5.7L mercruiser RPM fluctuates at idle

[tfret]

Cranked up the mercruiser this past weekend and have an issue I've never seen before. After warming up the RPM's at idle ramp up and down between 600 to 1000 back and forth. I shot a video of the tach but don't know how to post it. I'm guessing it's a carb adjustment. Maybe idle screws or possibly the choke. I didn't really have enough time to fully investigate. Was just wondering if anyone had possibly seen this same symptom before? FYI 1984 5.7L, electronic ignition, 4 barrel Q-jet rebuilt last season, electric choke.

 

[Scott Danforth]

Im going to suggest looking at the contents of your fuel filter.

may be an adjustment, however more likely crud in the idle circuits.

 

[tfret]

I've got 3 fuel filters on it, one is the water separator / filter, next an inline filter on the line, then the filter inside the inlet to the carb. Not to mention a screen in the pick up tube. So I doubt anything made it to the idle screws although anything is possible. I'll replace the fuel filters which is probably due any way and see what happens.

 

[TyeeMan]

If it ran well last year and now all of a sudden doesn't, I'm guessing you either picked up some water on the bottom of the fuel tank or have some varnish or other schmuts in the idle circuit.
I'd take it out and cautiously run it for a while, might go away on it's own.

Next tank of fuel add a fuel system cleaner.

 

[Scott Danforth]

if you still have water in the fuel, find out where your getting it. you could be buying it and pumping it into your tank

no need for 3 filters - you only need the fuel water filter. if you want to keep the inlet fuel filter, thats fine. the other just ads restriction to your fuel system. simply check the content of your fuel/water filter.

 

[tfret]

I actually ran all the fuel out last year. This is a fresh tank of 93 non-ethanol. Maybe I'll just take it for a good spin and see where that takes me. May just have some Winter cob webs that need blown out.