1988 90 hp Johnson Loss of 15% power at WOT

[waynehol]

Hi folks,
I searched old posts and found similar but different problems. Im looking for orderly steps to take to diagnose problem.

I have 1988 90 HP Johnson that runs (has) run great. It is on a 16 ft center console flats. Does 32 at top speed according to GPS (no tach). I keep the boat in salt water. Always flush with saltaway and remove periodically for hull care, etc. (Bottom has anti fouling paint and is currently clean)

I was running WOT after a day of fishing. Ran great as usual. All of a sudden, it dropped about 20% or so RPM. It was sudden and smooth, in fact I looked to see if my dog had knocked the throttle control back. It had not moved. It ran very smooth, no sputtering, no odd sound, just lower RPM and slower speed. I backed it down some, then put it back up, still maxed at the lower speed.

I came to no wake zone, throttled to idle, and it ran slow fine. Smooth, no roughness, nothing. I went just above idle for 8 or so minutes and opened to WOT when passed the bridge. Accelerated smooth, no sputtering, took a little longer to plane, but ran at same lower speed. It was almost like I advanced the control lever 1 inch short of full, but it was definately all the way forward, Ran like that rest of way home,

So, What things should I suspect and what order should I diagnose? I have a manual and am pretty handy,

Thanks in advance for your thoughts!!

 

[jonesg]

Re: 1988 90 hp Johnson Loss of 15% power at WOT

Compression test
spark gap test
and fuel flow in that order.

If compression is ok and even
and if it passes the spark test,
its fuel flow.

I think you'll find its dropping a coil.
If its heat related you can take a timing light on the water with you and test for ignition spark when it fails.

 

[kenmyfam]

Re: 1988 90 hp Johnson Loss of 15% power at WOT

Basic health check of the motor to begin with. Could be the start of a failing fuel pump. Do you still have a VRO system in place ??

 

[petryshyn]

Re: 1988 90 hp Johnson Loss of 15% power at WOT

I agree with jonesg. If it dropped a cylinder instantly and smoothly, its likely ignition. Also your overheat alarm didn't sound.

Thats the problem with multi-cylinder 2-strokes. They still run smooth with a dead cylinder. When you have a change in performance, its the engines way of saying "limp me back or get me towed". If you continue at WOT, you risk engine damage. If you lost ignition in a cylinder (most likely in this case) you haven't damaged the engine. If you lost fuel to the cylinder (plugged main jet/needle & seat) its likely your cylinder has suffered damage from the trip home.

I'm just trying to stress the importance of a tach and its monitoring, each time you WOT. Once you see a decline in performance, do not WOT until its rectified.


Good luck!.....

 

[waynehol]

Re: 1988 90 hp Johnson Loss of 15% power at WOT

Thanks, that makes sense. I hope to do the checks tomorrow afternoon. I;ll keep you posted.