1990 Johnson 70 Hp power problems

[timmermatt]

We have a johnson 70 hp, recently the ignition was left on overnight, since then, the engine runs as normal until it has been running for about 15 minutes, at this point if I try to run at full throttle it will only run to about 3000 rpm.
any ideas.

 

[Seasport]

Re: 1990 Johnson 70 Hp power problems

Leaving the ignition on shouldn't cause any problems. May be coincidental.

Your motor may be going into SLOW mode if it is overheating. When this happens the revs are limited to about 2500rpm and the motor will shake. This is controlled by the powerpack when it recives an overheat signal from the temp sensor on the powerhead (tan wire). You should also get an overheat warning buzzer if this is the problem.

Otherwise you might be dropping spark to one cylinder. Maybe a faulty coil.

 

[SawBum]

Re: 1990 Johnson 70 Hp power problems

I had a simular problem on a 120 hp, pulled each coil and cleaned the mounting surface on coil and block, I was getting an intermittent ground (corrosion) on 1 or 2 of them, or so I was told. has run fine ever since (about 3 years) until my recent fuel problem

 

[Flyfisher.1]

Re: 1990 Johnson 70 Hp power problems

What would be the cause of the overheating ... if it will still run in the SLOW mode? (I a'm having the same problem with a 2000 70 on my bass boat.

 

[ob]

Re: 1990 Johnson 70 Hp power problems

Excessively worn waterpump impeller and or crusty and sticky thermostat assy.Occasionally a faulty temp sensor can be culprit triggering the s.l.o.w. mode. You can verify overheat by a simple touch check of the engine when it acts up or use a 163 degree thermo melt stick on several locations on the engine block and head covers.Most likely a faulty waterpump or t/stat though.

 

[Flyfisher.1]

Re: 1990 Johnson 70 Hp power problems

Thanks!! ... Will check both and let you know the outcome