Some interesting comments from Steve O'Hara on the McCulloch Kart Engines.

These are Steve's comments not mine as i still have a heck of a lot to learn just hope i live long enough thats all. Regarding the carbs, the first thing I would say is that I'm not surprised you are not sure what is going on with the Mac carbs based on some of the stuff you have read on the internet sites. The largest source of info on the subject is an accumulation of postings on John Ts web site for Mac engines that are attributed to me but a good chunk of the material is not stuff written by me. What John assembled there is stuff from conversations that started like this one with posts by me and Pete Muller that really don't make that much sense unless you would have seen the questions and comments posted by others for context. That being said, I'll try to answer your questions without going on and on. First, the early Mac carbs and the Tilly carbs are very similar devices. Ignoring the fact that they look different, if you examine the function of the early Mac (flatback) carbs as used up to the 90/100 models and compare them with the Tillys they are nearly identical in the aspects that count. Each uses a crankcase pulse driven single stage fuel pump that can deliver approximately 5 gallons of fuel per hour. Each uses a metering diaphragm controlled arm activated inlet needle and seat to control the flow of the pump to the fuel chamber. Each uses a low speed circuit that is comprised of a tapered needle controlled orifice leading to a delivery passage that feeds fuel to an idle passage located just below the butterfly when close and a part throttle passage located just above the butterfly when in the closed position. Each uses a modest venturi restriction well upstream of the butterfly with a main fuel discharge that is controlled by the high speed adjusting needle. Both carbs are very simple and effective devices that meet the needs of the engines well if set up and tuned properly. The key to the proper peformance of the carbs is the relationship of their air flow rate to the size of the motors. They work well because the flow rate of the air is high even at low speeds and that is precisely why they limit the horsepower as well. As the kart engine development exploded in the 60s the use of multiple carbs flourished and we saw 2, 3 and sometimes 4 carbs stacked on the engines. With the addition of each additional carb the flow rate fell, particularly at lower engine speeds and with the lower flow rates came less effective metering. There were many creative methods employed to adapt such as progressive throttle linkages, hand operated secondary throttles, and even a cooling fan air pressure activated secondary throttle for the extra carbs. As you can imagine, four carbs with 8 fuel metering needles, and 4 idle adjustment screws became a lot of trouble to keep tuned and maintained so the logical step was to move to a single larger carb. The problem with one big carb is that you can't rig up progressive throttles to compensate for the poor air flow rate at low speeds so when Mac built the first "Big Bore" version they sought to address that problem by adding the duck bill circuit. In the very simplest sense, the duck bill circuit was designed to work similar to the accelerator pumps built into the Webber or Holley type carbs. The problem they sought to solve was the tendency of the carb to run lean or starve at low rpm when the high speed needle was set to deliver the correct amount of fuel at full throttle high rpm operation. Like any performance carb sized to maximise the performance of an engine at high rpm, the big Mac carbs would stumble pretty bad if full throttle were applied at low speed without the duckbill circuit. Unlike the carbs with accelerator pumps that "shot" an extra dose of fuel in the throat of the carb each time the butterfly was opened, the Mac approach was different and unique.... they designed a system that altered the air pressure on the dry side of the metering diaphragm at low engine speeds to assist the delivery of fuel to the throat of the carb. To understand what they were doing you must first understand the basic system as used in the early Mac, Tillly, Walbros etc. In all those carbs the metering diaphragm provides a seal that seperates the fuel chamber from atmosphere and the fuel moves from the fuel chamber to the venturi or throat only when the pressure at the point of the fuel delivery passage drops below atmospheric pressure. That drop in pressure is a function of the speed of the air flowing past the passage so the delta between the pressure at the venturi and the dry side of the diaphragm is directly dependent on the speed the air in the carb which is a function of the speed of the engine. If the flow rate is too slow, there is less pressure drop and the delta is inadequate to move fuel efficiently. One way to solve the problem would be to open the high speed needle to assist in delivering more fuel at low speeds but if you did that the engine would run too rich at high speeds. So the Mac engineers approached the problem from the other side.... if they could not have a large pressure drop at the venturi, they could create an artificially high "atmosphere" on the dry side of the diaphragm to create the delta in pressure needed to make the fuel move from the fuel chamber to the venturi at low speeds. To accomplish that, they created a passage from the case to the air side of the diaphragm and fitted it with a simple one way valve (the duck bill) and they carefully sized the air vent hole out of the air side chamber to cause the system to produce a pressure gain on the air side of the diaphragm when the throttle is opened. To make the sytem effective, they had to make the system non linear so they added another passage that is located and sized to cause the pressure gain on top of the diaphragm to decline as rpm (flow rate in the bore of the carb) increased. That is the small passage you see under the duck bill that runs to the bore of the carb. If you have read my various posts on the carbs you have seen that I advise the use of what most people in karting think are very high pop off pressure readings. Once you understand that the duck bill circuit is pumping pressure in on top of the metering diaphragm and trying to push the fulcrum arm down it becomes obvious why the pop off pressure must be kept high. Now, lets talk about the two versions of the big Mac carbs.... the first version had no venturi at all. I have to believe from the design and the literature that McCulloch put out about that carb, that the Mac engineers thought the duckbill circuit was sufficient in its influence on the metering to eliminate the need for a venturi but that is only speculation on my part. Whatever it was that they were thinking it was wrong because the carb never worked well at all. It may simply be that it was just too big and the flow rate was too slow even at high rpm. In any event, they junked the no venturi design and went to the BDC 14 model that retained the duck bill circuit but added a venturi to acclerate the air flow and also utilized a dump tube to place the fuel pick off in the center of the throttle bore where the flow is fastest (away from the drag caused by the surface of the bore). That one change alone would have been a huge step in improvement but they went much further in advancing the design by adding some other improvements that were not seen in any of the different brands. They are as follows... the idle circuit is provided an air bleed passage to assist in emulsifying the fuel before it passes into the bore of the carb and they created a part throttle compensating circuit that took fuel from the idle circuit and also draws fuel and air mixed together from the high speed circuit. In addition, they junked the old "ball check" fuel nozzle as seen in many Tillys and the "Big Bore" and replaced the control function with a simple but very effictive flapper valve under the circuit plate. So, in my opinion, the BDC line of carbs is a significantly more sophisticated design than the Tillys and I am distressed to see them so maligned by so many folks here and in other forums such as the saw groups etc. There is nothing wrong and plenty of things right about the BDC large bore carbs as long as they are properly set up and maintained. True, the duck bill valve itself can be a problem if it becomes affected by fuel and swells or shrinks from its intended size. Modern gasolines will swell the original parts up and make them useless but there are recently manufactured valves that hold their shape nicely in any kind of fuel. When I set up and tune BDC 14 or 16 carb I find that I have no problem with them meeting the needs of the engine all through the range and they idle like a motorcycle, start without any problem and respond nicely to settings on the needles. One other thing I like about the BDC line of carbs over the others is that by altering the relationship between the three duck bill circuit passages I can fine tune the fuel delivery curve to suit the needs of motor/pipe combinations that would otherwise have problems. For example, on JohnTs site you can see a picture of my multi Duffy winning enduro kart from the rear. If you look closely, you would notice that the exhaust on the kart has a very short header length... about 2" shorter than everyone else ran back then. The only way that pipe design could be used was with a carb significantly tweaked to provide a very rich mixture at the low end of the rpm band and I was able to obtain that from my carbs because I could manipulate the duck bill circuit. When others copied my pipe they just melted down their motors coming off the slow corners or they ran too rich on top if they tuned to meet the needs of the motor at the low end. I think they are great carbs! Very few share my affection for them  Regards, Steve O'Hara =============================================== Hi John, First let me say that my comments in the post above are in no way meant to be critical of the material you have assembled on your site. It is a very complicated subject and the nature of chat room posts leaves a lot to be desired when it comes to presenting technical topics in a clear manner. I know based on many messages I have received that your site has been very helpful to a lot of vintage karters and I appreciate what you have taken the time to put it together. About the straight bore carbs, I think the Mac designers overstated the ability of the duck bill circuit in the early literature by implying that the carb worked like fuel injection. Here is why.... in a conventional carb the fuel moves through the fuel passages and mixes with the air stream as a result of a pressure drop at the venturi. The greater the pressure drop, the more fuel moves thus the metering of fuel is responsive to engine demand. The greatest pressure drop occurs when the piston speed is highest on the way from BDC to TDC since that is the time when the volume of the case is expanding the fastest. If you look at the design of the duckbill circuit it is clear that it functions on pressure INCREASE in the case which only occurs when the piston is on the way DOWN the bore causing a compression of the charge drawn into the case. The system is effectively out of phase! If the pressure pulse from the case pushed through the duckbill were able to force fuel from the fuel chamber into the throat of the carb as a true fuel injection functions, the fuel would be arriving in the throat of the carb as the piston was on its way down the bore which is just about 180 degrees of crank rotation out of sync with the flow of air into the case. Obviously, that does not work and the marketing materials put out by McCulloch sounded impressive but were not very accurate. As many people who fiddled with the BDC model carbs know, they can be run with the duck bill circuit closed off but they won't work unless the popoff pressure is dropped way down to 10psi or less. Once those changes are made you basically have a Tilly or Mikuni carb. The big bore carb will run, but just barely, without the duckbill circuit because the pressure signal in the thoat is very week at low speeds. The BDC carbs modified in that manner work reasonably well as long as they are not asked to cover a wide range of rpm. The trouble starts when you try to get good metering over a wide power band where the air speed in the throat of the carb changes dramatically from low speed to high speed. Without the duckbill the carb will starve the motor and it will overheat or have detonation at low speed if tuned to run clean on the top end. If you richen the mixture to meet the needs of the motor are the low end of the range it will run way too rich at high rpm. The same is true of the Tilly or Mikuni carbs if they are too large for the motor. As explained above, the duck bill circuit compliments the normal function of the carb by altering the pressure on the dry side of the diaphragm in a non linear manner over the range with more assist applied at low speed compared to the higher speeds. The full explanation of exactly how it works is very complicated and I won't try to explain it here. For those reading this post that run a BDC 16 double pumper carb here is a little test you can run to see how much the duck bill circuit effects the carb... next time you run the engine on the track get the jets set where you like and then try this little test as you are going down the long straight..... take your finger and cover the little square hole right next to the high speed needle and see what happens. That little square hole in the side of the plate stack is the outlet path for the air from the dry side of the diaphragm and if you cover it and seal the hole the pressure from the duck bill circuit will build up and cause the motor to go very rich very quickly! If you have one of those carbs and you have found that it is hard to get it tuned you may be inadvertently covering the vent hole each time you reach back to the tweak the high speed needle and it will mislead you as to the effect of the change in the needle position. It is especially troublesome when you are rich and go to lean the high speed jet and you grab the needle and turn it in a little and the motor goes richer instead of cleaning out. You can solve the problem by adding some kind of needle extension to keep you finger off the side of the carb where the vent hole is located. Regards, Steve O'Hara ========================================== 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 ========================================== Andy, Your BDC 23 is basically the same carb as the more common BDC 14. In some cases it came fitted with a different throttle shaft and a choke. The 23 and 14 models are the best gas carbs that McCulloch offered. As you noted, the BDC 16 body is very similar except for the size of the dump tube, HS needle and inlet needle and seat. There is also a model either BDC 22 or BDC 24... I apologize that I can't remember... that is the same as the BDC 16 again except for the throttle shaft. The alky carbs are easily visually identified by the double pump stack that includes four plates as opposed to the two on the gas carbs. The BDC 16 carbs came on the 101A/A and 101B model engines. To the best of my knowledge no 100cc model was ever offered with the alky carb as OEM equipment. Regarding the 92 being run with the BDC 23 on alky... no surprise... everyone tried and some succeeded in using the BDC 14/23 with alky. In stock configuration, the 23/14 type carbs will run with 100% alky but they will cause engine damage in most circumstances due to the fact that the two plate pump simply can't deliver enough fuel to meet the needs of the engine when running alky. The BDC 14/23 model carb can be modified to perform very well with 100% alky but the carb is very sensative and must perform at 100% efficiency to meet the needs of the engine. If you wish to run alky, save yourself the grief and get a BDC 16 carb.... it can deliver more than enough fuel even when performing at less than full capacity. last... you can find the exploded diagram of the BDC 16 carb in the IPL for the 101B on this site. Regards, Steve O'Hara

Understanding the McCulloch BDC Carby .........