Please explain primer bulb function

[preventec47]

I have worked on many auto, boat, lawn equipment and generator motors etc but I am not
familiar with exactly how or what the primer bulb does. I know what priming means.... in the old
days just squirt fuel into the system to get it ready to run when usually cold. I have one chain saw
that says five squeezes of the bulb for cold start and another that says ten squeezes of the bulb for cold start. From the side of the bulb that would be squirting a massive amount of gas into
the intake guaranteeing flooding so i think something else must be going on. Where does the bulb squirt the fuel ? I dont really know how a chain saw carb works because I dont think
it has a carb bowl and carb float on account if the saw working in all different kinds of positions.

 

[Jedthro]

The vacuum created when you release the bulb draws fuel from the tank and through the carb pump chamber. Pressing the bulb forces the fuel back to the tank. This removes any air in the fuel system.

 

[frank_]

if it has a choke lever, then its just a purge bulb

 

[ChillyB]

Slurps fuel through the carb and back into tank. Impossible to flood the carb with a purge valve. The fuel is sucked through the "bowl" side. Nothing motivates the fuel to enter the idle or high speed jets.

 

[ammoaddict]

The vacuum created when you release the bulb draws fuel from the tank and through the carb pump chamber. Pressing the bulb forces the fuel back to the tank. This removes any air in the fuel system.

Good explanation here. There is no danger of flooding, you can press it 100 times if you want, actually an air purge, not really a primer.

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[SawTrash]

I have worked on many auto, boat, lawn equipment and generator motors etc but I am not
familiar with exactly how or what the primer bulb does.

My first thought when seeing the heading for this thread was "How could anybody not know what a damm primer bulb does ?"

Thanks to the answers given I now realize I didn't really understand what the primer bulb does even after pulling apart and rebuilding chainsaw carbs successfully many times.
Yes, I've changed diaphragms, adjusted metering levers, flushed solvents into every hole but never figured out what goes with all the passages hidden inside the carb body.

Now I do get it, so thanks OP

 

[preventec47]

Slurps fuel through the carb and back into tank. Impossible to flood the carb with a purge valve. The fuel is sucked through the "bowl" side. Nothing motivates the fuel to enter the idle or high speed jets.

So I should be able to infer that no matter how many more or less times I press the
rubber bulb, I will not or can not flood the engine. Right ? ? ? Somehow I thought I
was warned in a manual somewhere to not press the bulb too many times or at all when the engine is hot. Are they telling me that it is simply not helpful but could not
be harmful ?

 

[ammoaddict]

So I should be able to infer that no matter how many more or less times I press the
rubber bulb, I will not or can not flood the engine. Right ? ? ? Somehow I thought I
was warned in a manual somewhere to not press the bulb too many times or at all when the engine is hot. Are they telling me that it is simply not helpful but could not
be harmful ?

Yep, All it does is pull some fuel through the carb and right back into the fuel tank. You can push it until your thumb hurts and it wont hurt the saw a bit.

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[huskihl]

So I should be able to infer that no matter how many more or less times I press the
rubber bulb, I will not or can not flood the engine. Right ? ? ? Somehow I thought I
was warned in a manual somewhere to not press the bulb too many times or at all when the engine is hot. Are they telling me that it is simply not helpful but could not
be harmful ?

One of the reasons they use them is to pull cool fuel from the tank through the carb to prevent hot starting issues

 

[Jedthro]

Yep, All it does is pull some fuel through the carb and right back into the fuel tank. You can push it until your thumb hurts and it wont hurt the saw a bit.

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Assuming the needle valve and diaphram is good.....

 

[ChillyB]

Shouldn’t have to pump it on a hot saw unless run dry. Lawyers probably insist the manual writers say not to press the bulb when hot because it creates slightly pressurized fuel on the tank return line. Lawyers would have you make a sammich, sip some tea, wash the dog, then refuel and purge your cold dead saw.

 

[Vintage Engine Repairs]

Worth noting, if you’re getting fuel in the venturi when pressing the bulb or you feel resistance (both occur simultaneously generally) your duckbill valve isn’t sealing the incoming charge passage and forcing fuel back through the metering chamber and into the drillings, finally escaping out to the venturi. Then you can then flood it.

 

[dboyd351]

Assuming the needle valve and diaphram is good.....

And assuming someone didn't re-route the fuel lines so the "primer" bulb is pushing gas into the carb. Many, many people think this is how the fuel lines need to be routed when replacing the "primer" bulb, so they route the lines that way and then can't get the saw to run.

 

[KASH]

So what type works better and why?
Old style primer gives cold motor direct shot of fuel
New style removes fuel from carb and dumps it back in the fuel tank motor gets no fuel?
Kash

 

[huskihl]

So what type works better and why?
Old style primer gives cold motor direct shot of fuel
New style removes fuel from carb and dumps it back in the fuel tank motor gets no fuel?
Kash

Old…… As in how old? Everything semi-current would be the purge style. They don’t pull all the fuel out of the carburetor. They are hooked to the opposite side of the carburetor from the main fuel line and they pull fuel through the filter and fuel line up into to the carburetor to get rid of any air in the system so you don’t have to pull 10 times to get them started

 

[dboyd351]

From my understanding they work under different principles. The true primer bulb does give the motor a direct shot of fuel to help cold starting. The "new style" is really more correctly called a purge bulb because it's purpose is to purge any air from the lines, so the carb's metering diaphragm has plenty of fuel to pump and is not trying to pump compressible air to get the fuel flowing.

Shooting fuel into the motor may seem better, but if the carb can't pump fuel it will not stay running anyway, so I guess the idea is to get the whole fuel system pumping instead of just putting a shot of fuel in the cylinder.

This is my understanding of it. If any of it is incorrect I welcome corrections

 

[huskihl]

From my understanding they work under different principles. The true primer bulb does give the motor a direct shot of fuel to help cold starting. The "new style" is really more correctly called a purge bulb because it's purpose is to purge any air from the lines, so the carb's metering diaphragm has plenty of fuel to pump and is not trying to pump compressible air to get the fuel flowing.

Shooting fuel into the motor may seem better, but if the carb can't pump fuel it will not stay running anyway, so I guess the idea is to get the whole fuel system pumping instead of just putting a shot of fuel in the cylinder.

This is my understanding of it. If any of it is incorrect I welcome corrections

To me the purge system along with a choke is way better. With a primer bubble, you have no idea if you need to give it three pumps or eight. And you’re always wondering if you flooded it. Primers are the devil

 

[KASH]

Old as in most seventies eighties and nineties snow mobiles garden equip .I have installed many of these on garden tractors two cycle mowers generators etc etc even put one on a Pioneer 1450 with inject air.
My point is old style motor gets direct shot of fuel
new style motor has to pull fuel from the carb
Both systems work I just like my style better .
Each to their own.
Kash

 

[KASH]

Primer and choke is FIRST CLASS the only way to fly.
Kash

 

[Vintage Engine Repairs]

Lots of types and all have different advantages. Primer that will inject small amounts of fuel directly, primers that will push air into the carby to force fuel through the main jet and into the venturi, purge bulbs that will suck fuel through the carb and even a tickler that will push the float down to flood the carb bowl and thus go into the Venturi ect .

I don’t like any, if I was to be forced to have one, I’d go for a purge to remove any air in the whole carb.

I’ll take a choke and no bulb any day.