Replacement carburetter & reed cage experiments?

[John Miller]

Has anyone personally experimented or read any articles/web-sites on adapting a more modern reed cage plus a Mikuni or Keihin type carb to one of our McCulloch, West-Bend, etc. type engines?

I am very interested in hearing about anyones experiments & the results. Thanks.

[ted johnson]

John, as far as the reed goes, this is vintage, so why bother, we're not racing? You can buy the modern reed cage for the 820 from U.S. Motor Power, or maybe from Dave Bonbright. You'd need to build your own for a Mac. If you're talking about a modern slide carb, again, why bother, as it wouldn't be legal at T.B.O. anyhow. If you're talking about the diaphragm Mikunis, they came out early enough to be legal (1972), and I run alky versions almost exclusively. They're easy to tune, if you get your fuel tank properly vented (as I did not last year...). Good luck! T.J.

[Rob Voska]

http://www.rearenginekarts.com/tilly%20on%20mac.htm

[Dennis Turk]

Hi All

Not sure how a motor cycle carb will run on a cart given the corner G forces on the fuel in the float bowl but I can tell you that they work marvelously on mini bike engines. I have been using Honda carbs on small four stroke motors for over 40 years now and I am always amazed at how wonderfully they run with a good carb. The very first one I installed was a Honda scrambler 250 carb on a Wisconsin rototiller motor back in 1965. The last one we did was on a Clinton two stroke chain saw motor converted to an E-65 for mini bike use. Runs like a champ. Always starts on the first pull and will idle at less than half the speed of a stock carb. Some times a little jet work is needed but usually the adjusting of the needle up or down takes care of tuning. One nice thing is you don't have to mess with trying to hook a throttle cable up to a lawn mower motor carburetor.

Some of you guys that were at the Medford Oregon race this last September seen a Bonanza chopper that ran like a watch. That little 1940's Brigs had a stock Honda Z 50 carb on it and one of you guys young son got a chance to ride it.

Turk

[John Miller]

I realized before posting, that my question may not be welcome as far as maintaining vintage accuracy. I had searched the forum already & found that the few rare instances that any questions remotely like mine came up. They had mostly remained unanswered. I hesitated several times before I finally posted. The problem is, that the guys who are most likely experienced enough to answer the question with real information, are on this board.

If you guys hang out on some other forum that would be more suitable. Please let me know & I will move my questions to that location.

Much like most of you, (I assume) I have several various old 2-stroke (and 4-stroke) engines in various states of tune & running (or not) condition. Most of the older engines biggest limitation is the carburetion. Most of them were designed to only really perform 2 functions well. Provide a decent idle & to feed the engine a fairly correct mix of fuel & air at the designed steady state operational speed. With the 4-strokes, that is usually 3600 RPM with a governor designed to provide an approximate 400 or so RPM high/low swing to provide for generator load swings. Same with the 2-strokes intended for generators. The chainsaw type applications are much the same. Provide a good idle & the correct fuel/air ratio at "WOT" full throttle. Plus the chainsaw carb has to be able to perform these functions at any angle or even upside down.

O.K. So enough with the background information that I’m sure most of you are more knowledgeable about than myself anyway.

What area of motorsports can we draw from that has had extensive & ongoing research & development on small engine carburetion & intake tract design? Motorcycles, Snowmobiles, Jet-Ski’s & Sea-Doo’s. These carbs need to not only provide a good idle & the proper WOT operation, but more importantly, they must also provide for proper mid-range operation as that is where the majority of these engines operate.

So, if someone was more interested in trying to provide the optimum in fairly stable & maintenance free carburetion for his old Mac to play with in his kart or minibike vs. vintage authenticity, what are the possibilities? I’m thinking that the Honda CR type bikes & the Yamaha Banshee’s may have some of the finest designed reed cage assemblies due to their long time dominance in their perspective forms of racing. Both the Mikuni & Keihin carbs are undeniably “Good Carbs”. Has anyone experimented along these lines & if so, what were the results? Thanks.

[Rob Voska]

That's a good question but I doubt if you get much help as few have expierence with them.
Karts run diaphram carbs almost 100%. Some early karts had float bowl carbs.

I put a float on my minibike with a MAC 91. Ran bout the same as the diphram carb I ended up with on it.

Shifter karts run float carbs.......sorta. Most run a pump-around system that puts fuel in one side and has a pump sucking fuel out the other side trying to maintain a fuel level in the float bowl that always keeps the main jet covered 100%.

Karts run pretty much wide open. Idle is not a priority. In fact I never run idle in any of my engines. The only time they are really less than WOT is at start up or line up. We don't need driveability like a motorcycle. We are always trying to find more power and it's not very often we worry about anything else.

[Chris Vary]

Its hard for use new guys to get all the vintage goodies so we can play too. If we can come up with a new part that will help out I don't think that we should be looked down on. I have had to have parts made on a C.N.C. machine to make my vintage power products engines run. Remember the new guy are what will keep this sport alive.

[steveohara]

John,

Truth be told, there is really nothing "new" in the way of reed cages today compared to the vintage karting era that most define as ending 1975. If you were to look at the reed cages in the very best european kart motors made today you would find that they are virtually identical to the reed cages used in some Italian motors from the mid 70s. The same design was used by Hartman in their aftermarket dual carb manifolds for the Mac kart engine. Looking at the motorcycle side, thier cages more closely resemble some of the early Mac cages as you see in the Mac40/70 manifolds. One neat thing about early karting was the variety of ideas that were explored and reed cages and intakes were one of the areas where the creativity and variations were greatest. If you're interested in the subject there is no need to look to "modern" parts from other uses.... vintage karting has a huge array of designs to experiment with and you won't hear anyone fuss over the use of "non vintage" parts. Regarding carbs... again, early karting has a lot to offer. The very earliest european imports came with Dellorto float type carbs and there are dozens of models of Tillotsens, Mikunis and Mac carbs to play with. Some have no pump, some have single pumps and others have double pumps built in. There are many variations in venturi shape, fuel delivery nozzle design and mixture control arrangements. The later model Mac "BDC" carbs are some of the least undestood and most impressive of the lot. They feature design elements that are not duplicated in any other brand and those features offer the knowledgable tuner the abilty to manipulate the fuel delivery curve in way not possible in the other brands. Used and setup properly, the BDC genre of carbs offered on the Mac engine can open the door to variations in pipe design and engine setup that produce more power and greater reliability that simply won't work with Mikuni or Tilly carbs.
In short.... no need to go shopping for "modern" bits when the vintage karting era produced more variety and creativity than the years since.
Regards,
Steve O'Hara

[mcbob]

All we need now Steve is a better source of diaphrams for the BDC and we're back in business.

Mc Bob.

[steveohara]

Bob,

So true.... I just rebuilt a BDC 24 for a fellow vintage karter here in California and the kit he sent me was pretty marginal. The gasket quality was nothing like the originals and the circuit plate gaskets simply did not fit.
G'day old feller,
Steve