Stihl primer bulb

[Mr Mower Man]

I just replaced a broken primer bulb assembly on a customer's TS-420 cutoff saw. It starts up and runs fine, but I noticed that the primer bulb doesn't get any fuel in it. It's one of those with the compression spring inside. And this happened with another piece of Stihl equipment a year or two ago, with the same style primer bulb. Is this some kind of dry prime system, or is there something wrong with it?

[KE4AVB]

From what I have seen they are the same as a standard remote primer with the exception of the spring.

If I am reading the ipl correctly it is the standard Zama priming setup for that C1Q-S167(a). This carburetor should have two fuel fittings. With one coming from the fuel tank directly and one returning to the primer. Usually in cases like either the primer is connected backwards, a leaky main nozzle or a line that is not tight fitting leaking air. I had several Zama Carbs on Stihls last few that would not prime because the main nozzle which is a one check valve not sealing completely allowing air to sucked back into the carburetor via the main nozzle. They would not even partially fill the primer. The primer main job is to just remove the air in the carburetor though access fuel does enter it and sometimes fills it on it way back to the tank.

BTW the service manual is of no help on the fuel line plumbing as only shows connecting to fuel tank grommet area.

[wristpin]

The term primer is generally used to cover a starting system involving a "primer bulb" regardless whether it's a true primer pumping fuel through the carburettor or an air purging system allowing atmospheric pressure to do the job. An appreciation of the system in use and the way that it operates facilitates problem diagnosis.

[Fulltilt]

According to my Stihl service manual, the short spigot on the primer ( stihl calls it the "manual fuel pump" ) is connected to the hose coming from the carb.( suction)
The long spigot connects to the hose going to the tank. ( return)
Which, FYI, is pretty much standard for most remote primer bulb hookups, regardless of brand, as they pretty much all have a short and long spigot.

An easy test is to depress the bulb, and put your finger over the short spigot, the bulb should stay reasonably collapsed if it's good.

[thomd51]

there is a check valve in the idle circuit also TOM.