150 Johnson won't rev past 2500 rpm

[Paul 598]

I have a 2000 model 150 Johnson and it will not rev past 2500 rpm and also shakes badly.. I have changed plugs(L78YC), drained carb bowls checked plug wires.. I'm lost...

 

[Daviet]

Re: 150 Johnson won't rev past 2500 rpm

Welcome to the forum.
Have you checked the compression?
Spark jump a 7/16" air gap?

 

[phipps]

Re: 150 Johnson won't rev past 2500 rpm

Cleaned the carbs?

 

[boobie]

Re: 150 Johnson won't rev past 2500 rpm

Is it overheating and going into SLOW ?? No hot horn maybe ??

 

[fireman57]

Re: 150 Johnson won't rev past 2500 rpm

You guys overwhelmed him before you told him welcome to iboats. Must've scared him off. I'm with Daviet, think he dropped a cylinder or two.

 

[Paul 598]

Re: 150 Johnson won't rev past 2500 rpm

Thank you daviet.. First, what is air gap? I have not checked compression, the motor runs good under 2500.... A little hard to crank when cold but runs good.
I took carbs off and removed the bowls and everything look very clean. I've head about this s.l.o.w. mode and it sounds like that is what is happening to my motor. I just recently purchased this boat and I don't think the engine warning gauges are working but I do t think it is overheating because it has the problem at start up..

 

[Daviet]

Re: 150 Johnson won't rev past 2500 rpm

The spark needs to jump a 7/16" gap. You can get an adjustable gap tester at most auto parts store.
Did you remove the orifices in the carbs and make sure they were not resticted or plugged?
Basicly the s.l.o.w. mode shuts down cylinders if the engine overheats.
It might be a good idea to invest in a FACTORY manual, they have trouble shooting guides that might help you find your problem.

 

[Paul 598]

Re: 150 Johnson won't rev past 2500 rpm

I will try the gap tester.. If it doesn't pass the 7/16" test then what could be the problem? I did Check to see if they where firing with the plug in the plug wire and it was kinda weak but it did jump to the electrode.. I will get a factory manual as well.. Thanks

 

[Daviet]

Re: 150 Johnson won't rev past 2500 rpm

Testing spark using the spark can be misleading. Try it with the gap tester and see what results you get.

 

[Paul 598]

Re: 150 Johnson won't rev past 2500 rpm

Also this motor has the optical ignition system. Is the jet you are referring to on the outside of the carb beside the bowl?

 

[Paul 598]

Re: 150 Johnson won't rev past 2500 rpm

Motor pass the 7/16" spark gap test.. Compression is 110-120

 

[bob johnson]

Re: 150 Johnson won't rev past 2500 rpm

launch your boat with a cold motor, get everything ready at the dock...start the motor and take off and get it up to WOT as soon as possible...like in less than 1 minute.....if the motor wont rev past 2500 then..... then it isn't the SLOW mode....slow mode activates when you reach about 210 degrees I think.... you cant get that hot in less than 2 minutes, unless you have SERIOUS problems...bob

 

[clanton]

Re: 150 Johnson won't rev past 2500 rpm

Buy the book, check the shift switch, Strb side of engine, by the control cables.

 

[Paul 598]

Re: 150 Johnson won't rev past 2500 rpm

Finally got motor to start on trailer. Back it down into water, cranked it immediately put in gear and same problem. Will not run past 2500 rpm while in gear or neutral..

 

[Paul 598]

Re: 150 Johnson won't rev past 2500 rpm

What do I check for on the shift switch?

 

[Paul 598]

Re: 150 Johnson won't rev past 2500 rpm

Is there any difference between the L78YC That are in motor and the QL78YC?

 

[bob johnson]

Re: 150 Johnson won't rev past 2500 rpm

never run the motor in neutral past 1200-1500 rpms!!!!!!!!!!!!!!!!! for future troubleshooting as well as the current issues....get yourself a small hand held Infared Thermometer.......it is the best way to verify the motors temperature....is your motor getting hot???because for the test I was thinking, you don't want to warm the motor up!!! you maybe getting hot fast and thus your SLOW mode kicks in.... and remember you need to get the temps below about 180 AND turn the key off.....to reset SLOW mode. http://victorylibrary.com/mopar/champion.htm. you HAVE TO HAVE THE "Q" for your plugs!!!!!

 

[Paul 598]

Re: 150 Johnson won't rev past 2500 rpm

Motor is not hot.. The problem starts at startup when motor is cold..

 

[bob johnson]

Re: 150 Johnson won't rev past 2500 rpm

the books troubleshooting guide says this: No spark any cylinders: go to Stop circuit test...if that's not the issue go to Power Coil test, if that's not it, go to timing sensor test...if that's not it, go to power pack test..... good luck because it is intermittent...I suspect a shorting wire..... unfortunately that is like finding a needle in a haystack..... I had a black yellow wire go bad in the middle of my harness!!! whaich was inside a thick solid walled plastic wire hose.... and the coating on the wire wasn't broken!!!, but once I cut it open I could see it had blackened the inside where it was making contact off and on... do an ohms test on each wire from control to the motor..... I am not sure what it is, but at some point you have to start getting very systematic... eliminating one thing at a time....bob

 

[Paul 598]

Re: 150 Johnson won't rev past 2500 rpm

Found wires that where cut going to alarm.. Crank motor and I get a solid tone..