1977 55hp starving for fuel at idle?

My engine is having a tough time at idle due to a lack of fuel the lower cylinder.
Here are the facts...

135 psi on top cylinder, 150 psi on bottom cylinder.
Spark present at idle
If I pop the choke several times the second cylinder will fire and the idle jumps up, way up!
If I feather my hand over the lower carb while running I can keep it at idle
Tore the carb off tonight and opened them up, cleanest carbs I've ever seen on a motor that sat for a couple years!
Needles move freely, floats set to level when inverted, passages all clear and clean.


I am considering a rebuild of the fuel pump?

Anything else I should look into?

Thanks in advance for your help!!!!

Pump the primer bulb once and see what happens.

A ruptured fuel pump diaphragm will cause one cylinder to run rich--just the opposite of what you are experiencing. OR, if it is not supplying enough fuel it should affect both cylinders.

I'd suspect a major crankcase leak. Just guessing.

Boobie said:

Pump the primer bulb once and see what happens.

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Forgot to mention that I tried that. Bulb was firm and remained firm with little or no change noticed in run ability

Thank you for the suggestion!

Does spark jump a gap of 7/16" on both leads , yes or no ?

fdrgator said:

A ruptured fuel pump diaphragm will cause one cylinder to run rich--just the opposite of what you are experiencing. OR, if it is not supplying enough fuel it should affect both cylinders.

I'd suspect a major crankcase leak. Just guessing.

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Being the fuel enters the upper carb to feed the lower carb, could low fuel pressure be the issue? A weak fuel pump? Don't like the idea of a major or minor crankcase leak!
I have some experience with snowmobile engines. Typically with those we saw high RPM's due to a lean condition when a crank seal was failing?

racerone said:

Does spark jump a gap of 7/16" on both leads , yes or no ?

Click to expand...

I will have to check that. I no longer have an adjustable spark tester but can say the spark does jump across to a loose plug wire when holding with a pliers. Just unsure of how far that is.