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Johnson boat engine, still has a problem? #5206750 09/06/19 11:44 PM
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wallyuwl Offline OP
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Have a new(er) to me Johnson 90 hp 1986 MY boat engine on a ski boat. Probably has 200 to 250 hours on it total, my parents bought it used in 1987 or 88 and I bought from them spring 2018. I have done some repairs and maintenance (new gauges, trim/tilt sensor, fuel filter, plugs and wires, thermostats, rectifier/regulator, a power pack, seafoam treatment, and "engine tune" treatment spray in the carb throsts, impeller).

Seems to run pretty well now, but not sure if something is an issue or not... if I give it throttle and keep the handle so it should stay at say 1600 rpm, it will jump to 2000 rpm for a few seconds before dropping back down for 15 or 20 seconds. Rinse and repeat.

It does this when warmed up, not just cold. It also does it at idle (but only changes rpm about 100). This is as tested in my driveway with the hose on it, have not had it on the water in over a month so not sure if it does it there since putting in the new thermostats (was running cold) and power pack.

Is this even a problem or is it typical of these engines? If not typical, any thoughts on what it could be(air in fuel lines, carbs problem, etc)?

Re: Johnson boat engine, still has a problem? [Re: wallyuwl] #5206816 09/07/19 04:53 AM
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sweezy722 Offline
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Make sure the engine is not starving for fuel. If the mixture gets too lean it can increase rpm and pop a hole in a piston.
I would start with the engine running on muffs and try squeezing the primer bulb when the rpm rises. If the rpm settles down you can look for several things like faulty primer bulb, fuel pump weak, or sucking air at a connection.

Another possibility would be something loose with the throttle linkage or timing.

Those are the first things I would check and they are easy.

Re: Johnson boat engine, still has a problem? [Re: wallyuwl] #5206851 09/07/19 05:56 AM
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jhs914 Offline
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Also check for water in the fuel. Drain some gas into a glass jar by taking the fuel line off and pumping the primer bulb and look for water. If you have a water separator filter take it off and dump the contents into a glass jar and look for water. And use ethanol free gas.

Re: Johnson boat engine, still has a problem? [Re: wallyuwl] #5206852 09/07/19 05:58 AM
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gonefishin Offline
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I agree with Sweezy. There are a lot of easy things to check first. I have a 1985 70hp. If it were me, I would buy a fuel pump kit. They are under $25. Your diaphragm may be failing.


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Re: Johnson boat engine, still has a problem? [Re: wallyuwl] #5206921 09/07/19 08:02 AM
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Linctex Offline
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If it is THAT CONSISTENT.....

I'm guessing:
1) slightly sticky or bonding throttle linkage parts somewhere.
2) Loose/moving parts in the timing advance mechanism
3) exhaust port carbon
4) some really weird vacuum leak


"The evidence demands a verdict".
(Re:VOA)"it's nearly impossible to actually know the particular additives that are in there at what concentrations."
Re: Johnson boat engine, still has a problem? [Re: wallyuwl] #5206946 09/07/19 08:46 AM
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Ursatdx Offline
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I'm have a Johnson 150 year 2000.
It never runs right on a hose.
Put it in and see how it runs.


"If there must be trouble, let it be in my day, that my child may have peace." --- Thomas Paine
Re: Johnson boat engine, still has a problem? [Re: wallyuwl] #5206961 09/07/19 09:13 AM
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Driz Offline
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Check out the iboats forums. They sell the parts and there is some very specific forums relating to all sorts of old engines. There’s a lot of experience with some of those guy there and they’re very helpful.

Re: Johnson boat engine, still has a problem? [Re: wallyuwl] #5206968 09/07/19 09:22 AM
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JohnnyJohnson Offline
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Sounds like your motor doesn't get all that much use. Try some good premium gasoline (Shell V-10) and see if the problem persists after a couple days of good skiing. Get some good new oil for your mix in the gas also. Oh and maybe a shot of Techron fuel system cleaner.

Last edited by JohnnyJohnson; 09/07/19 09:43 AM.

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Re: Johnson boat engine, still has a problem? [Re: wallyuwl] #5207067 09/07/19 12:26 PM
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ragtoplvr Offline
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My 115 v4 Johnson never ran smoothly on ear muffs. I needed the backpresure of water to run at no load smoothy.

Got a barrel?

Rod

Re: Johnson boat engine, still has a problem? [Re: Linctex] #5207390 09/07/19 08:57 PM
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wallyuwl Offline OP
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Originally Posted by Linctex
If it is THAT CONSISTENT.....

I'm guessing:
1) slightly sticky or bonding throttle linkage parts somewhere.
2) Loose/moving parts in the timing advance mechanism
3) exhaust port carbon
4) some really weird vacuum leak



Yes, it is very consistent.

Thanks everyone.

Can I use throttle body/carb cleaner in the exhaust ports? They are black, but not blocked.

I checked the linkages today and one nut was pretty loose, and the other a little loose. Tightened one screw a little but it wasn't really loose. Also found a grease fitting around that area that goes into the block (not sure exactly what it is for but seems to be related to linkage) that you can tell has never been greased before, so I put some in. Put some PB Blaster on the other points of the linkages.

Am going to try it on the water tomorrow. If it still does it I will try squeezing primer bulb to see if it goes away, and if it does will replace the fuel pump. VRO was removed long ago so I would think it has a regular fuel pump, one of the triangle shaped ones that are about $50, though I have not checked yet. Will also check for air leaks. If none of that works, will probably have carbs rebuilt (I am pretty mechanical but don't want to try that).

Re: Johnson boat engine, still has a problem? [Re: JohnnyJohnson] #5314281 01/05/20 09:04 PM
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another Todd Offline
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Originally Posted by JohnnyJohnson
Sounds like your motor doesn't get all that much use. Try some good premium gasoline (Shell V-10) and see if the problem persists after a couple days of good skiing. Get some good new oil for your mix in the gas also. Oh and maybe a shot of Techron fuel system cleaner.


In my experience, the older two stroke Johnson OBs do not run well on premium fuel. It seems the lower the octane (at the pump) the better. On higher octane fuel they seemed to be down on power a little bit, but definitely noticeable. Stick with regular and good oil.


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Re: Johnson boat engine, still has a problem? [Re: wallyuwl] #5314301 01/05/20 09:39 PM
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4WD Offline
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Had that same motor on an aluminum bass boat …
The carbs needed a rebuild when 6 years old despite low hours …

Re: Johnson boat engine, still has a problem? [Re: wallyuwl] #5335673 01/29/20 04:42 PM
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1978elcamino Offline
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Looks Tippy

Last edited by 1978elcamino; 01/29/20 04:43 PM. Reason: add

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