Mercury 4 HP starts but stalls after 5 seconds

[Freddy-O-Town]

I have a 1987 Mercury 4 HP OB (Serial OA828341) that will run for several seconds and will stall. Seems like a fuel delivery problem. The engine sat for several years and leaked gas (bad gaskets), so I replaced carburator gasket, pump gaskets, pump diaphrams, float bowl O-ring, drain screw gasket, fuel cock, gas filter, and spark plug (gap set to 0.040 inches). Also cleaned out carburator and checked fuel lines. Fuel cock works fine and gas flows freely out of fuel line when disconnected at the fuel pump. Choke and throttle seem to work fine. Using new gas and new Quicksilver 2-stroke outboard oil with 50:1 mixture per manual. Played around with mixture screw (tightened screw to seat, and backed off 1 1/2 turns and more). Engine will start easy but runs for only 5-7 seconds or so. Opening and closing the choke and moving throttle seem to prolong the run a few seconds, but it still dies whether no choke or half-choked. Wondering if it is fuel pump, float bowl or some other issue.

 

[fastjeff]

Fuel pump. If that motor has the old style pump, it can't keep up when the motor gets older. Merc acknowledged this and put out a kit to replace the pumps. Do several myself.

Jeff

 

[Freddy-O-Town]

Have you replaced the fuel lines? It can be several different things have you pulled the spark plug to see if it is wet from fuel? If it is wet see if it burns with a lighter. The idle air adjustment is real touchy these motors run alot longer than five seconds when you unplug the fuel line. Are you getting a spark that will jump a gap of at least 7/16 inch with a spark gap tester. How tight did you get the carb? With a sheet of 200 grit sandpaper lightly surface the flange on a glass table. Just pull the carb in one direction only until the entire surface shines the same. Then pull the carb in evenly with both nuts until good and snug, overtightening the carb is a common mistake. After a few complete warm up cycles snug it back down when the engine is cold. There are two timing specs to check but is not adjustable 5 degrees at 1000 rpms in forward gear and 30 degrees WOT. If it is out of time the flywheel key is probably partially sheared if you can see the mark. Maybe try turning the slow needle in small amounts and see if you can keep it running. You must make the final adjustment in the water in forward gear or it will stall every time you put it into gear. Adjust it out slowly until it coughs note that position then turn it in until it stumbles and split the difference.

Thanks. I'll give these things a try and let you know how it goes.

Thx,

Freddy-O-Town

 

[Freddy-O-Town]

Jeff:

Thanks for the info. That seems to make sense. The engine only has a few hours on it, but has been sitting around for many years. I also can seem to get the new fuel filter completely full, even if I remove the line from the pump. Wondering if that is a result of the weak fuel pump, or it is ok to have air in the fuel filter.

 

[Freddy-O-Town]

Fuel pump. If that motor has the old style pump, it can't keep up when the motor gets older. Merc acknowledged this and put out a kit to replace the pumps. Do several myself.

Jeff

Jeff:

Sounds like you have some expience with these engines. On the parts list, my manual recoil start is for models A834680 and below (my Mercury serial is OA828341) but the pawl is shot and it is now an obsolete part and cant seem to find a replacement. Do you know if I can use the recoil starter for A834681 and Up. It looks it could be substituted but it also looks like the flywheel is slightly different on the newer models, but not sure. Any insight?

Freddy-O-Town