120hp Johnson jerking and high rpm

herbee11

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Jun 13, 2011
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3
When I first take out running the boat starts good and runs across the lake at 5500 rpms. No problem at all. When I leave fishing spot an hour later and run it wide open it starts to jerk like it's cutting out, but when I cut back to 2500 rpm it doesn't do it that often. I have had the carbs rebuilt, the ignition coils changed, spark plugs changed, drained and clean the tank, replaced all gas lines. I'm clueless. I'm going to change spark plug wires as last resort. Help! Running out of money.
 

wilde1j

Vice Admiral
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Apr 15, 2002
Messages
5,964
Re: 120hp Johnson jerking and high rpm

Is it like hitting a log? If so, it may be popping out of gear due to worn clutch dog/fwd gear ot shift linkage out of adjustment.
 

Daviet

Fleet Admiral
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Sep 24, 2008
Messages
8,958
Re: 120hp Johnson jerking and high rpm

You could use an inductive tming light to check the spark on each cylinder while it is missing to make sure you are not loosing spark on a cylinder.
 

levi_tsk

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Dec 26, 2010
Messages
907
Re: 120hp Johnson jerking and high rpm

sounds kinda like a power pack to me seeing how it only does that once it warms up....

how about a model number on that 120hp?
 

Sixmark

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Jul 11, 2010
Messages
890
Re: 120hp Johnson jerking and high rpm

Take the time to test things in a step by step process, it's better to spend the extra time instead of guessing and throwing parts at it!!!!
 

levi_tsk

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Dec 26, 2010
Messages
907
Re: 120hp Johnson jerking and high rpm

true but im just saying thats were id start looking... check the output to each coil while the engine is warm - it will tell you ALOT i might go a step further and heat up the power pack with a hairdryer ....
 

Sixmark

Master Chief Petty Officer
Joined
Jul 11, 2010
Messages
890
Re: 120hp Johnson jerking and high rpm

true but im just saying thats were id start looking... check the output to each coil while the engine is warm - it will tell you ALOT i might go a step further and heat up the power pack with a hairdryer ....

I wasn't implying anything against anyone, it just happens a lot on here where people will post a problem and then only look at a few answers and go directly to that part instead of doing any actual diagnosis first, then they are back later saying "I replaced part "X" and it didn't solve the problem"
 

levi_tsk

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Dec 26, 2010
Messages
907
Re: 120hp Johnson jerking and high rpm

very true im guilty of that one once or twice a milligram of certainty is worth more alot more than 20 tons of guessing
 

herbee11

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Joined
Jun 13, 2011
Messages
3
Re: 120hp Johnson jerking and high rpm

It feels like the engine is shorting out and loosing power. Not really a hard jerk, like hitting something. I'm going to buy an 8 wire spark tester to check all spark plugs to see if its a bad ignition coil or a bad power pack. Will using that dryer really warm up the power pack? Took it out this evening and it started doing it when in neutral when I tried to rev up the engine. I really appreciate the help.
 

herbee11

Recruit
Joined
Jun 13, 2011
Messages
3
Re: 120hp Johnson jerking and high rpm

I did all the trouble shooting from component level. I started at the spark plug wires, ignition coils and then to the power power pack. The power pack was the problem. Put a new pack on and now it works great. Thanks everyone for the help.
 

kenmyfam

Supreme Mariner
Joined
Aug 10, 2006
Messages
14,398
Re: 120hp Johnson jerking and high rpm

I did all the trouble shooting from component level. I started at the spark plug wires, ignition coils and then to the power power pack. The power pack was the problem. Put a new pack on and now it works great. Thanks everyone for the help.

Glad to hear it.
 
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