Crusader 454 Vibration

[Orion35]

Hi, all. New to the forum, and looking for advice. Have a 1988 Mainship Mediterranean with twin Crusader 454's. (I know, not really a trawler...)
Recently the starboard engine (reverse rotation) developed a hard rhythmic vibration related to engine RPM's. There's no noise associated with the vibe, and no evidence of coolant in the oil. In troubleshooting it yesterday, with RPM near idle, I pulled/replaced one spark plug wire at a time from the distributor (Mallory aftermarket YLM63), and when I removed the #3 wire the vibration went away and the RPM increased about 200 RPM's.
I assumed that the #3 cylinder was firing early, possibly because of a shorted distributor cap or a "jump" from adjacent plug wire. I separated the wires making sure they were connected to the correct plugs; removed and cleaned the distributor cap, and replaced the #3 plug, which was quite heavy with carbon but properly gapped. I've ordered a new cap and tomorrow I'll do a compression check. In the meantime, if any of you have any advice on this, I'd appreciate hearing from you. Many thanks.

 

[bayview]

Good work. Check the compression while it is out, Double check the firing order.

 

[tiltrider1]

It’s very likely that you have a bad wire. You could swap the suspected cap, rotor, wire between the two engines and see if the problem follows. If it does then replace all caps, rotors, and wires. You didn’t give a year for the engines. Not knowing the year I don’t know if you have points or electronic ignition.

 

[Tolly Roger]

For reference, there is a really good CRUSADER forum.
Crusader Marine Engine Forum

Try your question there too!

 

[Orion35]

Thanks, all. Have double-checked the firing order and it's good. I'll replace the cap, rotor and wires (aftermarket Mallory electronic ignition) and see what happens. If that doesn't do it, I'll swap everything between engines and see if the problem moves, as well. I'll let you know.

 

[Orion35]

An update: My mech (finally) was able to isolate the problem to the #5 cylinder's exhaust valve being fully stuck open. Spring is fully compressed. Push rod is not bent. We're working with PB Blaster and other means to try to free it, but I expect to have to pull the head and have it re-done. If that's all I have to have done, I'll be happy.

Thanks to all that offered help. I'll keep you informed.