Loss of power at high speed

Merc 20 hp, 1976. Motor starts and idles perfectly for hours on end. Accelerates and goes like a bullet for about 2 minutes, then fades and surges, unless primer bulb is squeezed, then it will run like a rocket forever. Already corrected and eliminated many problems on this one, it has become a stumper. Email direct:

piratebob@webtv.net

 

Maybe stating the obvious, but could it be that you have a clogged fuel filter?
Or it could be a problem with the carburettor. Sometimes taking the carburettor apart and give it a good cleaning, makes a lot of difference.

 

Check your fuel fittings, you could be sucking air or the fuel pump is tuckered out.

 

hi speed starvation, merc 20, 1976

Starts and idles perfectly, accelerates like a rocket, goes like a bullet for about 2 minutes or maybe a mile. Tried swapping fuel line, tank, then I soaked, blew out and rebuilt the B-Series MerCarbcarb and integral fuel pump. Later, I took out fuel strainer, found and replaced leaking crankcase bleed line, tried all combinations of pump check valves in carb kit. Was not happy to see new style hollow plastic float with no drop adj.(motor had foam float with brass height and drop tangs) , but put it in anyway, and then again at higher end of spec (higher bowl level, which did prolong good running time) as described by insert sheet in float package. Does not flood. Starts instantly in Michigan 40° water. Does not miss or falter one single bit upon acceleration from idle, in gear. Idles at dead slow indefinitely. Boat will plane at half-throttle and will run a long time that way. Will run forever at full throttle as long as I give primer bulb a squeeze every minute or so. While faltering, engine will recover immediately when bulb is squeezed. New tank, new fittings, removed tank vent, 2 new bulbs (last one Merc branded), new Merc silver hose, all steel screw-type hose clamps. Also probed crankcase pulse hole with soft tool and shot aerosol throttle body cleaner through it. New carb base gasket. New plugs, L78V.

 

Fuel vaporising ? Hot fuel lines ? Sounds heat & fuel related.

 

Try running without the cover on, see if it eases the problem, better heat dispersion, might give a clue.

 

Ask here...

http://www.outboard-motors.us/mercury/

 

check the fuel hose attachment fitting, both the from the tank and mounted on the engine. A loose fitting will suck air and give the same symptoms.

 

Ken, he has swapped all that out so that *should* rule out those issues. One typical complaint seems to be a small split in the tank pick up, but if he has swapped the tank out (unless he has been really unlucky) that should have discounted that as a source of the problem. Ditto for the line and the bulb etc...

One suggestion I read was that the bulb should always be placed in the vertical position to ensure that the one way valve has the best chance of working. Sounds a bit dodgy to me but hey...

Cheers
Mbz

 

All possibilities, but the tank is brand-new, and the swap is a known working one I use on an Evinrude here. I also heard that bit about keeping the bulb vertical, but the check ball is on the tank side and that would aid gravity in keeping it closed (I tried it anyway!). But, I have never, ever had to fanagle a fuel line into any position for any reason in 40 years of running outboard motors. If it's that feeble, something else is wrong. I will try the cover off, although this is very unlikely. It is 50°F daytime here and 43° water temp. Inside the hood, the fuel feed loops from port to starboard, from the connector and over the carb and into the strainer cup. Kind of a radiator in itself, and spaced well enough away from the crankcase, intake (cool) side. But worth a look, thanks. The connector at engine is very tight, the line (female) fitting is brand new with O-rings intact over ball bearing and at the outer end where it engages the taper of male engine fitting. Bayonet pins are not worn. Disassembly showed no debris, intact spring, pin and O-ring. I may buy a new one anyway just to say I did.

Thanks to all, and keep 'em coming. I have more jobs to do on this motor later on.

 

What about getting a vacuum gauge onto the fuel line, seems self evident that vacuum will falter at some point but it would be good to know if when running well you are within normal tolerances.