Dumb question on carb priming.

[MacAttack]

I've made several posts on here related to carb tuning etc, creating the illusion I may know a few things about small engines, and even helping some people along the way! (By sheer luck).
Lately I was pondering something... in the absence of a primer bulb, with a dry carburetor, does the vacuum created by the choke also help to prime the fuel circuit? Or does it rely entirely on the fuel pump diaphragm to prime up?
This is in relation to a hard-starting saw with no primer that takes pulling the recoil until you're blue in the face when the carb is dry.
I can't seem to wrap my head around this, but I suspect it relies on either the fuel pump or a primer circuit.

 

[Vintage Engine Repairs]

The vacuum draws the fuel out of the metering chamber to then be refilled by the fuel pump.

Until the vacuum in the venturi has pulled the fuel out of the metering chamber and the metering diaphragm down ( so long as the pop off pressure is above the fuel pump pressure), you’re not going to get any more fuel.

The fuel pump and the vacuum on the metering chamber work in unison.

Edit: in relation to the last part of your question, check metering diaphragm, pump diaphragm, blocked fuel screen, blocked jets, adjustment of the fuel screws, metering lever height, fuel lines, fresh fuel, clean filter and good impulse. One of those isn’t correct and is why you’re struggling

 

[MacAttack]

Agree with everything you said, and I know I need to rebuild the carb, it's an old Tillotson that I threw a metering diaphragm in about 25 years ago, I have no doubt it needs rebuilt.
But my pivotal question is this....when the fuel pump and metering chamber are dry, does the vacuum of the choke itself assist with priming the fuel pump in any way? Or no?
I'm talking drawing fuel from the tank into a dry carb.
Thoughts?

 

[Vintage Engine Repairs]

For a chainsaw I’d suggest it would assist but not be sufficient. Simply because the vacuum has to act against a long distance to the fuel tank, through all the drillings, fuel line and filter.

If the vacuum was directly over an emulsion tube that goes right in to the fuel supply just like on a 4 stroke that has a tank overhead supplying the bowl with fuel then yes.

 

[sean donato]

Yes the choke forced it to pull fuel through the carb. Every time you pull it over your still getting a minimal pulse to the carb so the fuel and metering diaphragms still get pulsed. I'd actually look closer at your choke Plate and make sure it's closing the whole way. Was a common issue in the older stohls with the plastic choke shaft and finger that engaged the built in the air filter choke to not get the choke to seal completely and start really hard. My old 026 is like this. Simple solution is to pop the air filter off and give it a squirter of mix down the carb or put tour thumb over the carb and give it a few slow pulls to prime it.

 

[sean donato]

Few screen shots from walbro manuals. Same principals applies to all diaphragm carbs.

 

[huskihl]

With the choke closed, engine vacuum pulls through the main nozzle, pulling the metering diaphragm down. From there it’s a wide open circuit to the fuel tank. The engine vacuum is much stronger than the fuel pump, so yes it makes a huge difference as long as your choke butterfly is closing and your metering diaphragm is pliable

 

[Old2stroke]

Getting fuel into the carb with a dry line and no primer is all about how effective the choke is and has nothing to do with the fuel pump as it has very little effect from just a few pulses at cranking speed. With the choke closed and the throttle partially open (very necessary), intake vacuum is presented to the main jet and the idle ports and hence into the fuel chamber where it pulls down the diaphragm, opening the inlet valve so that it can suck fuel up the line, through the fuel pump and into the fuel chamber of the carb. The inlet valve is NOT opened by the fuel pump exceeding the "pop-off" pressure of the inlet valve. The pop-off pressure is always higher than any pressure the pump produces and is even supposed to be higher than any pressure from the tank (which can be much higher than pump pressure) just to prevent flooding conditions.
Too much cranking to get the carb primed can be caused by a diaphragm that is too stiff to be pulled down, a control lever that is set too low or a weak vacuum due to engine health or air leaks anywhere that reduce the vacuum strength.

 

[sean donato]

It doesn't run very long with the impulse unhooked.

 

[lone wolf]

It doesn't run very long with the impulse unhooked.

True story it runs a couple seconds at best.

 

[ZeroJunk]

Just something to throw out there. Some of the aftermarket filters that have the choke in the air filter do not seal properly. You can pull to you turn blue. Sometimes on vintage saws the linkage can get bent and even a tiny crack will defeat you.

 

[lone wolf]

Just something to throw out there. Some of the aftermarket filters that have the choke in the air filter do not seal properly. You can pull to you turn blue. Sometimes on vintage saws the linkage can get bent and even a tiny crack will defeat you.

And are hell on your arm!

 

[MacAttack]

Great input guys, thank you! I'm dealing with a 60 year old saw with a Tillotson HL carb. Im going to install a full rebuild kit and check the choke butterfly.
My question was mostly an interest in the theory of operation on these small pumper diaphragm carbs. You can always learn something new!

 

[Old2stroke]

True story it runs a couple seconds at best.

With the impulse line plugged (not just disconnected) the engine will start normally, the function of the choke will fill the fuel chamber of the carb and the engine will run until it uses up that fuel. With small saws they will run at idle for a few minutes (2-5) but die as soon as you touch the throttle and get the main jet involved. The fuel pump has to supply fuel to the carb's fuel chamber at the same rate that it is being used once the engine is running. Max fuel delivery is required when the saw in running at full throttle under heavy load, if your saw starts to run lean under this condition, the type and condition of the fuel pump diaphragm could be the problem.

 

[Fordiesel69]

Let me use a Husqvarna 225RJ trimmer as an example. No primer / purge bulb. Just one line down into the tank.

As you pull your shoulders out starting it, the impulse from the crankcase goes thru a small hole in the carburetor and operated the pumping diaphram (two flaps basically). This will draw fuel up the fuel line and depending on how good and flexible these flaps are in relation with their respective sealing surface of the carburetor body, will hold fuel for many days. Usually not weeks or months however. Each day it will just take 2-4 pulls. If you let it sit for months, it very well can take 12-20 pulls to get it to pop off.

Not only is a good metering diaphram needed (the round circle one) but also the opposite side pumping diaphram needs to be nice and flexible. Often people will refurbish the carburetor with new metering and simply ignore the pumping side.

 

[MacAttack]

. The fuel pump has to supply fuel to the carb's fuel chamber at the same rate that it is being used once the engine is running. Max fuel delivery is required when the saw in running at full throttle under heavy load, if your saw starts to run lean under this condition, the type and condition of the fuel pump diaphragm could be the problem.

This is exactly how the saw behaves, it wants to run lean on the top end but inconsistent, like it's not getting enough fuel. I will be ordering a rebuild kit very soon.
It's just curious to me that the saw also takes so much pulling to prime the carb, it's a very simple carb and choke that seems to be closing the butterfly, which is what prompted me to post this thread. We'll see what I find when I pull the carb.

 

[JimR]

I've made several posts on here related to carb tuning etc, creating the illusion I may know a few things about small engines, and even helping some people along the way! (By sheer luck).
Lately I was pondering something... in the absence of a primer bulb, with a dry carburetor, does the vacuum created by the choke also help to prime the fuel circuit? Or does it rely entirely on the fuel pump diaphragm to prime up?
This is in relation to a hard-starting saw with no primer that takes pulling the recoil until you're blue in the face when the carb is dry.
I can't seem to wrap my head around this, but I suspect it relies on either the fuel pump or a primer circuit.

In a case like that I prime the motor by giving it a squirt down the carb. The motor will start right up and stay running because the carb filled up during the initial start.

 

[JimR]

This is exactly how the saw behaves, it wants to run lean on the top end but inconsistent, like it's not getting enough fuel. I will be ordering a rebuild kit very soon.
It's just curious to me that the saw also takes so much pulling to prime the carb, it's a very simple carb and choke that seems to be closing the butterfly, which is what prompted me to post this thread. We'll see what I find when I pull the carb.

Did you check your inline filter in the fuel tank? When you pull the carb apart check to see if the screen in the carb is partially plugged up.

 

[MacAttack]

Did you check your inline filter in the fuel tank? When you pull the carb apart check to see if the screen in the carb is partially plugged up.

Inline filter is new, but the old carb does have a large metal mesh screen inside, which surely will be cleaned.
I think the diaphragms are probably just all stiffened up.

 

[JimR]

How many of you remember this 2 cycle oil? The bottle says it is good for all 2 cycle engines. I opened it up yesterday. The oil is a pretty blue just like the XP oil.