1985 Johnson 75 - temporary power loss

[markk900]

I've owned this boat/motor for 2 years now, and its been running great, but in the past 2 weeks had an unusual problem: after startup for the first time in a day (or after standing for a half day or longer), throttle up on my 1985 Johnson 75 works well, we get on a plane, and then without warning motor slows (like running out of gas). It doesn't stall, idles perfectly, but won't throttle up again. I do a quick check of the obvious (do I HAVE gas, is the vent blocked, is the fuel line kinked, etc). By the time I've checked these things, and try again, the motor is happy to throttle up to full revs again. And it continues to run fine the rest of the day.

I'm suspecting a piece of detrius in the gas tank or fuel line, but thought I'd turn here for other things to look for. I was worried about an overheat or mild seizure, but this usually happens before the motor has run for long, and doesn't happen later in the day while pulling a load tubing or whatever once the motor is really hot.

Any suggestions of where to start troubleshooting?

Mark

 

[jtexas]

Re: 1985 Johnson 75 - temporary power loss

you said "slows" to an idle, then "won't throttle up again"

Does this mean that nothing you do with the throttle control has any effect at all on RPMs?

How long are you giving it to warm up?

 

[markk900]

Re: 1985 Johnson 75 - temporary power loss

Correct: pushing throttle forward, no increase in revs. Now to be really clear, there is some response from idle to 2000 rpm or so, but will not increase beyond that, so its like the throttle has no effect from about 1/3 to WOT.

I then do my troubleshooting as listed above, and without having turned the motor off, try pushing throttle forward and everything works as normal.

Cables are secure, and for the rest of the day everything works as usual.

Mark

 

[jtexas]

Re: 1985 Johnson 75 - temporary power loss

Just the once, always after a cold start?
Is there a consistent time duration before it happens?
How long do you typically give it to warm up?

Have you tried pumping the primer ball while it's acting up? It should collapse (get sucked flat) if the fuel line is obstructed.

RPMs top out after 1/3 throttle or so...is just about right for one cylinder dropping out...hard to tell though...

I'm grasping at straws........

 

[markk900]

Re: 1985 Johnson 75 - temporary power loss

Sorry for long duration between replies - work gets in the way

Seems to only happen once, after a cold start, and not every time - perhaps 3 times in the past month, when I've used the boat around 30 times. My usual habit is to start it, let it run at fast idle for a minute or two, then start driving the boat. I don't normally go to WOT for another minute or two, but drive at half throttle, then throttle up to cruising speed.

No consistent time to when it happens....have not tried the primer bulb when it happens (good idea).

I've not been around the triples much, but I'm thinking its not dropping a cylinder because of the smoothness of the motor when this happens. It doesn't just "die", it slows down smoothly but rapidly (like I said, as if it ran out of gas), then idles fine until whatever I do that clears the issue - it then revs up smoothly. I'd expect a cylinder dropping to introduce some roughness into the mix.

Anyway, thanks for the extra straws to grasp at!

 

[jtexas]

Re: 1985 Johnson 75 - temporary power loss

They idle surprisingly well on 2 cylinders, but you'd probably notice. But if one drops out when you throttle up, it'll run along at around 2000 (something less than 3k, it's not enough to plane my boat), smooth as you please. Don't remember loosing one at high speed though.

They don't all run dry at once -- they go in order, 1-2-3.

Only 1 out of 10 times...guess you've checked the connections at the tank? Maybe one is not seated just right...checking the fuel line for kinks straightens it up?

30 times in the past month...I guess work doesn't get in the way all that often...

 

[markk900]

Re: 1985 Johnson 75 - temporary power loss

4 weekends x 5 trips per weekend is 20, so I did exaggerate a bit....but we've been lucky to get out a lot lately!

Thanks for the tips - I will look into the connections. Didn't know they ran dry in order and that definitely could be the issue (especially if there's a kink or something and its letting some gas through)....

Won't be near the boat for another 2 weeks so I'll report back later. Thanks!

 

[markk900]

Re: 1985 Johnson 75 - temporary power loss

Follow up on this one: problem did not reoccur for several weeks, then last time we used the boat it happened in a big way half way across the lake. To make a long story short, final diagnosis was no compression on bottom cylinder. Not sure why the temporary issues I had originally reported but she's toast now. Motor had been rebuilt at least once before I got it, and the mechanic looking at it says in all likelihood it has no more service life - not worth the cost of tearing down to find out.

My assumption is that when the problem originally started it was a fuel starvation issue at WOT, that lightly seized the one cylinder. Afterwards, it ran reasonably well but I believe it was all downhill from there. I did notice a drop of about 200-300 rpm at WOT when things were running, but attributed that to wind on the lake.

As reference, I run without VRO (50:1 premix), I did check the overheat alarm was working.

Thanks to those who tried to help out.