496HO high compression in #6 and #8 cylinders
 

Hello, I ran a compression test on 2003 496HO in Donzi 22C and found #6 and #8 cylinders at 190 psi. Not sure what could cause this other than carbon buildup, worn cam, or stuck exhaust valves which I imagine would have worn the cam anyway.

All other cylinders are at expected levels of 150-155 psi. If I had to guess, motor may have total of 350 hours most of that below 4000 rpm. Being a 2003, manifolds are probably aluminum, which concerns me given their potential for corrosion. Boat had saltwater use before we purchased in 2008.

I took a peak inside #6 and #8 cylinders with borescope. I have pictures, but system is not letting me post them (can email or PM). Scope is Chinese, so I got the resolution I paid for, but they don't look pretty and appear to have carbon buildup, though, I don't know if it's enough for 40 psi above spec. Any ideas on what the culprit might be and any recommendations would be greatly appreciated. Hoping not to have to grab my ankles and bend over on this one, but will pull the motor if need be.

Thanks,
Jim

https://cimg1.ibsrv.net/gimg/www.off...3aed2eeb4f.jpg
Cylinder 6_1
https://cimg2.ibsrv.net/gimg/www.off...ce1d1af40f.jpg
Cylinder 6_2
https://cimg3.ibsrv.net/gimg/www.off...fd7ba87404.jpg
Cylinder 8_1
https://cimg4.ibsrv.net/gimg/www.off...eac3fa3bb6.jpg
Cylinder 8_2

For comparison, did you use the scope to look at the cylinders that are at 150psi? Were they drastically different in appearance?

Got better scope shots of cylinders today. 6 cylinder is making 190 psi but it doesn’t look bad. Looks similar to the good cylinders. Top of 8 looks to be caked in oil or carbon. 8 is making 180 psi. Turned over today with 6 in the compression gauge. I noticed a distinct high pitched but soft squeal noise as i disengaged starter. Wondering if this is a valve issue, can issue, or leaking valve seals.
https://cimg2.ibsrv.net/gimg/www.off...a724c919c.jpeg
Six
https://cimg4.ibsrv.net/gimg/www.off...9af49ff53.jpeg
Six
https://cimg5.ibsrv.net/gimg/www.off...70d483df4.jpeg
Eight
https://cimg9.ibsrv.net/gimg/www.off...1fe0c593e.jpeg
Eight

About 4 years ago, I owned a 2006 496 Mag. No issues, but I measured the compression using a good Snap-on gauge when I had all the plugs out for replacement. The cam is larger in the 496 HO so perhaps it has lower compression. All 8 holes were in the 176 - 188 psi on mine.