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

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

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!

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!!

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

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.

Glad to see some content from ya. Thanks sir!

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

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

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.

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

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

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.

I love it when someone lays it out

Ur not the only one who try’s to avoid the forums. The grammar nazzi’s are thick there.

Hope all is well sir

rcf8367 0 seconds ago Good redo, you got it right this time! Always consult the specific carbs IPL for the correct parts.

Excellent video

Good video. I learned something

Thanks Leon, That was really informative

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

Hdc carbs are my nemesis, same with hdb...lol

Thank s

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👍

Are the 4 holes on the curcit plate open or just the 2 bigger ones?

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?

I hate the poulan 3000 carburetor rebuild mine didn't fix it clean it didn't fix it

What the hell is that pink Barbie chainsaw in background??