255 mercruiser runs rough

[gjr711]

I have a 1977 mercruiser 255 inboard, this year it started great but after long runs it stalls and wont idle but after a short time to cool off all is back to normal, i have tried everything i even bought a new carb it got a little better but still wont idle

 

[dubs283]

Re: 255 mercruiser runs rough

when was the last complete tune-up?

 

[Bondo]

Re: 255 mercruiser runs rough

Ayuh,... Is the fuel pump Delivering,..??
Is the tank giving it up..??

 

[gjr711]

Re: 255 mercruiser runs rough

tune up was just done

 

[gjr711]

Re: 255 mercruiser runs rough

the boat will run great all day wide open throttle but when its time to idle no go

 

[Bondo]

Re: 255 mercruiser runs rough

i have tried everything i even bought a new carb it got a little better but still wont idle

Ayuh,... Time to diagnose,...
When it Stalls,.... Pull a few sparkplugs,+ LOOK.....
Is there Fuel,..??
Is there Fire,..??

 

[gjr711]

Re: 255 mercruiser runs rough

it does seem to be loading up with fuel but what could make this happen only when it runs for a while then is fine in just a few short minutes

 

[Bondo]

Re: 255 mercruiser runs rough

Ayuh,... My WAGuess is the Carb or it's settings....

But I'm not there to LOOK...

 

[gjr711]

Re: 255 mercruiser runs rough

its a new carb and idle mixture screws are adjusted just fine . im starting to think maybe a float adjustment? just that is wierd when you first start it up it can run at an idle for so long. take it for a spin then bam its back.

 

[John3]

Re: 255 mercruiser runs rough

Had a similar problem. The coil was overheating. Ran fine again when the coil cooled.

 

[Quadruplecccc]

Re: 255 mercruiser runs rough

Did you replace plug wires? I just had the same thing happpen and swore my plug wires were new enough to not be the problem. I was wrong! They were arcing somewhere.

Coil ? Could be! Gotta check it all!

 

[Quadruplecccc]

Re: 255 mercruiser runs rough

Oh yeah. Did it happen to backfire on you? If so you could have blown out the power valve in carb. This will kill your idle.

 

[gjr711]

Re: 255 mercruiser runs rough

tried new coil wires etc going to look into the carb again