Won't idle smoothly. RPM's build, engine pops, rpm's settle: then repeat.

[MrLundt]

I have a 1988 Mercury 100 h.p. 2+2 that I was able to revive last year thanks to some help from this i-boats forum. I need your help again.

My motor is not idling well (in the driveway w/ mufs or in the lake). It idles where I want it for a second but the rpm's gradually build over a 4 to 5 second period until it makes a sound of a backfire and then the idle drops back down to where I want it. This is a repetitive, rhythmic pattern of idling at a good rate, gradually building, "POP" and back down again.

I don't have a tachometer so I can only guess that the rpm's start around 700 RPM and build up to 1200 RPM before the "pop" and it returns to 700. The idle mixture screws (on the top two carbs) are 1 2/3 turns out as specified in the manual (I do have a service manual).

Any idea what is causing this or how to address it?


Thanks much!

 

[MrLundt]

Re: Won't idle smoothly. RPM's build, engine pops, rpm's settle: then repeat.

Does this sound like timing or carb adjustment?

 

[j_martin]

Re: Won't idle smoothly. RPM's build, engine pops, rpm's settle: then repeat.

If I had to guess, I'd bet it's slowly getting leaner, till it coughs, which may make the fuel pump push a bit extra for a bit, etc. etc.

To test the theory, try squeezing the primer bulb and see if it holds the speed down.

Is there anything loose or out of wack in the carb linkage?


Problem could be fuel pump, air leak in fuel line, ......

hope it helps
John

 

[MrLundt]

Re: Won't idle smoothly. RPM's build, engine pops, rpm's settle: then repeat.

Thanks, John. "Cough" is a good word. Not really a backfire but a cough and then rpm's return to "normal" - whatever that is. Fuel pump was rebuilt about 6 yrs ago and the boat may have 50 hrs on it since. It sits a lot. I had all 4 carbs off last fall to dip in carb cleaner. Linkage is good. I'll try the squeeze method to see what happens. Is there any adjustment that needs to be made on the idle mixture screws. Would be nice if it was that simple. Thanks again. I'll keep you posted.

 

[j_martin]

Re: Won't idle smoothly. RPM's build, engine pops, rpm's settle: then repeat.

Thanks, John. "Cough" is a good word. Not really a backfire but a cough and then rpm's return to "normal" - whatever that is. Fuel pump was rebuilt about 6 yrs ago and the boat may have 50 hrs on it since. It sits a lot. I had all 4 carbs off last fall to dip in carb cleaner. Linkage is good. I'll try the squeeze method to see what happens. Is there any adjustment that needs to be made on the idle mixture screws. Would be nice if it was that simple. Thanks again. I'll keep you posted.

Normally you set the idle screws for best idle, then richen them up a tad or 2 till it acelerates good. Depends on the load at hole shot time.

hope it helps
John