Primer/purge bulb install question

[rwh963]

i've had a McCullough MAC 3200 chainsaw sitting around for months that was a dump grab. in very good cosmetic condition, came with a case. had a nice 16" oregon bar and chain. it needed at least a set of fuel lines, and the bulb was toast. the bulb is not just the bulb, but a complete replaceable unit. didn't have one on hand at the time, so shelved it.

FINALLY, doing some spring cleaning and projects, and decided to tackle this saw. didn't even know if the saw would run after the work. disassembled most of it, ran new fuel lines, and an oil line. ordered a pack of bulbs off amazon. did a pretty full cleaning.

after all back together, took it outside, fueled it, and started pushing the primer bulb. kept pushing it. no fuel filled the bulb, as typical. definitely put the carb line on the short nipple, and tank return line on the long one. decided to switch the saw on, set choke, and it fired up after two pulls. opened the choke, and it ran like new. bulb still dry.

stopped it, started again, perfect. so what is going on here. why is the bulb dry, but the saw runs great?

 

[HarleyT]

The "purge" bulb just pulls fuel through the carb, and then back to the tank. It is designed to go one way. Visible fuel in the bulb is not neccesary. Just make sure the bulb is pulling from the carb and pushes back into the tank.

 

[rwh963]

But isn’t pushing back into the tank the reason we see fuel in the bulb?

 

[HarleyT]

I don't know the physics involved. I have had equipment that didn't fill the bulb, but the system worked fine. It is designed to "purge" the air out of the fuel system.
Just like the guys working on their lawn mower, and not seeing gas in their clear fuel filter, I just sell them a filter that you cannot see through, and go back to business.

 

[Vintage Engine Repairs]

If it’s not filling the bulb it’s pulling air from somewhere, though it doesn’t always fill the bulb 100% full when it’s working as it should, it should fill it at least most of the way (the very least 1/2 - 2/3 full). If it doesn’t, typically it’s either a non sealing main nozzle, a gasket leak (especially when reusing gaskets) or less common but still happens - accelerator pump leaks. I’ll always find the leak and fix it before re reassembly as sucking air is never good in a carb.

 

[Old2stroke]

Plumb the primer with a couple of lines into a can of fuel and verify that it works. There are a couple of check valves that might not be sealing.

 

[rwh963]

No suction occurring. All new lines look good. Short nipple line to carb. Nothing happening using bulb. Can try another bulb. SW was unused for an extended period of time, so probably issue inside carb. Zama carb.

 

[rwh963]

well, after some additional fiddling, the bulb is now working, and there is fuel flow. i pulled the bulb off the lines, and was pressing it to see it blew air. nothing, then it started to make a squeaky noise. i reconnected it, and fuel is flowing, saw is running. maybe the bulb needed to be "broken in" before attachment.

 

[Patrick1903]

Perhaps check valve was stuck on the bulb base. That was what @Old2stroke guessed before you got it working.

 

[grizz55chev]

well, after some additional fiddling, the bulb is now working, and there is fuel flow. i pulled the bulb off the lines, and was pressing it to see it blew air. nothing, then it started to make a squeaky noise. i reconnected it, and fuel is flowing, saw is running. maybe the bulb needed to be "broken in" before attachment.

Next time this little Mac quits, do yourself a favor and return it to the dump, you can thank me later.

 

[Old2stroke]

Those check valves like to be wet before they work properly