Stihl 020 Carb Hack Advice

[Vintage Engine Repairs]

Hi guys,

I have two 020 carbs, one uses a small diaphragm and gasket under a metal plate on the metering side of the carb to meter the H side fuel supply.

The diaphragm in question serves a dual purpose - it lets fuel in and as it does, it seals a drilling that is open to the venturi (it’s the vacuum created that pulls the diaphragm down).

The diaphragm I have is very worn (understatement) and I tried an AM kit as OEM is NLA, but the material is slightly too thick and it won’t close off the drilling that is open to the venturi, thus you lose all ability to tune the H - it’s constantly pig rich as it gets unmetered fuel.

My question, is it possible to do away with this diaphragm and just put some sealant over that drilling that is there to actuate the diaphragm?

The red arrow points to the drilling that is open to the venturi and the pressures in there actuate the diaphragm and opens and closes the red arrow port. That is the one I’m wondering if I can seal? The green arrow is where the fuel flows and is metered by the H needle

 

[Vintage Engine Repairs]

Ok guys, for the benefit of anyone else in the future, DON’T block that valve drilling.

I was so close to doing so to test it (even had Jb weld out ready) then it clicked. Of course you can’t, there will be nothing stoping the air coming back through the H drilling. Also, you’ll have fuel flowing from the H circuit at idle as there is no nozzel to protrude out into the venturi, so any slight vacuum will pass over the hole in venturi and draw fuel from the H circuit.

 

[Vintage Engine Repairs]

I’ll also add, I wondered why this carb had such incredible off idle acceleration, when compared to the walbro alternative.

I believe that as soon as the throttle butterfly is even slightly more open than idle, there is enough vacuum to lift the diaphragm off its seat and the saw starts getting fuel off the H circuit - basically at the same time as the progressive drilling’s do. This means I don’t have to run the L circuit on the rich side at idle for the progressive drilling’s to be effective - it’s getting H fuel supply at the same time.

The other carb is the normal nozzel style that protrudes into the venturi, and it’s not until the butterfly is open wide enough about 1/2 throttle that the H circuit flows, there was always a dead spot between progressive and main - I always got poor off idle acceleration and had to have the L screw super rich!

All this is speculation of course.. I’ll take the air filter off and gradually open the throttle and see where the butterfly is positioned when it first lets fuel flow through the H - I’ll report my findings even if I’m wrong!