97 8hp no high speed intermittently

Hello, all--New member here--

I have a 97 Evinrude 8HP that I bought on Craigslist about 3-4 weeks ago that hadn't run for at least a year. It ran fine when the seller demoed it for me before I bought it. I got it home and first thing I did was replace the plugs and gave it new gas at 50:1. It starts on 2nd or 3rd pull when cold and runs great at speeds up to about 1/2 throttle. Then, when I advance the throttle, it will not increase speed. Still runs fine, just at only about half speed even though I may have advanced to WOT. I sprayed OMC Engine Tuner thru the carb as per instructions, and it cured the problem for a day or so, then after sitting idle and not running for a couple of days, problem is back. It is intermittent. Problem will sometimes go away when I have throttle advanced to 3/4 or so of WOT (but the motor is only running at RPMs of 1/2 the throttle setting), and it will sputter a 1/2 second, then RPMS will go to the 3/4 or whatever setting I have the throttle advanced to, and run fine at the high speed for awhile. Then, a day or so later, no high speed again. I have run the OMC Engine Tuner thru it about 3 separate times to have it run fine for a day or so, then back to its problem. I've got a new fuel pump ordered and should arrive in a couple of days, but the old one looked fine, and I don't think that is the problem. Should I try Sea Foam? Trying to avoid a carb rebuild.

Assuming the engine has proper compression, and that the spark plugs aren't fouling for some reason... and that (with the s/plugs removed) you have spark that will jump a 7/16" air gap..............

Sitting for that length of time, the carburetor should certainly be fouled somewhat, varnish flaking off internally etc. Normal procedure would be to remove, dismantle, clean and rebuild the carburetor with a new kit.

Before removing the carburetor.... any adjustable needle valves <-- carefully count the number of turns in it takes to gently seat (make a note of it) before removing them. No, you won't remember the turns... make those notes, take pictures.

Thanks Joe! Been struggling with this problem for at least 2 months now. I rebuilt the carb (twice), which looked clean in the first place. Cleaned the jets, and was sure to get the correct # turns on the needle. Tried new fuel pump, new fuel connector at the motor (the fitting at the motor that the fuel hose snaps into) cause the old one was loose. New gas, different fuel hose and different tank. Plugs look good. Checked compression and both cylinders are within a couple pounds of each other. Ran OMC Engine Tuner thru it again 3 days ago, and it didn't change the operation until it had run for about 10 minutes when it burped a couple times, and then ran to WOT OK for about 1/2 hour. Thought I had it solved. But I went home and came back today (2 days later) and it's back to its old tricks, but today running it for 10-15 minutes didn't cure it.

Something I've noticed recently is it appears to get to high speed fine when in neutral or reverse gear, but just not when under load in forward gear. Keep in mind this high speed problem is intermittent, and it starts on the first or 2nd pull and idles fine and runs at low speed fine. I'm thinking about doing another carb rebuild this time with NEW jets rather than just cleaning the old, but want to know if I've missed anything before I do.

As most folks do you have jumped all over the fuel system.-----But I doubt that it is a fuel related problem.------So does spark jump a gap of 3/8" ( 1 cm ) on a test device, yes or no ?------Or run with a timing light hooked up to see if spark is missing on one cylinder.-----Testing is the path forward.

Bought a spark plug tester today, but haven't had time to try it out on the ignition system yet. Probably tomorrow. However, a local shade tree OB mechanic mentioned the issue might be reed valve? I've never had to deal with reed valves, so I don't know if it might lead to my problem. One thing that might indicate yes, is that I've noticed the motor seems to be using more gas for an 8hp than I would think it should. Never measured the fuel usage-it's just a feeling more than anything. Also, when I tested the cylinders' compression the reading I got on the tester was 40 on each cylinder. Since both cylinders were the same, I didn't go any further--thought maybe I was reading the tester in metric or something rather than lbs per sq inch. Are reed valve problems worth pursuing with all these symptoms?

That compression reading doesn't sound right, maybe the tester is faulty or the fitting wasn't seated well. I don't know if you could get the motor to run at all on 40 psi.

How many times did you pull the rope to test compression? I think it takes 3 or 4 strong pulls to get a good reading.

Pulled it thru 4-5 times on each cylinder. I as well thought 40 on each couldn't be right, and have everything run so well (except the no WOT power). That's what made me think I was reading something other than lbs/sq in.

Model # of this motor is ?---A bad reed valve would not be intermittent---Your compression is low.-----Could be a bad test gauge.----Or blown head gasket.----Or worse as in scored pistons.----Run with a timing light to look for ignition issues.