History: Bought a 70 hp Evinrude from 1987 with top piston/cylinder scorched.
Rebored all three cylinders
Changed pistons and rings
Changed the wristpin and rod bearings
Replaced all powerhead seals and gaskets
Rebuilt carburetors
New waterpump
The engine starts but runs rough on the top cylinder. It spits back fuel through the top carburetor causing alot of lean popping. Cylinder 2 and 3 apears to be running just fine.
To eliminate the possibillity of faulty reeds I changed place of the reed box from cylinder one and two. After switch cylinder two still runs fine and cyilnder one is spitting back through carb and leanpopping. I used a 8 mm hose as listening device (stetoscope?) and put it through the carbs and spun the flywheel by hand (plugs out). Cylinder three and two sounds alike and there's no blowing sound as the piston goes down. On cylinder one there's quite a strong blowing sound as the piston goes down. This blowing sound is not from reeds or a faulty gasket but from a canal that starts just outside the reeds (towards carbs) and leads up to the top main bearing.
I guess that the idea is that some of the fuel/oil mix is picked up before the reeds (by negative crankcase pressure) and routed through the main bearing and thereby lubricationg it before it mixes with the rest of the fuel/oil (that took the normal through-the-reeds way) in the crankcase.
Somehow the flow in this canal is to great (at least one way) and much of the fuel/oil is blown back through it causing lean conditions. This have lead me to think that theres a restrictor or one way valve missing. Either that or the flow through the top main bearing is to great wich I think is less probable. My seloc repair manuel mentions nothing of above at all and from different crank case and manifold "break up diagrams" I've found on the web I can't seem to get any clue either.
Info: Hole in main bearing is about 5 mm, connecting hole in crankcas cover is about 8 mm on the bearing side and about 3x5 mm on the manifold side. And the routing in the manifold (behind the gasket) is about 2 mm wide and deep. This is obviously to big to restrict blowback. I'll try to post som photos of this.
Rebored all three cylinders
Changed pistons and rings
Changed the wristpin and rod bearings
Replaced all powerhead seals and gaskets
Rebuilt carburetors
New waterpump
The engine starts but runs rough on the top cylinder. It spits back fuel through the top carburetor causing alot of lean popping. Cylinder 2 and 3 apears to be running just fine.
To eliminate the possibillity of faulty reeds I changed place of the reed box from cylinder one and two. After switch cylinder two still runs fine and cyilnder one is spitting back through carb and leanpopping. I used a 8 mm hose as listening device (stetoscope?) and put it through the carbs and spun the flywheel by hand (plugs out). Cylinder three and two sounds alike and there's no blowing sound as the piston goes down. On cylinder one there's quite a strong blowing sound as the piston goes down. This blowing sound is not from reeds or a faulty gasket but from a canal that starts just outside the reeds (towards carbs) and leads up to the top main bearing.
I guess that the idea is that some of the fuel/oil mix is picked up before the reeds (by negative crankcase pressure) and routed through the main bearing and thereby lubricationg it before it mixes with the rest of the fuel/oil (that took the normal through-the-reeds way) in the crankcase.
Somehow the flow in this canal is to great (at least one way) and much of the fuel/oil is blown back through it causing lean conditions. This have lead me to think that theres a restrictor or one way valve missing. Either that or the flow through the top main bearing is to great wich I think is less probable. My seloc repair manuel mentions nothing of above at all and from different crank case and manifold "break up diagrams" I've found on the web I can't seem to get any clue either.
Info: Hole in main bearing is about 5 mm, connecting hole in crankcas cover is about 8 mm on the bearing side and about 3x5 mm on the manifold side. And the routing in the manifold (behind the gasket) is about 2 mm wide and deep. This is obviously to big to restrict blowback. I'll try to post som photos of this.
