18 hp merc miss under load

[east coast]

bought a 1981 18 hp merc it worked great hooked up to the hose so I bought it.. put it in the lake and it has a bad miss, one cylinder has weak spark switch the switchbox wires and it follows , coil wires are good . if I ease the gas as I bring it up to speed around half way it will seem to work ok but still has a bit of a missfire any more gas than half and it gets bad . The compression is 135 in each cylinder any ideas, I think its switch box

 

[CharlieB]

Re: 18 hp merc miss under load

Generally, when you switch the switchbox wires to the coils and the problem follows, then the switchbox is the problem.


BUT......

Does this motor have a charging system?
If so, disconnect the stator wires to the rectifier and restest spark. If no charging, then forget this part.

ALWAYS DOUBLE CHECK

Because there is NO RETURN on electrical parts, you really should VERIFY THE DIAGNOSIS of a bad module, and that the stator and trigger are good by DVA testing, just to double check yourself.

 

[east coast]

Re: 18 hp merc miss under load

Generally, when you switch the switchbox wires to the coils and the problem follows, then the switchbox is the problem.


BUT......

Does this motor have a charging system?
If so, disconnect the stator wires to the rectifier and restest spark. If no charging, then forget this part.

ALWAYS DOUBLE CHECK

Because there is NO RETURN on electrical parts, you really should VERIFY THE DIAGNOSIS of a bad module, and that the stator and trigger are good by DVA testing, just to double check yourself.

Thanks for the tip no it dosen't have a charging system. I found a diagram for the stator test but the wire colors aren't the same ? I don't see a blue one for example they are all black or brown with yellow greem or white tracers . DVA do you need a special meter for that ? if I use mine it the reading jumps all over the place on voltage . did some ohomes cheacks they seem ok . all conections are clean , any thing you can tell me is a big help I don;t know much about outboards cars are my thing lol ... Here in Halifax Nova Scotia there are alot of boat dealers but they don't share much info they want ya to bring it in so they can fix it. Even parts are expensive I can get the switch box on iboats for about half the price even after the shiping cost. If you know where I can buy a prop puller for the smaller outboards 9.8 and 18 hp I want to get one both of my props are stuch on and I don't want to make a mess getting them off the shop wants $80 to pull a prop

 

[Faztbullet]

Re: 18 hp merc miss under load

Check the stator as switchbox only has one internal capacitor and hard for it to cause weak spark on one cylinder.

 

[CharlieB]

Re: 18 hp merc miss under load

A DVA Meter or a DVA Adaptor for a standard Volt/Ohm Meter has an internal resister and capacitor wired in a specific series/parallel circuit which places a load on the charging coils and stabilizes the needle swing on the meter so as to get an accurate reading.

DVA testing has become the accepted standard for measuring voltages for stators and triggers, most all other means (including Ohm's testing) are far less reliable indicators of system output and reliability.

CDIElectronics.com has both DVA Meters and adaptors for VOM's at reasonable costs, they also have a free downloadable Outboard Troubleshooting Guide which list most all major outboard systems, with specific instructions how to test both DVA and Ohm's values. Again, they have pretty much set the standards for the industry, their parts are of THE finest.

 

[east coast]

Re: 18 hp merc miss under load

took me some time to get to the bottom of this problem, took it to the merc dealer they lake tested it and cleaned some grounds said everything was fine . I tryed it out and it was no better, I remembered when i had the prop changed there was a small peace of metal like tin in the exhaust, so I took the bottom end off and sure enough 6 more chunks of metal. It must be baffles I guess , it seems to work fine now thought I would let you know in case you wondered how I made out. thanks for your help