Stihl 020AV Carb Issue Fix

[Vintage Engine Repairs]

Chaps,

I’m just sharing this info in the hope that it helps others out in the future.

I have read a number of times, people having problems with their Walbro WA carbs in their 020av’s.

Not responding to the H screw, rich and unadjustable high speed RPM’s etc.

I have figured out the solution when mine was playing up - my saw was running very rich and no adjustment of the H screw did anything - I figured that the H side was getting unmetered fuel.

My hypothesis was that the AM carb kits that we are using (because they are NLA OEM) were the issue and the valve that acts as a nozzle check valve to stop air entering the metering chamber at idle also seals the H speed orifice due to the negative pressure from the Venturi.

Well the diaphragm in the AM carb kits is thicker than OEM.

AM= 0.24mm
OEM = 0.16mm

The negative pressure from the Venturi is enough to draw the OEM diaphragm valve down and seat it, but not enough to draw the AM valve down and seat. Therefore it opens and allows fuel into the H speed chamber but not enough to stop that fuel from going straight through into the Venturi.

The picture below explains this in more detail.

There is a diaphragm and gasket that sits over the channels that are currently exposed.

The orifice that the red arrow is pointing to is a drilling directly into the Venturi for the high speed circuit. The diaphragm SHOULD be drawn down as the pressure decreases in the venturi. This lets fuel into the H side chamber, but then forces the fuel to take the path via the orifice the green arrow is pointing to. The flow in this channel is metered by the H screw.

The issue with the AM kits is the negative pressure in the venturi is just enough to open the valve into the H speed chamber, but not enough to then draw the flapper valve down to seal the orifice that the red arrow is pointing to. Therefore fuel is free to pass straight into the Venturi without being metered.




Hope this helps others who were pulling their hair out. I ended up just reusing my worn OEM diaohram and it works perfectly.

 

[CausticUC]

Is that the kit that has the two different types of materials you sandwich?

Which one was on the bottom?

 

[Vintage Engine Repairs]

Is that the kit that has the two different types of materials you sandwich?

Which one was on the bottom?

Hey mate, correct, gasket goes on the bottom, diaphragm goes on the top.

 

[CausticUC]

Hey mate, correct, gasket goes on the bottom, diaphragm goes on the top.

Thanks for verifying, pretty sure it's the same issue for my P41. I did the same thing to try and solve its issue.

However it seems like I'll be doing a bit of gasket sanding to reduce the thickness if my micrometer echos what you found.

 

[Vintage Engine Repairs]

Thanks for verifying, pretty sure it's the same issue for my P41. I did the same thing to try and solve its issue.

However it seems like I'll be doing a bit of gasket sanding to reduce the thickness if my micrometer echos what you found.

Pleasure, just to confirm, It’s the diaphragm that’s the issue for me, not the gasket.

 

[rbrown]

I fought this issue for a long time and I used OEM kits. The red arrow hole needs to be plugged on the top as it is a drilling. Ones you fix this issue the Stihl 020AV is a “Wild and crazy guy”!

 

[Vintage Engine Repairs]

I fought this issue for a long time and I used OEM kits. The red arrow hole needs to be plugged on the top as it is a drilling. Ones you fix this issue the Stihl 020AV is a “Wild and crazy guy”!

Hey mate, I did think of doing so, but I avoided plugging it because it didn’t come from the factory plugged for a reason - I presume it is because it allows the transfer of negative pressure from the Venturi into the H speed fuel chamber to fully open the flapper valve.

 

[Drobnock]

I have an 020AVP for may years that ran well, no problems. In 2016 the carb (a WA series) acted as though it was not getting fuel. Took to dealer and was told that it was not repairable no parts for the carburetor. With some searching did find and tried a few generic kits but no luck. I located (2020) a Tillotson HU-40 on line, with some simple modifications in the linkage had it put on the saw. The 020AVP is now back in service.

 

[ajshoe]

I have an 020AVP for may years that ran well, no problems. In 2016 the carb (a WA series) acted as though it was not getting fuel. Took to dealer and was told that it was not repairable no parts for the carburetor. With some searching did find and tried a few generic kits but no luck. I located (2020) a Tillotson HU-40 on line, with some simple modifications in the linkage had it put on the saw. The 020AVP is now back in service.

the tillotson has another fuel line port - what did you do to modify this carb to fit the 020 ?

 

[Drobnock]

the tillotson has another fuel line port - what did you do to modify this carb to fit the 020 ?

020AVP - It has been a while since I looked at the conversion. I believe I blocked off the return line. I need to open up the saw and see what was done. More later.

 

[ajshoe]

020AVP - It has been a while since I looked at the conversion. I believe I blocked off the return line. I need to open up the saw and see what was done. More later.

cleaned my original tillotson carb h7 - works now but not a fan of this carb. zama seems like a better carb. still would like to know how to use this newer tillotson in the stihl 020. I have over 10 of these stihl 020 super saws (my favorite saw in the field). thanks

 

[NE WOODBURNER]

cleaned my original tillotson carb h7 - works now but not a fan of this carb. zama seems like a better carb. still would like to know how to use this newer tillotson in the stihl 020. I have over 10 of these stihl 020 super saws (my favorite saw in the field). thanks

Which Zara are you using ….great thread …just going through this **** now ….can’t wait to get this little guy in some wood

 

[West Aussie]

Chaps,

I’m just sharing this info in the hope that it helps others out in the future.

I have read a number of times, people having problems with their Walbro WA carbs in their 020av’s.

Not responding to the H screw, rich and unadjustable high speed RPM’s etc.

I have figured out the solution when mine was playing up - my saw was running very rich and no adjustment of the H screw did anything - I figured that the H side was getting unmetered fuel.

My hypothesis was that the AM carb kits that we are using (because they are NLA OEM) were the issue and the valve that acts as a nozzle check valve to stop air entering the metering chamber at idle also seals the H speed orifice due to the negative pressure from the Venturi.

Well the diaphragm in the AM carb kits is thicker than OEM.

AM= 0.24mm
OEM = 0.16mm

The negative pressure from the Venturi is enough to draw the OEM diaphragm valve down and seat it, but not enough to draw the AM valve down and seat. Therefore it opens and allows fuel into the H speed chamber but not enough to stop that fuel from going straight through into the Venturi.

The picture below explains this in more detail.

There is a diaphragm and gasket that sits over the channels that are currently exposed.

The orifice that the red arrow is pointing to is a drilling directly into the Venturi for the high speed circuit. The diaphragm SHOULD be drawn down as the pressure decreases in the venturi. This lets fuel into the H side chamber, but then forces the fuel to take the path via the orifice the green arrow is pointing to. The flow in this channel is metered by the H screw.

The issue with the AM kits is the negative pressure in the venturi is just enough to open the valve into the H speed chamber, but not enough to then draw the flapper valve down to seal the orifice that the red arrow is pointing to. Therefore fuel is free to pass straight into the Venturi without being metered.

View attachment 873905


Hope this helps others who were pulling their hair out. I ended up just reusing my worn OEM diaohram and it works perfectly.

I have had issues with my 50yr old stihl 020av after getting the carby overhauled by a chainsaw mechanic - all the same problem with rich and unadjustable high speed issues. I had pulled all my hair out and I do not have that much at nearly 80 years of age!! All the local mechanics here in Western Australia told me it was an air leak but I was not convinced. Found this site and found your solution. Put the original seal and diaphragm back in last night and started the saw this morning and it runs like a new one. Cannot thank you enough. What a great post - You're a LEGEND!!

 

[Vintage Engine Repairs]

I have had issues with my 50yr old stihl 020av after getting the carby overhauled by a chainsaw mechanic - all the same problem with rich and unadjustable high speed issues. I had pulled all my hair out and I do not have that much at nearly 80 years of age!! All the local mechanics here in Western Australia told me it was an air leak but I was not convinced. Found this site and found your solution. Put the original seal and diaphragm back in last night and started the saw this morning and it runs like a new one. Cannot thank you enough. What a great post - You're a LEGEND!!

Glad it helped contact me if you’re looking for a mechanic in Perth WA, I’m here too and run my own business repairing engines. Warm regards.