Evinrude carb gasket with wire mesh opening

[olcah]

On my 1939 Evinrude Handitwin the gasket between the carb and crankcase has a wire mesh center opening. When motor is running gasket leaks and blows out gas mix from the crankcase. I can use some gasket sealer but eventually will need a new gasket. This gasket is #191613 , used in lots of small Evinrudes from the 1930s. Looks like NOS may not be available. How do members make a gasket that includes the wire mesh? What is the mesh for? Just to prevent trash entering the crankcase or is it flashback protection against a carb fire in case of backfire?
Thank you.

[chas56x]

First thing is flashback only happens with 4 cycle engines, so that is not a concern. As far as the gasket goes, there are several methods you can use with it.
1. Soak the gasket in water for a few days to swell it up. Then dry it and put a small amount of non-hardening gasket cement, like Permatex #2 on it.
2. Make a new gasket without the wire screen and us it instead. I have a couple of old outboards without the screen and it works just fine.

[frankr]

quote chas56x:

First thing is flashback only happens with 4 cycle engines so that is not a concern. As far as the gasket goes, there are several methods you can use with it.
1. Soak the gasket in water for a few days to swell it up. Then dry it and put a small amount of non-hardening gasket cement, like Permatex #2 on it.
2. Make a new gasket without the wire screen and us it instead. I have a couple of old outboards without the screen and it works just fine.

I have to disagree with that statement. Two-strokes will very often "lean sneeze", where gas explodes in the crankcase. In motors with reeds or intake check valves, it does not blow back through the carburetor, but it certainly can in rotary valve, third port, and other types.

Having said that, I know nothing about the screen in question, so no comment on that.

[Buccaneer]

My 1940 Sportwin had a screened carb gasket as well.
I did not re-install it, but rather made a new gasket
without the screen. Hopefully it doesn’t catch on
fire when I’m ready for the first try starting it,
in a day or two!

Prepare to be boarded!

[olcah]

Bucchaneer, Please let us know if you do have any problem, I have a Sportwin too and it has the same gasket as the Handitwin.

[chas56x]

Frank, please explain to me what a "Lean sneeze" is. How can combustion flame get into the crankcase when ignition occurs a few degrees before the piston reaches top dead center and the intake ports and the exhaust ports are blocked shut by the piston skirt. The combustion flame would have to get past the piston rings, piston skirt to reach into the crankcase.

[frankr]

Dang. Here I sit, having a senior moment, can’t think of the whyfor. But I have heard motors lean sneeze about a million times. So have most others on this forum. Most of us have also been spit in the face as it blows back through the carburetor. It is the cough, or snort, or sneeze that occurs at idle speeds when the incoming fuel charge has too much air in ratio to the amount of fuel.

Ah, it’s starting to come back now. At the bottom of the stroke, piston uncovers exhaust and intake ports, there is still fire in the cylinder, which ignites the fuel coming in through the intake ports from the crankcase. I think I got that right. Somebody help me out of this if that isn’t right.

EDIT: A correct fuel/air mix doesn’t ignite as it enters the intake ports, because it is not a combustible mixture until compressed. Or something like that.

[crosbyman]

possibility x probability ==> will it happen often ? if not maybe best not to worry about it ….

Joining AOMCI has priviledges 🙂

[chas56x]

There is no chance of "residual combustion fire" left in the combustion chamber when the piston reaches bottom dead center and the exhaust ports and the intake ports are uncovered. If you look at the position of the intake ports in relation to the exhaust ports, the exhaust ports are higher up in the cylinder bore than the intake ports. The shape of the piston with the deflector on top forces the burnt fuel/air mixture to exit the combustion chamber before the intake ports are uncovered by the piston. By design, when the intake ports are uncovered, pressure in the crankcase forces the intake charge (fuel/air mixture) into the combustion chamber and the deflector on the top of the piston forces the fuel/air mixture up into the top of the chamber and actually forces any left over exhaust gasses out of the exhaust ports. The carburetor in the older Evinrudes are of the poppet type. The poppet valve in the carb moves up and down in relation the pressure/vacuum pulses in the crankcase developed by the up and down movement of the piston. During the downward movement of the piston, pressure builds up in the crankcase forcing the poppet valve in the carb to close with the assistance of a small spring on top of the poppet valve, thus forcing the fuel/air mixture up into the intake side of the cylinder. During the up stroke of the piston, a vacuum is created in the crankcase, unseating the poppet valve allowing a fresh charge of fuel/air mixture into the crankcase.
With the incorporation of reed valves, this negated the need for poppet valve carburetors. If you get any spitting back of fuel/air mixture out of the carburetor in a two cycle engine, it is caused by either a stuck poppet valve or a broken reed.

[frankr]

edit–

[Author]

Posts