A couple jetting questions for Andrew...
1) If I jet a bike for sea level, then bring it here to 5000 ft above sea level, it'll be running rich, right?
Correct. Less air = more fuel.
2) For a Keihin Carb (or any other for that matter), a smaller # jet means it's more lean, right? So going from a 50 pilot to a 40 pilot, you lean out the mixture, right? The smaller number means a smaller hole for the gas to pass through, right?
Correct. Whether it is main jet or pilot the smaller the number the leaner the jet
Remember there is no (or should not be) guess work in jetting. It can be very time consuming and every bike is unique which requires unique jetting no two are ever the same. Always start very rich and work your way down. By doing that you'll quickly find the bike will tell you what it is doing and what it needs. Erratic/sputter/miss/cutting out =
lean. Blubber/smoke/loading up/sometimes cutting out =
rich. If you're not sure try looking at the exhaust lots of smoke = rich. No smoke = lean on a 2T.
I like to start with the bottom end (pilot/fuel screw) jetting then work on the midrange jetting (needle) and lastly the top end jetting (main)
Tuning the pilot jet is easy. If I'm starting from scratch I install the factory recommended jet for this altitude and set the fuel screw at 1.5 turns out and go from there. Get the bike
completely hot and up to operating temperature not just a burn around the block. Does it idle at a nice low and steady RPM?
-If so: Turn the fuel screw clockwise very slowly. If you turn it in and it dies turn it back out to where it was and leave it alone. If you can turn it all the way and the bike does not die replace pilot jet with one size leaner then repeat. Ultimately you want the engine to die at .5-.75 turns out once you get it near that point back it out ccw until you get a nice smooth idle and call it good.
-If not or if it is a very erratic idle: Pull the choke. Does it idle now? If so turn the fuel screw counterclockwise no further than 3 complete turns out. Does it idle with the choke off now? If not add one size richer jet and repeat.
The fuel screw fine tunes the pilot jet. It is only efficient between .75 and 3 turns out. The pilot/air screw jetting is for idle thru 1/4 throttle. Old gas especially ethanol blend easily plugs this circuit in as little as a month from what I have seen. Don't attempt to jet unless your certain your carb and air filter are clean and gas is fresh.
Use a magic marker and write your settings on the swing arm, engine case or something that will always be with the bike like this for example:
Pilot- 52
A/S 1.5 (air screw 1.5 turns out)
Needle 1348NS / 3rd (needle number and clip position from top)
Main-155
Checking your spark plug will tell you a lot of things however the bike will need to be ran for a long period of time to get a good reliable color. Installing a new plug and riding around the block is not a good way to read a plug. Fwiw I rarely if ever check my plug when jetting.
Jetting 4T's are no different although they have a few more jets and accelerator pumps to tune in. Whoever said 4T's are less susceptible to jetting was high :brett:
Does this help at all or is it confusing? I could show someone in person easily but have a very difficult time trying to explain through typing. Anyway I'll try to break down needles and main jetting when I get some time..
