"Possessed" 40 hp Johnson Outboard

[bm1961]

Ok guys, I've got a 40 hp Johnson 2 cyl that I have been working on for a buddy of mine. Thing has not been ran for 4 years. I pulled the two carbs, cleaned all the junk out of the bowls, jets, needles, seats, went through the fuel pump, changed out the gas, installed new plugs. Fire it up and the the motor will rev up and down on its own and not hold a steady RPM, fluctuates RPM with no throttle movement. Today I plugged the fuel supply line to the electric choke solenoid (enrichner) thinking it may be leaking gas by the solenoid into the engine intermittedly. Anyway, when I crank it up, this dam thing does its "up and down" rev, then takes off wide open (like it has done several times). I cut the throttle, no change, cut the switch off, no change, pull both the plug wires, still running wide open. I pull the fuel line and the damm thing still runs for about 45 seconds until the carb bowls run dry, with neither plug wire attached to the plugs and the switch off. Anybody got any ideas what in the hell is going on with this thing? Any advice appreciated.

 

[ScottinAZ]

Re: "Possessed" 40 hp Johnson Outboard

if it runs on with the plug wires removed, decarb the engine. It has some carbon glowing inside the chambers that is igniting the mixture. This may help clear up some of the other problems as well

 

[bm1961]

Re: "Possessed" 40 hp Johnson Outboard

Wouldn't it be best just to pull the head, invest in a head gasket and clean the carbon off the pistons and head?

 

[tx1961whaler]

Re: "Possessed" 40 hp Johnson Outboard

Scary, isn't it? It's called "thermal runaway".
Decarb the engine first, then try to look for air leaks in the fuel lines, connectors, intake, carbs, etc. if you're sure the carbs are internally right. Spray around the gaskets with WD-40 or carb cleaner to see if it the RPMs change when you spray a particular area

 

[noelm]

Re: "Possessed" 40 hp Johnson Outboard

runaway is not always caused by heat, a very lean burn will do the same (remember what happens just before you run out of petrol?) you are trying to restrict the fuel flow to it, when in fact it is probably not getting enough fuel (too much air) when this occurs, no pulling off plug leads will stop it, you need to block off the air intake, or wait untill it does run out of fuel (if it doesn't explode first) redo the carbies, paying special attention to any jets or air passages, look for air leaks after the carby.