Fuel pump question

[SweeperForce]

I have a questions about the three ports on the pump housing. The first port is the intake and it has the word 'FILTER' on it. The second port is the output which is right beside the first. My question is what does the third port do? It looks like runs into the middle of the motor behind the carbs. Here are some pics to better explain what I am asking.

Thanks,
Tom

 

[Unleadedaddict]

Re: Fuel pump question

It's there incase the diaphram ruptures. It probably goes to intake vacuum so it wont drain into the boat.

 

[jerryjerry05]

Re: Fuel pump question

It's part of the fuel recirculation system.All 3/4/5 cylinders drain unburned fuel back into the pump.
If the diaphragm goes bad it will go direct to the cylinder through the pulse/vacuum.Making it run bad.J

 

[SweeperForce]

Re: Fuel pump question

Thank you gents. I also took my two pumps off the motor and took them apart to inspest the diaphragm. Both were still intact but the one looked quite worn, so I replaced it. Plus the check valve washer was all torn up which also need replaced. I though for sure my diaphragms were ripped due to all the backfires I encounted last season.

Thanks,
Tom

 

[Frank Acampora]

Re: Fuel pump question

WHOA! The third port is connected to the crankcase by a short hose about 3-4 inches long. It supplies pressure and vacuum to the smaller second stage of the fuel pump. Without it connected the engine will run but may not get enough fuel to run at full throttle.

In your case, you have a twin pump engine and if either pump has a failure, the engine may run but at reduced capacity. The lower punp is serviced by the lower cylinders and the upper pump is serviced by the upper cylinders. In any case, the third nipple or port as you call it is USUALLY connected to the cylinder 180 degrees apart from the first stage (in a 4 cylinder engine) or 146 degrees apart (in the case of a 5 cylinder engine). Can be different depending upon how the system is plumbed.

The lower pump supplies fuel to the inlet or filter of the upper pump which then supplies fuel to the carbs. You can--it is preferable-- add a cheap plastic filter AFTER the second pump and remove the screens from the two black plastic "filters". Less pressure loss and better filtration. In the second photo, you do have a cleanable glass filter so the screens can be removed and your pulse line appears to be attached to a different crankcase port than on older engines.

 

[SweeperForce]

Re: Fuel pump question

So I can remove the metal screens inside the pump to give me a slight improvment in power? If so that be awesome. I am however a little confused about the third nipple being 146 degrees apart from the first stage. Is this due to the design if the pump, or it this something I must adjust. I replaced hoses coming from the tank to the pump and the pump to the carbs last year. And this year I want to replace the rest of the fuel lines. Sorry for the simple questions but I don't know much about outboard fuel pumps.

Thanks again,
Tom

 

[Frank Acampora]

Re: Fuel pump question

Removing the screens will not give more power: Removing them will ensure better fuel delivery to the carbs because of less restriction.

Now, on to your 146 degree. I chose 146 degrees because I did not have a calculator in front of me and the crankpins on the 5 cylinder are 72 degrees apart (I thought 73). Thus the most difference available is 144 degrees.

Now, operation of that type fuel pump. It is the Chrysler "B" type pump. It has two stages: The first stage supplied with alternate pressure and vacuum through a small port or hole in the body of the bypass cover, under the large diameter portion of the diaphragm. The second stage (with the small diameter portion of the diaphragm) is supplied again with alternate pressure and vacuum pulses through the third nipple through the hose, from a different cylinder crankcase.

In the ideal world of the two or four cylinder engine, these pulses are 180 degrees apart because the crankpins are 180 degrees apart. Thus: While stage one is under pressure and pushing fuel out to stage two through the center button valve, stage two is under vacuum and trying to suck in the fuel.

In the less than ideal world of the three cylinder engine, the pulses are about 120 degrees apart and in the five cylinder engine, ideally they would be 144 degrees apart.

In your photo, pump one is feeding fuel to pump two which will help increase the delivery amount. This is as it should be. Both pumps derive their pressure and vacuum pulses from different cylinders, again, as they should.

The important thing is that the third nipple must be connected to a different cylinder than the pump body as your photo shows it is.

I have not worked on the fuel system of the 5 cylinder engine and did not study the pumps. Since the crankpins are only 72 degrees apart, the plumbing of the hoses will be different than on the 2, 3, or 4 cylinder engines.