help understanding 2-stroke fuel pumping

[r7000]

kind of a rudimentary question(s), I think I already know the answer some, but want clarification to be sure and a good understanding of the whole process ...

small engine 2-stroke, uses the downward stroke of the piston as a pressurization signal...

- which always goes to carb? this pressurization signal works the fuel pump (thing?) in the carb, how does the carb pump fuel?
- on small engine 2 stroke carbs there's [always?] 2 hose nipples? one is the intake from the gas tank that in fuel into the carburetor that is then metered by the carb and makes its way into the intake of the engine. The other nipple on the carb is what?
- Gas is sucked or pushed into the carb?
- If pushed then the gas tank must be sealed so it can be under pressure then correct, if so how much pressure and how is that pressure regulated?
- Does the gas tank need a vacuum break?
- Is the vent on the gas tank a vent to allow excess pressure out, if so at what value psi, or a vacuum break to allow air in as fuel as used?
- How does the primer bulb function, in what direction regarding pressure?
- wow can some 2-stroke engine designs that don't have a primer bulb work/start ? an old stil av011 chainsaw for example
- my understanding is the 2 "main" makers of carbs are ZAMA and WALBRO? all the aftermarket ones on amazon/ebay are just chinese knockoffs but operate on exactly all the same principles?
- anything on elaborate intake mechanisms on certain machines that supposedly increase performance, (something on my Poulan PL4218 and extra boot?) on how they work

examples - my 2-stroke poulan PL25 trimmer, stihl KM90r trimmer, stihl BR600 backpack blower, chainsaws, etc.

any information on anything related to what I mentioned, interested in getting a complete understanding.

 

[huskihl]

Impulse signal operates the flaps in the pump. The down stroke pushes one open while forcing the other closed. The up stroke vacuum pushes the other one open while holding down the opposite one. The dual action pulls fuel from the tank

 

[atpchas]

This fellow has several excellent videos explaining 2-cycle carb operation. Here's one:

 

[Bonez]

The fuel tanks are usually vented by the fuel caps on some equipment like trimmers BUT some equipment produce pressure as it runs in the fuel tank like chainsaws do to help with fuel delivery so they dont have vented caps , just depends on the equipment . On the intake/compression stroke of the piston it creates a vacuum that draws the fuel into the cylinder that same vacuum sucks up the diaphragm that opens the needle valve to allow fuel into the carb ready for the next cycle. the return fuel line is for priming and also to prevent vapor lock since these motors are mostly air cooled and run hot. I have a chainsaw that doesnt have a primer bulb carb, the diaphragm works double duty as a fuel pump on those models ( and I prefer those carbs over the primer bulb style carbs )

 

[huskihl]

The fuel tanks are usually vented by the fuel caps on some equipment like trimmers BUT some equipment produce pressure as it runs in the fuel tank like chainsaws do to help with fuel delivery so they dont have vented caps , just depends on the equipment . On the intake/compression stroke of the piston it creates a vacuum that draws the fuel into the cylinder that same vacuum sucks up the diaphragm that opens the needle valve to allow fuel into the carb ready for the next cycle. the return fuel line is for priming and also to prevent vapor lock since these motors are mostly air cooled and run hot. I have a chainsaw that doesnt have a primer bulb carb, the diaphragm works double duty as a fuel pump on those models ( and I prefer those carbs over the primer bulb style carbs )

Little bit of clarification here. The impulse signal created when the piston goes up and down moves the flaps on the fuel pump diaphragm and creates a pumping action which sucks fuel from the tank to the carb. The throttle being held open creates vacuum on the main nozzle drawing fuel from under the metering diaphragm. When the diaphragm gets pulled down, the needle gets lifted so more fuel can take its place. Impulse has no effect on the metering diaphragm and needle. Impulse also has no effect on pressure in the tank. Pressure inside the fuel tank is caused from fuel evaporating from being shaken, vibrated from the engine, or from being heated. Pressure inside the fuel tank is a byproduct, not a requirement. Mandated by the epa, tank vents are only allowed to vent in, not out. That is why pressure builds inside the tank when fuel tries to evaporate

 

[Bonez]

Little bit of clarification here. The impulse signal created when the piston goes up and down moves the flaps on the fuel pump diaphragm and creates a pumping action which sucks fuel from the tank to the carb. The throttle being held open creates vacuum on the main nozzle drawing fuel from under the metering diaphragm. When the diaphragm gets pulled down, the needle gets lifted so more fuel can take its place. Impulse has no effect on the metering diaphragm and needle. Impulse also has no effect on pressure in the tank. Pressure inside the fuel tank is caused from fuel evaporating from being shaken, vibrated from the engine, or from being heated. Pressure inside the fuel tank is a byproduct, not a requirement. Mandated by the epa, tank vents are only allowed to vent in, not out. That is why pressure builds inside the tank when fuel tries to evaporate

so, you dont think the pressure in the tank, since it cant vent out (only in) has nothing to do with fuel delivery to the carb? if that was the case you wouldnt need a needle valve to shut fuel supply on a vacuum/suction type carb. It plays a role atomizing fuel. There is also a reason why the primer bulb is supposed to be pressed 3, 5, 10 times on various equipment... it pressurizes the tank to an extent not just get fuel to the carb/purge lines. I can explain the bulb pressure because I had equipment that never started, was on a solid surface so not shaken, pulled fuel line off and gas squirted out quite a bit and wasnt from evaporation... was cold and within a very short time, mainly just the bulb was pressed no start and checked.

 

[hacskaroly]

How does the primer bulb function, in what direction regarding pressure?

Here is a good video explaining the function of the primer bulb:

 

[huskihl]

so, you dont think the pressure in the tank, since it cant vent out (only in) has nothing to do with fuel delivery to the carb? if that was the case you wouldnt need a needle valve to shut fuel supply on a vacuum/suction type carb. It plays a role atomizing fuel. There is also a reason why the primer bulb is supposed to be pressed 3, 5, 10 times on various equipment... it pressurizes the tank to an extent not just get fuel to the carb/purge lines.

No, not at all, to any of those. Like I mentioned above, tank pressure isn’t required. The fuel pump sucks from the tank and makes fuel available under the needle at low pressure. It works with or without the fuel cap in place.

All a purge bubble does is pull fuel from the carb and dump it back into the tank to purge air from the fuel lines. It’s a 1 for 1 trade with air or fuel. What comes out goes back in. There’s no pressurizing of the tank

 

[Duce]

so, you dont think the pressure in the tank, since it cant vent out (only in) has nothing to do with fuel delivery to the carb? if that was the case you wouldnt need a needle valve to shut fuel supply on a vacuum/suction type carb. It plays a role atomizing fuel. There is also a reason why the primer bulb is supposed to be pressed 3, 5, 10 times on various equipment... it pressurizes the tank to an extent not just get fuel to the carb/purge lines. I can explain the bulb pressure because I had equipment that never started, was on a solid surface so not shaken, pulled fuel line off and gas squirted out quite a bit and wasnt from evaporation... was cold and within a very short time, mainly just the bulb was pressed no start and checked.

Easy to prove this incorrect. Remove fuel cap and engine will run just fine. If vent is plugged, that is method to use diagnosing a bad vent. It will run just fine, or bag it.

 

[EFSM]

Easy to prove this incorrect. Remove fuel cap and engine will run just fire. If vent is plugged, that is method to use diagnosing a bag vent.

You are correct. I think you wanted to say "run just fine" though.

 

[EFSM]

Like I mentioned above, tank pressure isn’t required.

No, it's not. However, it can mask a poorly pumping carburetor if the tank is well sealed otherwise. I've already had a MTD mini-tiller I was working on that would start and run for a while but then start starving for fuel. Turns out that the impulse passage in the intake block had never been fully drilled. Tank pressure was forcing fuel into the carburetor for a few minutes.
BTW, you're spot on @huskihl. Not trying to say you're wrong.

 

[EFSM]

so, you dont think the pressure in the tank, since it cant vent out (only in) has nothing to do with fuel delivery to the carb? if that was the case you wouldnt need a needle valve to shut fuel supply on a vacuum/suction type carb.

The fuel pump in the carburetor does not have a flow perfectly correspondent to fuel requirement. If it did, then theoretically you wouldn't need a needle. Anyone who has tested a round black vacuum pump on a lawn mower engine will realize that they actually pump more fuel while cranking, while the pulse is more distinct, than at operating speed. I'm sure that a 2-stroke diaphragm carburetor is the same way.

There is also a reason why the primer bulb is supposed to be pressed 3, 5, 10 times on various equipment.

Fuel line length and carburetor capacity vary greatly. A few styles of MTD equipment (trimmers) actually have a combination primer/purge bulb, where some of the fuel is returned to the tank and some is squirted into the intake. We vastly overuse the term "primer bulb" today--most are ''purge bulbs".

I can explain the bulb pressure because I had equipment that never started, was on a solid surface so not shaken, pulled fuel line off and gas squirted out quite a bit and wasnt from evaporation... was cold and within a very short time, mainly just the bulb was pressed no start and checked.

GAS IS VOLATILE!!! Who'd have guessed? It evaporates easily down to -40*.

 

[huskihl]

No, it's not. However, it can mask a poorly pumping carburetor if the tank is well sealed otherwise. I've already had a MTD mini-tiller I was working on that would start and run for a while but then start starving for fuel. Turns out that the impulse passage in the intake block had never been fully drilled. Tank pressure was forcing fuel into the carburetor for a few minutes.
BTW, you're spot on @huskihl. Not trying to say you're wrong.

I understand and believe you’re 100% correct on the poorly pumping carb theory

 

[Bonez]

Easy to prove this incorrect. Remove fuel cap and engine will run just fire. If vent is plugged, that is method to use diagnosing a bag vent.

easy to prove as well . Prime it , start it , kill it and then pull the fuel line off and it will squirt for a good 3+ seconds... pressure builds in the tank and its not from evaporation, not that much pressure that fast. I can shake the gas can and open and still not have pressure like that. But again, Im no pro, just what Ive experienced.

 

[qurotro]

easy to prove as well . Prime it , start it , kill it and then pull the fuel line off and it will squirt for a good 3+ seconds... pressure builds in the tank and its not from evaporation, not that much pressure that fast. I can shake the gas can and open and still not have pressure like that. But again, Im no pro, just what Ive experienced.

With the southern heat, starting the engine create enough movement to let the fuel build pressure. The bulb itself is connected in a close circuit, using clear fuel lines you can see no air is getting pumped in the fuel tank hence not pressuring the tank. And you can pump a million times without bursting the tank also mean no air is getting in to pressure the tank.
Oh btw, shaking the gas can under like 100 F you can even see the tank getting ballooned before you can put it down fast enough.

 

[huskihl]

easy to prove as well . Prime it , start it , kill it and then pull the fuel line off and it will squirt for a good 3+ seconds... pressure builds in the tank and its not from evaporation, not that much pressure that fast. I can shake the gas can and open and still not have pressure like that. But again, Im no pro, just what Ive experienced.

It’s from evaporation…..

 

[Bonez]

Nothing to do with heat, I did this in the middle of winter in an unheated garage. I know everyone is barking the heat, evaporation...ect... but it doesnt create pressure like that, that fast. Ill get a video this winter when Im doing more rebuilds and show what it does. hell, even my gas cans dont build pressure like that in the summer and they are sealed cans both metal and plastic. And what I mean by dont build pressure like that is, I can open it ( yes the initial phissssss, then close it, work on what I need ( possibly an hour later ) open the can and have no pressure. but the equipment I can fill, pull a few times and pull a fuel line and it will squirt quite a bit out, I have read somewhere as well that they create pressure in the tank which helps with the fuel delivery, not sure if its just certain equipment but I do know it was on 2cycle equipment with these tiny carbs. Im no expert but just stating what Ive researched in the past as well as experienced. But learning as I go too.

 

[cus_deluxe]

it puts the pressure in the tank, or else it gets the fuel hose again.

 

[cuinrearview]

I'm really glad that someone cued me into this thread. Fun!

 

[cuinrearview]

It’s from evaporation…..

Wait.... So if you shake gas, it releases gas? Madness!!