72 Merc 850 Thunderbolt

[gyjs9]

I have a 72 merc 85hp on a 17ft glastron trihull.When i first put it in the water it barely ran above idle. I had the carbs rebuilt and it ran great,for about an hour and cut out.It restarted and ran rough back to the dock.I was told there was water in the gas so i drained the tank put in fresh gas cleaned the filters and checked the plugs. One plug looked like it was welded on the end, the others were good.Any help on what happened and what to do next would be greatly apprecated. It is my first boat and hope not the last.

 

[emckelvy]

Re: 72 Merc 850 Thunderbolt

Sounds like you had a severe case of detonation and the stuff you found on the spark plug was likely aluminum melted from the top of the piston.<br /><br />Time to remove all the spark plugs, reinsert them in the wires and ground the plugs solidly to the block; then take a compression check on all 4 cylinders to see if it's still alive. Readings on a healthy Merc should be no more than 10 psi different from each other, preferably less. If you see drastic variances from each other, or one cyl almost zero psi, that's a problem and the motor will need internal repairs.<br /><br />And also an thorough examination of timing settings, etc to figure out what happened in the first place.<br /><br />Good Luck............ed

 

[gyjs9]

Re: 72 Merc 850 Thunderbolt

I checked the compression on the cyl. and got 30psi.,120 on the rest. I asked the mechanic at the boat shop and was told it would be about 1800 to rebuild and would be a waste of money.Is this somthing i could do myself ,what would be involved and how much? when he rebuilt the carbs he said it was in real good shape, now he recomends I sell it for parts. Any advice would be greatly appreciated. Thanks

 

[jimmbo]

Re: 72 Merc 850 Thunderbolt

Which cylinder is the dud?

 

[gyjs9]

Re: 72 Merc 850 Thunderbolt

The third one from the top