Crank Case Pressure/Vacuum Theory

[XP_Slinger]

Here’s the diagram Dan posted in the 462 thread. With the needle held off its seat, I see a siphon path for the high speed circuit.

 

[XP_Slinger]

I'm not on board with that theory yet

I’m glad or the discussion would end lol

 

[RIDE-RED 350r]

I guess what's really being debated here is whether or not the impulse cycle still occurs at high speed. I contend that it does, and if it does I don't see the siphon effect happening. Look at he pump side of that diagram

 

[XP_Slinger]

I guess what's really being debated here is whether or not the impulse cycle still occurs at high speed. I contend that it does, and if it does I don't see the siphon effect happening. Look at he pump side of that diagram

I agree the cycle is still happening, not debating that. I think that’s what the twitch on the guage is showing us in Steve’s test. But the guage can’t show us an accurate reading of the actual pressure and depression that it’s reaching. And it has to happen because the piston is displacing volume.

I see what you’re saying about the diagram pump side. Regardless of speed, the impulse signal (pressure & depression) will still act on the pump diaphragm.

 

[RIDE-RED 350r]

Right. So if it's happening, it's still exerting it's force on those components. Wether it's happening slow or fast I don't believe matters a bit, it still occurs and those components should still react accordingly

 

[XP_Slinger]

Right. So if it's happening, it's still exerting it's force on those components. Wether it's happening slow or fast I don't believe matters a bit, it still occurs and those components should still react accordingly

I smell what you’re stepping in...that would also explain why pinching the impulse line killed the saw. I know where my thoughts got F’d up now. Forgetting that “impulse” is both pressure AND depression. Takes mechanical abilities of the pump to react out of the equation. Impulse is imposing its will on it.

 

[RIDE-RED 350r]

Persactly!

 

[XP_Slinger]

And the needle is “normally open” until there is enough fuel present to lift the metering diaphragm away from the metering lever...correct?

 

[RIDE-RED 350r]

To be honest, I'm not exactly sure what force opens the metering diaphragm. The popoff pressure test leads me to believe that pump pressure at least plays a role

 

[RIDE-RED 350r]

Think of the metering side like a bowl on a wheeler carb

 

[RIDE-RED 350r]

Ready to stand corrected on that one

 

[Clemsonfor]

Wow, this is in deep into 2 stroke running and theory!

Sent from my SM-G900V using Tapatalk

 

[XP_Slinger]

Ready to stand corrected on that one

Me too. I’m finding that what I thought I knew is completely bass ackwards to reality.

Call me an idiot but I freely admit that until this point, I thought negative pressure acted upon the metering diaphragm to open the needle.

Starting to feel lucky that I have been able to put a kit in a carb and have it function properly...lol!

 

[drf256]

My bet is that the pump flaps “float” at a given max rpm. That may be how the Venturi signal can pull fuel at WOT.

Impulse line leaks etc may not induce such a critical change at WOT as they would at start and idle.

Steve’s videos sure make one think. Great that he did it.

I wonder what would happen if the pump membrane was pulled and then carb reassembled if saw could be fed fuel to start and get up to WOT. Not sure if that a test that could even happen.

 

[XP_Slinger]

Metering diaphragm wouldn’t be subject to impulse would it?

No frigin clue now...gonna find out tomorrow when I have time to frig around at the bench.

 

[D B Cooper]

I wonder if an oscillo scope and a pressure transducer would be able to measure the impulse signal fast enough at full throttle.

 

[XP_Slinger]

My bet is that the pump flaps “float” at a given max rpm. That may be how the Venturi signal can pull fuel at WOT.

Impulse line leaks etc may not induce such a critical change at WOT as they would at start and idle.

Steve’s videos sure make one think. Great that he did it.

I wonder what would happen if the pump membrane was pulled and then carb reassembled if saw could be fed fuel to start and get up to WOT. Not sure if that a test that could even happen.

I was thinking the same thing Al. Wonder if a shot of ether would get it running then see if it would siphon to stay running.

 

[Fruecrue]

The membrane sitting over the ports serves as the intake and discharge valves of the pump.
Pump diaphragm provides the motion to operate the pump. Has an intake and discharge stroke (something like a piston pump).
The surface area of the diaphragm multiplys the impulse force (pressure x area) and the pump is capable of significant discharge pressure.
This is why we measure the pop off pressure.
We want to ensure that the pump can not overcome the normally closed needle valve and feed the engine when it’s not needed, only force applied to the metering lever can open that passage.
On the other side of the carb, the metering side, there is another diaphragm. This diaphragm is subject to atmospheric pressure on one side and pressure at the Venturi on the the other.
As air flows through the carb, the Venturi effect pulls fuel in and creates a pressure differential on the metering diaphragm which reacts and flexes in toward the metering lever. Metering lever opens and replenishes fuel to the reservoir.

 

[drf256]

Here’s the diagram Dan posted in the 462 thread. With the needle held off its seat, I see a siphon path for the high speed circuit.

View attachment 154918

The membrane sitting over the ports serves as the intake and discharge valves of the pump.
Pump diaphragm provides the motion to operate the pump. Has an intake and discharge stroke (something like a piston pump).
The surface area of the diaphragm multiplys the impulse force (pressure x area) and the pump is capable of significant discharge pressure.
This is why we measure the pop off pressure.
We want to ensure that the pump can not overcome the normally closed needle valve and feed the engine when it’s not needed, only force applied to the metering lever can open that passage.
On the other side of the carb, the metering side, there is another diaphragm. This diaphragm is subject to atmospheric pressure on one side and pressure at the Venturi on the the other.
As air flows through the carb, the Venturi effect pulls fuel in and creates a pressure differential on the metering diaphragm which reacts and flexes in toward the metering lever. Metering lever opens and replenishes fuel to the reservoir.

Look at the flaps in Dan’s pic. Fuel at WOT should be able to be pulled past this check valves no sweat. Not sure why clamping the impulse line cause such a quick stall.

It didn’t even sound like it leaned out in Steve’s video. Did it?

 

[Marshy]

There was a really nerdy guy that connected the diaphragm to a pulse generator and was able to measure its displacement on an oscilloscope. Cant remember if it was here or on AS. I'll find it and it will answer lots of questions.