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BDC Carburetor assy

Details

Part No 68886 BDC 2 carb assy Mc91,91 prefixed 10, 91A prefixed 11, 101 Prefixed 10-, 101 prefixed 11-.

Part No 82894 BDC 4 carb assy Mc 91c prefixed 12.101 prefixed 13-.

Part No 91044 BDC ?? carb assy MC 92, 93

Part No 84446 BDC ?? carb assy venturi added, 91b, 101a prefixed 14-.

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Part No ????? BDC - 14 GAS CARB STOCK SPECS

1. low speed fuel pick-off 0.020 ( NO 76 drill blank)
2. high speed fuel pick-off 0.105 ( no 37 drill blank)
3. high speed needle seat 0.078 ( no 47 drill blank)
4. part -throttle compensating orifice 0.042 ( no 58 drill blank)
5, main discharge nozzle 0.080 ( no 46 drill blank)
6. fuel inlet valve seat 0.074 ( no 49 drill blank)
7. part - throttle discharge nozzle 0.055 ( no 54 drill blank)
8. part - throttle discharge nozzle 0.033 ( no 66 drill blank)
9. idle discharge needle seat 0.039 ( no 61 drill blank)
10. low speed needle seat 0.064 ( no 52 drill blank)

The tech dimensions for the Part No ????? BDC-14 gas carb...
if it still meets the specs, it probably has not been modified for Alky and will run gas.
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Whats the chance of getting the rest of the BDC's covered like the BDC 14 ?

Mc Bob.

Steve i have asked a question on the rear forum about various jet and orifice sizes on the different BDC carbs
also which BDC's are set for running Alky ?

Bob.


The BDC 16 with the double pumper is the only Mac carb made to run alky as it came from the factory.
There was another part number that showed up at one time... I think it was BDC 21 or 22 and it was the same carb fitted with a choke.
If the carb only has the standard single pump it is not suitable for alky without modifications.

Regards,
Steve

A little more to add....

The early "Big Bore" mac carbs with no venturi are all really poor performers and I use them only on my "show" motors mounted on the wall. I do not recommend running then as they greatly increase the chance of engine damage due to poor metering.
The BDC 16 and the 21/22? models that came with the double pumpers had only three other differences.... the inlet needle and seat was much bigger and used an all brass needle, the circuit plate had a larger main fuel delivery hole.... as I recall it is about .125 rather than .105 and the brass dump tube/high speed needle setup is bigger to handle alky. The functional part of the HS needle on the 14 is about .090" in diameter while the 16 model has a needle that is around .120. In each case the ID of the brass dump tube is approximately the same as the OD of the adjusting needle.
One more thing that is very important to know about the alky version.... in many, but not all, the carbs were delivered with the dump tube cross drilled all the way through both sides which allowed the HS needle to pass through all the way to the aluminum body when fully closed. The effect was to cause the carb to require about one full turn off of full closed to bring the tip of the needle to flush with the ID of the dump tube and from there you had to turn it the additional amount necessary to get the correct fuel mix. So, if you had one that was not cross drilled and the HS jet ended up at 1.5 turns and then you got your hands on another carb with the dump tube that was fully drilled all the way across and you set the needle at 1.5 you would only have .5 of effective opening. Needless to say this situation drove a lot of folks crazy!! And probably led to a lot of damaged motors. If you have a BDC 16 it is simple to check... just remove the circuit plate and gaskets and shine a light in the bore of the carb and check to see when the needle covers the whole ID of the dump tube and then see if it will turn any further. If it is cross drilled the needle will turn another full turn in after the light is blocked.
All of the rest of the passages for the air bleeds, idle circuit, duck bill circuit, part throttle circuit etc are identical in both the gas version and thelaky version.
Regards,
Steve O'Hara