5.0 Mercruiser running rough out of no where (misfire)

[mhowt24]

Hello all,

I'm new to the forum. I can't solve this issue so I figured I'd reach out for some help. I'm restoring a century 8000 28 foot boat with two sterndrive 5.0 mercruiser v8 engines (gm 305). THis boat has been sitting unused for over a decade.

One of them was running good after the following work done to it:

-new starter
-new distributor cap
-new plugs
-new impeller
-oil change
-point cleaned off
-new gas (old gas syphoned out)
-carb kit installed and carb cleaned

I ran it on the hose water and it ran good. One issue I encountered with this engine in specific was that I needed to send a wire directly between the coil positive terminal to the battery to get it to fire up. Then it ran great. Then I took the wire off and it ran on it's own very nicely again. Then the next day I needed to run the wire between the battery and coil for it to fire up. Then it ran well on it's own WITHOUT the wire and seemed to for a few days.

I get the boat ready to bring to its first test on the water, as both engines were running good, and this engine started misfiring bad. It was idling super low and smoking bad. I hooked up the wire to the coil and battery to see if it would resolve itself again, but it did not. I am confused as to what could have possibly went bad between not running it for a day, moving the boat, and this being the result.

Since it started running poorly, I've changed the condensor and points in the distributor, I've put another fresh set of plugs in, and I'm using a new coil. Same symptoms appeared.

I am on my way to get new plug wires (they havent been changed yet) and an OEM coil to install. I cannot figure out where this issue may be coming from, and any help would be appreciated. Thank you.

 

[Rick Stephens]

Pull the wire to the tach off the coil.

 

[Scott Danforth]

did you clean the fuel system or simply siphon off the wet stuff leaving the sludge behind?

normally you need to remove the fuel sending unit and mechanically clean the tank if it sat for a while.

 

[mhowt24]

Thank you for the replies. I syphoned the fuel out with the tank tilted so that it pooled where I could suck the most out of it. The fuel was suprisingly clean and we actually have been running it through the lawnmower with no issues whatsoever. So I'm not concerned about any residual fuel. I replaced the fuel filter on the engine that's having issues, and the other engine is running great. So to me, that rules out a bad fuel scenario.

I have removed the tach wire from the coil and it still runs rough.

 

[QBhoy]

Water in the fuel just once can have lasting effects long after it’s been removed from the tank. It can really affect an engine but you can be lucky with others. Carbs usually always need completely stripped at the very least (you’ve done this I think you said), but in extreme cases it can all of a sudden cause the valves to stick or drop and contact the piston. Just a thought. Happened recently to a friend.

 

[mhowt24]

Problem solved. After all of this, it turns out the choke was stuck on. the heating element that slowly opens the choke when the engine gets power is shot. Thank you for your input all!