98 johnson 40 hp-bucking past high idle

Hi folks,
I have a 98 Johnson 40 HP that has been running great, used it several days the previous week. It cranks easily and idles without issue. When I give it plenty of throttle at first start-up, it gets on plane quickly and runs great for about two hundred yards and then dies back and begins bucking baddly until I go back to idle where it runs fine.

If I let it sit for a while and prime it up, it runs great?for a couple hundred yards and then the same thing happens. I doubted carburetor issues since it ran great at start up but I went ahead and cleaned and rebuilt both carbs thinking there may be trash in the bowls.

After the re-builds, the same problem, I?m still thinking gas issue but have no idea where to go next. I disconnected the fuel line going to the top carb and it?s pumping a good stream of gas/oil mix out of it. I can disconnect either spark plug and the engine idles fine but no high idle without bucking.

It still has the factory oil mix. Any ideas would be greatly appreciated.

Re: 98 johnson 40 hp-bucking past high idle

Sounds like a bad ignition miss. Coil, pack or ect.

Re: 98 johnson 40 hp-bucking past high idle

Thanks Boobie, I had not even considered an electrical issue. I did swap out the plugs with no success, coil packs were replaced a couple of years ago but I guess they can go bad at any time. I'll do some more checking.

Thanks again,

Re: 98 johnson 40 hp-bucking past high idle

Could be a spun hub on your prop. Btw, welcome to iboats

Re: 98 johnson 40 hp-bucking past high idle

Fireman57, a spun hub would have been the last thing I would have checked but it makes perfectly good sense. I'll check it when I get home this evening. Thanks for the welcome.
Thanks!

Re: 98 johnson 40 hp-bucking past high idle

Take a sharpie and strike a line from the back of the prop to the lower unit. After you run it and it does it make sure they still line up. If they do then we will consider other possibilities. Also, when you are out there, take a buddy and have him repeatedly squeeze the primer bulb when this happens. If it picks up and runs just replace your fuel pump.

Re: 98 johnson 40 hp-bucking past high idle

Thanks Fireman57, I will try as you have suggested this evening.

Thanks,

Re: 98 johnson 40 hp-bucking past high idle

I tried all of the recommended procedures with no luck. Took the vro out, found no damage, replaced several gas lines and replaced wire tie clamps with real ones. I cranked it up and everything ran great in the water at high idle. I cut the engine off, put the cowling on and cranked it up again, instantly it started running rough. I turned to go back and remove the cowling and caught the kill switch cable with by leg and pulled it loose by accident...the engine didn't die.

I turned the key off and replaced the kill switch and worked the switch a few times, pushing it in while replacing the kill switch. When I cranked the engine to retest the kill switch, I noticed the engine had quit bucking and was idling better than it had in sometime. I revved it up to high idle and let it run about ten minutes then went out on the lake and ran it for another ten or fifteen minutes without any problems.

I couldn't believe the switch was the problem. I remembered a few weeks ago, I switched the engine off by the key and it kept running so I sprayed contact cleaner in it and latter put some graphite in it. I guess now I need to buy another switch.

Thanks to all who offered advice.

th

Re: 98 johnson 40 hp-bucking past high idle

Glad it was something cheap. Enjoy the rest of the summer and don't be a stranger here.