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Johnson 100GT

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JeffHawk57

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I bought a used boat with a Johnson 100GT motor. I put fresh fuel in it. It has a heck of a time idling, at higher throttle setting, it runs smooth and great! But when I idle back, the engine stalls and I have a hard time starting it again. Im suspecting the low speed jets on the carbs, but not sure.. Any ideas from the group?
 
The first thing to do is a compression test.-----Post your numbers.------No point spending any time or beer tokens without a compression test.
 
racerone said:
The first thing to do is a compression test.-----Post your numbers.------No point spending any time or beer tokens without a compression test.
Click to expand...

I did a compression test, looks good. Starboard top cylinder, 115 psi
Starboard bottom cylinder, 120 psi. Port top cylinder, 110 psi. Port bottom cylinder, 120 psi.

Looks like I'm getting unburnded full and oil from the exhaust when it is running, and sounds like it's missing. I'm wondering if ignition coils and/or wire are at fault.
 
You might want to remove bypass cover on top port cylinder,( $4.00 gasket ) and look at piston and rings.----Does spark jump a gap of at least 3/8" on each lead ??-----Use a timing light and see if you have steady spark on all 4 leads.
 
Last edited:
racerone said:
You might want to remove bypass cover on top port cylinder,( $4.00 gasket ) and look at piston and rings.----Does spark jump a gap of at least 3/8" on each lead ??-----Use a timing light and see if you have steady spark on all 4 leads.
Click to expand...


I used a spark tester, all 4 cylinders show spark, can't really tell if it a strong spark with the tester. Maybe the float(s) are sticking and flooding the engine.
 
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