dead cylinder

[sassboater]

have a friend who has a 94 merc 40hp 4cyl. he says #3 cyl is dead. compression is 120 on all 4 cyl. coils and wires are good. he swapped coils around with no results. when you pull off #1,2or4 plug wire you hear a drop of rpm but not so with #3. the plug is emitting a sharp blue arc. someone told him it could be reed valves on that cyl.
i told him i would post it here because if anyone knows how to fix it one of you will. thanks for any advice you can give

gary

 

[Chris1956]

Re: dead cylinder

Gary, If he has the same compression as the other cylinders, chances are the reed valves are good. What does the spark plug look like? Wet? fouled? water droplets?

 

[sassboater]

Re: dead cylinder

chris1956, i'll find out from him tomorrow. anything else you want me to ask him? thanks alot

gary

 

[sassboater]

Re: dead cylinder

chris 1956: he can't remember what the plug looked like as h put new ones in last weekend. he is going to fire it up after work tonite and let me know how the plug looks.

gary

 

[diaric]

Re: dead cylinder

take a read of the troubleshooting on this one http://forums.iboats.com/showthread.php?t=200388

 

[tashasdaddy]

Re: dead cylinder

here are some test to perform, there are two pages pick what is pertainant to that engine. http://www.boatpartstore.com/page24.asp

 

[sassboater]

Re: dead cylinder

chris1956: he just told me the plug is oily wet-no water drops. switched plugs also.
thanks to you other guys also, i'll print them out and give them to him tomorrow. (only see him on weekends and no internet down there at campground.

gary

 

[Chris1956]

Re: dead cylinder

Gary, That is a tough one. You might put a timing light on number 3 to see if it really has spark when the motor is running. If it does have spark, I would think it is not getting enough fuel to run.

 

[studlymandingo]

Re: dead cylinder

If it is wet with fuel, then I would go with a spark issue, is it missing at idle, higher rpms, or all the way through?​

 

[sassboater]

Re: dead cylinder

it misses at idle and gets worse as rpms rise. believe it can only reach 2000-2500 rpms.

gary