96 3.0 merc carb alpha one

hardlearner

Seaman Apprentice
Joined
Nov 22, 2010
Messages
40
I am having terrible idling problems with my 3.0 mercruiser. I have rebuilt the carb. Cleaned the tank and filled with clean fuel. Tuned the motor. Installed new water filter. The boat runs fine on the water at 1800 or above. Smooth, good performance. It just does not want to idle. I also have a leak of gear oil coming out a hole in the foot just below the right side of the front bolt on the outdrive. When I say front bolt I mean the front one on the side not the very front one. Any ideas?
 

Bondo

Moderator
Staff member
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Apr 17, 2002
Messages
70,465
Re: 96 3.0 merc carb alpha one

I am having terrible idling problems with my 3.0 mercruiser. I have rebuilt the carb. Cleaned the tank and filled with clean fuel. Tuned the motor. Installed new water filter. The boat runs fine on the water at 1800 or above. Smooth, good performance. It just does not want to idle. I also have a leak of gear oil coming out a hole in the foot just below the right side of the front bolt on the outdrive. When I say front bolt I mean the front one on the side not the very front one. Any ideas?

Ayuh,... Carb rebuilds don't always work out, 'n it sounds like yer's Didn't,...

Try again, or swap out the carb...

There's several seals that could be leakin' in yer drive,...
Ya gotta look, 'n see which 1s...
 

andyspoppa

Cadet
Joined
Sep 26, 2012
Messages
22
Re: 96 3.0 merc carb alpha one

I am having terrible idling problems with my 3.0 mercruiser. I have rebuilt the carb. Cleaned the tank and filled with clean fuel. Tuned the motor. Installed new water filter. The boat runs fine on the water at 1800 or above. Smooth, good performance. It just does not want to idle. I also have a leak of gear oil coming out a hole in the foot just below the right side of the front bolt on the outdrive. When I say front bolt I mean the front one on the side not the very front one. Any ideas?

I bought a SELOC mercruiser book because without one it is hard to fix anything...

According to the book...(check for a vacuum leak at the Carb or Intake Manifold)...crank it up (muffs on and water hose attached) spray some carb cleaner around the base of the carb...if its leaking the RPM will shoot up...same for the manifold.

Don't know about your leak of gear oil...

Andyspoppa
 

wellcraft-classic210

Master Chief Petty Officer
Joined
Sep 22, 2010
Messages
839
Re: 96 3.0 merc carb alpha one

Carbs use different internal passages when at idle to deliver fuel than when at higher RPMS.

The idle passages are in use when your throttle plate is closed. The vaccum increases BELOW the throttle plate and draws fuel from the passage that exits the card below the throttle plate as well. You also need good vacuum for all that to work -- ( no manifold or carb base plate leaks via hoses, fittings and gaskets. ) ( did all the vac hoses get put back on ? )

If the idle passages are blocked it will create the scenario you describe. Cleaning them out with compressed air and gummout jet spray usually does the trick if that is the issue. ( disassesembly required ) ( you can usually hear /feel the air coming out of the other end of the passage(s) ) ( look for any debris in the bottom of the float bowl while your in their -- remove and debris and fix the filtration problem if its present.

When the rpms increase the fuel is pulled from the venturis above the throttle plate. These are activated as air of increased velocity passes through them. The accelerator pump helps make the transition a little smoother by adding a squirt of gas.

Good luck
 

hardlearner

Seaman Apprentice
Joined
Nov 22, 2010
Messages
40
Re: 96 3.0 merc carb alpha one

I have looked for vacum leaks and sprayed around the baseplate. This is a merc carb and there is just a small gasket at the base of the carb instead of a traditional baseplate. The kit I have called for this small gasket and the old one matched. The boat will idle with the muffs on but not once in the water. The boat runs really cool when in the water (120 to 140). I just checked and the thermostat in there is one in the boat. I thought maybe the boat was not getting to operating tempature. After the rebuild and a new water seperator the boat ran great but only for a short time. I thought something in the fuel tank or delivery system but just stumped as of now.



Carbs use different internal passages when at idle to deliver fuel than when at higher RPMS.

The idle passages are in use when your throttle plate is closed. The vaccum increases BELOW the throttle plate and draws fuel from the passage that exits the card below the throttle plate as well. You also need good vacuum for all that to work -- ( no manifold or carb base plate leaks via hoses, fittings and gaskets. ) ( did all the vac hoses get put back on ? )

If the idle passages are blocked it will create the scenario you describe. Cleaning them out with compressed air and gummout jet spray usually does the trick if that is the issue. ( disassesembly required ) ( you can usually hear /feel the air coming out of the other end of the passage(s) ) ( look for any debris in the bottom of the float bowl while your in their -- remove and debris and fix the filtration problem if its present.

When the rpms increase the fuel is pulled from the venturis above the throttle plate. These are activated as air of increased velocity passes through them. The accelerator pump helps make the transition a little smoother by adding a squirt of gas.

Good luck
 

Don S

Honorary Moderator Emeritus
Joined
Aug 31, 2004
Messages
62,321
Re: 96 3.0 merc carb alpha one

I have rebuilt the carb.

Don't take this wrong, but I heard this kind of thing all the time from customers. Only to find they just dumped it out, washed it with gas, and threw a gasket kit in and called it rebuilt. As a marine tech, a quarter of the carbs I ran into were not even rebuildable. The rest had about a 50% success when rebuilt. That includes using dip tanks of carb cleaner, and ultra sonic cleaners to help clean passages.
Then they have to have everything set and adjusted properly. Even to the point of adjusting the idle mixture when installed on the engine. The book settings are only a starting point for the mixture screw/s.

Have you done a compression test? It's also possible your engine isn't running right at higher rpm, you may just not know it.
 

fat fanny

Lieutenant Commander
Joined
Feb 9, 2006
Messages
1,935
Re: 96 3.0 merc carb alpha one

What Don replied is bankable! a re-build consists of a good soak in carb cleaner (maybe twice) thourough blow out with compressed air, re-assembly and the proper adjustments. There are other factors to consider and address timming,compression and a full tune up for this to be successful and even then it's 50/50. Good luck
 
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