help understanding 2-stroke fuel pumping

[EFSM]

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

Apparently this is totally shocking to some people.

 

[bogieboy]

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.

Try pulling a fuel line off a cold 2 stroke... will still spurt 99% of the time... you can have a tank with the cap on tight, and the carb not connected, and it will slowly push fuel out of the fuel line without any outside pressure aside from the evaporative properties of fuel....

 

[huskihl]

Try pulling a fuel line off a cold 2 stroke... will still spurt 99% of the time... you can have a tank with the cap on tight, and the carb not connected, and it will slowly push fuel out of the fuel line without any outside pressure aside from the evaporative properties of fuel....

Squirts out really fast when you pump it up like a Coleman lantern

 

[cus_deluxe]

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

im no pro, but ill get a video this winter, based on my experience but im learning as i go, im no expert, but its all about that initial phissssss

 

[cus_deluxe]

Squirts out really fast when you pump it up like a Coleman lantern

thats what she said

 

[cuinrearview]

its all about that initial phissssss

thats what she said

Ahem...

 

[cus_deluxe]

Ahem...

 

[Woodpecker]

Squirts out really fast when you pump it up like a Coleman lantern

Title of your sex tape?

 

[huskihl]

Title of your sex tape?

They have names?

 

[Woodpecker]

They have names?

They do now

 

[cuinrearview]

They have names?

I accidentally named at least six today

 

[huskihl]

I accidentally named at least six today

I can remember that far back

 

[Bonez]

They have names?

 

[lilspenny]

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.

After many years of operating Stihl equipment I was looking in the carb of my br800 while I squeezed the trigger. Suddenly I felt like the dunce I was. I saw fuel squirting. After 3 decades and doing all of my own repairs. Eureka! So now I never use my chokes. I prime the carb well, squeeze the trigger 8 times, pull the rope twice, and presto! That's for cold starts. No more choke on, choke off, flooding or not flooding, pulling the plug to see bull. Works on fs131's, fs100's, br800's, and probably many other Stihls. Two pulls, every time, unless just one.

 

[edisto]

Title of your sex tape?

I think that I have a copy of that on VHS...

 

[EFSM]

After many years of operating Stihl equipment I was looking in the carb of my br800 while I squeezed the trigger. Suddenly I felt like the dunce I was. I saw fuel squirting. After 3 decades and doing all of my own repairs. Eureka! So now I never use my chokes. I prime the carb well, squeeze the trigger 8 times, pull the rope twice, and presto! That's for cold starts. No more choke on, choke off, flooding or not flooding, pulling the plug to see bull. Works on fs131's, fs100's, br800's, and probably many other Stihls. Two pulls, every time, unless just one.

That works for carburetors with the dreaded accelerator plunger. It is a little piston that is pushed by an eccentric on the throttle shaft; Stihl/Zama has had a lot of trouble over the years with the o-rings failing on them due to not using an ethanol-resistant rubber, although they have been using better rubber recently.

 

[edisto]

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.

There are some saws that move bar oil by pressurizing the tank. The impulse line passes though a duckbill valve that closes on the negative impulse.

An impulse line to pressurize the fuel tank would run to the fuel tank, not to the carb. In fact, as was mentioned earlier, a bad tank vent will stop a saw from running because of negative pressure in the fuel tank created by the pump drawing fuel from the tank.