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25hp merc one cylinder intermittent spark

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Corvairdave

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Newbie here. I have a '97 25hp merc outboard. Bottom cylinder runs well, top has very intermittent spark. Here's what I know; 120psi compression on both cylinders, coil packs, spark plugs, spark plug leads, CDI box all good. Bad spark follows coil lead for top cylinder. Pulling top lead when running makes slight difference, pulling bottom one kills it. I disconnected the kill switch wire last night and it made no difference. I'm going to clean up every ground on it, but it seems I'm missing something. I'm a decent mechanic, but have no outboard experience. Would love some help-suggestions! Cheers, Dave
 
Does spark jump a gap of 3/8" or better on top cylinder ?-----If you have good spark then perhaps there is a reed valve issue on top cylinder.
 
Be looking at upper crankshaft seal for the issue. Reeds usually show up as sneeze back through the carb throat that cant be controlled through mixture adjustment assuming the carb doesn't have blockage issues from varnish or other restrictions. Do you see any oil running down the block from under the flywheel?
 
Post #1 contradicts everything you say about good spark? If you have an itermitant power issue that will not be a seal or reed valve. I would look to the switch box.
 
Update; Spark is excellent on both leads. Easy 7-16" gap. Put a timing light on each and they both show a steady rhythm. Not under load, running on the muffs. Top cylinder is not pulling it's weight. Pulling the top lead and you can hear the 'miss' go away but it still runs. top lead back on and pull the bottom lead and it dies. Does one set of reeds feed one cylinder exclusively? Following the 'compression, spark, fuel' troubleshooting motto, the only thing missing is fuel
 
flyingscott said:
Post #1 contradicts everything you say about good spark? If you have an itermitant power issue that will not be a seal or reed valve. I would look to the switch box.
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Yes I did. In my earlier troubleshooting, I tried 3 switch boxes, all with the same result. Last post is the latest. Post should read intermittent firing!
 
You need fuel.----Ignition.-----Compression in the cylinder.----Compression in the crankcase.------Yes each crankcase has a set of reeds.-----Your motor has 2 crankcases.
 
racerone said:
You need fuel.----Ignition.-----Compression in the cylinder.----Compression in the crankcase.------Yes each crankcase has a set of reeds.-----Your motor has 2 crankcases.
Click to expand...
Thanks so much for the response. I am going to pull the carb, and reed plate if necessary and check it out. Motor sat for an undetermined amount of time, so am hoping for an ultrasonic clean, then off to the races. Thanks again! Cheers, Dave
 
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