Some questions about McCulloch

[steveohara]

Hmmmm,

I don't what Jay was smokin when he wrote that last sentence.... may have been the effect of inhaling smoke from boiling 90 weight gear oil after we did a quicky case harden on a new left hand clutch nut in the shop yesterday
The duck bill circuit is the same in the "Big Bore" (straight bore) Mac carbs and the BDC models except for some of the early carbs had a slightly smaller hole in one spot. The most significant difference between the two designs is the venturi and dump tube that were added in the BDC models. One other interesting note.... even a well tuned Big Bore carb that is dialed in at top speed will not produce more HP than the BDC model even with the much larger bore in the early model. I have never flow benched one vs the other but I would expect the Big Bore version to flow a lot more on a flow bench since the BDC venturi knocks the opening down by around 20%. That would seem to indicate that the Big Bore carb is simply way too big for the size of the motors at least in their stock port configuration. It might be a different story on a punched out radical ported 101 where the engine could possibly put the extra intake area to good use.
Steve O'Hara

[jmendoza]

Who did you say the left hand nut was?

What I think I meant was that the straight bore reacted to swollen duckbill by leaning out coming off a turn, whereas the BDC reacted by going too rich coming off the turns with Elder rubber duckbills.

The differance is probable due to the fact that the duckbills in the BDC were replaced with new ones every run, and the ones we used in the straight bore were taken out and allowed to dry out, then re-used( we had no spare new ones until the next day.) The different reactions were probably due to the extent the duckbills were swelling? I did notice that the circuit plate beneath the diaphragm is different on the BDC, that is the different circuit I misstakenly identified as being part of the duckbill circuit.

In any case, the straight bore with the new gasoline proof duckbills runs good enough to start the engine, idle, and drive the kart, at least I finally was able to make it work somewhat... and sold it on a MC-91A to Casey! The BDC quit acting up rich as soon as we put in the new style duckbills. Regardless, I'm still sticking with the BDC carb now that I have several.

If the gear oil was that good, I'de still be in Steve's garage, the price sure was right, let me tell you!

[Dean Seavers]

Not meaning to throw in another complication, but has anyone had success with plugging the duckbill orifice with epoxy and readjusting the popoff setting to a lower pressure?

[Tom Smith]

Kendick (Dick) brags about drilling his "magic hole" that makes the big mac carbs work. The flat on the butterfly shaft uncovers it as the butterfly opens. I've heard Mcculloch designed those carbs for a US Army target drone powerplant. No wonder they don't work.

[steveohara]

Tom,

The "magic hole" you refer to was conceived to bypass the inlet needle and seat in the era when IKF rules for Reed Lt and Hvy classes required the carbs to be run in factory stock condition and every dimension and orifice was subject to post race tech. It allowed the use of 100% alky in an era when getting that much fuel through a stock carb was very tricky. It was outright cheating and taints the results of those drivers who used carbs with the extra passage. Of course we don't care about those rules now so that modification is of no use at all today since we can simply drill a larger hole in the seat for the inlet needle. Or better yet.... just use a BDC 16 carb for running alky and then you don't worry about the pump not keeping up or the rubber tip on the needle swelling up.
Regards,
Steve O'Hara

[Rob Voska]

Dean
I have tried to make a duckbill free carb work. Spent hrs talking to a old engine guy that told me it worked..........I got it to work after hours and hours but it's not consistant nor was it fast. I could change back to a stock Mac carb or a Tilly and they were noticably faster and user friendly.

If I get some more time I might give it another try as I have some new ideas.

[Bill Johnson]

Comet Kart use to fill the duckbill hole with epoxy and drill a smaller hole in it. They had a name for it but it escapes me now. I ran one but it didnt run as well as the one I had converted at the local kart shop. They enlarged the hole in the circiut plate, took a drill bit and wallered out the length of the long channel under the plate, drilled the hs needle area and dump tube. Best alky carb i ever ran on a stock engine. Others wanted to borrow it all the time. Wish I stll had it. It would not flow enough for a ported engine. It would starve and you would have to run twin Hartman 293's.

All the other converted BDC14's have been run of the mill carbs.