4.3 Mercruiser vapor locks

[Frankjones28]

My 95 4.3 runs great all day. When it sits overnight it cranks just fine but won'y start. I replaced water separator. Still wouldn't fire and no gas in carb. I sprayed a little starter fluid in carb and boat starts and runs fine. Next day, same thing, fuel separator was 3/4 filled when removed. I fill it to the top. Spray a little starter fluid and it starts again and runs fine all day. Thoughts?

 

[alldodge]

fuel separator was 3/4 filled when removed

It's not vapor lock, my guess is your fuel pump is failing. Get a vacuum fuel pump pressure gauge and put it inline between the carb and fuel pump.

 

[thumpar]

Doesn't sound like vapor lock. Vapor lock is when the engine is hot and heats up the fuel in the line to a boiling point. Is your fuel bowl empty too?

 

[NHGuy]

Carbureted engines must be pumped before cold starts. Have you been pumping the fuel a few times?
Is your choke closed when the engine is cold? or ever? Some folks just set them to stay open in boats and deal with a little roughness on starts.
If it's not closed when cold, adjust it to be closed cold, and open warm.
But truthfully there's more chance that what AllDodge says is the solution. It's just easy to check the choke.

 

[alldodge]

Ya know being 3/4 full you might have a leak and its drawing air in along with the fuel

 

[Frankjones28]

where would the leak be. in the fittings? I went out today. I hadn't been out since Sunday. Once I got it started. It ran fine at all speeds and started after being shut off for 30 + minutes. Thoughts.

 

[NHGuy]

If it sits overnight can you crank it for 15 or 30 seconds and get a start? If that's the case your fuel bowl is emptying overnight. Like a leak or just evaporation. Where are you writing from? Somewhere dry?

 

[alldodge]

where would the leak be. in the fittings? I went out today. I hadn't been out since Sunday. Once I got it started. It ran fine at all speeds and started after being shut off for 30 + minutes. Thoughts.

For the filter to once be full and then be 3/4 later, I'm leaning more to a leak in either the fuel line, a fitting, or the filter gasket. The pump is trying to pull fuel thru the lines and filter. If there is a slight leak in the path, it will be sucking air along with fuel as it works.

 

[achris]

Sounds like the standard 'Weber on a V6' problem. Those Weber 4bbl carbs are well known and documented for their habit of dribbling fuel until the float bowls are empty. I had a 94 V6 with same carb, and the same problem. Took me years to figure it out, but when I did, it's not fixable. I was contemplating swapping the Weber out for a Edelbrock 1409, but in the end, I opted to change out the whole engine, to an MPI.

Here's my thread from 12 years ago (I think it was my first ever post. Look where it lead me :facepalm.... Enjoy the read. http://forums.iboats.com/forum/engin...carb-on-4-3-lx

Chris.........

 

[Rick Stephens]

I added an inline check valve between water separator and fuel pump - part of the reason was I run a kicker off the same water separator and it would pull the electric fuel pump dry. I would add one to your setup in a second, keep the fuel from draining back into the tank. You might also just pull and clean your anti-syphon valve, which does the same thing unless it has crud in it not allowing it to seal off back flow.

Rick

 

[achris]

It's not fuel flowing back from the carb, it's the carb continuing to allow fuel through the main discharge cluster after shutdown. Years ago I had a Weber with the top off on the bench. I filled both float chambers and left it overnight. Next morning, both still full. I then blew air down ONE primary throttle barrel, simulating a running engine. I refilled the float chamber for that barrel and left it overnight. Next morning that chamber was empty and all the fuel was on the floor. The other chamber was still full. I then refilled the chamber and blew air down it again. When I remove the air flow, the fuel continued to dribble from the main venturi cluster. It was dripping into my hand! That's when all the 'experts' told me that can't happen. I watched it happen! Only conclusion I could come up with was capillery action. When I examined all the drillings and passages they are all VERY VERY small. I even contemplated drilling them out to stop the action, but realised I'd need a flow-bench to set the jets and tubes to get it all back to the original calibration.

Chris........

 

[Rick Stephens]

It's not fuel flowing back from the carb, it's the carb continuing to allow fuel through the main discharge cluster after shutdown.
Chris........

I can't disagree with that. I would only add that the OP noted his filter was half full. That is more likely flow back and prevents the pump from getting primed.

Rick

 

[Frankjones28]

Rick,
By adding the check valve, would Ithat replace the steel tube between the separator and the pump? I think the back flow is the problem as the pump is dry each morning,

 

[Rick Stephens]

Yeah. I used rubber hose and push on brass fittings with clamps. My separator is on a mount I made. If yours is on the motor you may want to mount the check valve right to the out port on the separator.

 

[Bondo]

I would only add that the OP noted his filter was half full. That is more likely flow back and prevents the pump from getting primed.

Ayuh,.... For the same reasons a carb can't back-flow/ siphon out, neither can the fuel filter,....

A siphon can't jump an air gap,....

A 1/2 full filter is more indicative of an air/ vacuum leak, 'tween the fuel in the tank, 'n the filter,....

'n the only check-valve ya need goes on the tank's outlet, We call it the anti-siphon valve,.....

 

[Rick Stephens]

IF the anti symphony valve is tight and squeaky clean it will prevent back flow. If there is no properly working back flow device, as soon as the needle valve opens the fuel will start back.

 

[Bondo]

Ayuh, but the filter can Not siphon out, as the fuel enters, 'n leaves from the top of the filter,.....

As soon as any gas in the gas line to the carb back-flows to the point it's level with the top of the filter, the flow stops,....

Gas can Not jump the air gap to the top of the filter to get out,.....

I stand by the filter bein' 1/2 full indicates a air/ vacuum leak, 'tween the fuel in the tank, 'n the filter,....

Vacuum leaks don't necessarily leak gas out, only air into the system, causin' a loss of prime,....

 

[hogbare35]

I agree with Rick & Bondo . You should only need the anti-siphon valve at the tank . Is probably an air or vacuum leak . I have had no experience with weber carbs but I have with edelbrocks. They are simple to work on and very reliable .

 

[Bondo]

I have had no experience with weber carbs but I have with edelbrocks. They are simple to work on and very reliable .

Ayuh,.... They are quite literally one, 'n the same carb,....

I don't buy Merc parts for 'em,....

I buy Edlebrock parts,.... Much cheaper, Better choices, 'n options,....

 

[Frankjones28]

ok. pulling it out today. i will let you know. i would love to correct w ant siphon valve and skip the new fuel pump.