MerCruiser 3.0 Carb Idle Mix Adj

[Jacalore]

Q: The carb mix screw has no effect after about 1-1/2 turns out. The rpms rise about 3-1/2 turns out... any further adj has no affect. Turning idle screw puts the idle back down around 650-700 rpm. Idles a little rough. This doen't seem right? Any help appreciated. Thanks

 

[Don S]

Re: MerCruiser 3.0 Carb Idle Mix Adj

Idle mixture and idle speed are two different things. You have to adjust the idle mixture at idle rpm.
If you adjust the idle mixture and the speed is above idle, adjust the idle speed down and readjust the mixture until both are happy.

It could be the carb needs rebuilt and simple adjustments won't work.

 

[Jacalore]

Re: MerCruiser 3.0 Carb Idle Mix Adj

Thanks, Don, that's the method I used. And this is second carb, same results.
I think I'm at the end of my capabilities...

 

[NHGuy]

Re: MerCruiser 3.0 Carb Idle Mix Adj

That's not the end of the world. Below 1 1/2 turns it stumbles and above 3 1/2 turns it speeds up. I normally expect a stumble when the mix is too fat, but a speed-up is at least a reaction. Split the diff at 2 1/2 turns, full warm and at correct rpms.
ps. a smart check is to shoot some carb cleaner around the carburetor sides, it's base gasket and any intake manifold gasket joints. If the rpms change you could have a gasket leak.
If it speeds up when you shoot cleaner at the outside of the throttle shaft or the choke shaft complain to the place that sold it to you. It should be air tight there.

 

[airshot]

Re: MerCruiser 3.0 Carb Idle Mix Adj

You will get best results if you do this in the water while underway, that is how I had to adjust mine to get it right.

Airshot

 

[Jacalore]

Re: MerCruiser 3.0 Carb Idle Mix Adj

New carb gaskets. Seems to be a linkage problem, in that when throttle is returned to idle, rpm don't return to 700, have to manually push the linkage to get it to return. But that's prolly a separate problem. Still working on stalling at higher rpm. Idle mix adj seems to have an effect on the rpm at which it stalls. Set timing, which was off, but now feel like I'm gpiing in circles.

 

[NHGuy]

Re: MerCruiser 3.0 Carb Idle Mix Adj

Resetting timing requires rechecking idle and mix.

 

[Jacalore]

Re: MerCruiser 3.0 Carb Idle Mix Adj

Yes, thanks, but we did that all afternoon... like a dog chasing his tail.