1987 15 HP Johnson Carb Tuning Issues

I have a 1987 15 HP Johnson that starts easy and runs great for 10-15 seconds with good throttle response then bogs down and dies. It has even compression at 95 lbs per cylinder, rebuilt and cleaned carb, new plugs, good spark, checked fuel pump and adjusted link and sync to where it should be. I backed out the needle 1.5 turns then started from there. When it starts to die I give it a bit more throttle and half choke and it runs better for a couple seconds then dies. I have tried adjusting about 1/4 turn at a time each way and doesn't help. Bulb seems fairly firm after running for a short time. Any tips on fine tuning this carb? Thanks.

Run with another fuel tank and hose as a test.

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Agree. Sounds like you are having a problem with the fuel flow. To confirm this, when it bogs down, give the primer a squeeze and if it revives it, then we are correct.

Fuel flow problems can be a number of things. You can try removing the tank cap to see if venting is an issue. It can be your fuel hose, either the check valves in the primer or the seals around the connectors. It could be the fuel pump. Make sure the 4 screws on the pump are good and snug. Make sure the fuel pump filter screen is not clogged with crap. It can also be your inlet valve on the carb sticking for some reason.

I am from norway and my englich is not good but i try.....Check that the needle valve is in order, if the tip of the needle have damage/wear it will not "fall" down and open for the fuel, the valve is closed..the engine will then run on chocke a short periode until it gets to ritch.
Buy a new needle valve set.

Balder said:

I am from norway and my englich is not good but i try.....Check that the needle valve is in order, if the tip of the needle have damage/wear it will not "fall" down and open for the fuel, the valve is closed..the engine will then run on chocke a short periode until it gets to ritch.
Buy a new needle valve set.

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You can also just tap on the side of the carb with a wrench and if it starts running better yes the float needle is sticking closed.

I tightened up the fuel pump and lines and tried another tank and fuel line (like new) and it died again after a bit. I then kept the primer bulb firm by constantly pumping it and it ran like a top. I know the carb needle is good because it looked brand new with no groove around it. Does it sound like a bad fuel pump? I did go through it because the last guy had it rotated wrong and it didn't let any fuel through. Thanks for the help!

Yes the fuel pump ( little square one ) can be assembled wrong.-------I believe the factory repair kit comes with the proper instructions to get it corrected.

I double checked the fuel pump and it looked good. I ended up replacing the gasket behind the fuel pump to the crankcase and that was it. The gasket was not sealing to the crankcase. Thanks for the help.