1989 Johnson 40 horse running rough

nathansr06

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Mar 30, 2014
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15
I am new to boats but have a decent understanding of engines. I can turn a wrench and try to do most things on my own. I bought a pontoon boat with a 1989 Johnson 40 hp vro engine. 2 stroke 2 cylinder. I heard it run on muffs before I bought it. It sounded a little rough at idle but not terrible. I took it out today after completely rebuilding the console. It fired right up and I took off...not so speedy. I know it's a pontoon with a 40 horse but it should run faster than 5 to 10 mph. I can Rev the engine to over 6k rpm no problem but in gear it tops out around 2500 rpm. I got it back on the trailer and popped the hood. The bottom carb wasn't connected so when I was giving it fuel it was only to the top on. I hooked it back up and backed it back into the water. As soon as I took off it ran great and started getting to full speed. It ran great for about 100 yards then the rpms dropped back down to 2500. I looked and the bottom carb was still hooked up and working right so now I'm at a loss of thoughts.

Anyone have any ideas?
 

racerone

Supreme Mariner
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Dec 28, 2013
Messages
36,047
Re: 1989 Johnson 40 horse running rough

It is a mistake to rev that engine to 6000 rpm in nuetral.----Vheck to see if there is spark on both cylinders.
 

boobie

Supreme Mariner
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Nov 5, 2009
Messages
20,826
Re: 1989 Johnson 40 horse running rough

Revving a mtr to 6000 rpm in neutral is asking for an explosion.
 

175se

Petty Officer 1st Class
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Mar 6, 2014
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274
Re: 1989 Johnson 40 horse running rough

was it shaking badly, if it was, did it clear when restarted?
 

nathansr06

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Mar 30, 2014
Messages
15
Re: 1989 Johnson 40 horse running rough

It would shake a little while idling every once in awhile. I'm headed out to the shop now to see what I can get done on it today.
 

Daviet

Fleet Admiral
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Sep 24, 2008
Messages
8,958
Re: 1989 Johnson 40 horse running rough

Start with the basics.
Check the spark, does it jump a 7/16" air gap and is it bright blue?
Have you checked the compression, what kind of numbers did you get?
There is a high speed orifice in the carb, it is plugged or restricted it can cause loss of the cylinder at high speeds.
 

nathansr06

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Mar 30, 2014
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Re: 1989 Johnson 40 horse running rough

Ok so I finally got back to it and just checked compression. 130 in the top cylinder and 135 on the bottom. I haven't checked the spark yet because I haven't had a helper to crank it while I watch. I got some new spark plugs. The old ones didn't look too bad. I pulled the carbs and cleaned them up. If that doesn't work I will rebuild them. I went ahead and replaced all the fuel hoses except the one from the tank to engine. When I pump the fuel ball should it get really hard to pump at some point? This one has never gotten completely hard. Was going to take it to the lake but we're about to have some storms here in oklahoma. Any help will be greatly appreciated!
 
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nathansr06

Cadet
Joined
Mar 30, 2014
Messages
15
Re: 1989 Johnson 40 horse running rough

I got it out on the water today and it ran a lot better but still not like it should. Out of the hole at full throttle it would gain to 3800 rpm and then drop off to about 3400. If I back off the throttle it would gain and run fine at 3600. What should the rpms be at wide open? I tested spark and I've got good spark to both cylinders. Carb rebuild?
 
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