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Tuning “Dependant” carburettors.

Vintage Engine Repairs

Vintage Engine Repairs

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Aug 12, 2019
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3,591
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Australia
Back story -

I was given a cheap Chinese saw because it wouldn’t oil, wouldn’t run properly and the purge bulb was broken.

I fixed all the issues with spare parts I had around the workshop and gave it a tune, but couldn’t get the H lean enough without it affecting the L side off idle.

Long and short is two fold - this it what you get with a big box store Chinese saw, where they just stick a generic carby (although in this case good walbro) on to their equipment without really caring about how it runs, but secondly it is a dependant carb - all the fuel flows through a single drilling in the carb floor controlled by the H screw. This fuel then passes the main jet and then flows to the L needle which then controls the fuel flow through the idle and progressive idle drilling’s.

This is the first time I have worked on a dependant system that I’m aware of (could very likely had it in other carbs but they were matched to the correct powerhead etc).

Anyway, here you go, a quick video showing how the H affects fuel supply to the L.

I’m sure many of you already know this, but it may also be new to some too.

Thanks to those who shared advice on the topic already, it’s much appreciated :)

It has been a great opportunity to learn something new, to add a leaf to the book so to speak and get hands on experience with it.

i’ll re tune it back and just have it run a little rich on H and good at and off idle - it starts, runs and cuts just fine. Not a refined tool, but, you can’t expect much and it gets the job done for the average homeowner pruning their garden. I just thought someone may enjoy seeing it in a video.

 
Old2stroke

Old2stroke

Never too many toys
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975
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I have modified a few of these carbs by blocking the low speed fuel feed passage from the main jet area and drilling a new hole in the carb body where it can supply fuel directly to the low speed circuit just before the L jet. Makes the L and H adjustments completely independent and makes it easy to tune with no bog when transferring off the low speed circuit.
 
Vintage Engine Repairs

Vintage Engine Repairs

Addicted to ArboristSite
Joined
Aug 12, 2019
Messages
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Location
Australia
I have modified a few of these carbs by blocking the low speed fuel feed passage from the main jet area and drilling a new hole in the carb body where it can supply fuel directly to the low speed circuit just before the L jet. Makes the L and H adjustments completely independent and makes it easy to tune with no bog when transferring off the low speed circuit.
Fiddly stuff!! How did you get between the L screw drilling and the H nozzel to block it? Also how did you drill it so accurately? That’s pretty cool
 
Old2stroke

Old2stroke

Never too many toys
Joined
Jan 24, 2016
Messages
975
Location
Ottawa, Canada
Describing it takes more time than doing it but here is the essential part of it. This is for the Walbro MDC series on a Mini Mac 110
My solution is to create more isolation between the two fuel circuits so that the low speed is not effected by a stuck open check valve in the high speed or by any adjustment of the H screw. To do it, you have to remove the fuel control diaphragm, both mix adjustment screws and the welch plug covering the high speed jet. You will notice there is a fuel well just inboard from where the high speed adj screw does its control function and down in the corner of this well there is a small drilled passage heading off to the low speed adj cavity. This is how the low speed gets its fuel and you will want to block it, but first you have to provide the low speed circuit with a different source. Stick a sewing needle in this passage and lay a flexible straight edge on the carb, lined up with the needle, and scribe a line on the floor of the carb over to, and past the low speed screw location. Now stick something straight and larger (one of those nice round tooth picks?) in the threaded hole where the low speed adj screw was and again, use a flexible straight edge lined up with the centre line of the tooth pick and scribe another line to intersect with the previous one. The point where they intersect is directly over the spot where fuel has to be introduced to the low speed adj screw cavity and you have to drill a small hole here. Before drilling, mask off everything so that shavings don't get into places they shouldn't. A drill press makes things easier and be very careful as you break through so that shavings don't follow the drill into the passage below. The material is quite thin and easy to drill, don't use anything bigger than 1/16"or a #50 drill. Shine a light down the hole or stick something bright down it to verify the hole is in the right place. You should now have opened up a new passage between the fuel reservoir under the diaphragm and the low speed circuit just before it gets metered by the adj screw. You can use a larger drill to produce a slight countersink cavity on the entrance to the hole (not really necessary) and be very careful to make just a SLIGHT cavity. Clean out any drill shavings and using an air hose, locate the idle port just in front of the closed throttle disc and blow air back through this port just incase drill shavings have ended up in the low speed cavity. Now you have to go back to the original fuel passage from the high speed circuit and block it off. Clean any residual oil off the entrance to the passage with your favorite degreaser, then squeeze a bit of epoxy down it to seal it. Use a good grade of overnight cure epoxy like J.B.Weld. Do not use any fast cure epoxy, they are the garbage of the epoxy industry. Replace the welch plug and you're done.
 
Vintage Engine Repairs

Vintage Engine Repairs

Addicted to ArboristSite
Joined
Aug 12, 2019
Messages
3,591
Location
Australia
Describing it takes more time than doing it but here is the essential part of it. This is for the Walbro MDC series on a Mini Mac 110
My solution is to create more isolation between the two fuel circuits so that the low speed is not effected by a stuck open check valve in the high speed or by any adjustment of the H screw. To do it, you have to remove the fuel control diaphragm, both mix adjustment screws and the welch plug covering the high speed jet. You will notice there is a fuel well just inboard from where the high speed adj screw does its control function and down in the corner of this well there is a small drilled passage heading off to the low speed adj cavity. This is how the low speed gets its fuel and you will want to block it, but first you have to provide the low speed circuit with a different source. Stick a sewing needle in this passage and lay a flexible straight edge on the carb, lined up with the needle, and scribe a line on the floor of the carb over to, and past the low speed screw location. Now stick something straight and larger (one of those nice round tooth picks?) in the threaded hole where the low speed adj screw was and again, use a flexible straight edge lined up with the centre line of the tooth pick and scribe another line to intersect with the previous one. The point where they intersect is directly over the spot where fuel has to be introduced to the low speed adj screw cavity and you have to drill a small hole here. Before drilling, mask off everything so that shavings don't get into places they shouldn't. A drill press makes things easier and be very careful as you break through so that shavings don't follow the drill into the passage below. The material is quite thin and easy to drill, don't use anything bigger than 1/16"or a #50 drill. Shine a light down the hole or stick something bright down it to verify the hole is in the right place. You should now have opened up a new passage between the fuel reservoir under the diaphragm and the low speed circuit just before it gets metered by the adj screw. You can use a larger drill to produce a slight countersink cavity on the entrance to the hole (not really necessary) and be very careful to make just a SLIGHT cavity. Clean out any drill shavings and using an air hose, locate the idle port just in front of the closed throttle disc and blow air back through this port just incase drill shavings have ended up in the low speed cavity. Now you have to go back to the original fuel passage from the high speed circuit and block it off. Clean any residual oil off the entrance to the passage with your favorite degreaser, then squeeze a bit of epoxy down it to seal it. Use a good grade of overnight cure epoxy like J.B.Weld. Do not use any fast cure epoxy, they are the garbage of the epoxy industry. Replace the welch plug and you're done.
That’s about as involved as I expected, wow! Well all I can say is, great ingenuity and very welll done mate that’s cool!!
 
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