Johnson 100hp 4 cly 2 stroke

Jtorg98

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May 16, 2022
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Engine starts easy, runs smooth at higher rpm’s runs rough at idle and low rpm. Compression is 103 to 105 on all cylinders. At full throttle (and I did already check linkage) it doesn’t have a lot of power like it used to. I found a bad ignition coil and replaced all 4 cause they looked bad. The engine used to go to high rpm and now under load it won’t go past like 3500. Thought it might be not getting enough fuel . I tap the electronic choke and it immediately shuts the motor down. So I’m thinking it getting plenty of fuel. And I know that’s not the right way to do that so don’t hate on me. Lol. Anyway I’m thinking maybe it could be reed valves?? Any suggestions. This boat used to be real peppy and quick. Now it’s a slouch. Made no prop changes. Put new plugs in it last year but I will be checking those again anyway. But last year had same problem even with the new plugs.
 

racerone

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Dec 28, 2013
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38,819
Test compression with another gauge.----Those numbers are low,-----Sounds like it is running on 3 of 4 cylinders.-----When was a new water pump impeller installed ?------Does spark jump a gap of 3/8" ( 1 cm ) on all leads ?-----Some will argue, but I would pull bypass covers to have a look at piston and and rings.----Cost you 4 cheap gaskets.----Model # of the motor is ??
 

Jtorg98

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Joined
May 16, 2022
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3
J1009995 is the model number. Water pump and lower unit seals last year. I rebuilt a 140hp for my fish and ski and after I was done it had 110lbs in each cylinder. It took that boat to 65mph no issues.
 

Jtorg98

Recruit
Joined
May 16, 2022
Messages
3
Test compression with another gauge.----Those numbers are low,-----Sounds like it is running on 3 of 4 cylinders.-----When was a new water pump impeller installed ?------Does spark jump a gap of 3/8" ( 1 cm ) on all leads ?-----Some will argue, but I would pull bypass covers to have a look at piston and and rings.----Cost you 4 cheap gaskets.----Model # of the motor is ??
Also I think they u might be right about the 3 of 4 clyinders. Will be looking at that tonight.
 

racerone

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Dec 28, 2013
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38,819
Sounds like a 1979 EVINRUDE model # to me.----Stop running it until you figure out the issue.
 

jimmbo

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May 24, 2004
Messages
13,895
J1009995 is the model number. Water pump and lower unit seals last year. I rebuilt a 140hp for my fish and ski and after I was done it had 110lbs in each cylinder. It took that boat to 65mph no issues.

65mph? How tiny was the boat 13 ft?
 
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