Chainsaw Repair

Because both the H and L circuits of the carb feed from the same metering chamber, the barometric pressure of the metering chamber must be kept below atmospheric barometric pressure or the fuel stops flowing.  The high speed nozzle which is fed by the H circuit, has a check valve contained somewhere in its system to prevent atmospheric barometric pressure from seeping past its check valve and back into the metering chamber.  Some of these valves are screens that are wetted by fuel, and others are of a mechanical check valve style.  The mechanical check valve style nozzle has been more problematic than the screen type.  All of the screen type systems are serviceable, but the mechanical check valve types are not.  The reason is some of the components are not sold by the carb manufacturer.  I guess they want you to buy a new carb?  Navigate over to Walbro's website and look up the carb.  The parts listing will show which components are used.  Before I bought any carbs or parts though I would go with what Eccentric posted and replace the seals and all of the fuel line.                   

Good info.  The HDA164 carbs come to mind when talking about problematic mechanical check valves.

When I worked for Sears Service in the early 1990's, a Poulan rep came by the shop one day.  One of the topics he covered while talking with us was the WA/WT Walbro carbs used on basically all of the Poulan manufactured 2-stroke machines that Sears sold.  I remember him admonishing us to NOT blow out the carb bodies with high pressure compressed air, as the non serviceable mechanical check valves in those carbs can easily be damaged that way. 

We also discussed the large number of WA/WT series carbs that we were having unsuccessful rebuilds with.  The failure rate was high enough that we were replacing carbs on warranty work instead or kitting them (and were attempting to talk non-warranty customers into replacing the carbs rather than rebuilding).  Our list cost for the carbs was about 2X the price of the kits.  The book labor charge for replacing the carb was 1/2 the book cost for rebuilding the carbs.  For the customers, it was basically a wash..............and we could bang out the job while they waited.