Miss at High RPM's

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jlw1972

Seaman Apprentice
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Feb 15, 2009
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39
Hi all. I've got a 1997 Johnson 115 crossflow. Motor starts and runs great. I've owned new motors that didn't run this good. My problem starts around 4500 Rpm's and continues up through 5500rpm's. It will start to have a miss like a cylinder is losing fire. And it doesn't do it until the engine has run for a while. My plug wires is where I'm thinking it is so I ordered new ones tonight. If that doesn't fix it, what would be my next move? Maybe coils? Any advise? Thanks.
 
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emdsapmgr

Supreme Mariner
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Dec 9, 2005
Messages
11,551
Weak ignition components tend to fail once they heat up to normal operating temps. Run fine when cold, then crap out when warmed up. When it's warmed up, you may need to check some of the ignition components, esp the stator. You can find some excellent ignition troubleshooting info for your engine at this website: cdielectronics.com. An automotive timing light may be helpful to determine which cylinder is misfiring, or if they all are, randomly. The flashes from the gun will show weak spark, intermittents, etc. Be sure the engine is misfiring (warmed up fully) before start the test. The coils are the most reliable of the ignition components, but they can be swapped with each other for testing purposes.
 
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racerone

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Dec 28, 2013
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38,461
Possible that a coil wire is shorting to one of the metal brackets of the cowling.---------As stated run with a timing light to pinpoint the fault.-----------A compression test should be done on these motors as well.----------------Yes they run very well when in good condition.
 
J

jlw1972

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Feb 15, 2009
Messages
39
Thanks for the input. I'm pretty sure compression is fine. The powerhead only has 48hrs on it since I rebuilt it. Block was machined to .020 over on all cylinders and I was very attentive on break in period. Even ran double oil for the first 20hrs. Motor runs and idles great but I'll check compression anyway.

I am unsure how To test the ignition components the way y'all are describing since the only time it has the stutter is when it's running 35-40 mph? Can you clarify? Thanks.
 
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emdsapmgr

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Take a buddy with you to the lake. Have him drive. You can test it on the lake-with the cowling off. Sit at the back and put the timing light on each plugwire. See what the flashes from the gun tell you. If you are uncomfortable with this, take it to a dealer that has a test tank or a propshaft load test machine.
 
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boobie

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Turn the timing light around and look into it. When you see black spots, that's where the miss is.
 
J

jlw1972

Seaman Apprentice
Joined
Feb 15, 2009
Messages
39
Just wanted to update. I found a wire from my stator that had a nick in the insulation. I put a small piece of electrical tape on it and my motor runs like a new one. Thanks to everyone here who pointed me in the right direction.
 
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