Carb primer bulb air lock/pressure building

[Old2stroke]

You have a bad check valve in the carb body. When you operate the primer it is sucking air back through the check valve rather then fuel up through the carb and that is the source of air to pressurize the tank. If a saw has a primer bulb, there will be two check valves in the carb, one will be a nozzle check valve to stop air bleeding back from the high speed circuit into the idle circuit, the other will be a check valve in the body that is just there to allow the primer bulb to function and can be removed with no affect on the saw except that the primer won't work.

 

[hotshot]

, the other will be a check valve in the body that is just there to allow the primer bulb to function and can be removed with no affect on the saw except that the primer won't work.

Can you remember what carb model you have found this “second“ check valve on?

That means that there is another fuel feed out of the metering chamber, but I’ve never come across one on a purge type carb.

 

[Old2stroke]

With this type of carb where the primer is not attached to the carb, the check valve that allows it to function is a small round brass plug (about 1/4" dia.) with a hole in the centre located in the main fuel cavity and is the source for fuel to the idle circuit (sometimes high speed too). Cannot be removed without destroying it. The high speed jet nozzle check valve looks like a solid brass plug blocking a passage angled up to the venturi part of the carb and can be removed by punching it through into the venturi. Gets it's fuel feed to the metering screw either through a small hole in the fuel cavity or from a passage fed from the body check valve. Some carbs have a different type of main jet check valve so it may not look like this one but there will be one there somewhere in MOST carbs although there are always the exception, some older carbs on big old saws had enough isolation between the idle and high speed circuits that they weren't needed.

 

[hotshot]

With this type of carb where the primer is not attached to the carb, the check valve that allows it to function is a small round brass plug (about 1/4" dia.) with a hole in the centre located in the main fuel cavity and is the source for fuel to the idle circuit (sometimes high speed too). Cannot be removed without destroying it. The high speed jet nozzle check valve looks like a solid brass plug blocking a passage angled up to the venturi part of the carb and can be removed by punching it through into the venturi. Gets it's fuel feed to the metering screw either through a small hole in the fuel cavity or from a passage fed from the body check valve. Some carbs have a different type of main jet check valve so it may not look like this one but there will be one there somewhere in MOST carbs although there are always the exception, some older carbs on big old saws had enough isolation between the idle and high speed circuits that they weren't needed.

No carb make & model in all that reply?, that’s all that I asked...

 

[Old2stroke]

Zama C3A, Walbro WT-402B, Walbro WTA-30, Happy now?

 

[CFOD]

Zama C3A, Walbro WT-402B, Walbro WTA-30, Happy now?

It's a C3A

 

[hotshot]

It's a C3A

Good deal!
Can you post up a pic of the metering side, with the cover & diaphragm removed?

 

[Old2stroke]

Easy test for check valves. Plug the fuel inlet port to the carb and apply a vacuum to the port going to the primer, if it won't hold a vacuum, there is a bad check valve.

 

[CFOD]

Good deal!
Can you post up a pic of the metering side, with the cover & diaphragm removed?

Will do later this week, my final exams for university are in a few days so I'm taking a hiatus from chainsaws

 

[hotshot]

Thanks, & good luck with your tests!

 

[Okie]

As grizz55 indicates, you can bypass the carb to see if the bulb will circulate gas back to the tank.
You probably know this, the bulb sucks gas from the carb and sends it back to the tank on the return line.
If any of the lines in this picture are reversed you will be SOL.

 

[hotshot]

Will do later this week, my final exams for university are in a few days so I'm taking a hiatus from chainsaws

Still in the midst of final exams?