Johnson 4HP 86' dies after 45 seconds at WOT

This is killing me - niggly problem just wont go away!
History is that the motor would not run without the choke in (as purchased by my father) it was put in for service and dealer could not find problem except that when he swapped the carby body with another, it worked OK. He deduced that the body had a hairline crack and replaced the carby body (dont ask how much). The motor was not used for 6 months and now exibits the problem mentioned. The dealer in question is over 400K's away and a return is out of the question.
I first thought lack of fuel flow but with the carby drain plug open the fuel flow seems more than enough (would empty the tank in 5 mins if left). - I cleaned the carb thoroughly and there are no blockages. I have also adjusted the float level a little higher to allow the float valve to be open a little further when the float is dropped right down - figured this is the biggest restrictor to fuel flow as the needle only drops a very small way from the seat when the float is fully down.
I also emptied the bowl and firmly held it in place so I could see how long it took to fill. I opened the tap for two seconds and removed it - the bowl was already full.
I tested the reassembled motor in a small tank but cant make it repeat the fault as the tank isn't big enough to run a full minute at WOT as it aerates the water to the point where there is no load - also worried the stand may fail and the seahorse will cut off my legs
It will run all day at part throttle opening or at idle
Just hoping there may be something I have not thought of before heading to the lake for another try.
Thanks for any help,
Wayne

Re: Johnson 4HP 86' dies after 45 seconds at WOT

Take the fuel petcock out on the (it screws out) tank, and clean it throughly. Clean the screen, and soak the living daylights out of it (over night) in good carb cleaner. There is a VERY small opening in the tap, and it's plugged with gum/crud/who knows. Just got done with the same problem on a 2hp Rude "Mate". It might flow fine playing with it, but at WOT, its starving out. Also check to make sure the cap vent is working OK. Cut a new hunk of fuel line too. Might be swelled up on the inside somewhere.

Re: Johnson 4HP 86' dies after 45 seconds at WOT

Hi Scaaty- Thank for your help.
The little Johnny doesnt have a petcock on the tank but does have one actually mounted on the carby but the exploded view doesn't show a screen - The petcock does seem to have very small passages though - I didnt dissasemble it when I done the carby - I will give that a clean and also replace the hose - It's cheap and you never know .
The vent cap is a good point too- I made sure it was open but I didnt positively check it was working - Could be blocked.
I'll give these things a go too and let you all know how it goes.
Thanks again for the tips
Wayne

Re: Johnson 4HP 86' dies after 45 seconds at WOT

Don't know about your outboard, but farm tractors have a bowl that attaches to the bottom of the main carb casting. If you take the bowl off and look up into the bottom of the carb casting there is an extremely fine mesh screen pressed in there the same size as the diameter of the bowl.

Won't take but a minute to check.

Other thing could be tank venting, or a restriction/malfunction in the primer bulb. Check these things and if it persists, hit the prime bulb the next time it does it. If you have to pump to keep it going might investigate fuel pump and cracked fuel lines.

Mark

Re: Johnson 4HP 86' dies after 45 seconds at WOT

disconnect the fuel line at the carb and blow air into it back toward the tank, could be debrie blocking the fuel pick up in the tank.