Johnson Sea horse 9.9 only runs with one spark plug removed....o.O

[RaceGeeks]

hello, i have a really freaky situation and i figured id present it to a boaters forum.. first off... i grew up in the automotive world so two stroke motors kinda boggle my mind, my nitro style rc car stumps me as well.... that being said... this problem seems to get me wierd looks from all my car buddies because they are equally stumped... my parents have a johnson sea horse 9.9 on their pontoon boat with an electric start. my dad thinks its a 1985 but we arent sure.. last weekend it was running perfectly fine he says and then all of a sudden it stopped running on him. i went down there this weekend to see what i can come up with.. you know checking wires and spark plugs.. making sure its all good and making good connections etc. well i get the cover off , try to start it a few times and wont start so i pull the top spark plug to check if its getting good spark and the engine fires up and runs fine on the bottom cylinder only while the top plugs out... when i put that plug back in and remove the bottom one to see if it works vice versa, nothing. wont start at all. wont run with both plugs in, or with the bottom one out... only when the top plug's out..... it completely has me stumped and i cant really get to the muskie from the dock!

thanx guys
Steve

[Haffiman]

Check if both plugs spark at the same time?
If so, change power pack.

[freddyray21]

will it not turn over with the top plug out? and is the fuel pump mounted to the top cylinder? If so I suspect the diaphragm is bad and hydro locking the top cylinder.

[RaceGeeks]

yeah it turns over fine regardless if the plugs are in or out... i can spin the fly wheel by hand and get the normal compression feel when the cylinder is compressing so i dont think hydro lock is an issue but i could be wrong.... ill have to check the pump next time im down there.... im still stumped... i mean its getting fuel because i can smell it and the bottom cylinder runs fine with the top plug out... wouldnt a bad diaphragm starve both cylinders and not just one?

[Haffiman]

If a 'bad' pump membrane, fuel will leak into and drown cyl 1 (upper) when using the primer bulb. With running engine, the pump will not work and engine stops! If engine keeps on running on cyl 2 and fuel not drowning cyl 1 when priming, most likely not the pump. However easy to check:
loosen pump (2 screws), press hard on the primer bulb and check for fuel in the pulse hole.
Did you check if both plugs are sparking at the same time? (one piston on top - the other at bottom position)

[jmendoza]

OK, You have described what happens when the power pack fails and you loose spark to the top cylinder. It runs because with the top plug removed, it can run on the lower cylinder without having to fight the drag and load of the top cylinder, which is not firing. Swap the plug wires top to bottom, and if the problem reverses(runs with bottom plug out), you need to check/ replace the power pack. Check the grounding of the ignition coils: make sure the bolts holding them to the block are tight, as sometimes they loosen up. If that is fine, then the power pack is probably shot.

[the machinist]

I am waiting for the cure