Primer Bulb Operation

[DanAyo]

Recent chainsaw troubleshooting lead me to the primer bulb operation. An Echo CS-303T is the saw in question.

1. Is the fuel tank pressurized during operation?
2. How does the Primer Bulb work? There are 2 lines that run from the primer bulb. One (suction side) goes to carb and the other primer line goes to 1 of 2 barbs going into the fuel tank. The other barb located just to the side is attached to what I think is the pulse line. There are no tubes in the tank that are attached to the primer barb and the pulse line (?) on the top, only 2 small holes that correspond to the primer tubes above.

The only way I see gas being used to initially prime the carb is through the fuel pickup line.
1. Does the primer bulb use the pump side of the primer lines to pressurize the tank for pushing fuel up through the pickup line?
2. The primer bulb only works when when the suction line is attached to the carb.
Thank in Advance for any help,
Danny

It is becoming very obvious that 4 stroke and 2 strokes are very different types of engines! The more I learn about 2 strokes, the more I am amazed by their simplicity but also the complexity of their design.

 

[ammoaddict]

These are not my words, copied from another forum, but a good explanation.

Quite a few smaller saws and trimmers have a primer bulb that lets you get the thing running with fewer pulls after it has been unused for some time, however the bulbs seem to have a short lifetime (ethanol again?) and I have seen problems with their replacement. There is a misconception of how these things work. On modern engines they do NOT inject fuel into the engine and they cannot flood the engine if they are pressed too many times. The primer base has two check valves in it that keep fuel moving in one direction only, and the device is meant to suck fuel up the fuel line from the tank, fill the metering chamber in the carb and then dump any excess fuel back into the tank. In operation, the bulb should be pressed until you can feel fuel being squished through it, then you know the fuel line and carb are full and the engine should start on full choke with a couple of pulls. When replacing these things, it is important that the suction port of the primer be connected to the carb and the other one goes back to the tank. You can easily find the suction port by hooking a short fuel line from a container of fuel to each port in turn and squeezing the bulb to see which one sucks. If it is hooked up backwards, it could either suck fuel out of the tank and flood the engine by forcing fuel through the carb, or depending where the tank fuel level is with respect to the primer return line, could seriously lean out the air/fuel mix.
A side advantage of a primer is it makes it easy to drain all the fuel out of a saw. Just drain the tank, pump the primer until it is just sucking air, then drain the tank again. The carb and fuel line is now empty but if you are really fussy, it might start up on choke and run for a short while to get rid of any residual fuel or fume

 

[DanAyo]

This link seems to confirm my thoughts.


The Repair Specialist (YouTube Presenter) has been a great help in my 2 cycle learning journey.

Things are always what you think them to be… especially with 2 strokes!

 

[DanAyo]

A side advantage of a primer is it makes it easy to drain all the fuel out of a saw. Just drain the tank, pump the primer until it is just sucking air, then drain the tank again. The carb and fuel line is now empty but if you are really fussy, it might start up on choke and run for a short while to get rid of any residual fuel or fume

Thanks for a great tip! Empty the tank, the pump the primer to evacuate all gas from the carb. “Genius”.

The video above is a great learning tool. The illustrations help. The only nuance to the video is having to visual the primer bulb and lines being located near the carb and not attached to it. But no big deal.

 

[ammoaddict]

You're welcome, but like I said, not my words. I copied and pasted from the forestry forum from a guy called old2stroke.

 

[thenne1713]

Recent chainsaw troubleshooting lead me to the primer bulb operation. An Echo CS-303T is the saw in question.

1. Is the fuel tank pressurized during operation?
2. How does the Primer Bulb work? There are 2 lines that run from the primer bulb. One (suction side) goes to carb and the other primer line goes to 1 of 2 barbs going into the fuel tank. The other barb located just to the side is attached to what I think is the pulse line. There are no tubes in the tank that are attached to the primer barb and the pulse line (?) on the top, only 2 small holes that correspond to the primer tubes above.

The only way I see gas being used to initially prime the carb is through the fuel pickup line.
1. Does the primer bulb use the pump side of the primer lines to pressurize the tank for pushing fuel up through the pickup line?
2. The primer bulb only works when when the suction line is attached to the carb.
Thank in Advance for any help,
Danny

It is becoming very obvious that 4 stroke and 2 strokes are very different types of engines! The more I learn about 2 strokes, the more I am amazed by their simplicity but also the complexity of their design.

Pretty sure the SUCTION should goto the TANK, and pressure to the CARB, UNLESS the TANK line is the RETURN? (short line inside tank w/ NO filter? I fought one (start, run 15-sec, die) issue on one saw, finally trimmed 1/8" off the 1-inch long line between primer and carb, and CURED Issue. SLIGHT bend/crimp would allow Prime, but restricted Free-flow. MUST be able to free-flow. (Reread your post Since fuel normally pulled to carb by piston suction, Primer just assists getting fuel to carb/cylinder w/ FEWER Cranks... vacuum sucks fuel into carb, and spits it back to Tank RETURN. p.s. Not uncommon for old fuel line inside tank to crack/ break/ fall off, so make sure you have good, uncracked line inside tank w/ filter attached, and attached to fuel supply line?

 

[grizz55chev]

Pretty sure the SUCTION should goto the TANK, and pressure to the CARB, UNLESS the TANK line is the RETURN? (short line inside tank w/ NO filter? I fought one (start, run 15-sec, die) issue on one saw, finally trimmed 1/8" off the 1-inch long line between primer and carb, and CURED Issue. SLIGHT bend/crimp would allow Prime, but restricted Free-flow. MUST be able to free-flow. Luck to ya

As stated, the primer is for drawing fuel through the carb then returned to the tank, it does not pressurize anything.

 

[huskihl]

From the fuel filter to the fuel line to the carburetor to the purge bulb and back to the tank.

The tank usually gets pressurized some, but it’s from fuel warming, evaporating, and expanding in the tank.

Older fuel tank vents allowed air to pass either way. On newer models, it’s only supposed to be able to enter the tank. Therefore, the warming fuel inside the tank will build pressure on newer models

 

[DanAyo]

Pretty sure the SUCTION should goto the TANK, and pressure to the CARB, UNLESS the TANK line is the RETURN? (short line inside tank w/ NO filter? I fought one (start, run 15-sec, die) issue on one saw, finally trimmed 1/8" off the 1-inch long line between primer and carb, and CURED Issue. SLIGHT bend/crimp would allow Prime, but restricted Free-flow. MUST be able to free-flow. (Reread your post Since fuel normally pulled to carb by piston suction, Primer just assists getting fuel to carb/cylinder w/ FEWER Cranks... vacuum sucks fuel into carb, and spits it back to Tank RETURN. p.s. Not uncommon for old fuel line inside tank to crack/ break/ fall off, so make sure you have good, uncracked line inside tank w/ filter attached, and attached to fuel supply line?

I was operating under the assumption that the primer line going to the carb was the pressure line, pushing fuel. But in fact that line is the suction. It sucks all the way through the carb, into the carb intake, and finally into the fuel tank.

Now, the next thing blowing my mind is, it appears the tank is a sealed system and the pulse line goes into the tank and not the carb. Can this be correct? If this is, there must be some type of check valve, otherwise the tank will be pressurized one stroke and vacuumed the next.

 

[DanAyo]

Another incorrect assumption! In a previous reply to this thread I assumed that one of the barbed fittings going into the fuel tank was for the pulse line. From what I’ve recently learned (I think), it is not the pulse line but the external fuel tank vent. It appears the pulse line is not a line at all but a passageway through the carburetor flange that mounts to the engine.

If any statements I’ve made are not correct, please let me know. I want to learn…

Danny

 

[pioneerguy600]

Impulse signal comes from the crankcase and has no connection to the fuel tank. How the impulse gets transfered to the carb differs from saw to saw, older saws used passages through the carb to cylinder flanges but more modern saws changed to the carb being connected to the cylinder by a rubber manifold, some manifolds still used a passageway in them for impulse but most use a separate hose or rubber line to connect the carb to the impulse nipple on the crankcase.The impulse operates the fuel pump flaps inside the carb.

 

[DanAyo]

Impulse signal comes from the crankcase and has no connection to the fuel tank. How the impulse gets transfered to the carb differs from saw to saw, older saws used passages through the carb to cylinder flanges but more modern saws changed to the carb being connected to the cylinder by a rubber manifold, some manifolds still used a passageway in them for impulse but most use a separate hose or rubber line to connect the carb to the impulse nipple on the crankcase.The impulse operates the fuel pump flaps inside the carb.

Thanks Pioneer. My 028 Stihl has a pulse line going from the crankcase to the carb. I think this newer Echo uses a through passage from the crankcase that enters the carb through a hole in the carburetor flange.

I think my best bet to learn more is to get my hands on a shop manual. I’ll have to go beg for one -)

 

[Old2stroke]

I was operating under the assumption that the primer line going to the carb was the pressure line, pushing fuel. But in fact that line is the suction. It sucks all the way through the carb, into the carb intake, and finally into the fuel tank.

Now, the next thing blowing my mind is, it appears the tank is a sealed system and the pulse line goes into the tank and not the carb. Can this be correct? If this is, there must be some type of check valve, otherwise the tank will be pressurized one stroke and vacuumed the next.

A key point when sorting out primer bulb issues is you cannot force fuel into the carb by pressurizing the fuel tank, you would have to exceed the "pop-off" pressure of the inlet valve and that should be at least 10psi which is much higher then any pressures to be achieved in the tank. Many saws that have been "worked on" have had the bulb lines reversed so that the bulb is now a "primer" rather than a "purge" and is injecting fuel directly into the fuel chamber, bypassing the inlet valve. A sure way to flood the engine, and even if the engine starts, the check valves in the bulb are pointing in the wrong direction and can act as a variable uncontrolled fuel source causing flooding or, if the exit point of the bulb line in the tank is above the fuel level, an uncontrolled source of air to lean out the fuel in the carbs fuel chamber.

 

[DanAyo]

A key point when sorting out primer bulb issues is you cannot force fuel into the carb by pressurizing the fuel tank, you would have to exceed the "pop-off" pressure of the inlet valve and that should be at least 10psi which is much higher then any pressures to be achieved in the tank. Many saws that have been "worked on" have had the bulb lines reversed so that the bulb is now a "primer" rather than a "purge" and is injecting fuel directly into the fuel chamber, bypassing the inlet valve. A sure way to flood the engine, and even if the engine starts, the check valves in the bulb are pointing in the wrong direction and can act as a variable uncontrolled fuel source causing flooding or, if the exit point of the bulb line in the tank is above the fuel level, an uncontrolled source of air to lean out the fuel in the carbs fuel chamber.

My learning evolution is a little slow, but steady. Is it correct that the primer pump sucks through the line that goes to the carb, and pushes (via one way valve in the primer) the air evacuated from the carb into the tank via the second line?

The carb is filled with gas as the primer bulb sucks through passages in the sealed carb and finally draws gas into the carb through the fuel inlet barb. Correct?

“or, if the exit point of the bulb line in the tank is above the fuel level, an uncontrolled source of air to lean out the fuel in the carbs fuel chamber.”
The pressure line on the CS-303T terminates on a barb above the fuel tank. Upon inspection there is a small hole between the barb and top roof of the fuel tank. There is no line extending down into the fuel. The pressurized air (initially) and ultimately the fuel exist the hole and is sprayed into the tank.

 

[pioneerguy600]

My learning evolution is a little slow, but steady. Is it correct that the primer pump sucks through the line that goes to the carb, and pushes (via one way valve in the primer) the air evacuated from the carb into the tank via the second line?

The carb is filled with gas as the primer bulb sucks through passages in the sealed carb and finally draws gas into the carb through the fuel inlet barb. Correct?

“or, if the exit point of the bulb line in the tank is above the fuel level, an uncontrolled source of air to lean out the fuel in the carbs fuel chamber.”
The pressure line on the CS-303T terminates on a barb above the fuel tank. Upon inspection there is a small hole between the barb and top roof of the fuel tank. There is no line extending down into the fuel. The pressurized air (initially) and ultimately the fuel exist the hole and is sprayed into the tank.

Yes.

 

[ammoaddict]

A key point when sorting out primer bulb issues is you cannot force fuel into the carb by pressurizing the fuel tank, you would have to exceed the "pop-off" pressure of the inlet valve and that should be at least 10psi which is much higher then any pressures to be achieved in the tank. Many saws that have been "worked on" have had the bulb lines reversed so that the bulb is now a "primer" rather than a "purge" and is injecting fuel directly into the fuel chamber, bypassing the inlet valve. A sure way to flood the engine, and even if the engine starts, the check valves in the bulb are pointing in the wrong direction and can act as a variable uncontrolled fuel source causing flooding or, if the exit point of the bulb line in the tank is above the fuel level, an uncontrolled source of air to lean out the fuel in the carbs fuel chamber.

I hope you don't mind me reposting your info from forestry forum. I thought it was good information that the op might find helpful.

Sent from my moto g(7) power using Tapatalk

 

[huskihl]

My learning evolution is a little slow, but steady. Is it correct that the primer pump sucks through the line that goes to the carb, and pushes (via one way valve in the primer) the air evacuated from the carb into the tank via the second line?

The carb is filled with gas as the primer bulb sucks through passages in the sealed carb and finally draws gas into the carb through the fuel inlet barb. Correct?

“or, if the exit point of the bulb line in the tank is above the fuel level, an uncontrolled source of air to lean out the fuel in the carbs fuel chamber.”
The pressure line on the CS-303T terminates on a barb above the fuel tank. Upon inspection there is a small hole between the barb and top roof of the fuel tank. There is no line extending down into the fuel. The pressurized air (initially) and ultimately the fuel exist the hole and is sprayed into the tank.

Post #8

 

[501Maico]

I was operating under the assumption that the primer line going to the carb was the pressure line, pushing fuel. But in fact that line is the suction. It sucks all the way through the carb, into the carb intake, and finally into the fuel tank.

Now, the next thing blowing my mind is, it appears the tank is a sealed system and the pulse line goes into the tank and not the carb. Can this be correct? If this is, there must be some type of check valve, otherwise the tank will be pressurized one stroke and vacuumed the next.

I worked on a small Homelite a while back and noticed an impulse line going into the oil tank. Curious, I read up about it and it's not a very reliable system. The oil tank gets pressurized by the impulse line through a rubber duckbill style check valve. Oil is forced out of the tank onto the chain. The fuel tank also has one but that was for the primer bulb IICR. Over time the duckbills stop closing all the way and cause problems on both systems.

 

[redunshee]

I hope the following helps: The fuel line from the tank goes to the fuel inlet nipple on the carb. The suction side of the bulb(short nipple) connects to the other nipple on the carb. Consequenlty you are sucking fuel from the cylinder, into the carb and then into the bulb. The excess fuel returns to the tank via the other longer nipple on the bulb. That line returns directly to the tank.

 

[Old2stroke]

. Consequenlty you are sucking fuel from the cylinder, into the carb and then into the bulb.

I'm hoping you meant to say "fuel tank" and not "cylinder" ?