Evinrude firing on top cylinder only

[peskate33565]

Hi, I have a 1984 50hp Evinrude that is only firing on the top cylinder. I have spark on both plugs and compression is reading 140psi on both cylinders. The bottom plug is clean leading me to believe there is no fuel getting to the plug. The fuel lines are not clogged, the carb has been cleaned, what else could be the problem? It couldn't be the fuel pump since the motor will run on the top cylinder right?

 

[Joe Reeves]

Re: Evinrude firing on top cylinder only

Did you manually clean the high sped jet that is located in the bottom center portion of the carburetor float chamber, that is.... clean it with a piece of single strand steel wire?

Fuel must flow thru that jet freely before it can flow to any other fuel passage way.

Spark, with the spark plugs removed, must jump a 7/16" gap with a strong blue lightning like flame.... a real SNAP! Does it? (testing spark with the spark plugs is a waste of time). The gap is important.

 

[peskate33565]

Re: Evinrude firing on top cylinder only

Do you happen to live in Plant City? I saw your location and your last name, I think I know your son Tyler... The high speed jet is the one running through the middle of the carb correct, you can see it when the "flap" is open? If so then yes I cleaned that thoroughly. I tried to understand your spark tester but couldn't figure out how to make it, but the old fashion spark test gave me spark. If you do live in Plant City you think you could take a look at it?

 

[Joe Reeves]

Re: Evinrude firing on top cylinder only

No, that narrow brass tube is a slow speed passage way, the larger brass tube is a high speed nozzle. The high speed jet is a 1/4x20 threaded brass jet about 3/4" long and is located in the center bottom portion of the float chamber (not in the upper carb body). It is accessible by removing the float chamber drain screw/bolt.

Sorry, I don't do that type work any longer nor do I live in Plant City.

 

[peskate33565]

Re: Evinrude firing on top cylinder only

Oh ok, I'll clean that out and give it a try.

 

[boobie]

Re: Evinrude firing on top cylinder only

You can go to Auto Zone and get an adjustable spark tester for around $7.00. It's a cheap investment if you plan on doing your own work. The spark has to jump the 7/16" open air gap. Checking spark with the old spark plug trick tells you nothing.

 

[James R]

Re: Evinrude firing on top cylinder only

Had problems with a 1987 50HP like yours. You need to check if it is a fuel problem. Use a spray bottle with some 50:1 mix and spray it into the carb with the motor running. If it picks up it is a fuel problem. My motor would not suck fuel in properly at Idle. Ended up replacing the carbs. The only thing that I found was that the butterfly spindles were loose probably allowing air to be sucked in and not pulling air over the jets drawing the fuel in. These OMC motors don't have a good suck. The only other thing, if it is not the carb, would be an air leak at the carb or manifold, a crack in the crank case or a bad crank seal.
If it doesn't pick up then look to your ignition.

 

[ba_50]

Re: Evinrude firing on top cylinder only

I'm having the same trouble with some '86 48hp carbs. They sat for about 2 years and only idled most of the time. They keep needing cleaned again and again even after soaking. The crud must keep breaking loose and plugging t up. Seafoam in the gas got the power back, then went back to idle. It must be ethanol. Stubborn.

 

[James R]

Re: Evinrude firing on top cylinder only

If the motor sat then the fuel tank also sat. Shelac can form on the sides of the tank as the fuel evaporates. It then breaks off and washes into the fresh fuel. It ends up in the carbs. I am assuming that you are using the same tank of course.
If it is a plastic tank then replace it. If it is aluminum and more expensive then it may be worth trying to clean it.
Always use Stabil in your fuel and for your size motor consider a water trap filter.

 

[geoffwga1]

Re: Evinrude firing on top cylinder only

Hi, I have a 1984 50hp Evinrude that is only firing on the top cylinder. I have spark on both plugs and compression is reading 140psi on both cylinders. The bottom plug is clean leading me to believe there is no fuel getting to the plug. The fuel lines are not clogged, the carb has been cleaned, what else could be the problem? It couldn't be the fuel pump since the motor will run on the top cylinder right?

Seeing as you're running on 1 cylinder,fuel must be getting through the carburettor,therefore both plugs should show signs of fuel.If they don't I would suspect a reed problem,where fuel was getting to 1 cylinder but not the other.This happened to me with a 75 stinger .I was getting no fuel to the bottom cyl but had to run it for 20 mins or so to get to dock.Therefore also no oil.Result was that despite sorting reed problem,three or four trips later I lost that cylinder spectacularly and destructively so don't run it more than necessary before finding the fault and fixing it
cheers.Geoff69

 

[99yam40]

Re: Evinrude firing on top cylinder only

If you have not replaced the fuel pump lately or at least taken it apart to see if it has a pin hole in the diaphragm, then do that first.
Too much fuel being sucked into the crankcase through the pump vacuum/pulse port is just as bad as too little in the cylinder to fire properly