1970 johnson 25hp fuel problem

[Mattster]

I'm having fuel issues with my 25hp johnson. The motor runs great for a minute but runs out of fuel. I already replaced the fuel pump tried a different carb, fuel hose and tank. Any ideas?

[SlappyWhite]

Did you check the vent on the fuel tank?

[Mattster]

Did you check the vent on the fuel tank?

Yes. Not that. Would a crank seal cause it? Or maybe a reed stuck open?

[milliesdad]

Pump the fuel bulb when engine dies. Did you clean the carb or just put a dirty one on?

Check the compression, check the spark wirh a spark tester, check the coils for cracks?

Start with the basics.

[Daviet]

I know you said you replaced the fuel pump, but did you do a pressure and flow test on the fuel pump? How about the pulse to the back of the pump?

[Mattster]

Pump the fuel bulb when engine dies. Did you clean the carb or just put a dirty one on?

Check the compression, check the spark wirh a spark tester, check the coils for cracks?

Start with the basics.

I can keep it running when I pump the ball. The carb was off a good running motor. Very strong spark and compression.

[Mattster]

I know you said you replaced the fuel pump, but did you do a pressure and flow test on the fuel pump? How about the pulse to the back of the pump?

What is the best way to pressure test? Can I just pull the pump off to check the pulse?

[Daviet]

I just plumb in a pressure guage, you can also take the line to the carb and put it in a container to check for flow.
If you remove the pump, the passage behind it should pulse when the piston goes back and forth. You can use a pressure/vacume guage to check it.

[Mattster]

What does it mean if there is no pulse or very weak?

[AlTn]

1.crankcase is leaking...2.piston ring / piston skirt clearance to cylinder wall is too great...3.upper/lower crankshaft seals are leaking....4.reeds aren't sealing properly ....basically anything that produces or contains the pressure or vaccum pulses can lead to a weak or no pulse...do you notice any other symptoms such as leaking fuel mix, oil around the seals, spitting back through the carb, etc?..w/o the plugs in...can you hear the reeds opening with a whooping sound when you pull the motor through?