'85 model 90 HP Fluctuating RPM's on WOT

[Margarita Man]

My '85 Johnson 90 HP was having problems today with the motor RPM fluctuating. I would be running on WOT for some time and then the RPM's would drop about 1,000 to 1,500 for several minutes and then the motor would again rev back up to WOT. This was going on constantly all day and I was just hoping the motor keeps running back to the dock since I was pretty far out. Also when I have it on WOT and back down because of a passing boat's wake it would not go back to WOT for a long time. I just had the carbs tuned up by a Johnson dealer so I do not know where to start looking for the problem. I switched to a reserve tank thinking it might be a fuel problem but no luck. Has anyone had the same problem?

 

[chbailey]

Re: '85 model 90 HP Fluctuating RPM's on WOT

You should start by testing your compression and spark. You should have a 100+ compression and all cylinders should be within 10% of each other. Spark should jump at least a 7/16 gap on the tester. Post the numbers to let us know. Sounds like carbs but it could be something else, so start with these test first.

 

[Margarita Man]

Re: '85 model 90 HP Fluctuating RPM's on WOT

CHBAILEY,
I followed your instructions. Compression was 125 on all four cylinders. Spark gap was very strong well over 7/16". What do you suggest I try next?

 

[HighTrim]

Re: '85 model 90 HP Fluctuating RPM's on WOT

Im by no means one of the experts on here, but it does sound like a fuel problem, your compression numbers and spark jumps sound great. Go over all your fuel lines, looking for leaks, listen for them, or pump the bulb and carefully observe. If you pump your bulb while its dying, does it pick up, if so you may need to change your fuel pump. Possibly antisiphon valve on fuel tank? Hopefully others will jump in with other ideas. GL

 

[Margarita Man]

Re: '85 model 90 HP Fluctuating RPM's on WOT

I called a Johnson dealer that rebuilt my carburator and he said check the stator but really has no clue since he cannot test the boat in the water. Any body else out there with any suggestions before I start replacing expensive parts? Thanks for your help.

 

[jtexas]

Re: '85 model 90 HP Fluctuating RPM's on WOT

1. Try pumping the primer bulb when you lose RPM's, if that brings it back up, rebuild the fuel pump.

2. Consider whether a float valve or two is hanging - but from WOT to 1000 RPM sounds like more than two cylinders.

3. Closely inspect the wiring coming out from under the flywheel - look for a chafed wire that might be grounding out sometimes.

 

[tjt1980]

Re: '85 model 90 HP Fluctuating RPM's on WOT

I have a 2000 Johnson 90hp and I have been experiencing a similar problem. Last weekend, i was going down the lake at 80% throttle and all the sudden the rpms would drop about 200rpm for a few seconds and jump back up. This happened at sporadic intervals. The length of time that the rpms were down increased until it stayed that way. My first thought was that i had a spark plug foul and lost a cylinder, but i pulled all the plugs yesterday and they didn't look that bad. Any ideas?

 

[jtexas]

Re: '85 model 90 HP Fluctuating RPM's on WOT

tjt, it'd be better if you put your question in a new thread, so we don't get confused..........

 

[tjt1980]

Re: '85 model 90 HP Fluctuating RPM's on WOT

tjt, it'd be better if you put your question in a new thread, so we don't get confused..........

That's what I normally do, but I never get a reply so I figured i'd try a related thread. Thanks.

 

[Don Tauer]

Re: '85 model 90 HP Fluctuating RPM's on WOT

Well fella's that makes three of us with almost the same problem. Today I took and replaced the plugs all 4 (it is a V4 89 Johnson and I posted a thread yesterday afternoon.) At the recommendation of several folks,I replaced the primer bulb, and went one step further I replaced the connector "O" ring, placed into the fuel a STP carb cleaner and everything seems to run normal for about 10 miles of traveling. Tomorrow it may just be a serpent in disguise and rear it's head again. Together we will eliminate the problems together.
Don