1984 70hp evinrude bogging down at full throttle

[afalkowsky]

Hey guys I have an 84 evinrude that goes down to crawling speeds after about 5-10 minutes of running at full throttle. It starts and idles perfectly fine and never stalls but with the throttle fully engaged it will always bog down to a crawl eventually. If I leave the throttle fully engaged it will sometimes kick back up to speed but mostly it stays at very low speeds. Any suggestions on where to start would be greatly appreciated. Would like to hit all possible issues before spring.

 

[lmuss53]

First thing I'd look for is a clogged vent in the tank. Look for a mud dawdber nest in the vent for a built in tank or a plugged vent in a portable tank. When it is bogged down, look at the ball. Is it collapsed? If you pump the ball does it pick up and run better?

Look for a collapsed lining in some of the gray Atwood fuel hoses, I saw that more than once last year. The clear lining in the hose separates from the outer hose and either collapses or gets pushed up into the hose and creates a blockage.

It could also be a pinched hose somewhere, a leak in a line letting it suck in air somewhere, or a restriction or block in the pickup hose in the tank.

It sounds like it is getting some sort of a blockage that causes a fuel starvation after a while. Because it runs for quite some time before it quits, my first guess would be the vent. I would try it on a different, known good tank with known good fuel and a different hose. If it runs OK then you know you have some sort of restriction like I described. If it still bogs down then you will have to look to the hoses under the cowling and the fuel pump or carbs, or possibly some electrical issue.

 

[Joe Reeves]

In reading over your post a few times, I assume that the engine "does" hit full throttle for a few minutes then gradually slows down tp a crawl... if so, that sounds like a fuel restriction... does the fuel primer bulb have a tendency to go flat? If it does, check the following.

(Fuel Anti Siphon Valve)
(J. Reeves)

Many of the later OMC V/6 engines incorporate a fuel restriction warning via a vacuum device attached to the powerhead. If the engine overheats, or if you have a fuel restriction, the warning is the same.... a steady constant beep.

NOTE... Only the V/6 & V/8 engines have the above "Fuel Restriction Warning". The warning horn will not sound on the other models.

The fact that a engine is not overheating, but the warning horn sounds off with a constant steady beep, and that the rpms drop drastically would indicate that the engine is starving for fuel due to a fuel restriction. Check the built in fuel tank where the rubber fuel line attaches to the tank fitting. That fitting is in all probability a "Anti Siphon" valve which is notorious for sticking in a semi closed position. It will be aluminum, about 2" long, and the insides of it will consist of a spring, a ball, and a ball seat. If this valve exists, remove it, knock out those inner components which will convert it to a straight through fitting, then re-install it. Hopefully that cures the problem.

The above procedure will cure a restriction problem with the anti siphon valve as stated. BUT, it may also allow fuel to drain backwards to the fuel tank when the engine is not running (siphoning backwards) due to the fact that the carburetors/fuel pump etc are higher than the fuel tank. This condition is not an absolute as the valves in the fuel primer bulb usually prevent this backwards siphoning problem. However.... if this does take place, the cure would be to install a new anti siphon valve.

NOTE: There has been cases when the output valve in the fuel primer bulb would come apart, and the inner portion of the valve would actually reverse itself and be drawn back into the primer bulb's output valve body. This in effect would create a shut off valve and result in a fuel restriction. If this is the case, you should be able to feel something laying in the bottom of the primer bulb when held horizontally.

 

[boobie]

Do a cylinder drop test on it. That may tell you something.