1982 260hp mcm runs rough when hot

[Triangleboater]

Mercruiser 260hp mcm carb running rough
Hey all,

Posting in other forums too but like the wealth of knowledge here too



My gm 350ci 260hp mcm 1983 vintage is running rough after it warms up. Runs great all through throttle range until 10ish mins and is like it's missing a cylinder very few revs. The Rpms surge.



Any ideas? I just did the cap, rotor, plugs and plug wires as I thought they might be the culprit. But, alas. Does this engine have a ignition module? If so. Anyone have any ideas on how to test?



Thanks!

 

[Bondo]

The Rpms surge.

Ayuh,... I'd start by checkin' the contents of the fuel filter, for anything other than fresh clean pure gasoline,....

Sounds like a fuel delivery issue to me,...

 

[alldodge]

Runs fine until it warms up, then runs crappy, my first guess is fuel.
Check fuel filter contents, pour the filter into a baggie and see if there is stuff in there but clean gas.
Next guess would be carb needs cleaning
Then check fuel pressure, should be 4-7 psi

 

[Triangleboater]

Very cool, I will check both fuel/water separator and the fuel filter and report back. Thanks!

 

[CamaroMan]

probably running rich? is the choke opening? a rich condition is just like this - fuel delivery wd be anywhere regardless of temps imo. it runs fine when cool because the extra fuel is acting like a chke, making it rich. When warming up its too much -

doa hard run, switch off motor and pull a plug - dry black powder is too much fuel.. id def start there -

 

[Triangleboater]

Ok all so o have some info. Thanks to all that have chimed in so far.
Took out plugs 7 and 8 as these engine sometimes had water intrusion after thermostat opening but they work fine been firing fine.



Also first video is before surging about 4 or 5 mins after startup. Running smooth.

https://m.youtube.com/watch?v=C2Mo3w1nTf8

This is after surging starts. About 11 mins after startup.

https://m.youtube.com/watch?v=Rg4GF6APNk0

As you can see the starboard side jet isn't spitting any fuel at all. It was upon startup and a little when you give it some throttle. Also the engine smooths out a bit upon manually choking it a bit. As seen in the video. I am now thinking the idle passage ways or jets in the carb may be a bit clogged.

What's your opinion?
Thanks in advance!

 

[alldodge]

Agree with your diagnosis, carb needs cleaning, idle circuit is a bit clogged up

 

[Triangleboater]

Agree with your diagnosis, carb needs cleaning, idle circuit is a bit clogged up

Looks like there may be different carb rebuild kits. Can someone point me in the right direction for the correct rebuild kit?

 

[alldodge]

Looks like there may be different carb rebuild kits. Can someone point me in the right direction for the correct rebuild kit?

Need to know which model carb you have and there should be a number stamped on the side or a tag