1967 Merc 500 Thunderbolt running issue

[Yurri]

Hey guys I have recently developed a problem. I have a 1967 Mercury 500 thunderbolt. I took my boat out joy riding and after about an 14 mile ride, I suddenly lost power. It will run but only with the throttle about midway to WOT. I managed to limp it back and load it up to bring it home.
It sounds like it is missing a cylinder, so I checked my plugs (all produce a strong spark). So I looked at the tops of each cylinder and cylinder 2 looks wet. The other 3 look relatively dry. How could it be wet if the plug is firing and where should I go from here? Thank you gents for any help.

 

[Lessonlearned]

You might want to try to put a timing light on each cylinder to make sure each plug is firing.

 

[Yurri]

The motor has been running like a top for a year now. Will timing screw up like that instantly? Forgive me, I have only been boating for about 1 year now.

I do have an interesting new piece of information to add to this equation. I removed my starter this afternoon to check out my carbs. I found that a bolt had backed itself completely out of the powerhead. I was able to put it back in and tighten it back down. It will idle at a lower rpm now but still not correctly and still has a wet 2nd cylinder. It was a 3/8"x1" bolt located right behind my top carb on the starboard side of the motor. Does anyone know what this bolt could be? There is one right behind the lower carb also but it was tight.

 

[Yurri]

Nevermind the timing question. I see what you're getting at and yes sir I have good spare on all four plugs.

 

[Yurri]

Ok, have pulled my powerhead and found my problem. The bolt I was asking about held the reeds in place. On this particular motor the reeds are attached to the crank shaft. (Aweful design by the way.) Since the bolt decided to back out, the top reeds spun around and has destroyed it's seal. Now the hard part is finding more reeds and on top of that, finding them in good condition. Hopes this helps someone else.