1989 Mercury 80hp 3 cyl

[Glastron1987]

This motor looks almost like new. It also starts cold and runs great, but then after is has warmed up a while it starts to lose power and stalls out. It's easy to start again at this point but has no power and stalls out especially if you try to accelerate. Try another day and it is fine again, then the cycle repeats. I was pretty thorough making sure the fuel system from tank to carbs is healthy. I'm thinking something like the switch box or coils warm up and then start to faulter. Any advice?

[rgsauger]

If the switch box is original or very old, it would be good to replace it. You are looking at about 150 or $175 for a good Sierra brand. Also double check the pulse fuel pump and float levels in the carburetors.

[Glastron1987]

If the switch box is original or very old, it would be good to replace it. You are looking at about 150 or $175 for a good Sierra brand. Also double check the pulse fuel pump and float levels in the carburetors.

Thanks. I think the floats and pump are good, but will triple-check them after trying a new switch box.

[Glastron1987]

Replaced the switchbox and fuel pump, also verified the float level on one of the carbs. No dice. Still runs good for a while then I need to keep pumping the ball, and this is sticking with an external tank that I know works. Guess I better check the floats on the other carbs.

[rhardwick]

Stator, buy a Mercury stator, stay away from after market ignition parts.

[rock]

Check compression.

Rock

[Glastron1987]

Stator, buy a Mercury stator, stay away from after market ignition parts.

Not that I don't believe it. but how would pumping the ball help with a bad stator?

[Glastron1987]

Tried another set of carbs. Runs the same. Runs for a while then seems to starve of fuel and pumping the ball brings it back. Using good external tank and new fuel pump.

[Glastron1987]

Check compression.

Rock

Just checked compression and it is 120-110-120. Wouldn't think that is bad, however the fuel pump does run off of #2, the lower one. You think that could be making it act like this?

[James45157]

could be a bad bulb had it happen before on a omc v4 walmart stuff did not supply enought fuel try mercury bullb

[rhardwick]

Since it runs when you squeeze the bulb and you have tried a known good fuel supply you may want to check the reed valves. I'm thinking with a broken reed the crankcase pressure needed to drive the fuel pump may not be there. You are correct the fuel pump runs off #2 cylinder.

[Glastron1987]

could be a bad bulb had it happen before on a omc v4 walmart stuff did not supply enought fuel try mercury bullb

Tried three different fuel sources (i.e. internal tank, and two external tanks) with separate bulbs and lines. All should be good and also tested with a special attachment for testing. All produce similar results.

[Glastron1987]

Since it runs when you squeeze the bulb and you have tried a known good fuel supply you may want to check the reed valves. I'm thinking with a broken reed the crankcase pressure needed to drive the fuel pump may not be there. You are correct the fuel pump runs off #2 cylinder.

Reed valves are new CCM. Examined them after running quite a lot with the issue. All reeds looked good and nothing noteworthy on their seating. Actually then looked quite good to me.

[Glastron1987]

So I put a pressure/vacuum gauge at the pulse entry into the fuel pump, and also at the output of the pump. While running, the pulse entry cycled between roughly 5psi and -12psi. This seemed normal to me. On the output end there was 0 (zero) pressure, unless I pumped the ball, then I got around 2psi momentarily. Now this is a brand new fuel pump, not just a kit rebuild. So I took the original fuel pump and applied a rebuild kit, and output pressure is now roughly 4-6psi. The engine seems to operate normally now on all three of the fuel tanks I was testing with. All this testing was on the trailer, so now I need to take it to the lake.

[rhardwick]

Is this a Mercury fuel pump or aftermarket? I would only trust a Mercury fuel pump.

[Glastron1987]

Is this a Mercury fuel pump or aftermarket? I would only trust a Mercury fuel pump.

The zero pressure was on a brand new non-merc fuel pump. So sounds like you're right to trust only the Mercury one.

So the test at the lake proved that the rebuilt Mercury pump had corrected a problem. It ran fine on the external tank at all speeds and sustained it.

When I hooked it up to the internal tank it failed again. So obviously there is still a problem with that system somewhere. I started with replacing the ball and immediately adjacent lines. Recently I had already replaced the pickup tube in the tank and the anti-siphon valve. This all seemed to pump by hand OK, but several locals are telling me to remove the anti-siphon valve entirely... even though it may be brand new they are bad news.

Anyone else have an opinion about anti-siphon valves?

[rock]

Unless your motor is lower than the fuel tank remove it.

Rock