Walbro HDC Carburetor Function Discussion Using Walbro Manual & Correction To Check Valve Function!!

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Күн бұрын

I owe the Arboristsite guy a thanks for correcting my error! I don't like making mistakes and never intentionally spread bad information. This video is meant further everyone's understanding of the Walbro HDC carburetor and the major function(s) of the main nozzle check valve.
Walbro HDC Service Guide: www.walbro.com...

Пікірлер: 42

@Man-Time 11 ай бұрын

Amazing job by the engineers who designed this system ~70 years ago. Engine experts still trying to figure out the “magic” that makes it work. As an engineer I strive to be on that level

@OOOOOO12345 11 ай бұрын

Thank you, Leon. Appreciate you sharing your knowledge with all of us.

@thomasfrost6993 11 ай бұрын

Always learning. Yup, that's why I watch your channel. For Homelite chainsaw repair, your the best and most thorough I've experienced and you've taught me a lot. I'm not as timid as I used to be in tearing into one now and I've had some great success because of you. Thank you!!

@jimbotapp2849 11 ай бұрын

Great job Leon, you give us awesome content! Keep up the good work. I am a chainsaw “power saw” hobbyist and I really enjoy learning from men like yourself. Vintage saws are amazing to bring back to life and you give us the info we need. Thanks!

@notajp 11 ай бұрын

I think where you are getting confused is that on idle, the throttle valve is NOT entirely closed. There is still a slight gap around the throttle plate, which is set with the idle speed screw. That slight gap allows just enough air flow to draw fuel thru the slow speed passage.

@LeonsChainsawPartsAndRepair 11 ай бұрын

Absolutely. There's also a small machined area on many carbs that prevents the throttle plate from ever being completely closed.

@robertsimard735 6 ай бұрын

Excellent tutorial. Thanks Leon for another great video for us amateurs. Comment section also laid out some great points. Thanks everyone.

@MrRmh3481 11 ай бұрын

Good Job Leon. Here are a couple things to think about; 18:32 Use of the fuel pulls the needle off the seat. The diaphragm changes position by the engine drawing in the fuel. The low pressure (Baro) on the piston side is what draws the fuel into the engine. Low pressure air moves into and out of the Venturi based upon the amount that the throttle plate is open. You see this as blue on the diagram. None of the idle ports will flow until the plate uncovers them because of where the low pressure air is located. Same thing with the Hi Speed Check valve, the fuel is drawn into the engine by the low baro pressure traveling back to the check valve. Notice the position of the check valve in relation to the Venturi. In order for the air to flow thru the Venturi, the air must speed up. When the air speeds up the Barometric Pressure falls because of the Venturi effect on the Bernoullis principal.

@LeonsChainsawPartsAndRepair 11 ай бұрын

Makes perfect sense...I had just never wrapped my mind around the concept of the fuel circuit dropping below atmospheric pressure and that being the force that actuates the metering diaphragm.

@marksarkaquariums9056 11 ай бұрын

Awesome video Leon,I have a better understanding of the whole 2 stroke walbro carb check valve.You really detailed the function of the system and made it a lot clearer and easier to understand.

@jimball816 11 ай бұрын

i just finished a mcc pm650 with hdc. very helpful video. deeper understanding. learning is not always easy.

@steveswift9028 11 ай бұрын

I love it when someone lays it out

@jimkillen1065 11 ай бұрын

I do appreciate the video..i not a expert on these carbs so i like to learn all i can . Sure helps a lot . This is one of those videos i may want to watch twice lol

@tedmiller1745 11 ай бұрын

Glad to see some content from ya. Thanks sir!

@simonilett998 11 ай бұрын

The forum guy has it mostly correct. However, the main jet check valve is also very important for machines that have a purge bulb. If the check valve doesn't seal, the purge bulb will draw air in through the check valve instead of removing air out of the carb to bring fuel up from the tank, and also can allow atmospheric pressure into the metering chamber, which then prevents the metering diaphragm from working to open the fuel inlet needle. Refer to the fuel inlet needle being opened by fuel pump pressure, your intermission statements are still not quite correct. The fuel pump doesn't create pressure to lift the inlet needle and allow fuel into the metering chamber. Rather, as the engine draws fuel from the metering chamber via idle circuit and/or main circuit, this creates negative pressure in the metering chamber on the fuel side of the metering diaphragm, and atmospheric pressure is essentially pushing from outside on the other side of the diaphram, the little spring under the inlet needle lever is creating a force/balance resisting atmospheric pressure opening the inlet needle until enough negative pressure/vacuum exists in the metering chamber to overcome the spring. This negative pressure in the metering chamber draws the metering diaphragm inwards until the nipple in it's centre pushes on the inlet needle lever, which then lifts the needle off it's seat allowing the now pressurized fuel from the pump to enter. Once a certain amount of fuel has entered the metering chamber, the internal pressure equalises with atmospheric pressure and pushes the metering diaphragm nipple away from the inlet needle lever, so then the inlet needle closes again until the cycle then repeats👍

@TheGreasyShopRag 11 ай бұрын

I really enjoyed the video but I don't think either you or the arborist site guy got it completly right. The metering diaphragm has atmospheric pressure on one side and as the engine runs and pulls fuel, low pressure on the other side. That low pressure moves the diaphragm to open the lever and allow more fuel in (we all know that). At idle, if the check valve is bad, atmosheric pressure is allowed to pass directly into the metering chamber. Now you have even pressure on both sides of the metering diaphragm and it will never move to open the metering lever and allow more fuel in, creating that lean condition you described. Whuddaya think?

@LeonsChainsawPartsAndRepair 11 ай бұрын

Okay, so the vacuum of the engine is pulling the the diaphragm "down" through the main nozzle and then opening the inlet needle? I can also see your point on the atmospheric pressure on both sides when the throttle valve is mostly or all the way closed.

@TheGreasyShopRag 11 ай бұрын

@@LeonsChainsawPartsAndRepair Yes thats how the needle opens. If you put a pressure pump on the fuel line it would take over 10 pounds to unseat the needle. The fuel pump could never make that kind of pressure.

@Drottninggatan2017 11 ай бұрын

We can read in the Zama service manual: "The main nozzle check valve is another important component of the idle circuit. This one way check valve located in the main nozzle is under atmospheric pressure at idle. Without it air would flow from the main nozzle into the metering chamber. This will cause an air leak in the metering chamber and there would be no drop in pressure. The diaphragm will not move and the engine will die lean when idling. This check valve also serves the same function during the priming operation." The symptoms I have noticed with a bad check valve is not necessarily starving on idle but idling very funky, and it will die as soon as you touch the throttle trigger. But it will run fine on full throttle, if you can get it there. I believe Zama claim that the check valve is an no servicable item. If it is bad then you need a new caburetor. But at the same time there is a perfect explanation on how you change it in the service manual.

@TheGreasyShopRag 11 ай бұрын

Thanks. A more detailed description of what I said straight from the source. @@Drottninggatan2017

@KensSmallEngineRepair 11 ай бұрын

And it's Scott for the win!!! All good stuff, from everyone. I'm learning everyday and I've been doing small engines for 24 years!! Well, 2 strokes for about 7 years, was always afraid of them! Thanks guys!

@oologahgardentractorandcha3055 11 ай бұрын

Thanks Leon, That was really informative

@flavorwaster 3 ай бұрын

Great explanation. Now, if you could kindly explain to me how the posi trac rear end on a Plymouth works, I'd have all the answers I need in this life.

@philliphall5198 10 ай бұрын

We all forget and have to remember It’s just part of it 😢 Don’t beat yourself up over it 😊

@tnbasscat 11 ай бұрын

Great Channel..... keep up the good work.....

@diggindiggenit6540 11 ай бұрын

In the walbro parts and service manual it covers everything your showing on the screen, first time i looked at the service manual it's like a dictionary of walbro carbs