1985 Merc 50hp loses power

[MSBrown253]

The last two times I've taken my boat out, I've had this problem. It will start up, idle fine, and accelerate to full speed. Then after 30 seconds to a minute, it falls flat on it's face. It won't stall, just won't get above 2000-2500 RPM. Anyone experience this, or have any thoughts? Compression is great, 125psi on all 4 cylinders, plugs were new yesterday, gas is fresh.

 

[Mike722]

Re: 1985 Merc 50hp loses power

I would check spark on all 4 cylinders first. If good it sounds like lack of fuel. Check/rebuild fuel pump, check fuel lines for air leaks, check tank for water/debris, and check /rebuild carbs.

Hope this helps
Mike

 

[lisfisher]

Re: 1985 Merc 50hp loses power

Check your spark and coils. If one of your cylinders is not fireing this will happen.

 

[j_martin]

Re: 1985 Merc 50hp loses power

Pump the bulb when it's draggin' down. If it picks up, check/replace, in this order 1. fuel filter. 2 fuel pump (kit) 3. Vacuum hose to fuel pump. 4. All the other fuel hoses. Then go to the expensive stuff above.

Most likely cause is a plugged or misfit (oem) fuel filter. If the fuel pump hasn't been gone through yet, it has rubber check valves in it. Alcohol in the gas wrinkles these up. The kit replaces them with nylon.

Be sure the suction hose, bulb, bayonet fittings are all in good condition. Sucking air kills fuel pressure real quick.

You could T in a fuel pressure gauge just before the carbs. It should hold 3 to 7 lbs of fuel pressure. A real healthy system seems to hold about 5-6 lbs. That would help the troubleshooting.

Get it fixed. Starving yer engine could make a piston problem out of a fuel problem. :^

hope it helps
John

 

[MSBrown253]

Re: 1985 Merc 50hp loses power

I tried squeezing the bulb yesterday, and it didn't seem to have any effect. The carbs on this motor have the fuel pump (with strainer integrated) built right onto the side. Has anyone ever experienced problems with this arrangement?

 

[fishingdan]

Re: 1985 Merc 50hp loses power

I have had this problem on the same engine.

Once the symptoms occur, check for spark while the engine is running (use a simple inductive spark tester you can get at Sears or other chain auto store - you just laying against the wire and it will flash if there is spark through that wire - less than $10).

I bet you are losing spark on two cylinders. In my case, the power pack (the electronic ignition box on the starboard side of the engine) was failing. Simple to replace, but not cheap.

You can fake out the fuel system to determine the problem in that subsystem if you have spark on all 4 cylinders (pumping the bulb, squirting fuel in the carb throat, etc). However, never waste your time with the fuel system until you confirm spark on all cylinders.

 

[MSBrown253]

Re: 1985 Merc 50hp loses power

fishingdan,

You were right on the money. The switchbox is bad, and has no spark on two cylinders! I found a used one for about 1/2 price of new. Not my favorite thing to do, but I already sold the boat, and a new one is sitting in the driveway. I thought I had a simple fuel supply issue to fix before the guy picked it up, so I'm not crazy about dumping a ton of money into it.:|

Thanks for all the advice everyone.