Need to pump throttle even when warm

H

headnsouth

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Joined
Apr 5, 2007
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25768
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53
98 Mercruiser 5.7 with carburetor.
After sitting for a weekend I expect to have to pump the throttle to get fuel to the carburetor.
But after it's all warmed up I still have to pump the throttle to get it to start.
Is the fuel draining back that quickly?
Why is it doing this? BTW it's always done this. Choke has been adjusted.

No performance issues besides this.
Would an inline check valve help?
Fuel pump is original.
 
Sounds like the carburetor float is not working. Might be time for a rebuild kit.
 
I though that is how Carb works, when cold, pump the gas pedal to do two things, set the choke and give the intake manifold some gas to distributes to cylinders, when warmed, pumping gas pedal(not as much as when it's cold but stills needs) to give some gas to the manifold/cylinders. Just like fuel injected engine(no choke but IAC instead of), instead of pumping gas pedal when cold/warm, the injectors would spray some fuel to the manifold when the key is turns to on. I may way off base here since the last time I play with Carb was back in the early or mid '80.
 
quote:
Originally posted by DeeVee

I though that is how Carb works, when cold, pump the gas pedal to do two things, set the choke and give the intake manifold some gas to distributes to cylinders, when warmed, pumping gas pedal(not as much as when it's cold but stills needs) to give some gas to the manifold/cylinders. Just like fuel injected engine(no choke but IAC instead of), instead of pumping gas pedal when cold/warm, the injectors would spray some fuel to the manifold when the key is turns to on. I may way off base here since the last time I play with Carb was back in the early or mid '80.

Click to expand...

I never had to pump the pedal on my vehicle's unless there was an issue.
 
When fully warmed and you are about to try to restart, 1st take off the flame arrestor, hold the choke and throttle plates open and pump the carb throttle once.
Can you see a strong spurt of gas down the throat from the accelerator pump?

If so, you have sufficient gas sitting in the bowl. If not, perhaps you have classic Quadrajet leak down in which case the cylinders may actually already be flooded . If you suspect the latter, without pumping at all try moving the control throttle lever to say 3/4 or more throttle position and see if it starts faster ( being ready to reduce to idle position immediately.)

If you don't see the squirt of gas, your 1998 likely has an electric fuel pump so instead of pumping try cycling the ignition key from OFF to ON/RUN several times without trying to actually start.
That should cycle the pump and hopefully ensure fuel pressure is sufficient to the carb.

Then move the control lever all the way forward and back & try starting with no or maybe one pump .

Are plugs and dist. cap and rotor in very good condition and timing to spec. to optimize chance of initial ignition? And fuel is fresh, not degraded?

Your boat's fuel supply system likely already has a "check-valve" ( besides the carb float valve) in an anti-siphon valve (ASV) at the top of the tank where the distribution line attaches. It is required any time the fuel supply line dips below the level of the top of the tank at all on its way to the engine's fuel pump. FWIW- A little debris in the float valve & ASV could allow drainback.
 
Thanks for the information. It's starting to warm up here in Georgia so as soon as temps allows I'll check all of the above.
Pretty sure I have a mechanical fuel pump.
 
Try just pushing the throttle all the way forward just once and then return it to a position just off idle before starting.
Repeatedly pumping shouldn’t be necessary and results in flooding causing the cold engine to run rough or stall.
 
I'm hoping for the weather to warm up here soon so I can get on this project. Thanks to all that have put in their suggestions. I'll check it out as soon as I can. And I'll report back.
 
A slight chance the choke is adjusted too tight. When warmed up, check that the choke plate is fully open.

Another thing to check, air leak in intake side of fuel delivery between carb and filter..check clamps. Won't get to the anti siphon valve yet...starting with difficulty of access.
 
Cold start this morning in the driveway after going out yesterday. Pull the flame arrestor off pumped the gas and plenty of gas squirted into the carb. :) Mid 80's temps and no choke movement. Choke has voltage going to it. Adjusted the choke and freed it up a bit. Not much restriction on movement but I figured less is better. Won't have time this weekend to give it a test drive but I'll report back with my findings.

Thanks for of the help.
 
If its a '98 then it should be a Mercarb/Weber carb. Try just pulling out the shift handle to disengage the shift system and putting the throttle forward about 1/4 or so. It should start then. Every carbed car I ever had was like that (always opened the throttle a bit to start hot) and so is my OMC (4bbl Q-Jet). Not pump just set it at 1/4-1/3rd throttle open.
 
It may also be a two barrel carburetor, he never answered that question. I have a 98 MerCruiser and mine is a two barrel. I have found the carburetor to be very reliable.
 
You already confirmed that your electric choke is getting power?
If you are starting the cold engine properly (throttle forward all the way just once then pulled back to just above idle before turning key) it should start.
You mentioned that it’s also giving you a problem starting when warm.
You shouldn’t have to pump it when it’s warm. Just advance the throttle a bit to start a warm engine.
You’ve already verified that you are getting a spray of fuel when you pump.
Continually pumping will only flood it and make starting even harder,
Might be time for a rebuild again.
 
If you have not changed your water/fuel filter recently, change it. If you have never changed the internal carb filter, change it. You must get the right one and don't just remove it. You might find it at West Marine or Pep Boys. Clean all of the linkage with carb cleaner and spray some on the butterflies as the engine is running at about 1000 RPM so it does not stall. I had some water in my fuel once and it ran crappy but after I got through the crappy fuel, the carburetor has been flawless without any rebuild. Remove the flame arrester and spray carb cleaner in it from the inside of the fins out. Shut it off and then come back the next day when it is cold. Remove the flame arrester and carefully open the choke butterfly so you can see down the carb. Do not turn on the ignition so the fuel pump does not operate. Push the throttle and the fuel pump should squirt gas in the carb. If there is no squirt of fuel then you might need a rebuild.
 
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