1974 15 hp Johnson performance issue

[bently]

I have been working on my dad's older 15hp Johnson and I have replaced the Thermostat & gasket. The motor starts and idles fine but once you put it in forward gear and run up to WOT after a bit it seems to bog down and then when you try to increase speed it seems to only run on one cyclinder. I have read Randy's Ramblin and added the "pee hole" to which I get a good streem while the engine is running. I have now removed the side water jacket cover and ordered two new gaskets for the out and inner surfaces. There was alot of debris in the lower area which I will be cleaning out. How do you test spark at high speed or is there an ohm test I can perform on some of the electrical components?

Thanks

 

[racerone]

Run with a timing light hooked up to check ignition performance.

 

[OptsyEagle]

I would do an external spark test to determine if the ignition is strong enough to jump a 3/8" gap on both cylinders. I would also do a cylinder drop test to see if both cylinders are working properly. This is done by starting up the motor and letting it warm up. Now bring it to a fast idle and with insulated pliers pull one spark plug boot at a time. The motor should be able to stay running on one cylinder so if the motor dies when a plug boot is pulled you know that the other cylinder was never firing or is very weak. If the motor does die when a plug boot is pulled, switch the plug boot to the opposite cylinders and switch the clips for the coils that come out from under the flywheel and do the cylinder drop test again. If the problem now switches to the other cylinder you now know it is a bad coil. If it stays with the same cylinder it could be a few things. Just report back on your findings.

If you have a compression tester, a measurement of that can be very useful at these times as well. Stop looking at your cooling system, since it sounds like it is working fine. Also, if you are not using NGK spark plugs, then put in some NGK B7HS plugs. That series of motor hates champion spark plugs.

 

[wrmdunker]

I had a similar issue with my 1974 70hp last week. Would not get up to speed.....just plowed water. I checked the ignition system and found bad wire connections at both ends of #1 cylinder. The wire actually just pulled easily out of the coil. I cut both ends and reattached and then it ran fine. On my coil the post is twisted and the wire has to be screwed on to the post. I am not as experienced as most of these guys but maybe this will help.

 

[bently]

I have the NGK plugs gapped to.030 and I will test the above once I get the new gaskets and side covers back on.

Thanks alot.........

 

[bently]

I believe I have the motor running well now. The timing plate that is located under the flywheel was gummed up and did not advance like it should. The float in the carb did not move freely by itself and was tight against the hinge pin, loosened its grip and the float moves up and down freely now. motor starts and runs well under power now. Most of these issues were from just sitting and not being ran periodically.

Thanks everyone