Mercruiser 3.0 running to rich

[davidhenson06]

I have a 99 mercruiser 3.0 that is running too rich. I blows lots of black smoke. Idles rough but accelerates fine and seems to run good at higher rpm. Any suggestions?

 

[GA_Boater]

Welcome aboard.

Before tearing anything apart, make sure the choke plate is fully open after the motor is warmed up.

 

[thumpar]

Check the choke and it may be time to do a carb clean/rebuild. The kits are cheap. If the float is bad it will dump fuel also.

 

[davidhenson06]

I opened the choke all the way. Tried adjusting the 2 screws on the side of the carburetor, started off at 1 1/4 turns out and then probably 3 more turns. Hasn't made any difference. I replaced distributor cap and rotor and cleaned plugs with wire brush. After about 5 minutes running they were black again.

 

[JustJason]

Black smoke is a sign of low compression. Do a compression test and report back your results.

 

[alldodge]

Sounds to me that the carb is flooding.

 

[harleyman1975]

Is the carb dripping/dribbling fuel down the throat when idling? if so you have either a stuck needle and seat or a float issue.

 

[bruceb58]

Tried adjusting the 2 screws on the side of the carburetor, started off at 1 1/4 turns out and then probably 3 more turns. Hasn't made any difference.

Those are only for idle mixture. Is your problem at idle or when normal running above idle?

 

[Mercruiser420]

Put a vacuum gauge on it and let know what its doing

 

[fishrdan]

Tried adjusting the 2 screws on the side of the carburetor, started off at 1 1/4 turns out and then probably 3 more turns.

cleaned plugs with wire brush.

3 turns out, you went the wrong way to clear up rich. Turn the idle mixture screws (one at a time) inward from 1 1/4 and see what that does, it should start stumbling from a lean condition, then back out the idle screw(s) to get the best idle. What is the idle RPM? FWIW, I just had to rebuild my carb due to ethanol crud, last rebuild worked for 5 years, but it was idling really bad at the end. Pine-Sol soak + a rebuild kit, and it's running great again.

Cleaning plugs with a wire brush can foul the insulator, put a metallic layer on the insulator that could cause mis-fires. I'd replace the plugs even though it may not be the root cause of your problem....

 

[davidhenson06]

This afternoon I pulled the top off the carb and adjusted the float. I put it back together and reset the mixture screws at 1 1/4 turns out and it didn't help at all. I've only had the boat 3 weeks and the previous owner said the carb was new. After looking inside I can tell it's not new but very clean so I'm assuming it's been rebuilt. The only real carburetor experience I have is putting a kit in a quadrajet 20 years ago and if I remember correctly I couldn't get the needle valve to seal and wound up putting the old one back in. Might just get a kit for this one if I can identify it. Did notice a bow tie stamped on top.

 

[airshot]

While it does sound like a carb problem, why not be smart about this and do the compression test first before spending your hard earned money as they are free.
If the compression is good and spark is good, then look at the carb.

 

[davidhenson06]

What should the compression be?

 

[bruceb58]

You didn't answer my question about when is it running rich. Is it at idle or during regular running?

 

[davidhenson06]

At idle.

 

[RaceCarRich]

Are marine carbs supposed to have Chevy bowties on them? My 2000 3.0 didn't. Wonder if it's the wrong carb that the prior owner replaced which is why he said it had a "new" carb. Just a guess from a boat rookie.

 

[bruceb58]

Are marine carbs supposed to have Chevy bowties on them?

Since the Chevrolet division of GM doesn't make carbs, I would be very surprised if they did.

 

[thumpar]

Make sure you have a marine carb that is made for your engine. They all have different jetting and marine carbs don't have a vacuum line. Does it have the clear line from the fuel pump to the carb?

 

[Fun Times]

What is your engine serial number?
Can you post a photo of your carb?
From new in 1999, your carb should be called a Merc carb and look similar to these first 3 photo'd,http://www.ebay.com/sch/i.html?_fro...TR0.TRC0.H0.TRS0&_nkw=3310-8M0045397&_sacat=0

http://www.mercruiserparts.com/Merc/...OMMON/8166.gif

 

[davidhenson06]

It does have the clear plastic line. Engine s/n 0L097637.