more leak down through the valve if it has crap on it..a human hair could cause that. A decarb was performed after the new valve springs were installed. Another leakdown test was performed, it was <5% on that same cylinder...the rest of the cylinders were <2%.
I am sure once the motor is ran for a bit, another leakdown would show the same cylinder <2%, as the valve open and closes properly, and tightens up a bit.

If I were you I would do a decarb, it certainly will not hurt anything and you might get some of that crap cleaned off of the tops of your pistons.