Johnson Outboard Fuel Pumps: Any Real Difference?

[70hpjam]

Hello All,

I've searched hi and low on the topic of fuel pump compatibility and interchangeability. All of the three port pumps (gas in, gas out, vacuum) have the exact same physical size, the diaphram is the same, etc. Apart from the size of the nipples, what could possible change the flow ratings between these pumps? I've got a 385784, the replacement is a 348559, a Sierra 18-7353 looks exactly the same. Plenty of people say "no problem - use it!", others say "won't flow enough... WOT will suffer... or worse, not enough oil; your engine will blow!". There seems to be no conclusive answer. Does Sierra really not make a three port pump for applications larger than 30HP?

I've got a 1974 70hp that needs a fuel pump, and I'm looking at a brand new Johnson 438557 for a good price. Unless somebody can tell me this absolutely won't work, I'm very tempted to go with it.

Thanks for your help!

 

[hidef]

Re: Johnson Outboard Fuel Pumps: Any Real Difference?

If you don't flow enough fuel you will impact performance. The 557 is used a lift pump on larger outboards and buy itself on smaller ones the 98 50 used the 557. The 559 is used as a stand-a-lone pump on on the 70 to 140s and is used as the secondary pump on the V6s. Will the 557 do the job for you I don't know? You can try it but I can't say one way or another.

 

[70hpjam]

Re: Johnson Outboard Fuel Pumps: Any Real Difference?

Thanks Hidef.

Actually, I have looked up the applications (all? I doubt it...), but it is pretty hard to tell. In those dual pump set-ups, how does one tell which pump is doing the heavy lifting, or are they sharing the load more or less evenly? 438555 is another application that looks as though it should work. Do the daphrams actually deflect more in some pumps in order to move more fuel? There can't really be that much difference if the pumps are physically the same. I mean in all honesty, if you didn't know the model numbers you would not be able to tell them apart.

By the way, you're pretty close by; I'm near Cornwall, ON. Guess I'll have to add all my personal info.

 

[kodibass]

Re: Johnson Outboard Fuel Pumps: Any Real Difference?

I have put the smaller pumps where bigger ones were and visa versa, upside down , sideways, big nipple reduced, little nipple enlarged etc. ect. never had a problem, the fuel pump sends fuel to the carb bowl, the bowl via needle/seat controls flow rate, I also use minkoni, and briggs pumps and they also worked fine, maybe I have just been lucky

 

[hidef]

Re: Johnson Outboard Fuel Pumps: Any Real Difference?

The 559 does the heavy lifting I believe. The 557 in most applications get the fuel to 559 which makes sure the carbs get filled with fuel constantly. I don't know if the 557 will flow enough fuel it might but I just don't know.