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1967 Johnson 20hp losses power under load

C

Cjcyrus

New member
Motor starts and runs great, revs up fine, after a few minutes in gear running at speed it goes into a low power state. Take out of gear and revs up fine. Have rebuilt carb installed new fuel pump, new gas tank line w/ bulb. Checked gas tank dip tube and screen is clean, tube is fine. After it sits for a while the same thing happens. I’m going to do a compression check tomorrow, any help appreciated.
 
These motors are famous for cracked coils.----They are either cracked or a new set has already been installed.-----Answer this question.---Does spark jump a gap of 1/4" to 5/16" on a testing device, yes or no ?
 
Good. Also make sure the main jet was properly cleaned and opened up to original size. It may look opened but it may be reduced flow because of deposits within the orifice.
 
Changed out both coils and same thing with slight variation. Before changing them the engine would run under load for 5-10 minutes now it doesn’t. Throttles up high and good when not in gear but under load seems like running on one cylinder. Will do gap test to engine and observe spark. I didn’t change plug wires that, compression test next. And suggestions appreciated. Thanks
 
Changed out both coils and same thing with slight variation. Before changing them the engine would run under load for 5-10 minutes now it doesn’t. Throttles up high and good when not in gear but under load seems like running on one cylinder. Will do gap test to engine and observe spark. I didn’t change plug wires that, compression test next. And suggestions appreciated. Thanks
 
The same video I used changing coils, then points, plugs, condenser, plug wires and new boots with spring connections. I ohm tested all old parts and they are ok.
 
Done and the same. Changed plugs and wires with boots and spring connections. Tested all old components and they test fine, I’m losing my mind, but not quitting.
 
Does spark jump a gap of 1/4" or more on both leads , yes or no ?-----If it does then you perhaps have a blown head gasket, reed valve problem , fuel pump diaphragm ruptured , scored cylinder.----Should be easy to figure out here.
 
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