80's Johnson 90HP issues

[Jungleboat]

Recently acquired and 'did up' a very old boat. I cannot find any numbers on the engine but it's a V4 Johnson, 90HP, which I'm led to believe is from around 1980+.

Since I had it there has been a problem with a 'spun' prop, in that if you opened the throttle too wide or fast the engine would race - but not the boat.

You could maintain a reasonable speed, but after awhile you'd hear a rumbling beat the engine then it would suddenly rev up high.

Well, I've changed the prop. It now has a brand spanking new SS prop of the same size and pitch as the original. It's an OMC "Viper", if that means anything to anyone.

Funny thing is, the boat now behaves just as before, purrs along to a reasonable rate of knots, say around 15 to 20mph, then, sometimes, you hear a rumble and "blatty" kind of sound, the revs shoot up - except now the boat lurches forward and goes faster, instead of just spinning inside the loose prop' hub.

So the worn prop is fixed - but there is still a problem with intermittent power.

She cruises along smooth and steady, then that noise and a sudden burst of power, which then dies off again.

Max speed, according to my GPS, WITH the spun prop, was 24.7mph. That may have had some tide helping it, plus most of the time I couldn't nurse it above 19mph.

Today, testing it out with the new prop, I couldn't get it above 15mph, so it's actually worse than before. Apart from those brief moments where it seems to come to life, develop a lot more power, surge forward for a brief moment - then slows down again.

The other difference is I fitted new spark plugs (NGK B8HS 10, which is what was in there). She started a lot easier (2nd turn of key) with the new plugs.

The engine idles OK, burpling away quite happily.

Interestingly, when I removed the old plugs they were all dry and a light tan color, ie apparently running perfectly.

Before, when the prop would slip, you could really hear the engine roar. I was looking forward to hearing that, with the prop connected this time...

I'm an engine noob really but my thinking so far:

Lack of fuel getting through - quite possible but not sure how to check. The fuel line has a little manually pumped rubber bulb thingy. Not sure if there is any other form of fuel pump?

Not running on all cylinders - I'd like to think that, the change in engine note and surge of power would suggest it - but there's absolutely no sign of misfiring or lumpy running. Seems to run nice and smooth, just feels like it needs more throttle - then the sudden surge, which doesn't last at all.

Having written that, it does sound like fuel, doesn't it?

Any idea if this engine has a fuel pump, beyond the rubber bulb thingy?

BTW, yes, the fuel tank vent was open. That was the first thing I checked.

I'd love to know exactly what model this is, so I could get a manual for it, but there's no numbers on it anywhere.

Any ideas anyone?

 

[bktheking]

Re: 80's Johnson 90HP issues

Sounds like intermittent spark to me, maybe a failing powerpack, stator going bad, bad coil. Get yourself a spark tester and check all 4 for spark, how big is your boat, my 16.5 footer does almost 40MPH with my 90?

 

[Jungleboat]

Re: 80's Johnson 90HP issues

She's 20ft, relatively light (compared to, for example, a 20ft Bayliner)

I'd guesstimate her top speed, if she were trying hard, at around 35mph.

What confuses me is how smooth and steady the engine runs. In a sense I was thinking "This is the wrong prop, she's too high-geared and can't rev" - yet every now and then there's a distinct kick and surge of extra power, albeit only for a second or so.

I didn't realise the power wasn't maintained, because every time she did that before the prop-shaft spun within the hub and the engine would race, causing me to immediately close the throttle.

Now it seems I didn't need to close the throttle, as the power dies away again anyway.

I'm vaguely familiar with 2 strokes from my mispent yoof on 2 wheels but what's a "stator"?

Coils I do know, though my experience with dodgy coils has usually been a motorcycle that wouldn't start or would cut in and out. This engine is running smoothly, doesn't cut out at all.... Having said that, when she was first launched she did indeed keep cutting in and out. Hasn't done that since and I put it down to old fuel and too much oil.

Now I carefully measure out the oil at 50:1 and like I say, the old plugs showed perfect running (the same plugs were black and sooty when I first got it, suggesting the previous owner was a bit too happy with the oil).

Coils are sneaky things I know, fine one minute, dead the next. I think trying to source and replace the coil would be a good step anyway, as they give no warning when they're about to die completely.

Silly question alert - does this engine have an air filter? I can see where air is sucked in, no obvious sign of a replacable filter though? Just little vent thingies at the rear of the engine. A blocked air filter could perhaps explain the lack of power, if not the intermittent element.

OK, google tells me a stator is something stationary inside a magneto type arrangment.

Mmm.

There is, apparantly, some local mechanic guy around somewhere. Think I'll have to track him down and see if he can give the whole electrical and fuel system a good going over, replacing pretty much everything replacable.

In the meantime she still purrs along enough to get me fishing, which is the main thing


To be helpful, the symptoms again, plus more detail:


Engine fires easily from cold, then cuts out, required the fast-idle lever to be fiddled with to keep it going. I'm guessing that's normal.

Ticks over (idles) very happily once warm.

Starts very easily while hot. Like my car, turn the key, vroom, no problem.

Real pain in the neck to start when somewhere between hot and cold! Won't start with choke, won't start without it. Actually flattened the battery the other day. Only wanted to move closer to the bank, so left it for 2 hours and continued fishing. Tried again when stone cold and she started OK.

Previously seemed powerful but the prop couldn't handle it due to worn hub.

Changed plugs and prop, to the same as fitted when I got it.

Now runs like the throttle won't open enough, no slippage from the prop any more but no more power available than before either.

Around once every 3 minutes or so, noticed a different engine tone (deeper) for around 5 seconds followed by a brief burst of power, enough to lift the nose up higher, then it would revert back to the low-power cruising speed (at full throttle).

The surge of power lasts for around 1.5 seconds max, just enough to lift the bow, then it drops back again.

I think the real clue lies in that change of engine note. You can hear when it's about to surge because the engine sounds deeper and gruffer for a moment, yet no difference in speed. THEN you get a brief spurt of extra power.

Then returns as it was.

Does that still sound electrical?

The more I write the more I'm convincing myself it's fuel starvation...


Just in case it is the wrong prop, lemme go check the prop specs:

13.88 x 17


Does that sound right for this size boat and engine?




JB

 

[bktheking]

Re: 80's Johnson 90HP issues

It could be fuel, you need to test for spark on all 4 cylinders. Diagnosing a problem is more of a process of elimination then guessing at what could be wrong. If motor is down a cylinder the rest of the cylinders will try to make up for it but simply won't. What you are describing by changes in engine sound and lack of speed sure sounds like spark coming back and leaving. Generally when carbs are the problem the issue stays around, they typically don't get dirty then work then get dirty again. As for the stator, it's the beast that powers the powerpacks and provides charging to the battery via the rectifier/regulator. If it's failing it could be causing the issue as well, timer base can also mess up the plugs firing as well.
Servicing the carbs doesn't hurt as well, after all, they provide the life blood to the motor.

 

[Jungleboat]

Re: 80's Johnson 90HP issues

Yeah, it's that process of elmination thing that makes me feel I should basically go over everything.

Apparently it's supposed to take Champion QL77JC4 plugs, which would be a different NGK equivalent, ie BZ7HS-10, rather than the B8HS-10 that are in there. I know the "10" means 1mm gap. Not sure how much difference it would make but it certainly won't hurt to put the right plugs in there...

Seems to be very hard to find Champion plugs around here.

I agree a carb won't foul then clear then foul again (though possible, if an obstruction is bouncing around) but I'm wondering if fuel isn't reaching the carbs as fast as it should in the first place.

However if you say the change in engine note followed by a brief surge of power sounds electrical then that's where I'll start.

Yesterday was the first time it ran with new plugs, so I'll go up the boat club again today and try popping them out, see if there's any sign of any not running as much as the others.

Very difficult to work on the thing, when the tides out it's on knee-deep mud and when the tides in it's floating on 6ft of water. My trailer died of rust.

Ah, there is one other distinct difference I've just remembered - last time I filled up with 97 RON fuel. My wife then laughed and asked why I didn't use the 95 RON, which is significantly cheaper...

So this time I filled a 20 liter can with 95, which was added to the existing half tank (45 liter or 12 gallon tank) of 97.

Does the Johnson come under the heading of a hi-performance engine that needs 97 - and would mixing the 2 cause a problem? I wouldn't have thought so... but be nice to know I'm putting the right freakin' fuel in it...

One thing that did concern me was being such an old engine it was obviously built in the days of leaded fuel - but I'm figuring without valves and having lubricant in the fuel that shouldn't be a problem? You cannot buy leaded gas here now.


Thanks for the help




JB

 

[Jungleboat]

Re: 80's Johnson 90HP issues

Mystery solved


Did I mention the previous owner had managed to wire the "reconditioned" starter motor backwards (necessitating a replacement)?

Well he managed to do the same kind of thing to the HT leads!

Cylinders 3 and 4 had the leads crossed. Amazingly C3 was working, but C4 was not.

Managed to source the correct Champion plugs and all were sparking like little good'uns. Fuel seemed to be getting through... twas a mystery.

Until I removed the plugs and spotted C4 showed no sign of being used at all.

"Ah, your power pack no good" I was told. Apparently a "power pack" is the same as a distributer?

Remembering the trouble I'd had with the previous owner's wiring of the navigation lights and starter motor I tried the unthinkable, swapping the wires.

Now she starts easily and on full throttle roars down the river in a serious hurry.

 

[Jungleboat]

Re: 80's Johnson 90HP issues

Oh, and the gruff engine note?

Once the problem was identified, everyone at the boat club became full of wise old wisdom and assured me it was fuel igniting in the crankcase... doesn't sound right to me though. How?

I still can't explain the intermmitent surge of power but today, on a rather short test run (plus an hour's fishing, of course) she ran very smoothly.

I'd guestimate the top speed at around 35, though you could feel she was picking up even more speed before I decided it would be nice to breath again and slowed down. Wasn't a fair test though, as it was late in the evening and the river was absolutely calm. Calmest I've ever seen it.


I'll add something else - she was running a little rough until she had a damn good thrashing. I think this engine likes being revved up a bit. Doesn't seem happy if you keep her running slow..? I'm guessing it's 2-stroke fouling or something?

Anyway, all seems good for now. Thanks again!



JB