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Gordon Duax
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| Joined: | Mon Dec 21st, 2009 |
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| Posts: | 28 |
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Posted: Wed Aug 11th, 2010 02:16 am |
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When that carb first come out on the Mac kart engines, an old boat racer/repair shop owner/tech school instructor told me it was a variation of a carb that had already been in use on the Scott outboard boat engines (Scott was owned by McCulloch back then).
Comet kart sales sold a replacement, that was nothing but a hard plastic plug that closed off the circuit. It took major tuning to get to run, and then it never did run right.
Remember, the very first versions of that carb did not have a venturi, they were strait bores, and were actually considered a pulse injector. But they would tend to run lean at high rpm, as the pulse circuit just couldn't keep up, so McCulloch/Walbro added the Venturi to help out. If the duckbill circuit is plugged off, you have a normal carb, with a venturi that does not supply enough of a vacuum signal for mid range, as it was never intended to do so. Probably why the plug that Comet sold didn't work.
I've tried a main venturi discharge nozzle check ball assembly out of a Tillotson, it almost works, but because of the inertia of the plastic ball, I can't say that it's 100% fool proof.
I've seen a write up by someone who had done major work/tuning with them somewhere on the web, and he mentioned changing the size of the bleed-off hole from the chamber that the duckbill fed, to get around rich/lean problems at different tracks. Along that line of thinking, adding a miniture needle valve coming off that bleed hole (after inlarging it), one could then 'tune' the pulse circuit instead of changing out plates that had different diameter bleed holes. The miniture needle valves are available from any industrial pnuematic valve distributor, and come with 10/32 threaded ports. They also have the threaded hose barbs in that size. I have some set asside, and when ever I get the itch to dig out my old reed open 91A, that carb will get the mod.
The major anoyance that I have had with the carbs, is that they seem to be way over senitive on adjustments. You can be blubbering at the end of the strait, and just 1/8th turn on the high needle will almost be too lean. I have wanted to find a needle with more of a taper that would fit, or be made to fit the Mac carb. Maybe that is what the guy that was changing bleed holes was dealing with.
They are quite the challange, and fun to play with. But wise men dump them, and put on a Tillotson :-)
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David Luciani
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Posted: Thu Aug 5th, 2010 01:47 am |
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wow shane,
i'm surprised i missed this thread.
would love any and all info you have.
many attempts at "fixing" the duckbill were made.
not much came of it.
dave
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Doug Tenney
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| Joined: | Sat Apr 25th, 2009 |
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Posted: Fri Jul 30th, 2010 08:39 pm |
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Shane, this is a really old post, so who knows if you'll ever see this... By accident I came across a book that seems to explain what McCulloch and Walbro were thinking. See if this makes sense:
First, in the early 60's, Walbro was desperate to get into the diaphragm carb business, but in order to do so, they'd need to skirt Tillotson's iron-clad patents. Meanwhile, Brown Carburetor was started by a former Tillotson engr, and tried hard to compete with Tillotson. (I think that Brown's only customer was Clinton, and they were constantly teetering on bankrupcy too.) Brown succeeded in getting some number of his own patents, probably in part so that he could horse-trade with Tillotson and thus avoid getting crushed by Tillotson. Eventually that is exactly what happened, and Brown went under. Walbro wisely bought the assets of Brown, (primarily, their patents), and THAT was enough to get them going in the diaphragm carb business.
Here is the info for the book. (I might be the only person who ever bought a copy!)
http://www.amazon.com/Imaging-Organization-Forrest-Walpole/dp/0738817929/ref=sr_1_5?ie=UTF8&s=books&qid=1280522002&sr=8-5
OK... I have a hunch... and I may be wrong... that the duckbill circuit was created to skirt one or more of Tillotson's patents.
Steve O'Hara has given eloquent explanations of how the duckbill circuit works; see http://macbobaust.com/steve1.html
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Shane Cash
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| Joined: | Tue Nov 3rd, 2009 |
| Location: | Deshler, Ohio USA |
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Posted: Thu Nov 12th, 2009 06:25 am |
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Hello, first I would like to begin by sharing a little info. I myself am a mechanical engineer. I have many hobbies including guns, muscle cars and even CB and HAM radio. I prefer antiques and vintages over anything at all. I am also a dog trainer who specializes in gaurd, police and search and rescue I have a modest sized kennel and I breed Dobermans, German Shepherd Dogs and my favorite Belgian Malinois. I believe that I get most of my will to learn more from my Grandpap, and quite truethfuly this post is more about him than anything.
My Grandpap was an engineer for McCulloch Motors and his hands were involved in many of McCullochs designs, he was the go to guy. I would say my Grandpap was a genius, but he would quitely dissagree and say he was just a learned man. In truth he was obsessed with learning, he spoke six languages and constantly studying everything from mechanics, electrical all the way to quantum physics. After his death I was given his books and all his notes and as I was going through his things I realized he kepy absolutely everything. More importantly to the karting world he kept what looks like every note, idea and designs he had made while at McCulloch. See my Grandpap use to carry a pen around absolutely every where he went and was constantly jotting down ideas no matter where he went even at church. He use to call his shiny silver pen King Arthurs Sword and believed that there was always an answer to every problem. His notes are written on everything from paper, napkins and even a shirt sleeve. I am trying to go through all his works typing everything and am more than willing to share what I have with everyone who may be interested.
As of right now I came across something very interesting and this brings me to the what is known as the "Dreaded BDC Carb" and the Duckbill Valve. I myself know very little about Karts but am realy digging into them now so this info I would like to share comes from a dozen pages of his research. I have scoured the net looking for an answer but have came up empty so I felt this would be the best place to ask. I would like to know if anyone has modified or have seen a modified carb to replace the duckbill circuit? The reason I had asked was because in his research it looks as if he has figured out a way to change the circuit and replace the duckbill with a different valve. Grandpap was a hands on person but unfortunetly my step Grandma had a sale shortly before his death and sold off almost all of his modified McCulloch parts, engines and saws. So I started looking through the handful of parts that my father and I have I even pulled the carb off of his wildly moddified Super 101 converted into a chainsaw with no luck. My father called me a couple days later to ask me if I checked my kart, Grandpap had gave my father the carb on it. I felt it was a long shot but was excited to find that it was indeed modified and as far as I can tell exactly as his notes described. Now I have been running this kart hard durring the last couple of weeks and it runs like a mad dog, on my super smooth asphalt drive the kart will break loose at 20 to 25 mph sliding sideways. On the road it will snap the front wheels off the ground and pull hard up to atleast 50 mph after words I have checked the plug to find the motor is not running lean or rich. I have a few other BDC carbs and I am going to try to copy his modified carb and notes then I will begin testing it out too. More than anything before I do all this I just wanted to know if any one else has managed to do this or is this a first?
Anyhow I apoligize for the long post I just thought that I should share this info that way in the future as I start posting other findings you will remember who was truely responsible for the work. Thanks, Shane
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