Engine appears to be running on 3 cyl's after running for 20 minutes

[sconoy1183]

97 Johson 150 Fastrike. Fires up real easy. Runs great. But after 20 minutes of running up on plane the engine really struggles. Seems like it is running on 3 cylindars... Brand new Gas. Brand new Plugs. Im the original owner. I change the spark plugs and lower unit oil..and winterize it myself. The motor has only been serviced one time for a new impellar(2 years ago). The motor has never been in for a tune up. Doesnt seem to need one. It really starts well and runs well even in cold weather. Please advise. I think the vin number is 18WRT192V1D64625

[ChampioNman]

Check your shift interruptor switch for binding. It is under the shift linkage on the motor. That will knock out one bank of cylinders if it is sticking. Your model number would be a J150GLXXX.

[sconoy1183]

Thanks for the quick reply. I have scheduled an appt with the dealer that i bought the boat from. He feels that it is the water impellar not working properly which causes the boat to go into SLOW mode. Q: Does a 97 Johnson even have slow mode? And does SLOW mode cause the boat to run like it really is struggling ie.running on 3 cylindars. Just a note that kind of supports his thought is that after running the boat for 10 minutes trying to get back to the boat ramp...then it all of a sudden takes off and runs like nothing is wrong. Is it possible the boat cools down after 10 minutes and the SLOW mode dissengages and runs in normal mode.

[ChampioNman]

You should have gotten an alarm and a light on your system check gauge if it went into SLOW mode. Do you have a water pressure gauge on your boat? How is the water stream coming out of the Tell Tale? SLOW mode will kick you down to approx 2500 RPM. Let us know what your dealer finds.

[225RUDE-TRITON]

check one of your coils it might have a slight crack on the back of it.....it runs good cold but when it warms up the crack will widen and cause a miss......start it up with muffs on at dark and let it get at running temp.....sometimes you can see the coil arc of the block....thats just my 2 cents