FOR THE LEAD
"Trigger" your way to the Front
If you've been following the excellent articles on the Walbro WB3A carburetor by David E. Close (NKN July 1997 & August 1997) you know that this carb, standard issue on Yamahas and other Piston Port 2 cycles, delivers a relatively constant supply of fuel in the upper RPM ranges. Unfortunately, your engine requires a steadily decreasing fuel supply as RPMs increase through the 13,000 to 14,000 range and beyond. At the upper end of the RPM range, the workload on the engine diminishes and, among other changes, this lessens it's need for fuel. On top end, the WB3A just delivers too much fuel for peak performance. For years savvy tuners and drivers have worked around this by constantly adjusting the hi-speed needle, richening the mixture in the corners and leaning it down on the straights. You've probably seen drivers reaching across to tune the carb, corner after corner, lap after lap.
This has raised some questions, both from a driver concentration perspective, and as a matter of safety. Finding a way to allow the driver to tune for maximum performance while keeping both hands on the steering wheel led the clever folks at RR Racing (Reggie, Randy, and Rick Fulks) from Jacksonville, Illinois to develop what they call the Kwik Set Carb Adjuster. Most racers just call it the "Carb Trigger." Let's take a look at it and how it mounts.
You can install the Kwik Set with the carb installed on the engine. but it's a lot easier with the carb off. While you're at it, slip the pulse hose off the back plate of the carb too. Now begin by removing both the hi-speed and low-speed adjusting needles. You'll be replacing the hi-speed needle with a modified one that comes with the Kwik Set kit. You'll need to make sure this new hi-speed needle turns easily, so first we'll need to reduce the spring tension on it. Try taking the hi-speed needle spring and compressing it fully with a pair of pliers. Now slip it on the needle and screw it in finger tight. If it's still hard to turn, remove it and hold it in a direct flame until it's cherry red. You can use a propane torch for this, but a cigarette lighter or even a match will work. Let the spring cool back down and install it again. This flame annealing of the spring should make it quite soft. Don't dunk it in water to cool it or it will become brittle and it'll just snap. Just let it air cool. You can also get some softer springs ready made for this application from Two Cycle Technology in Wisconsin. Anyway, once the spring is handled, screw the modified hi-speed needle in finger tight. Don't forget to be sure the packing o-ring and the little brass washer are in place first.
Before you go any further, here's a little trick the guys at RRR missed. Use a very thin cutting tool or saw (I like the cut-off wheel in my Dremel Tool kit) to cut a thin screwdriver slot in across the blunt end of the modified hi-speed needle. This will make it a lot easier to adjust the needle position, both when you install the Kwik Set trigger, and later if you need to adjust the trigger adjustment range.
Now it's time to slip the Kwik Set pulley over the modified hi-speed needle. You want the little clip that holds the spring and the string on the pulley in the 3 o'clock position and the machined slot curving around the bottom. Properly installed, the pulley should look like it's smiling at you. Slip it as far down the needle as you can without it interfering with the throttle arm on the throttle shaft as the throttle moves through it's full range of motion, fully closed to wide open. Now lightly snug down the set screw securing the pulley to the modified needle. Grasp the pulley and check how easily is turns. If it takes any effort at all, go back and work on the spring some more. It needs to turn quite freely for the trigger to work properly.
Once the pulley is all set, screw the Flex-T low-speed needle back in through the "smile" slot in the pulley. You may find you need to trim some of the plastic coating off the shaft of the Flex-T to get it to turn smoothly. Again make sure the pulley turns freely. Now it's time to remove the back (pumper side) plate from the carb body. This is also a good time to check the condition of the pumper diaphram and to clean the fuel inlet screen.Toss the 4 screws you remove in the old parts bin and mount the Kwik Set backing plate onto the carb back plate using the longer screws provided. I like to fit 4 little washers into the recesses in the carb back plate to get the carb trigger back plate to rest flat and pull down evenly. Properly installed, the brass compression fitting should be lined up directly with the 3 o'clock side of the pulley. Snug the 4 bolts up and thread the string through the brass compression fitting on the carb trigger back plate.
When the first carb triggers came out there was a problem with the return spring tension pulling against one side of the needle and chewing up the threads in the carb body. The Kwik Set unit now features it's own little 3rd bearing support, a piece of self-lubricating plastic mounted to the back plate that has a hole that fits over the end of the hi-speed needle to support part of the spring load. Set that up next, being careful to make sure the needle continues to turn freely once the support leg is in place. Don't let it "lean" against the side of the needle as you tighten it up or it will bind the needle.
It's time now for the final initial adjustment of the needle position. Determine how far you want the hi-speed needle to be closed when the trigger is fully pulled, then loosen the set screw on the pulley and, holding the pulley in it's fully clockwise position with the Flex-T as the stop, use a little screwdriver in the slot you made to first close the needle completely, then back it off the desired amount. (I told you that slot would come in handy.) In general, I'd set the hi-speed needle to be between 1/4 and 1/8 open when the trigger is fully pulled to start with. Experience will show you if you want it richer or leaner as time goes on. Now retighten the set screw and you're ready to hook up the return spring.
As you rotate the pulley counterclockwise, the string should feed onto the pulley and the spring attached should come into position over the 1/4" hole in the backing plate. Feed the spring through the hole and take the little square tin retainer and catch the slot in it in the spring about 4 or 5 coils from the end. Slide the retainer through until it stops and let it rest against the backing plate. If you pull on the string now, the hi-speed needle should close smoothly. Release the string and the needle should pop back to the richer position. If it doesn't, go back and check to find what's binding it up. It has to move smoothly to work properly. Sometimes a little more tension on the return spring is needed. Once the tension is right, bend the edge of the spring retainer over the edge of the backing plate to keep it in place.
All that's left now is to mount the handle in a spot that's convenient, mount the other compression fitting so it lines up with the handle on the steering wheel, and install the string housing plastic tube just like throttle cable housing. Be sure the handle moves freely regardless of the steering wheel position. Then thread the string through the tubing and secure it to the handle. Just like a throttle cable, you'll want it fully activated when the handle is pulled all the way to a comfortable grip. Don't let the Flex-T become the "stop" for the handle. Give it a quick final check. You should be able to pull the handle easily and smoothly to close the hi-speed needle. Releasing the handle should snap the needle back to the open position. Remember, you've given up the option of moving the hi-speed needle setting the old way, so make sure it works easily with the trigger. With the carb trigger working, you can richen the low-speed Flex-T needle to as much as 2 to 2 ½ turns open and still get the engine lean enough to "sing" down the straights.
You'll be amazed at how easy it is to develop the touch of keeping the carb mixture right where the engine wants it after just a few laps of practice. Pull it too slow and the engine will be sluggish in the upper mid-range. Pull it too fast and it will "sag" down for lack of fuel. Unless you're completely ham-fisted, you'll feel that "sag" in plenty of time to avoid sticking it by just feeding the trigger back out.
There you have it; a nearly perfect device for optimizing carb and engine performance without taking your hands off the wheel. Using it can be as simple, or as complex, as you wish. Next time we'll talk about the nuances of using the trigger to anticipate when your engine will want more fuel, using it for quick "flash-cooling" and other subtle techniques. See you then.