1987 Merc 150 Idle problem

[Steve1987]

Motor was working well for about 15 hrs of operation this year. Then it began to idle rough. Starts a little harder but not a lot, idles fine at about 1/2 high speed idle or higher. As soon as you take off the high speed idle it will stall. You can actually hear the miss begin at the 1/2 way mark and if you leave it there it will alternate between idling fine and rough. Above that speed motor runs fine, and after it is really warmed up it does improve a bit on the idle, but still not as well as it used to. Have changed plugs (NGK) and filter. Do you think this is electrical or fuel related?
thanks

 

[Chris1956]

Re: 1987 Merc 150 Idle problem

It is likely dirty carbs.

 

[Steve1987]

Re: 1987 Merc 150 Idle problem

It is likely dirty carbs.

Thanks, they were done last year. Should I try some kind of carb cleaner on them? or have them done again?

 

[1nebel0]

Re: 1987 Merc 150 Idle problem

You need to confirm good spark on all cylinders.. first thing I would do along with compression check to rule out issues there...after that carbs........

 

[Steve1987]

Re: 1987 Merc 150 Idle problem

You need to confirm good spark on all cylinders.. first thing I would do along with compression check to rule out issues there...after that carbs........

Update, while idling rough, if I squeeze the bulb it smooths out for a while. Fuel pump??

 

[Chris1956]

Re: 1987 Merc 150 Idle problem

Could very well be fuel pump. Rebuild it and see...