For Pops and the others reading this forum -
The carbs on Sam's bike started as GSXR 34mm OEM CV units. You add a Dynajet stage 3 jetting kit to get them working well on your cb750's. The bore spacing is too wide, so you disassemble them to allow machining on the carb bodies to allow the linkages to be brought closer together. Reweld the angle iron mounting pieces to reflect the closer spacing. The biggest challenge is the center linkage where the carb cables go. I ended up just using one cable (pull) with an added spring from the linkage arm to an available hole on the block to ensure that the carbs close quickly when you come off of the gas. You end up with near stock jetting*. If you get the jetting right, when it gets colder than normal, you slow down.(too lean) Most people leave the jetting richer to prevent the need for rejetting to conditions.
*Racers use electronic gauges that take in the temperature, humidity, and barometric pressure readings. That information is converted over to a density altitude reading (in feet above/below sea level) As everyone knows, the higher you go, the less dense the air is. The density altitude value is the theoretical distance above sea level that you are if it were standard temperature, humidity, and barometric pressure. To over simplify, the bigger the number, the less air is available to mix with the gas to form combustion. Since you want a set air to fuel ratio for maximum power, the higher the density altitude, the smaller the main jet you want to run. So, if you run your bike in Denver, Colorado (which is about 5,000' above sea level) you will use a smaller main jet than you would at sea level. You can see 2,000' changes in density altitude on a day that starts cool and gets warm in the afternoon. I'm not going to get into how water grains can displace oxygen molecules, altering your jetting changes. Most racers know all this stuff, but a lot of bracket racers just jet the bike rich and leave it there. I set up the carb jetting for about a 2,000' density altitude, which is roughly correct for the southern drag strips that Sam is running this spring. You jetting will change, depending on what the density altitude is where you race. A good test of your carbs setup is to run a gear from say 1,500 rpm to redline with the throttle wide open. If the bike doesn't cough, spit back, or stop accelerating at any rpm, you've got the jetting in the right ballpark. Sam can launch his bike in high gear and run it down the track without any problems. Adapting carbs from one bike to the next can be very challenging. Milder street bikes can use the Seca 32mm oem carbs that I'm told are a bolt-in. I haven't personally tried that yet, but have read of others who have used them. They fit the stock intake rubber manifolds. The 34mm CV carbs work better with aftermarket intake boots. There's a guy periodically on Ebay with aftermarket "K" head intake manifolds from Germany. They are the ones that I prefer. (also cheaper than the OEM ones that are getting pricey)
I've offered to help Pops and any other Hondamatic racer. I started running Hondamatics in 1985 in bracket racing after losing to one in 1984. There still is nothing better out there for bracket racing. That's why they are so universally disliked. Richard has too many stories of how Hondamatics are discriminated against at various tracks in the US. I've had my Hondamatics banned at my local track for the last 7 years. Why? Because I won too much. They are afraid that I'll discourage competition! I'm still watching the Dragbike.com forums and this one. There are so many helpful people here for people to use. I'm thankful to Sam and others who let me post here!
Jon Weeks
