496 HO Makes Fairly Loud Tapping and Occasional Back/Mis Fire
 

I just purchased a Donzi 28ZXO last month and was running the boat at the lake last weekend when I accidentally hit the Corsa Selectable Exhaust switch at about 3500 RPM. Instantly heard a bang so I stopped the boat, lifted the hatch and heard a loud tapping and occasional bang or popping coming from the intake area. No Exhaust leak and sounds with stethoscope as tapping is coming from intake manifold near a middle cylinder on the port side. Pulled all spark plugs, looked pretty black, and did a compression check and all cylinder have good compression. Any Ideas?

Really, nobody has any ideas what might have happened?

Sounds to me like you have more than one problem, black spark plugs - your running way to rich, the noise and backfire suggest to me that you may have jumped a tooth on your timing chain. Check your timing. What is the variance on the compression from cylinder to cylinder?

First of all, does the Corsa still work? Does it open and close properly and still fully diverts the exhaust in the direction it should both opened and closed?

Thanks for the responses!

Sounds to me like you have more than one problem, black spark plugs - your running way to rich, the noise and backfire suggest to me that you may have jumped a tooth on your timing chain. Check your timing. What is the variance on the compression from cylinder to cylinder? I think the spark plugs may have been original from new. However I agree, to rich. I do get a considerable amount of black soot on the transom. Will check timing and the variance on the compression is no more than 10 PSI. Did not do cylinder leakage but may try that also.

First of all, does the Corsa still work? Does it open and close properly and still fully diverts the exhaust in the direction it should both opened and closed? The Corsa still works properly.

Maybe it bent a pushrod or an intake valve.

That's what I was thinking, but wanted some feedback before I started tearing into this thing. Man theirs a lot of stuff to remove just get to the valve covers.

Anybody else agree with this possibility?

When I saw the thread title, my mind went straight for sticking intake valve.

What is usually involved with that? Bent valve, push rod or lifter. I'm kinda thinking a lifter might be involved. Any thoughts?

Took the port side valve cover off and found nothing unusual. So took the Starboard valve cover off and the #6 exhaust rocker arm stud has striped out of the head. To clarify, the threads on the head are gone. Not sure of the cause, but just back pressure seems a little extreme. Anyway, does anyone think the head can be re-tapped with a larger thread, say 3/8"? I think the current thread size is 7/16". If this is possible does the stud exist?