Basic Mercruiser 4.3 Carbureted Fuel System Questions

[jtw]

Hi All! I have a few basic fuel system questions about a 1998 Mercruiser 4.3 LH I/O.

Is it true that the electric fuel pump does not work until there is oil pressure? In other words, the electric fuel pump doesn't run until after the engine has started.

When you pump the lever before starting the engine, where does the accelerator pump get its gas from? The bowl in the carburetor?

After a long layup, how does the bowl fill up?

Under normal conditions, how many weeks would gas remain in the bowl?

Thanks.

 

[alldodge]

Re: Basic Mercruiser 4.3 Carbureted Fuel System Questions

Hi All! I have a few basic fuel system questions about a 1998 Mercruiser 4.3 LH I/O.

Is it true that the electric fuel pump does not work until there is oil pressure? In other words, the electric fuel pump doesn't run until after the engine has started.

No and Yes, an electric fuel pump is turned on when the key is turned to start while cranking. The newer systems allow the pump to turn on for a short time before the engine is cranked the first time. Otherwise, once the engine starts the key is released to the run position and the pump is turned off until oil pressure rises. The oil pressure rises in a few seconds and with the main fuel system being charged it will last until the pump returns to full system pressure. For the short meaning, no one notices.

When you pump the lever before starting the engine, where does the accelerator pump get its gas from? The bowl in the carburetor?

The accelerator pump gets its gas fro the fuel bowl. If the fuel bowl has gas the accelaerator pump will push gas when depressed.

After a long layup, how does the bowl fill up?

When you crank the engine the first time it will take a bit to fill the fuel bowl up and start the engine.

Under normal conditions, how many weeks would gas remain in the bowl?

I would say no more then a few days

Thanks.

Hope this helps

 

[jtw]

Re: Basic Mercruiser 4.3 Carbureted Fuel System Questions

OK. But I think a 'normal' starting procedure is to pump the lever completely twice, leave the throttle a little forward of idle, and turn the key. So, if the boat has been sitting for a week and the bowl is empty, it will never start on the first try. With an empty bowl, those two 'pumps' of the throttle will do nothing to 'prime' the carb. Does that sound right?

 

[Don S]

Re: Basic Mercruiser 4.3 Carbureted Fuel System Questions

I never had problems with my boat not having gas in the carb after 2 or even 3 weeks. Two pumps of the throttle and a little throttle and it started on the 2nd or 3rd revolution every time.

If you don't have fuel in the bowl for 2 weeks, then you should fix the leak.

One additional note the only time the fuel pump comes on before the cranking starts is with EFI engines. The pump runs for 2 seconds to prime the injectors.

 

[jtw]

Re: Basic Mercruiser 4.3 Carbureted Fuel System Questions

Here' the latest:

The boat has been idle for over a month and won't start. When I pump the throttle, the accelerator pump does not squirt gas into the carb. When I prime the carb with gas, the boat starts, then dies. Strangely, the fuel pump seems to be running with the key in the 'on' position.

So it seems bad fuel pump or dirty carb. I guess I should check the fuel pump discharge pressure first. (Is there one that I can leave in place for future troubleshooting?) And if the pump is ok, rebuild the carb. Any easier or cheaper suggestions?

 

[Don S]

Re: Basic Mercruiser 4.3 Carbureted Fuel System Questions

Well, the pump shouldn't run with the key on. That is one problem that needs fixed, but not the running problem. BUT, if the pump is running and not filling the carb. Did you check the fuel pump pressure? How about the water separating fuel filter? Are you getting fuel to the pump?

 

[jtw]

Re: Basic Mercruiser 4.3 Carbureted Fuel System Questions

Thanks for the help, guys. The problem was a plugged up fuel pump. No gas to the carb, no fuel in the bowl, so when I pumped the throttle, there was no gas squirting into the carb.

The fuel in the pump seems to have turned into a 'gel'. When I left the pump in the hot sun, all of the fuel came out as a liquid. The pump is working again.

The pump running with the key in the 'run' position and the engine stopped seems to be a one time glitch. I can't reproduce it.

 

[oldjeep]

Re: Basic Mercruiser 4.3 Carbureted Fuel System Questions

Thanks for the help, guys. The problem was a plugged up fuel pump. No gas to the carb, no fuel in the bowl, so when I pumped the throttle, there was no gas squirting into the carb.

The fuel in the pump seems to have turned into a 'gel'. When I left the pump in the hot sun, all of the fuel came out as a liquid. The pump is working again.

The pump running with the key in the 'run' position and the engine stopped seems to be a one time glitch. I can't reproduce it.

Was the gel red?