Carb Tuning 101
You must first understand the three main circuits of a typical carb.
Main- This is the master control of the fuel system. This is tuned at wide open throttle (WOT), above about 6k rpm (for any 10k motor). Keep in mind that CV carbs are vacuum operated, so WOT at the wrist is not always WOT at the slide. You open the butterfly and vacuum opens the slide. Typically, you will only adjust the main jet, though it is also affected by other elements, such as air jets and needle taper. Air jets are usually only modified when there is a big change in displacement, cam profile or overall operating range (max rpm). Needles are the primary driver in mid-range tuning but can have a miniscule affect on main jet flow because the needle never lifts completely out of the main jet passage, therefore a thin needle allows more fuel to flow than a fat one at wide open throttle. It’s a very minor thing, so just be aware.
Mid-range- This is the heart of your carb tuning because the vast majority of your driving is here. It is the part throttle range, usually between 2k-3k and 6k rpm. It is managed with a fuel curve, generated by the taper of the needle versus the main jet. Timing of the curve is a function of how high the needle is and the ramp of the needle taper.
Idle- This is the range between starting and the first movement of the slide, usually around 2k-3k rpm. Above 3k and the idle mixture has almost no impact on mid-range tune. Pilot jets also play a role, but would normally be unchanged unless you have a big change as outlined above for the main jet. Mixture is adjusted by screws. On some carbs, the screws are air screws (turn them out to lean), on others, the screws are fuel screws (turn them out for enrichment). In either case, a starting point of 1.5 to 2.5 turns is normal. If you get very far off this, you have a clogged passage or need to change pilot jets.
IMPORTANT: You will have a lot less hassle tuning a motor that is well sorted. Everything is vacuum dependent, so carbs out of sync will be a disadvantage. Carbs are not easily synchronized if valves need adjustment. It all comes down to regular vacuum pulses. If you have a noisy valve-train when cold, or had trouble synchronizing your carbs, start by performing a valve adjustment. Be prepared to revisit carb synchronization after a first pass of jetting. It should be the last thing you do, as well as the first.
Tuning order: The main jet affects everything else, so it must be dialed in first. The idle circuit dovetails with the bottom of the mid-range tune, so it comes last. Perform all tuning with air cleaners and exhaust baffles installed as desired. Leaving either off will alter your results.
Main jet: If we all had a dyno and A/F gauge, we probably wouldn’t need this write up, so let’s keep it old school. You want to read mixture at WOT, under a load. This means getting out on the road (closed course or desolate highway, be careful, yada yada). Pack your tools, spare jets, plugs, etc and bring your lunch. You need to achieve WOT, above 6k rpm for 30-60 seconds to color a plug. That eats up a lot of road. Watch out. Third gear is good. You also don’t want any part throttle operation after coloring the plugs, so conclude a run by hitting the kill switch and pulling in the clutch. Coast off the road and park where you can pull a plug. There are great articles on reading plugs elsewhere on this forum and the internet, so I’ll skip that. When you see how rich or lean it is on top, make a jet change in the desired direction and repeat the process to read the plugs. Don’t make drastic changes, and take notes. Camera phones are great for this. When you start getting close, be sure you look at all your plugs. Keep in mind that a flooding condition from a stuck float needle, or a vacuum leak on a manifold can ruin your results. If you make a huge change in the main jet, it may require compensation in the mid-range if you put in or take out enough fuel to adversely affect the mid-range. After dialing in the main jets, you should have the right coloration on the plugs when you make a run as described above. Make a longer run to verify. When satisfied, move to the midrange.
Mid-range: You are probably in the ball park if you followed the instructions of whoever provided your jet kit, though We used to piece it all together ourselves. The process here is much as for the main jets, but now you will adjust fuel with a change in needle taper (thinner = richer) and/or height (shim or clip to raise needle for richer). Go out and do your plug
Readings again, just as you did for the main jet, but this time hold your rpm at about 3500. This should give you a basic mixture read by plug color. Again, remember to cut the motor, pull in the clutch lever and coast to a stop. It would be surprising to be very far off, and then you would make a needle change of major clip change. Now do some acceleration testing, eyes focused on the tach. Look and listen for the flat spot. If it’s big and wide, you probably need a different needle, perhaps an air jet. Your kit instructions are helpful here. Experiment. Once close, move the needle up and down a couple of notches, and test again. When it starts feeling good, you are close. Some kits include diaphragm springs, too. These might allow for a snappy acceleration, but can also be too much. Some kits include a drill bit to open the slide lift hole to match the springs. Follow the kit instructions carefully. If you want to un-do a drilled hole, take it home and break out the JB Weld. When you get the mid-range dialed in, head for the house. You’ll need vacuum gauges (manometer) for the idle tune.
Idle: As mentioned above, the base setting on your idle mixture screws is just a starting point. Regardless of whether you have air or fuel screws, the process is the same. Hook up your gauges to all four cylinders and get the best sync you can. Don’t try to figure out the average reading with a bouncing needle or fluid. Look at your peaks. Idle the engine down as low as you can, and tweak, idle down, and tweak. When it seems you can go no lower, adjust all four mixture screws in ¼ to ½ turn increments, keeping them equal to one another. What you want here is to adjust for highest idle/highest vacuum (they should be the same). Idle the engine down to spec or a little lower, the tweak for highest idle/vac, repeat. Once you have found the sweet spot, you can blip the throttle to check your results. Set the idle speed at spec, then give the throttle a good tug (2k rpm or so). Watch the tach carefully. If you blip and the idle comes down slowly, you are still rich at idle. If it comes down below idle speed and then recovers rpm, you are lean. The right tune will blip up and quickly back to idle speed. You’ve heard it on race bikes. Crisp. Watch the tach.
Now look at all your gauges and make sure you like your carb sync. Pull the gauges, assemble the loose pieces and go ride! Remember to make some more plug readings when the weather changes. If it’s hot and humid, expect rich. If it’s cold and dry, expect lean (this should feel fast!) If you see undue heating on the temp gauge, you were probably on the lean end and are getting into trouble. Plan on a minor bump up in the main jet. Keep your operating rpm in mind when reading plugs. Realize that the carb is not perfect. It will have rich and lean peaks. You’ll kill yourself trying to fix them all. Instead, aim for smooth transitions across the power band. Fix stumbles and flat spots. If you identify one, note the rpm at which it happens. This will tell you what circuit you need to look at, and you can focus a plug reading here if it’s significant. It will probably only need a minor change on the needle clip position or shims.
Good luck!
