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1992 Johnson 150hp Fast Strike only runs half speed.

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AllenMcKee

New member
So I need help, my Johnson 150hp has low hours, all six cylinders measured 100 compression. The first time in water it usually runs great, starts easy and idles perfect. Then get on down the lake and fish, go to leave and it idles like half the cylinders are not firing. Boat will only run half open then after some amount of time, the motor just launches out of water and is running perfect again. I have replaced the coil packs, spark plug cables, and plugs. The carbs were rebuilt by mechanic. Any idea what might be causing this? The time before it relaunches and bout scares you to death varies from a few minutes to not recovering before trolling all the way back to the dock at about 10 or 15mph.
 
100 is pretty low, but lets assume your tester is off, as 100 be noticeably hard starting. I've seen testers off as much as 30 PSI. I have 3 testers and each one test different. One will read 90 when the other two are between 125-130.

As strange as it might sound, you might check your flywheel magnets. Should be 6 of them all evenly spaced apart. I've seen a few now that one or two of the magnets slide over. The strange thing is, the owners claim the same symptoms. Idles and runs great for a little while, then it will start acting up, like it is dropping cylinders.

I had an 92 Evinrude 150 that 3 of the magnets were slid up against the others and it would idle and run decent for about 20 minutes and then it would start acting like it dropped a cylinder. Then it would get worse and worse as it drove more. Owner replaced all the electrical components and when he went to replace the voltage regulator (separate issue), he took the flywheel off and turned it upside down on his bench. That is when he noticed the magnets were out of place. He epoxied the magnets back in place, evenly spaced out and that 150 runs like it is brand new.
 
100 psi is about normal on a 60° as it has decompression ports to assist in smooth idle and starting RPM. Sounds like a stator or pack dropping a bank of cylinders.
 
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