FUEL PUMP OUTPUT - just need to verify this with the experts

[Seahorse16]

I've been having problems with my 1976 Johnson 15hp fuel supply. If I pump the bulb it will fill the carb bowl and stay hard, but once I fire it up, it will die after a minute or so if I don't keep pumping it. I read through several other posts on this issue and they provided the following troubleshooting list:

Air leak in hoses; kink in hoses; vent in gas tank; cylinder compression on fuel pump cylinder; needle value and float sticking; spring on metal flappers in carb; and fuel tank cracked tube; and output of fuel pump.

I recently cleaned and rebuilt the carb (FYI I was having this issue fuel issue before I rebuilt the carb), replaced the filters on the fuel pump, replaced all the fuel lines, bulb, and connections, and just checked the gas tank vent for airflow and for cracks in tube (internally). Assuming the carb needle valve and spring are ok (was careful with these and tested them), that leaves the fuel pump, cylinder compression, and possibly the springs on the carb valve flappers.

I removed the fuel line from the carb and pulled the starter 6-7 times. Only once did a little bit of gas come through the line. Does this indicate my fuel pump is the problem??? Or could it be compression too? It runs great when first fired up so doesn't that indicate that compression would not likely be a problem?

I guess I'm just wondering if I should order a fuel pump for $67 before doing anything else.

Trevor

 

[boobie]

Re: FUEL PUMP OUTPUT - just need to verify this with the experts

Check the compression on it and if that's okay, order a fuel pump.

 

[racerone]

Re: FUEL PUMP OUTPUT - just need to verify this with the experts

But the fuel pump runs from crankcase compression pulses and that is not the same as cylinder compression.

 

[Bosunsmate]

Re: FUEL PUMP OUTPUT - just need to verify this with the experts

But the fuel pump runs from crankcase compression pulses and that is not the same as cylinder compression.

But if you didnt have compression happening in the bottom cylinder (i think thats the one with fuel pump hole in it) you wouldnt get any pulse happening on the pump diaphram would you?
I would think that with no compression there would also be no movement of air/fuel in the lower crankcase to provide the pulse into or out of lower crankcase fuel pump port.

 

[Bosunsmate]

Re: FUEL PUMP OUTPUT - just need to verify this with the experts

When you say it runs great do you mean at wot in the water or just at idle on muffs at home?

If you dont have a compression tester try putting your thumb in each plug hole and checking you get a good force in both cylinders especially notice if the bottom is much less

 

[racerone]

Re: FUEL PUMP OUTPUT - just need to verify this with the experts

If the bottom crankcase seal is KAPUT then you will have low crankcase compression.-----Compression at the sparkplug will be just the same as the top cylinder !

 

[Bosunsmate]

Re: FUEL PUMP OUTPUT - just need to verify this with the experts

If the bottom crankcase seal is KAPUT then you will have low crankcase compression.-----Compression at the sparkplug will be just the same as the top cylinder !

Yes yes lower seal can go and you will lose crankcase compression, i dont know an easy way to test for that, but i still dont understand how crankcase compression can be ok if compression at the spark plug was 10psi for instance? Are you saying the state of the piston and rings has nothing to do with crankcase compression?

I still think its his fuel pump but worth checking for compression and the lower seal too before he forks out all that money if you know a tip for checking lower seal on what is i think a one carb motor???

 

[Seahorse16]

Re: FUEL PUMP OUTPUT - just need to verify this with the experts

It runs good in a water tub in neutral. I will check the compression tomorrow. Is it important to warm up the engine before testing? What ballpark should I see. I know comparing between the two cylinders is most important. I will post my findings tomorrow and I really appreciate any and all advice.
Trevor

 

[Seahorse16]

Re: FUEL PUMP OUTPUT - just need to verify this with the experts

Yes, single carb motor

 

[Bosunsmate]

Re: FUEL PUMP OUTPUT - just need to verify this with the experts

Maybe Trevor could also put his finger just over the fuel pump hole and feel the pulses? Or watch the diaphram move in and out and feel for any holes in it?

 

[Bosunsmate]

Re: FUEL PUMP OUTPUT - just need to verify this with the experts

No dont bother warming it up Trevor.
That seems a bit steep a price for a fuel pump.
Ive a 60hp, the renewable parts are about $20

 

[Bosunsmate]

Re: FUEL PUMP OUTPUT - just need to verify this with the experts

I was thinking to test lower seal you could pump the carb full with the bulb, pull out the choke, ground the two HT sparkplug leads on to the engine and then pull the starter handle two times. Then remove the bottom plug. If its wet with fuel your bottom seal and compression are probably ok to run the fuel pump so it would suggest its just your fuel pump.
Have a good day!

 

[w2much]

Re: FUEL PUMP OUTPUT - just need to verify this with the experts

Raise your fuel tank and hose above the carb and let it gravity feed the carb . You may want to bypass the fuel pump and feed the tank straight to the carb if you can adapt the hose to fit at the carb . This will eliminate the fuel pump issue. If it runs well the pump is bad.

 

[kbait]

Re: FUEL PUMP OUTPUT - just need to verify this with the experts

Agree with w2much^. Once your sure you have no connector/hose leaks, elevate tank above carb and test run. Should keep running and keep carb bowl full. Now, place fuel tank below carb (where the bottom of boat would be) and see if it dies from no fuel to carb bowl. If it dies w/empty bowl (you have to pump bulb to re-fill), replace/repair your fuel pump.

I wouldn't get all caught up w/crankcase pressures.. 99.9% of the time it's just a bad pump (if tank/hoses/connections are known good).

Good luck!

 

[Seahorse16]

Re: FUEL PUMP OUTPUT - just need to verify this with the experts

Ok, so before when my gas tank was in the bottom of my boat I needed to keep pumping the bulb to keep fuel flowing into the carb bowl and it would die after 20-30 seconds. I raised the gas tank above the carb and it did keep running (although I still need to play with high speed and low speed adjustments). Attached is a link to a video of it running with the gas tank raised. So does this definately suggest fuel pump issue? That being said, I thought my bulb was staying hard, but upon further inspection it is stays firm but not rock hard. Could this also be because the fuel pump valve is shot?

I did do a compression test. I was using an old school tester that only has a rubber stopper end and does not thread into hole, so it may not be the best, but I had 68-70 PSI in both cylinders. I don't know what "normal" should be. I saw a post for a 15 HP mercury and he was getting around 100 PSI.I will point out that a few times I didn't have it sealed good enough and it shot the tester across the garage!

So, if you guys recommend fuel pump "Bosunsmate" thought $67 was too steep. I cannot find the rights part here on iboats for my year and model. ********** sells the entire pump for $67. Do I only need the inner seal?
Johnson Fuel Pump Kits
Johnson Fuel Pumps
http://www.**********/parts/search/BRP/JOHNSON/1976/15R76R/FUEL PUMP/parts.html

Thanks again everyone for your suggestions!

 

[Seahorse16]

Re: FUEL PUMP OUTPUT - just need to verify this with the experts

Also, I've read about running "Engine Tuner" through the motor. Since I don't know the history of this motor would you recommend that? Is this something I just put in with the gas?

Thanks

 

[racerone]

Re: FUEL PUMP OUTPUT - just need to verify this with the experts

Just got in 4 diaphragms ( 331307 ) to repair these pumps , just a few $3 each.

 

[Seahorse16]

Re: FUEL PUMP OUTPUT - just need to verify this with the experts

Racerone so this is the same part I need? So the diaphragm should be all I need? Is that off iboats?

 

[racerone]

Re: FUEL PUMP OUTPUT - just need to verify this with the experts

Inspection is needed to determine what parts you would require.----Not much can go wrong with these simple pumps.----Make sure you catch the wee parts when it comes apart.

 

[racerone]

Re: FUEL PUMP OUTPUT - just need to verify this with the experts

Part # can be found on -------shop.evinrude.com