McCulloch BDC Stock Specs .........................

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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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 the alky version.
Regards,
Steve O'Hara

McCulloch Kart Carb Part Numbers.

Mac 1 84050
Mac 3 67895
Mac 6 84260

Mac 49c 67895 s/n prefix 13- flatback

BDC 2 68886 used on the Mc 91, 91a prefix 11- , Mc 101 prefix 11-
BDC 4 82894 used on the 91c, 101c prefix 13- same as 68886 but with choke.
BDC 14 84446 used on 91b, 101a prefix 14- venturi added.
BDC 16
BDC 20 101D
BDC 22
BDC 23

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Mac 2 48542
Mac 5 47830
Mac 6 47830b
Mac 7 48890
Mac 6 48890a
Mac 9 48890b
Mac 10 50070c
Mac 20 48890
Mac 30 48890
Mac 40 "A" used on 48890a
"B" used on 47821
Mac 45 48890
Mac 45d 48890b x 2
Mac 49 m/c 89664
Mac 49c 68795
Mac 70 48907a
Mac 75 48890b

BDC Carb's

Mac 91 without prefix 68886 prefixed 10-, 91a prefixed 11-, 91c prefixed 12-, 91b prefixed 13-, a12-.
Mac 91a 68886
Mac 91c prefixed 12- 82894
Mac 92 91044
Mac 93 91044
Mac 100 48890
Mac 101 without prefix and 10- and prefixed 11- 68886
Nac 101 prefixed 13- 82894
Mac 101 prefixed 14- 84446

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In the BDC line of large bore carbs for Mac engines the single pump versions can't deliver enough volume of fuel to run 100% alky no matter what you do to the jets and other passages in the carb. If you run straight alky in the single pump carb the fuel pressure will drop down to near 0psi at around 11,000 if you have jets large enough to flow the required amount of fuel. The BDC 16 version with the double pump setup will deliver more gallons per hour so it can maintain reasonably steady fuel pressure all the way up to over 13,000 rpm. To give you an idea of the flow rates... we used to run alky through the single pump version and had to limit the top rpm to around 11,000rpm on the 91 series engines and the fuel consumption was about 3.75 gallons per hour. If we geared to turn more rpm the engines would just run out of fuel when the revs went over 11,000 no matter where we set the jets or how we modified the passages. Using the BDC 16 double pumper carb on the 101 engines we would burn 6 gallons in an hour and the high speed jet was open less than 1 turn so I could flood the engine if I opened the HS jet at top rpm.
In the late 70s I developed a modification to the single pump version that was internal and allowed the single pump to deliver enough fuel to turn the 91 series engines up to 13,000 but it was still not as good as the double pump version. If you run gas then the single pump is all you need and the double pump offers no benefit but for alky the double pump setup is the only way to go.

Hope this helps.
Steve O'Hara

Bob,

They are all gas carbs unless they have the double pumper stack. The BDC 16 is the first one with the double pumper stack... after that they made one that was a 22 or 23 that was the same with a different throttle shaft. If you are not sure what pump arrangement came on the carb body then you can look at the high speed needle and the inlet needle..... those are the only things changed in the alky carbs except for the pump. On the alky version the inlet needle is much bigger and it is solid brass with no rubber tip. In addition, the high speed needle is much bigger in diameter so it can cover the larger ID in the dump tube. The non threaded part of the HS needle is about .085 on the gas carb and about .110 on the alky version.
Regards,
Steve