1995 Johnson 70 HP Issue

This year, I'm having issues with the motor sputtering or chugging at high RPM, especially when warm.  It basically will not achieve higher end RPM's and will chug along revving up and down and not maintaining a consistent powerband.

When the issue first started, I did the easy stuff:  fuel filter and spark plugs.  That didn't work.  I started to worry bad gas or moisture in the fuel.  I completely emptied the tank and put all fresh fuel in.  It ran great for two outings.  Then, the last outing, after it was warmed up and had run fine at 3/4 throttle for some time, the issue returned.

I'm at a loss and not sure where to start.  Is this a fuel delivery issue (fuel pump) or an electrical one?  

EDIT:  I know there were some oil injection issues with this vintage.  However, the prior owner had the oil injection system removed, so I've been running 50:1 mix since owning the boat.

Glad it worked out, it’s always nice to know that you found the smoking gun! 

Just to wrap a bow on this, it ended up being the power pack.    

I had a 1989 Evinrude 70HP that I was running 50:1 just like you that had this issue as well. It would start and idle fine, plane my boat fine, but after 30-60 seconds at WOT, it would bog/surge and would about give you whiplash. I tried everything...drained the fuel (thinking moisture in the tank), changed plugs, checked spark plug wires/boots for cracks...eventually took it to the marina...TWICE...had the carbs rebuilt/fuel lines replaced inside the motor...EVERYTHING! Still did it! 

One night I gave up on the fuel delivery and started looking more at electronic components and located a corroded connector where the power pack connected to one of the coils. Cleaned it up with a wire brush, put dielectric grease on it and never had an issue until something let loose inside the powerhead a few weeks ago. My guess was there was enough resistance in that connector with the corrosion once those electrical components got warm when running at WOT that there was an intermittent misfire? Not sure, but my message is to look at the ignition system VERY well and check the connections on the power pack and coils for corrosion. My guess is if you've exhausted all possible fuel delivery issues, it has to be electrical/ignition. Start with the easy/inexpensive things like connections, then move to coil packs and power pack. Those engines are pretty simple. I just replaced a power pack on my brother in law's 1987 Evinrude 150 last week that was only firing on one side of the block (left cylinders would fire, right would not). The OEM power pack was $175, but took less than 10 minutes to swap out. Motor now runs like a top! Start simple, then look into coils and power pack. Good luck!

To eliminate a cracked diaphragm in the fuel pump, try pumping the primer bulb when the motor is misbehaving.  

If it begins to work, the issue is likely a crack in your fuel line, or a bad diagram in your fuel pump.

The above said, I don’t believe that is your issue, usually the motor just quits, and then you pump it, and it restarts instantly.

Just narrowing possibilities based on the post before mine...

Try rebuilding your fuel pump and carb worked for me

I pulled the plugs the first time this happened, and they all looked good.  I replaced them anyway, and haven't pulled them since.  I will check them again.  Thanks.

Dude,

I just posted this on a different thread, but the issue seems similar to me...

You may losing a cylinder due to a failing power pack. 

The power pack stores voltage from the stator to release it to your ignition coil when the trigger tells it to. Often a faulty Power unit will see intermittent or no spark on cylinders and may only breakdown when your engine warms up.

Pull your spark plugs, and physically inspect them, they should all look the same, if one of them is fouled, you found your answer...