Fuel dripping from venturi over 2000 rpm??

[IanFS]

'88 Mercruiser V6 4.3L Alpha One

I've been investigating a hestiation/engine miss at idle and also what I can only describe as harsh running at higher revs like 2000 and above, not seriously bad but not smooth.

Plugs seem nice and dry and a good colour with a little soot around the rims. Someone suggested it may be an air leak so I've done the old brake cleaner spray around the joints but no change.

What I have noticed though is that at 2000 and above up to 3000 (didn't want to rev much higher on muffs) the venturi drips fuel very slightly. Is this correct?

 

[Bondo]

Re: Fuel dripping from venturi over 2000 rpm??

'88 Mercruiser V6 4.3L Alpha One

I've been investigating a hestiation/engine miss at idle and also what I can only describe as harsh running at higher revs like 2000 and above, not seriously bad but not smooth.

Plugs seem nice and dry and a good colour with a little soot around the rims. Someone suggested it may be an air leak so I've done the old brake cleaner spray around the joints but no change.

What I have noticed though is that at 2000 and above up to 3000 (didn't want to rev much higher on muffs) the venturi drips fuel very slightly. Is this correct?

Nope, there shouldn't be any drippin',...

Sounds like the carb needs Rebuildin',...

 

[IanFS]

Re: Fuel dripping from venturi over 2000 rpm??

Nope, there shouldn't be any drippin',...Sounds like the carb needs Rebuildin',...

I rebuilt the carb last year with all new bits, however did I get the ball bearing right?

 

[alldodge]

Re: Fuel dripping from venturi over 2000 rpm??

Re-check your float level setting, it may be a bit to high

 

[NHGuy]

Re: Fuel dripping from venturi over 2000 rpm??

If you are talking about the check ball my guru says to take your old check ball and use it to form a new seat by giving it a tap or two with a punch. Then toss the old ball and put the new one in there.
That's something you could recheck. But it should not be doing anything at part throttle. All it does is keep the fuel in the accelerator pump chamber ready to go for when you tip in the throttle.

 

[NHGuy]

Re: Fuel dripping from venturi over 2000 rpm??

Primary drip...hmm. The primary fuel supply comes through the main jets in the bottom of the float chamber and it's rate is regulated by the power piston in the center of the carb. The power piston has a spring on it and is kept down/closed by engine vacuum. When vacuum falls such as under increased throttle openings the piston rises in it's bore allowing more fuel to the primary venturies. When you have the carb apart make sure the piston moves up and down smoothly. And make sure there is good engine vacuum in the intake manifold or the carb won't do it's job.
On carbs as old as ours the vacuum break on the side of the carburetor could be bad. It's easy to check. Push in the link and cover the vacuum port, if the link stays in it's holding vac., if the link moves back out it's a vacuum leak.
Also, before anything, spray some CARBURETOR cleaner around the carb and intake. Brake cleaner won't burn.
AllDodge is on the right track too. The float level needs to be correct or things go wrong.

 

[IanFS]

Re: Fuel dripping from venturi over 2000 rpm??

Thanks everyone, this is a great help. I rebuilt the carb last June with a friend who works with Japanese cars remapping them etc. I bought a Carb kit and we replaced the pump, power jet, idle jet and gaskets. As you can see we were very careful to check the float drop/height, but of course that doesn't mean it is correct now. :facepalm:




I'll check the vaccum first then I may have to strip the carb again. Oh and I can't find any reference in the manual to find the vacuum break on the side, it is a 2BB carb, maybe not the same as yours?

I also don't get this bit, where is the vacuum port, do you mean the Air Horn? And can this be done without taking the carb off.

"On carbs as old as ours the vacuum break on the side of the carburetor could be bad. It's easy to check. Push in the link and cover the vacuum port, if the link stays in it's holding vac., if the link moves back out it's a vacuum leak."

Also which link are we talking about? Also you refer to a Power Piston, do you mean the Power Valve Assembly as this was replaced last year in the kit? Sorry if I'm coming across as a bit stupid.

 

[nola mike]

Re: Fuel dripping from venturi over 2000 rpm??

One thing to mention, I noticed when I rebuilt mine that the float level/drop specs were different in the merc manual v. the supplied (sierra?) instructions. Mine is a mercarb, don't know if yours is rochester or not. IIRC, the merc manual specs were wrong.

 

[nola mike]

Re: Fuel dripping from venturi over 2000 rpm??

See this thread. Sorry, I didn't update what I actually did! But I guess it worked :facepalm:

 

[IanFS]

Re: Fuel dripping from venturi over 2000 rpm??

I sprayed carb cleaner between the float chamber and the throttle body (the middle and lower parts of the carb) and there was a hint of hesitation and revs lowering so I did an extended spray in the area and she almost cut out. This has now made me think there is an air leak at the gasket which was replaced last year.
However would this lead to fuel dripping from the venturi's?

 

[xjdriver]

Re: Fuel dripping from venturi over 2000 rpm??

I sprayed carb cleaner between the float chamber and the throttle body (the middle and lower parts of the carb) and there was a hint of hesitation and revs lowering so I did an extended spray in the area and she almost cut out. This has now made me think there is an air leak at the gasket which was replaced last year.
However would this lead to fuel dripping from the venturi's?

Ya you have a vacuum leak, i would fix that first, but i dont think it would couse the fuel drip.