Mercarb? Need some carb help

[jayhanig]

I've got a 1985 GM manufactured 120 HP Mercruiser engine that has way too fast an idle speed. Looking in the Seloc manual, I think my carb is a Mercarb 2 barrel. I have two pictures of what I believe is the idle adjustment screw that is low on the port side of the carb.



This first one is from the bow looking aft. The red pointer is to what I think I'm looking for.



Here is another view from the port side of the boat looking to starboard.



Finally, this is from the stern looking forward. Nothing I did with linkages would slow it down from 3000 rpm short of cutting it (the engine) off. FWIW, I have to manually push the choke valve in place to start it.

I know the carb looks rough. What do you want? It's 26 years old. I've only owned it since last August.

 

[Bondo]

Re: Mercarb? Need some carb help

I know the carb looks rough. What do you want? It's 26 years old.

Ayuh,... 'n runs like a turd too...

Rebuild it...

The red arrow is pointin' at the idle air mixture screw...
The last picture shows the idle setting screw, on the aft side of the carb...

Once you've Rebuilt the carb, 'n Fixed the choke...
You might have to extend the control cable, or not...

What's the history on the hi-idle anyways,..??

 

[Jmunk]

Re: Mercarb? Need some carb help

I recently rebuilt my Merc carb and man it looked 10x better then yours before the rebuild. I bet if pull the top piece (air horn assembly) the bowl area will be covered in sediment and other nasty things.

 

[Don S]

Re: Mercarb? Need some carb help

I bet if pull the top piece (air horn assembly) the bowl area will be covered in sediment and other nasty things.

Never judge a book by it's cover. If the inside looked like the outside it wouldn't idle at all.

To the original poster.

I've got a 1985 GM manufactured 120 HP Mercruiser engine that has way too fast an idle speed.

Disconnect the throttle cable from the linkage at the carb, then adjust the idle speed of the carb, then adjust the cable to slightly preload pressure on the carb linkage to the idle position.

 

[jayhanig]

Re: Mercarb? Need some carb help

Ayuh,... 'n runs like a turd too...

Rebuild it...

The red arrow is pointin' at the idle air mixture screw...
The last picture shows the idle setting screw, on the aft side of the carb...

Once you've Rebuilt the carb, 'n Fixed the choke...
You might have to extend the control cable, or not...

What's the history on the hi-idle anyways,..??

If the red arrow is pointing to what you say is the idle speed adjustment screw, please note it's not actually touching that cam. It seems to me that if I were to screw it in, it would increase the idle speed, would it not?



As for the history of the idle, the last time I cranked it, it wanted to idle between 2000-2500. Today was even worse; it went right to around 3000 rpm no matter what I did with the carb or the throttle cable.

To further cause me pain, the last time I started it before today, I had to use a charger with the start function. It did crank right up... that was two weeks ago. Today, the first time I tried to start it, the solenoid just clicked. I got the charger and put it on start mode. The engine cranked right up... but with the very fast idle. I shut it down after screwing with it for a couple of minutes. The next time I tried to start it, nothing. No click; no nothing... although the gauges came to life. I guess my solenoid finally gave up the ghost as well.

It's one damned thing after another with this boat although I can't say I wasn't warned. Still, I've gotten fond of the old girl and so I'll get all of this fixed as I identify the problems.

 

[dubs283]

Re: Mercarb? Need some carb help

the linkage on the mercarb is designed to allow for a cold start high idle after you "set" the choke before initial startup by pumping the throttle

that is what the cam is for

once the engine warms up and the choke is open the cam rotates allowing one to set the idle to proper "warm" engine speed

your issue is there is WAY too much preload on the throttle cable

disconnect it and start from scratch - as Don S has stated

my guess is that it has been YEARS since the last FULL tune-up and people have just been compensating for crappy running by continually cranking up the idle speed - which has probably wreaked havoc on the clutch dog in the drive

 

[73Steury]

Re: Mercarb? Need some carb help

If you can't find a Solex manual try an early GM car manual. The is a GM/Rochester 2 barrel carb and was used throughout the 60's and 70's. You may find that the rebuild kit will be cheaper from you local auto parts store verses a marine supply.

 

[fishrdan]

Re: Mercarb? Need some carb help

Check the throw of your shift cable linkage, where it connects to the carb. The linkage should be pulling the carb further closed than it appears. From what I'm seeing, either the linkage, cable shifter is holding the carb open too far (IE: 3000RPM) or the carb is not able to close all the way, rust corrosion, obstruction.

Something else to add to your laundry list, the manifold to riser gasket looks like it's in pretty rough shape. If that goes out the engine could start ingesting water...

 

[Don S]

Re: Mercarb? Need some carb help

the linkage on the mercarb is designed to allow for a cold start high idle after you "set" the choke before initial startup by pumping the throttle

that is what the cam is for

Sorry, there are no high idle cams on marine carbs. That is strictly automotive. Look close, the high idle cam is the same size all the way around.

 

[dubs283]

Re: Mercarb? Need some carb help

the high idle cam is the same size all the way around.

oops!!

my bad - was thinkin of a different setup