Mercury 40hp 4 cyl issue

bgsupra

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May 29, 2008
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I have a 92 Mercury 40hp 4 cyl that is hard to start as well as boggs and misfires above 2000rpms. At 4000 rpms it will run for about two minutes before shutting down completely. I have rebuilt carbs, replaced fuel lines, fuel pump, filter and installed new plugs. I used an inline spark tester which it appears to have spark on all cylinders. I am still leaning towards an electrical problem. Upon removing new plugs I noticed the top two and bottom plug are fouled while the third from top is not. Any suggestions would be greatly appreciated. Thanks in advance.
 

CharlieB

Vice Admiral
Joined
Apr 10, 2007
Messages
5,617
Re: Mercury 40hp 4 cyl issue

The neon light flash-tube in-line spark testers are not an adequate test of spark intensity. They are good in checking to see if there is ANY spark, but for good performance at throttle the ignition must be capable of throwing a nice hot blue arc across a 7/16 inch gap. Any less will limit WOT RPM.

Download and study the FREE Igniton Troubleshooting Guide from the Support page at CDIElectronics

Their guide will walk you thru the tests to ensure your ignition problems gets cured.
 

bgsupra

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Joined
May 29, 2008
Messages
2
Re: Mercury 40hp 4 cyl issue

Thank you for your reply. I will check the trouble shooting guide. Given these symptoms what would be your from the hip guess to the problem? The only reason I ask is the boat is close to 4 hrs away is a rurual area and I would prefer to take whatever I can in hopes to fix it. Thanks again.
 
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