1979 Evinrude 85 hp misses at high rpm

[ChristianMariner]

Hello All,
I purchased a used boat and insisted that the PO would take it on the water to show that it was in working order. It is an open bow trihull 16 ft Newman Cam II with an Evinrude 85 hp motor.
We took it out for about 20 minutes. It started ok, had to try it 3 times before she fired, and then it warmed up and we took it out. Went for about 8 minutes through the 5 mph zone then out to the open water, got it up on a plane, no problem. She ran beautifully. Came back through the 5mph / no wake area smoothly and I bought her.
Now, my first trip after buying her, I was plagued with motor problems. First, took forever to start. Very unlike the previous experience. Then, once running had a bad miss at low spees going through the 5mph zone. Once out in the open water, the boat smoothed out a little after giving some gas. Then wouldnt go above 2k rpms. Runs very smooth at WOT but wouldnt go above 2k rpms. Then, every once in a while, would just start trying to, and then would bounce between 2k and 2.5k and then sometimes would hit 3k. Then would fall back to 2k. It really is jerky when doing this... Crusing at wide open throttle and starts jerking forward as the rpms jump and fall off. I thought maybe I had fouled the plugs by starting it incorrectly and choking to much. I changed the plugs and she started way faster and smotther, but ran the same way on the water. Nothing over 2k, and then sometimes the jerking and missing up around 2.8k then back to 2k.
Next, I changed the fuel line from the tank to the motor, and removed the squeeze bulb and put in an electric fuel pump there instead. After about 15 minutes of the exact same bad behavior, the RPMS wouls go up to 2.5k consistently and after an hour like this I could get up to about 3.5k to 4k at wide open throttle. Sometimes really smooth, sometimes doing the same jerking and hesitating near the 4k mark.
So, I took it out again with the same setup, hoping it would eventually get better. It did not. It is still starting ok, going smooth at low RPMs through the no wake zone, and accelerating nicely upbto about 3k but then it sometimes stays there and sometimes bounces its way to 4k and sometimes not.
I am a total newbie and am hoping this kind of behavior is something that points to a common problem that I am just unaware of. I have scoured the net and also the OMC sticky page of this forum, but havent seen anything that sounds like my situation. Thanks for reading and hopefully someone know what would cause this.

 

[racso]

try mixing a can of seafoam in the fuel , maybe the fuel system is duty , my was doing kind of the same I saw a video on youtube, and i mixed a can in a gallon of fuel and run it until i cut off that helped it don't give any problems no more , but i also rebuilt the carbs

 

[Roj115]

It sounds like you're not firing on all cylinders. That engine should get to around 5,500 rpm at WOT, unless it's way overpitched (propeller is too large). It also sounds like the fault is intermittent. The reason I say that is that it sounds like the problem cylinder(s) are working for short periods of time and this is what is causing your revs to suddenly increase for short periods of time.

I would first disconnect one spark plug lead at a time to see which cylinder(s) are not firing. It won't make any difference when you disconnect the lead from a non-firing cylinder but it will run way more rough when you disconnect the lead from a firing cylinder. To do this, shut the engine off, disconnect a plug lead, start the motor up and if it starts, take it out for a test drive to see how it performs. Don't try connecting or disconnecting a plug lead when the engine is running. You could get a nasty shock doing that.

It could be as simple as one or more bad spark plugs. Maybe a crack in the electrode insulator. I've seen it where a plug with a cracked electrode insulator will spark normally at idle and under light load but when under heavy load, the resistance increases inside the ignition chamber and this can cause the plug to misfire sideways through the cracked insulator.

If you can isolate the bad cylinders, start working your way up through the ignition system by replacing the spark plugs, then the spark plug leads and then the coil(s). To test a coil, you can swap the suspected bad coil with one on another cylinder that you know to be working and see if the problem shifts to that other cylinder. Replacelent coils for that engine should be really cheap to get on eBay.

Hope this helps

 

[emdsapmgr]

It is very possible that you have an ignition problem. Weak ignition components tend to fail once the engine heats up to normal operating temps. I like the idea of pulling the spark plug wires off the engine. I'd purchase (or borrow) a set of automotive insulated pliers and pull the plugwires off the plugs when it's running. Then replace them. You will find out quickly that way just which ones are firing. That engine will seem to run very smooth, when firing on only 2 cyls. You need all 4 firing when the boat is in the water and under load (in forward gear.)