Skipping motor - black plugs

[viper1216]

Boat is a 1988 Four Wynns with a 3.0 Mercruiser.

End of last season, I replaced the head gasket and had the Carb rebuilt. Finished it up this season and it just hasn't run right since. Has a very bad skip at 2k rpm and will not idle. Brand new AC Delco plugs, cap, rotor and wires are all new as well. Also has a brand new Igniter II ignition and Flame thrower coil. I put the new plugs in and ran the engine, Ran great for about 15 seconds....then got a slight skip, and then got bad again. I pulled the plugs and they all black which leads me to believe she's running very rich. The only carb adjustment I see is a screw on the bottom, which I'm pretty sure is the idle adjustment screw. Carb is a mercruiser carb if that helps.

 

[thumpar]

Are you running a full 12v to the Ignitor and coil?

 

[viper1216]

Are you running a full 12v to the Ignitor and coil?

Yup. I ran a wire to the the choke wire to get the full 12v to the coil. I read that on the forum somewhere.

 

[saltybutwet]

did you have these issues before you installed the new head gasket, and the carburetor rebuilt?
lots of things could cause this issue...need more info

 

[viper1216]

did you have these issues before you installed the new head gasket, and the carburetor rebuilt?
lots of things could cause this issue...need more info

Sorry, I explained it a bunch of times on other posts, and forgot I didn't do it here. It wasn't skipping like this prior to the head gasket. Reason for replacement and rebuild was the only time I took it out, it ran great for about 15 minutes, then died and would not restart. Got it home and it fired right up. Found that I had a leak between cylinders 2 and 3 which is why I replaced HG....carb was full of crap so had it rebuilt while I had it off. Been getting the skipping issue since then. I'm thinking it has something to do with the carb. Based on what I saw on the brand new plugs after 20 seconds tells me it's running very rich. Just not sure what would cause that in the carb itself. I read somewhere that the Mercarbs are horrible....hoping I dont need to replace it.'

 

[NHGuy]

Black plugs usually are from very rich mix. Pull the plugs & dry them, no sandpaper though.
Find the Mercarb mix screw info. Or lacking that put it down softly to the stop and then 1.5 turns off bottom. See if it idles there. Once you get a nice low idle turn the mix down to a spot where it stumbles and then back off part or half a turn. See if it accelerates there. Carbs are a lot of feel.

 

[alldodge]

Sorry, I explained it a bunch of times on other posts, and forgot I didn't do it here. It wasn't skipping like this prior to the head gasket. Reason for replacement and rebuild was the only time I took it out, it ran great for about 15 minutes, then died and would not restart. Got it home and it fired right up. Found that I had a leak between cylinders 2 and 3 which is why I replaced HG....carb was full of crap so had it rebuilt while I had it off. Been getting the skipping issue since then. I'm thinking it has something to do with the carb. Based on what I saw on the brand new plugs after 20 seconds tells me it's running very rich. Just not sure what would cause that in the carb itself. I read somewhere that the Mercarbs are horrible....hoping I dont need to replace it.'

I'm thinking you have 2 issues. The first is as all mentioned your running rich. Your running so rich that the idle screw is not going to fix the issue. Either your float level is not set correctly, needle valve/seat problem or there is a leak in the carb, I would guess it's the float level. The next issue is you had a head gasket leak and you just replaced the gasket, did you check to head for flatness, and if so how much was it off?

 

[viper1216]

I'm thinking you have 2 issues. The first is as all mentioned your running rich. Your running so rich that the idle screw is not going to fix the issue. Either your float level is not set correctly, needle valve/seat problem or there is a leak in the carb, I would guess it's the float level. The next issue is you had a head gasket leak and you just replaced the gasket, did you check to head for flatness, and if so how much was it off?

I did have the head checked and machined. I honestly don't remember what the amount taken off was, but I do remember thinking, wow, that was a lot. I'm leaning towards float as well. I had a buddy who is a small engine mechanic do the rebuild for me. I'm thinking he did not measure the float height like I'm learning should have been done. So hopefully that's it. Gonna play around with it this weekend. I'll report back as to if adjusting the float helps.

 

[Bt Doctur]

Try adjusting the mixture screw first.

 

[viper1216]

Try adjusting the mixture screw first.

Are there any mixture screws? If so, where the heck are they!!! All I can find is that one screw on the bottom, which is for idle. Can't find any other screws anywhere.

 

[havoc_squad]

On the Mercarb 2bbl carb used, there is only one idle mixture screw and that is at the base of the carb. It will NOT be near or attached to anything involving the throttle body.

You first need to make sure your Venturi clusters are not dripping at low to mid idle and not dripping after the engine is turned off.

If they are, your idle mixture screw is the least of your concern, it will either be float level or drop incorrect, bad needle and seat, missing seat gasket, etc.

Also, make sure fuel pressure is 5 to 6 1/4 PSI at 1800rpms or lower to the carb before completely disassembling your carb.