Hi speed bypass

[cease232]

I'm the first to admit I know jack about carbs so I need some help. I bought a brand new wj76 carb for a project but I can't get it to lean out. Seems to be just right if not a little rich with the needle turned all the way in. I looked on the google and found a similar thread on AS. Someone mentioned that early walbro WJ carbs had a Hi speed bypass (what is that). Is that what's going on here? What is going on here?
Thanks
Jeremiah

 

[wcorey]

The 'H' needle regulates the fuel flow into the high speed (main) jet and on some/most of these type carbs this is the only route for fuel to get there. So it can be shut off entirely by the needle and then will most likely cause a fatally lean condition at large throttle openings. I'm sure you know this already...

The bypass circuit is an additional fuel path to the same high speed jet that has no external adjustment and typically has a fixed inlet jet that can be found in the metering side of the carb. I think the intent is to prevent the uninformed users ability to completely adjust out the fuel supply to the high speed jet.
Usually starts as a little round brass insert that can be soldered or peened in to block it and then drilled back out to a smaller dimension if need be.

 

[cease232]

The 'H' needle regulates the fuel flow into the high speed (main) jet and on some/most of these type carbs this is the only route for fuel to get there. So it can be shut off entirely by the needle and then will most likely cause a fatally lean condition at large throttle openings. I'm sure you know this already...

The bypass circuit is an additional fuel path to the same high speed jet that has no external adjustment and typically has a fixed inlet jet that can be found in the metering side of the carb. I think the intent is to prevent the uninformed users ability to completely adjust out the fuel supply to the high speed jet.
Usually starts as a little round brass insert that can be soldered or peened in to block it and then drilled back out to a smaller dimension if need be.

Great info thank you.

I'm assuming it's the brass insert in the upper left hand side.
I think I'll just block it off completely.


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[RIDE-RED 350r]

I believe you are referring to the check valve in that pic

 

[drf256]

Be careful.

2 things,

Run the saw with no air cleaner, can you lean it out? Sometimes a restricted air cleaner can cause a saw to not lean out.

Use some carb cleaner and (put safety glasses on) squirt some in the holes and see what comes out where. My point is to try and trace the circuit before you plug it. If you plug it, you may not have enough flow through the other jet alone to get it rich enough. You can try installing a thin piece of bent wire through the hole as a test. It will limit flow there and will be removable.

 

[cease232]

I believe you are referring to the check valve in that pic

Yes I was haha. I've been up reading the stihl carburetor manual and I think I'm starting to get it. It's the removable screw in the picture. I can get smaller ones from baileys.


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[cease232]

Be careful.

2 things,

Run the saw with no air cleaner, can you lean it out? Sometimes a restricted air cleaner can cause a saw to not lean out.

Use some carb cleaner and (put safety glasses on) squirt some in the holes and see what comes out where. My point is to try and trace the circuit before you plug it. If you plug it, you may not have enough flow through the other jet alone to get it rich enough. You can try installing a thin piece of bent wire through the hole as a test. It will limit flow there and will be removable.

This is what I found

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[cease232]

The walbro wj76 has a .074 fixed jet which is quite a bit bigger than the standard 066 jet which seems to be .062-.064. (Wj69).


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[Deets066]

The walbro wj76 has a .074 fixed jet which is quite a bit bigger than the standard 066 jet which seems to be .062-.064. (Wj69).


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The 74 jet is rather large, hold onto that jet because you can't buy just the jet anymore. They work well in some ported 66's.

Smaller jet will definitely help you

 

[cease232]

The 74 jet is rather large, hold onto that jet because you can't buy just the jet anymore. They work well in some ported 66's.

Smaller jet will definitely help you

Thanks Deets. The saw is the same displacement as an 066 plus I hacked on it so thought it would work but apparently not.
Baileys still lists them for sale.
https://www.baileysonline.com/Parts/Parts-By-Walbro/Jets/JET---FIXED-0-74.axd


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