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165 inline 6 runs rough after 30 mins

D

dougido

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"I have a mercruiser 165 in a

"I have a mercruiser 165 in a 1980 sea ray. I just had the head done, risers are fine, all new gaskets. Motor starts easily, and runs like a dream for about the first 30 min on the water, but then it begins to misfire. It is very much like you would see with water in gas, and that is what I suspected it was, but I am worried that it only begins after the engine is hot. Bad gas should show up at all times rght? Oil looks like new, no depostis or signs of water. I'm stumped!"
 
Do you have points in your dis

Do you have points in your distributor?
 
"Sounds a real lot like classi

"Sounds a real lot like classic weak coil symtom. But there is a remote chance that it is the condenser. Go ahead and swap out the condenser for good measure. If it's still bad, get a new coil. Be sure to verify if you need a coil that needs external ballast resistance or is one that does not need it. Also check and clean all battery cable conections including cleaning and britening the spot on the engine block where the ground cable connects."
 
"The misfiring takes place at

"The misfiring takes place at most all speeds. When it starts I am usually running at 1/2 to 3/4 throttle. When I back off of the throttle it still continues to stutter all the way back to idle, and usually just before getting to neutral, it will die. Always starts right back up though."
 
"i can't tell from your ex

"i can't tell from your explanation if you your problem is fuel or ignition. Could be many things on a boat that old. Sounds like ignition though.

Would start by going over your ignition. Good points and condenser, gapped properly, timing set correctly with advance working properly, fresh cap rotor and plugs with the correct heat range.

Then move to the fuel system. Make sure you have fresh gas, no trash/water in the filters, etc."
 
Doug: Is the mechanical advan

Doug: Is the mechanical advance working? Rust will make it hang up and throw off the timing. I would consider dumping the points ignition with an upgrade to electronic components that replace the points/condenser and fit directly in the distributor. Price is reasonable from Sierra. Guy
 
"Thanks to all for the input!

"Thanks to all for the input! this a great forum, and being a fairly new boat owner it is a great feeling to have so many folks interested in offering help. At this time I think that the ignition solution is the best place to start. I have been called out of town on business so it will be until next week before I can hit the water, but I have left the boat in the hands of my trusted boat guy. He will install a fuel/water sep. filter and a new coil. the other ignition parts were new when we did the rebuild on the motor last month. Again thanks to everyone for the help, and i'll be sure to follow up."
 
"Would definately go with a so

"Would definately go with a solid state ignition and get rid of the points and condenser, but that's not necessarily the solution to your problem.

Rusted mechanical advance would make it sluggish but not likely misfire.

We live for this stuff, Doug!"
 
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