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(under construction only partial repost by) STEVE O'HARA MCCULLOCH TECH & TIPS
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STEVE O'HARA MCCULLOCH TECH & TIPS
 
IKF Champion Steve O'Hara (hand on carb) and his Father at Adams Kart Track Riverside CA 2005
BIG MAC CARB - OVERVIEW
The center of the carb bore at the venturi is the location where air velocity (and hence; pressure drop due to velocity) is most representative of the demands of the engine. Trying to meter fuel based solely on the velocity changes at the wall of the carb is a losing proposition. McCulloch had the design correct... and we figured it out and tuned it to perfection for the application (methanol on a go-kart).
A 2-cycle engine's fuel demands are not linear. There is an overly large demand for fuel right around peak torque (the mixture needs to be richer). Most any carburetor will simply feed a given amount of fuel for a given airflow through the venturi.The compensation circuit (i.e. duck bill)  banks on one thing: that the crankcase pressure will be highest in the range of peak torque (which it is). It simply uses that "indicator" of peak torque (actually: peak efficiency) to trick the carb into thinking that atmospheric pressure is higher than it really is. It's a brilliant design... we just didn't understand it 30 years ago
Duck Bill Operation
The "duckbill circuit" (for lack of a better name) is simply a method by which the pressure above the diaphragm can be raised slightly above atmospheric. It's not a pump at all... rather it "tricks" the carb into providing more fuel at certain points by changing the [pressure] differential above/below the diaphragm. The "jets" are simply a means to fine tune the already existing holes. Drilling out the existing holes, and then tapping them... allowed an installation of a "jet" where there was a hole in the stock carb. This made changing hole size very simple... because all that had to be done was to remove one "jet" (a drilled setscrew, to be exact) and replace it with one that had a different hole size. The "bleed" hole was in the throat of the carb, and ran at an angle into the fuel passage feeding the dumptube (high speed).
To take it a step further, the passage I called the "bleed" hole was there to progressively eliminate the effect of the "duck bill circuit" as the engine speed increased. By varying it's size we could make the effect fade away at different rpm and thereby fine tune the mixture through the low and mid range while leaving the actual air/fuel mix needles set for optimum top end power. With regard to the question about changing the taper of the needles... no, we didn't change the taper as the Mac carbs didn't actually have a "taper" on them. Rather, they had a radius on the end and they were very touchy until we fine tuned the main dump tube size and got them just right. When it was all sorted out we ran the high speed needle at around 5/8 to 3/4 turn and 1/8th of a turn would change the cylinder head temp by around 20 degrees.
Duck Bill Circuit  and Fuel Pump
On the air circuits on big MAC carbs there were two critical areas that I worked on a lot. The first one was the air bleed hole on the leading edge of the venturi which McCulloch engineered into the carb to produce a progressive "lean out" effect as revs increased. The stock configuration was intended to let the engine have plenty fuel in mid range where the loads were high and then lean out for good clean performance at high revs. As our setup evolved and the pipe design became more radical over time the needs of the engine actually reversed and we needed more fuel on top end where the motor was running the majority of the time so the air bleed circuit at the venturi was eliminated. Many people thought that it was intended to help atomize the mix like an emulsion tube in a Weber but that wasn't the intent of the orifice at all. The more complicated system to sort out was the part I will call the "duck bill circuit". When the rules allowed modification inside many of the engine builders eliminated the duck bill altogether and that was exactly the wrong thing to do. Other crazy things I saw were all sorts of silly springs under the flaps of the pumper or in the pump section itself and of course there were the carbs with giant fuel passages and ball checks in place of the inlet needle and seat! None of that stuff was effective and most made the carbs behave worse, not better. I took a different approach and in the end with one exception our carbs looked completely stock and you would only know they weren't if you had a spec sheet on the stock carb and a set of drill blanks to check the small changes made to the size of several holes. Anyway, back to the "duck bill circuit".... the purpose of the duck bill circuit was to enhance fuel delivery at low speed so that the motors would not overheat when at extreme loads. To understand the duck bill circuit you would have had to know what it was like to run a Mac engine with a big bore carb on gas because you would never understand it's effect if your only experience was using alky. Remember, the basic problem running alky was not the size of the fuel passages themselves, rather it was that the fuel pump simply could not keep up with the needs of the engine at high rpm. The result of the weak pump was that everyone turned the jets out to 2 or more turns in order to suck the last little bit of fuel through down the straights and when the engine slowed down or you were off the throttle for a few seconds the pump would catch up and the 2 plus turns would be way too rich, thus causing extensive loading up in the turns. One solution many used to help the high speed starving problem was to raise the fulcrum arm and lower the pop off pressure way down which simply made the low speed loadup even worse. Once we modified the pump to deliver more fuel at high rpm  we found that the stock dump tube size was more than adequate and we could make the engine "four cycle" down the front straight at Ontario with a high speed needle setting of around 1.25 turns. See how we modified the pump in the diagram below. Click on them for a larger view.
  
I use Marine Tex epoxy as the filler material.... it is resistant to any fuel I ever threw at it.
If I leaned it out to run clean down the straights at high speed we no longer had a load up problem at low speed. It was just the opposite, the motor would overheat making the long hard pull out of a slow corner. At first we struggled to understand why the engine would rattle or detonate pulling out of the turns when we knew it had plenty of fuel at top speed but we figured it out pretty quickly. At low speeds where the stock pump could keep up with the needs of the engine, two plus turns on the high speed needle was too rich but one turn was too lean for the low speed no matter what pump we were using. If we opened the jet to keep things under control out of the turns then we were way too rich down the straights.... just the opposite problem most with alky had but exactly the problem encountered if running straight gas. After considerable study and a lot of trial and error we finally came to understand that the "duck bill circuit" was designed to work much like an "accelerator pump". At low speeds the air flow in a big bore carb was simply to slow to "draw" the fuel efficiently and the engine would run lean since there wasn't sufficient pressure drop at the venturi to pull the fuel in. If we lowered the pop-off to help the problem then the engine would run too rich in the mid range. The duck bill circuit would create greater than atmospheric pressure on the dry side of the metering diaphragm to help push the fulcrum arm down and open the inlet needle and seat thereby simulating the effect of lowering the pop-off pressure at the lower engine speeds. 
The duck bill circuit was comprised of an inlet hole where pressure pulses from the crankcase could enter the dry side cavity of the metering diaphragm through the duck bill which was a simple one way valve, an outlet hole in the bottom of the "middle" plate which formed the dry side cavity and a pressure bleed off hole that ran from the crankcase side of the duck bill cavity to the throttle bore. 
By changing the size of the inlet, outlet and bleed off we could manipulate the degree of above atmospheric pressure effect on the metering diaphragm and the rpm at which it stopped. Once we had the sizes all sorted out we could run 16 the 18 pounds of pop off pressure and keep the fulcrum arm set so there was no leakage at idle or low speeds and that's what allowed us to have the engine idle like a chain saw or motorcycle for as long as we wanted too. The final setup was so close to stock it was hard to believe that it made as much difference as it did. The inlet hole from the case to the duck bill ended up about .005 larger than stock, the bleed off hole to the throttle bore was around .010 smaller and the exit hole out of the center plate was our fine tuning spot where we used drilled set screws as adjustable "air jets". If we were too rich out of the turns we would increase the size of the exit hole and if it was too lean we would make it smaller and increase the pressure on the metering diaphragm. We had set screws with every size hole from around .020 through .060 to play with different combinations of in and out air flow in the duck bill and venturi air bleed circuits. Once we had the sizes set we just drilled the stock carbs out or lead filled the holes and re drilled if the holes needed to be smaller. The track layout would usually dictate the setup and weather would play into it as well.

I did modify the pump to meet the needs of the engine at higher revs and that was the key to allow the fine tuning that followed. Without the improved pump output there is no way you could make the carb work properly with 100% alky. The diagram and picture show the modification.... it was very simple and effective but not really relevant to vintage karting since you could just bolt on the double pumper setup from a 101B and the problem is solved. Back in the days of the "spec" Reed classes we were allowed to modify the carbs in any way that could not be seen from the outside so the modified pump was legal but never seen by any tech person so our competitors never caught on. By making the carb able to keep up with the fuel requirements at high rpm the door was opened to develop much more aggressive pipe designs and we were able to run close to 2,000 rpm more than the competition. Needless to say, my motors sounded a lot different than the rest of them and it sure had people scratching their heads for several years!
My carbs had many other fine tuning modifications to the various passages but the most important change was the pump. Adding the pump modification to an otherwise stock carb was not a bad setup but it would not work with my special pipes without overheating problems. In all, there are about 30 changes made to turn a stock carb into one of my 100 cc versions.
Our carb setup was so dialed in that in the last two years we would set the jets in the pits at 5/8turn on the high speed and 1.5 on the low speed and we could start the motor and let it sit and idle like a finely tuned motorcycle.... no loading up, no fingers in the throat, no pinched fuel lines. Sometimes I would have my Dad start the motor before I sat down in the kart and my competitors would just stare in disbelief!
Reed Cage Assembly
The last major development was a method of re- arching the reed guards. Our re-worked guards allowed the reeds to open at only a fraction of the force that it took to open them with the stock guards. The effect was amazing. The engines didn't sound anything like anyone else's as they had a deeper, throaty sound and they made so much more power we ended up all the way back to the 4 to 1 gear ratio everyone was running except we were still at 12,900 while the competition was at 11,200. The real neat thing about it was that the engines became ultra reliable once we sorted them out and I didn't have to run them lean or hot to get the power.
Some of these tweaks really are simple stuff by today's standards but not in that era. Even today, the conventional wisdom is that reeds need to be tight and stiff but I figured out by accident that it wasn't so. I was racing at Ontario Motor Speedway in 1977 and I heard the motor make a strange noise and hesitate a bit while flat out on the front straight. It kept running and didn't lose any speed but the sound changed. At first I thought maybe the header had broken or the pipe was loose. When I got to the tight section of the course and lifted off the throttle I really noticed a big difference in the sound of the engine and the power was down exiting the corner but the engine climbed all the way back to normal rpm down the inside straight. I could tell there was a problem through the twisty parts and figured I would be able to continue until I reached the scoring chicane and when I slowed way down for the tight turn the motor died and I coasted to the pits. When I took the motor apart I found that about half of one reed petal was gone. That started me thinking about the fact that the motor was still able to produce full power on top end even with an opening of about a half inch square with no reed at all!!! In those days there were aftermarket manifolds around like the GEM V12 and the Hartman twin carb manifold that used reed cages with no guard at all.... just little metal strips to keep the screws from killing the reed material. The length of those reeds was shorter than the Mac reed if you just looked at the reed petal itself but when you looked at the affective length of the Mac reed with the metal plate it was shorter and it took more force to open. The first test was to eliminate the guards completely and just run the metal strips and we saw immediate results but the reeds would break right away. After that I started changing the arc and the position where the guard separated from the reed material until I found the best shape and adequate reed life. It is interesting to note that the reed design had a noticeable effect on the optimum pipe shape and header length as well as the jet settings on the crab. It's strange to note that two of the major discoveries I made along the way were the result of parts that broke while racing at Ontario.... the other was the points. Who knows if I would have ever paid much attention to the reeds if that event had never taken place.
The change in sound does not come from the reeds banging against the cage or anything like that, in fact I believe they hardly touch the cage at high rpm.  The change in sound is the result of the better breathing of the motor much like a 4 stroke changes in sound when a cam with more lift and duration is installed.  When a big cam is installed in a four stroke the motor usually runs poor at low rpm and the benefits of the cam are only seen when air speeds in the intake tract reach a level where inertia in the air acts to offset the problems caused by valve timing overlap. The same thing is going on with the two stroke... the modified reed cage acts like a big cam that lets the intake valve open sooner and stay open longer.  At the engine speeds we run with kart clutches the engines never really operate at "low" rpm so we don't see any of the negative effects of the longer duration if there are any.  Because the reed responds to the needs of the engine and is not mechanically set like a rotary valve or piston port, the effect is like variable valve timing so we don't see a big drop off in performance at low rpm.  I would be interested to see how a back to back comparison of one of my modified reed setups would compare with the stock setup on a direct drive kart.  We might see a bit less throttle response and less power at the very low end of the power band. However, at the top end there would be no comparison, the modified reed setup will breath much better and the engine would rev much higher
Ignition - Points
I am certain that Mac points will float at some rpm no matter what the spring tension is set at. My belief is based on the fact that a 5 degree flywheel produced a substantial performance boots and change in operating temperatures at top end over a standard flywheel no matter what point tension we ran. If the point tension was too low we would get overheating or detonation in the mid range around 10,000 rpm so we ran the point tension as high as we could make it with standard points that met the spec rules. With a standard flywheel we never had the mid range detonation but no top end either. We didn't do much dyno work in those days but on one occasion we set up a timing light to look at the timing while running and it appeared that the timing was around 30 degrees at high rpm. I never placed much confidence in the crude measurement of the timing.... at 12.000 rpm the light is pretty much looks solid. I never gave much thought to trying to understand the flux or sign wave characteristics of the Mac ignitions as it was something we were not allowed to modify under the IKF rules. One interesting experience I did have happened in 1977 at Ontario motor speedway.... I was leading a race there by a long stretch and all of a sudden I started having heating problems coming of the low speed turns. I really had to nurse the thing along by tuning the carb several times a lap.... rich for the turns, then lean it back to normal settings for the long straights. I ended up winning the race and as I headed to the pits after the race the motor died and when I came to a stop in the tech area we tried to start the motor and it would not fire. When the motor was torn down for tech I was amazed to see that there was no internal damage to the engine.... I thought the reeds might have broken off or maybe a ring land had blown off. When I got the motor back to my pit spot I started looking for another explanation and I finally found the answer when I took the flywheel off to look at the points..... the spring was broken and the points were just sitting there wide open!!! It's funny how we learn things by accident some times.... that event caused me to have a much better understanding of the impact of the static timing and the importance of the flywheel / coil relationship to performance throughout the range of rpm. I never fooled around with electronic ignitions, in fact I have never worked with any other type of ignition on a Mac engine as I was always running the "spec" classes where the engines were completely stock so everyone would have the same amount of power.... in theory
Compression Ratio - Rod & Piston
We used to run a Mac 45 with 20 to 1 compression back in 1968! That was pretty radical and it was before the expansion chambers arrived on the scene but it was not a problem with 100% alky. The important part is getting the head close to the piston and the timing advanced with a 5 degree wheel. If you don't, the motors will self destruct at high revs. With alky you need to get the flame going early to build enough combustion chamber pressure to reverse the direction of the piston. Most Mac engine failures are the result of the mass of the piston trying to keep moving toward the head while the connecting rod, wrist pin, crank etc are stopping at TDC and heading the other way. To help people understand I like to use the example of.... " a perfectly tuned motor does not need the bottom half of the big end of the connecting rod." If all is working as it should, there should always be downward pressure to keep the rod in contact with the crank. If the engines don't have enough timing or compression the combustion pressure rises to slowly and the inertia of the piston has to be reversed by the rod and crank which leads to failures. Here's a good example....a Mac 91A. The motor looked like it was almost brand new but when I took it apart I noticed a little discoloration on the big end of the rod but the bearing surface still looked nice and smooth. Just to be sure, I measured the big end and it was .003 larger in diameter in the direction from top to bottom than across the bore perpendicular to the length of the rod.... it was junk! That is exactly what happens to them before they blow up but you usually don't get to measure one because half of it is out on the track somewhere! LOL You will find the same kind of telltale wear on the wrist pins when things are not tuned right. Next time you pull a wrist pin make sure you mark the orientation before you take it out.... once it is out, look for signs of wear or heat on either end where it rides in the bearings.... if the wear or heat is on the side of the wrist pin that was facing away from the head you have a problem... the wrist pin and bearings are having to stop the upward movement of the piston. If you see wear or signs of heat on the side of the wrist pin facing the head, chances are you are getting some detonation or pre-ignition and it will lead to ring land failure or a hole in the piston. If your wrist pin shows no wear or signs of heat after an hour or two of hard running it is a good sign that your motor is set up pretty well and should last a long time
Exhaust & Rear Axle
My National winner chassis with one of my custom built pipes.... notice the large aluminum block around the torque tube.... it was insulated from the chassis by dense rubber so there was no third bearing on the axle to cause more drag. Also note the large diameter and steep cone angles on the pipe and how short the header was. The header measured only 5.5" which was about 2" shorter than any of my competitors ran. When the tried the copy it they burned down motors in a hurry.
 
Piston Pin Removal 
Here is a pictorial sequence of the process
 
 
 
101 Model
91 Model 
These are examples of factory 91 and 101 pullers. See a Small Engines Service Manual for diagrams on how to use them or see these factory 101 instructions by clicking HERE.
The  101 version is for pistons 2.280 - 2.310 and the 91 version is for 2.165 - 2.195 pistons.

Dave L.
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picture issues again,         never mind....
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