cold start on carbureted mercruiser

[Jcris]

I've been having problems starting my carbureted 4.3 LX when cold. After reading this forum I decided to install a momentary switch to run the fuel pump prior to starting the motor. For the first time in too long the motor fired right up. I wondered though, how long will the fuel pump run after I press the switch? And will it shut off automatically when the fuel bowl is full?
Thanks ,
Jcris

 

[Bondo]

I wondered though, how long will the fuel pump run after I press the switch? And will it shut off automatically when the fuel bowl is full?

Ayuh,..... It'll run, so long as there's power to it,...

It never shuts off when the motor's runnin', it just dead heads against the needle valve,....

 

[Scott Danforth]

if your carb is losing fuel, you have a carb issue that you are masking by running the pump prior to cranking.

fix the issue.

 

[Jcris]

So the pump continues to run but fuel only flows when the needle valve unseats?

 

[stonyloam]

Just like with a mechanical fuel pump.

 

[Scott Danforth]

or you have a weak needle and seat causing the fuel to continue to flow, then without the motor running, you wash the cylinder walls down with gasoline because the pump keeps running.

 

[SeaDooSam]

fix the issue.

I agree why don't you fix the problem instead of making an elaborate set up to mask it?
Have you tried using the cold starting throttle only button?

 

[Jcris]

So then , rebuild the carb?

 

[TyeeMan]

Jcris - Do you by chance mean you are having a hard time starting the boat cold after it sat for a few days between uses? If so I might call that normal as the fuel is likely evaporating out of the float bowl in the carb. So when you go to start it, you pump it once or twice but there is no fuel in the bowl to get squirted into the venturies when you pump the throttle. So you need to crank it a few seconds to refill the float bowl, then pump it a couple times, then start.

I had a Volvo 4.3 GS with electric fuel pump and 4 barrel. That was the same way. If I used it every day like on vacation, pump it once, touch the key and she's off and running.
On the other hand, if the boat sat for 3+ days and especially if it was really warm out I would have to crank it for a few seconds to refill the float bowl, then pump it once or twice and I was off.

I was going to do the exact thing you want to do with an electric switch but never did, , I traded this spring. If you do wire in a switch you want to use a momentary push button switch so as soon as you take your finger off the switch the pump stops. It doesn't have to be a push button, but it NEEDS TO BE a momentary switch. As apposed to a "maintained" switch which will stay on until you shut it off.

I was going to wire it such that when I pushed the button that would power the fuel pump relay which would turn on the pump. But like I say, I never did it.

My new to me boat is carburated. Same story except that now I have a mechanical fuel pump so I couldn't wire in a switch if wanted to.

 

[TyeeMan]

I re-read your original post. I see you have a momentary switch. You should be good to go, and as Bondo said the pump will run as long there is power to it, , , or as long as your finger is pushing the button.

 

[Jcris]

TyeeMan,
Yup I installed a switch temporarily. And it does correct the cold start issue. And yes this only happens when the boat sits for several days. I'm tempted to rebuild the carb but for now this works until I park the boat for the winter months. Mine has an electric fuel pump so it was easy to install the momentary switch.
Jcris

 

[JoLin]

Jcris - Do you by chance mean you are having a hard time starting the boat cold after it sat for a few days between uses? If so I might call that normal as the fuel is likely evaporating out of the float bowl in the carb. So when you go to start it, you pump it once or twice but there is no fuel in the bowl to get squirted into the venturies when you pump the throttle. So you need to crank it a few seconds to refill the float bowl, then pump it a couple times, then start.
.

Precisely. Both my engines act the same way, and I use the same starting procedure.

 

[TyeeMan]

You can rebuild the carb but I'll bet you a gallon of gas it will be a waste of time because the fuel will still evaporate out of the float bowl and you will still be using a push button "primer" switch.
My new to me boat has a Mercury Remanufacturing Plus Series engine which comes from Merc with all new accessories, manifolds, alternator water pump even the carb. When I bought the boat this spring the engine was basically just out of it's break in period, now there's probably 50 hours on it.

What I'm getting at is, this new carb (Weber 4 barrel) still acts the same way. If I use the boat consecutive days she pops right off in practically 1 revolution cold. Let it sit a week and I crank and crank and crank. To top it off the Merc manual says to NOT pump the throttle, just crank until it starts. Figure that one out.
The first cold start I did with it I pumped it once and promptly flooded the berjeebers out of it.

It's one of those deals where it is what it is.

Paul

 

[Jcris]

Hello Paul,
There are so many posts on this issue it's kind of confusing.
The previous owner took very good care of this boat. That's obvious the more I get to know it. I'm used to fuel injected motors that are started without pumping the throttle. I'm inclined to agree it would be a waste to rebuild this carb. This Mercarb 2 barrel shows no other symptoms or issues. In the past I've fallen victim to replacing parts that weren't bad. For now I'll be using the momentary switch and just getting used to how it runs. Thanks everyone for the help.
Jcris

 

[achris]

Nothing about rebuilding the carb or anything else will 'fix' this problem. It's a 'design feature' of the Weber carbs used on these engines. They dribble after shutdown until the bowls are empty. I had this problem on one of my engines for many years. Merc won't acknowledge it, carb specialist stop talking to me as they tell me what I see can't happen..... Fitting a momentary switch is a good work-around. Just be aware that pressing the switch also powers the igntion system (due to the way the wiring works).

Chris...............

 

[Jcris]

Chris,

"Just be aware that pressing the switch also powers the ignition system (due to the way the wiring works)."
In what way could this be an issue?
Jcris

 

[achris]

Not an issue, just that someone working on the engine needs to be aware of that. Just as using a remote starter switch on these engine when doing a compression test that person needs to be aware that once oil pressure comes up, the ignition system will be powered. Can lead to a 'surprising' result. ;-)

 

[Jcris]

Thanks Chris,
A couple more questions, please.
When I prime the fuel bowl using the momentary switch, how long is appropriate to fill the bowl? 5 seconds? Is it true that the needle will seat and prevent overfilling the bowl? No matter how long the pump runs? Also, does the pump cycle on and off to fill the bowl or does it run all the time? Dose that make sense? I'm trying to get up to speed on how this carburetor should work.
Jcris

 

[TyeeMan]

You will need to play around with how long to hold your pump switch on. Too long wont be a big deal, too short and you'll just have to crank or push the button to get more fuel into the bowl.
Time between uses of the boat is another factor. I'll guarantee you'll find that if you use the boat the very next day you won't have to run the pump at all as there will be enough fuel in the bowl to start.
Yes, once the float raises the needle high enough to seat it, it will shut off the fuel flow, it's a positive shut off. The perfect example is the valve in your toilet tank. When the tank fills and raises the float high enough the water will shut off and stop filling. There is still water pressure on the back side of the valve but the valve won't let it through. A needle and seat is just another version of an on/off valve.
If your needle and seat are not shutting off properly you will have run-ability issues like the engine running rough/running really rich at idle. Also if it's bad enough you might be able to see fuel dripping down the venturies if you take the flame arrestor off right after you shut down.

The fuel pump will run constantly (just like your car/truck) as long as there is 12 volts going to it. Weather it be from your push button switch or when the engine is running, the pump will run constantly.
BTW - it's not a high pressure pump like in your vehicle. Your car/truck fuel pump will put out 60-80 psi somewhere around there. The pump in your boat is like 6 psi or something like that.

It just takes a bit of playing around and time to learn the best way to start your boat. Unfortunately cold starts take longer to learn cuz your engine doesn't cool down to cold unless they sit for a while.

 

[achris]

Thanks Chris,
A couple more questions, please.
When I prime the fuel bowl using the momentary switch, how long is appropriate to fill the bowl? 5 seconds?

If there is still fuel in the line between the pump and the tank, then 5 seconds would be plenty. If the pump needs to pull fuel from the tank, anything up to 20 seconds.

Is it true that the needle will seat and prevent overfilling the bowl?No matter how long the pump runs?

Yes.

Also, does the pump cycle on and off to fill the bowl or does it run all the time?

No, the pump runs continuously. It's designed to only be able to create flow at a head pressure less than about 7psi. Once the needles close and the pump becomes 'dead headed', although it is still running, it can't overcome that pressure. Pumps create flow, not pressure. Resistance to that flow is what builds up pressure. Once the maximum pressure of the pump is reached, it can't produce anymore flow.

Dose that make sense? I'm trying to get up to speed on how this carburetor should work.
Jcris

These carbs work exactly the same as a car carb. Understand those, and you understand a 'marine' carb. A marine carb is basically the same as a car carb, but with a few safety features. The fuel bowl is vented to the throttle barrels, not open air, there is no vacuum advance, and there is a port open to the barrel for a 'fuel pump diaphragm rupture' line (for the old mechanical fuel pumps)..... That's about it. Jetting is different...

HTH,

Chris...........