Walbro HDA 175

[Armosh]

ANyone know what purpose does the plastic spacer serve that is under the cover?

 

[J D]

ANyone know what purpose does the plastic spacer serve that is under the cover?

Which part are you referring to?

 

[Armosh]

View attachment 1033531
Which part are you referring to?

I had a look at several of those before I posted and I don't see it in any of the schematics. It is a plastic spacer underneath the metering diaphragm cover.

 

[Chevboy0167]

Well... of course the Walbro parts look up page is down. It seems they changed the layout and navigation. Shame.....

 

[Chevboy0167]

Just some info...

https://www.walbro.com/wp-content/uploads/2016/01/HDAseries.pdf

 

[J D]

What saw is it on? Usually the spacer would be there on a compensated carb. Does the cover plate over the metering diaphragm have a hole in it? It's always possible someone has rebuilt the carb with parts from a carb that was slightly different

 

[Armosh]

What saw is it on? Usually the spacer would be there on a compensated carb. Does the cover plate over the metering diaphragm have a hole in it? It's always possible someone has rebuilt the carb with parts from a carb that was slightly different

It is a 2004 Husky 359.

 

[Chevboy0167]

Available extras is what caught my eyes...

 

[Chevboy0167]

LOL.... they called it just a Plate....

https://www.partstree.com/models/359-husqvarna-chainsaw-2001-01/carburetor-parts-2/

 

[a. palmer jr.]

I think that plate is for venting among other things. Probably the metal cover doesn't have a hole in it? I'm sure the engineers had a reason for this configuration..

 

[J D]

 

[J D]

The plastic plate vents what would usually be the atmosphere side of the metering diaphragm back to inside the air filter through a passage in the carb body. The idea behind this is to compensate for a blocked air filter (the more restricted the air filter gets the lower the pressure behind the metering diaphragm, causing the metering needle valve to release less fuel).
Unfortunately the passage is to restrictive & this can lead to a variety of tuning issues.
Tree monkey on YouTube does a good spiel on the subject.
My recommendation would be mod the carb to a regular "vents to atmosphere" style if you're having any tuning issues (particularly at or around idle). This can be done with no additional parts by removing the plastic spacer, flipping the gasket so it seals the vent passage, & drilling a hole in the cover plate

 

[Armosh]

The plastic plate vents what would usually be the atmosphere side of the metering diaphragm back to inside the air filter through a passage in the carb body. The idea behind this is to compensate for a blocked air filter (the more restricted the air filter gets the lower the pressure behind the metering diaphragm, causing the metering needle valve to release less fuel).
Unfortunately the passage is to restrictive & this can lead to a variety of tuning issues.
Tree monkey on YouTube does a good spiel on the subject.
My recommendation would be mod the carb to a regular "vents to atmosphere" style if you're having any tuning issues (particularly at or around idle). This can be done with no additional parts by removing the plastic spacer, flipping the gasket so it seals the vent passage, & drilling a hole in the cover plate

I did drill a hole in the cover plate and that did improve it's behavior however I didn't know about removing the spacer and flipping the gasket. I'll give that a try when I get a chance. Thank you.

 

[Armosh]

The plastic plate vents what would usually be the atmosphere side of the metering diaphragm back to inside the air filter through a passage in the carb body. The idea behind this is to compensate for a blocked air filter (the more restricted the air filter gets the lower the pressure behind the metering diaphragm, causing the metering needle valve to release less fuel).
Unfortunately the passage is to restrictive & this can lead to a variety of tuning issues.
Tree monkey on YouTube does a good spiel on the subject.
My recommendation would be mod the carb to a regular "vents to atmosphere" style if you're having any tuning issues (particularly at or around idle). This can be done with no additional parts by removing the plastic spacer, flipping the gasket so it seals the vent passage, & drilling a hole in the cover plate

I should add that I am indeed having trouble tuning it at idle. The saw when tipped on its sides did change idle which would suggest an air leak if I'm not mistaken (needs to pressure/vac test it to confirm) but as I said when I drilled a hole in the carb cover that behavior got better so I'm leaning more towards the carb being the problem.