this is my...boomstick!
Tuning Jet kits
So, You Want To Tune Your Jet Kit, Hmm?
Wow, where to start. It seems daunting. Tuning this new jet kit you just installed. Don't worry; if you take a systematic approach and take good notes, you can do this.
A disclaimer first: If you don't have access to a dyno facility with a qualified operator, you may be doing some testing on the street. I don't advocate this and am not responsible for the trouble you get yourself into, if you choose to do that.
You've obviously started tuning your jet kit by following the manufacurer's baseline recommendations for your bike.
It's possible your bike may run really well on the manufacturer's baselines but, in most cases something will not be as you like it.
If you're not happy, here's where to start. First, create a table with these columns: Main Jet, Pilot Jet, Pilot Screw Position, Float Height, Needle Clip Position.
Then, note down the existing setup you are starting with.
First, you want to make sure you have the right main jet in there. The jet that gives you the highest top speed, WOT, in a gear is the right one. Also note if the bike runs harder, WOT, at low engine temps and softens as temps rise. This will indicate you are too rich. The opposite is true if it's slower when cooler than when hot. Make your changes one jet size at a time, maintaining the stagger the stock jets had. Main jet stagger is the richening of the inner two cylinders on an inline 4 or the rear cylinder on a twin. This is to help those hotter cylinders stay cooler.
Once you have the right jet in place, the rest follows from that so, make sure it's right. Let's move to the opposite end of the running spectrum and work on the pilot jets and screws. Some jet kits come with pilot jets, some don't. There is a bit of "play it by ear" here. Usually, if the manufacturer wants a new pilot jet installed, it's ok to do it. If the bike is exceptionally soggy off the bottom and adjusting the pilot screws does little or nothing, go back to the stock pilot jets, which will be leaner, in most cases.
Pilot screw position affects the engine throughout it's operating range. Blip the throttle LIGHTLY and watch it's response. If it hangs up before dropping to the factory idle specification, screw the pilot screws in, in 1/4 turn increments, until it drops straight to the set idle speed. If it drops below set idle and struggles back up, turn the screws out by 1/4s, until it goes away. Take complete notes of everything you do on the chart you made.
Let's move on to the midrange, now that we have a handle on the low end and WOT. The midrange is between 4000 and 7000 rpm on your average sport bike. Needle height is controlled by clip position on the needle. Raing the needle (lowering the clip) richens the midrange transition by introducing fuel sooner. This can eliminate the dreaded, and very common, 4-5000 rpm hole that many bikes have. The reason the hole is there is because manufaturers delay the rise of the needle as long as they can to meet emissions regulations in other nations.
Lowering the needle (raising the clip) will lean the transition to the needle/emulsion tube enrichment by delaying the entry of fuel in the midrange. Lowering the needle is typically not what is required as the needle taper on jet kits is more accurately machined and less radical. Usually, a little fattening is required, if the bike is flat when the throttle is opened 1/2 way at 4-5000 rpm, especially if it runs strong below and above that. Once again add clear notes to your chart to document your changes.
Next, let's move on to the last and most irritating aspect of tuning a jet kit: Float height setting. Float heights are adjusted as per the instructions in my previous article. Float height will tend to affect low rpm/high load operation because typically, float heights are too high in stock position. Low speed/high load running (WOT, 2500-3500 rpm) will be richened by high float heights with no corresponding high rpm benefit. Run the lowest float height you can without affecting high rpm, WOT operation. Once again, keep clear notes of where you are. Lower it in 1 mm increments until the performance window above clears and runs best.
Lastly, wait for your engine to cool, pull your sparkplugs and give them a good clean and gap before riding. That is, assuming they are low mileage examples to start with. If they are more than 10,000 km examples, it's probably a good idea to change them before you tune the kit, to eliminate any possibilty of tuning around a slight misfire.
When you pull the plugs, look at them. I mean, really look at them. Compare them with any number of charts in your manual or online for what plug colour means what. The plugs are your point men for trouble and will tell you alot, if you listen to them.
I know it sounds pretty time consuming, especially if you have to yank the carbs for every change, but, the results are worth it in every corner or straightaway you encounter while you ride that bike.
Crotch, thanks for posting these carb help tutorials. You've cleared up many questions I had, and I'm sure I wasn't alone in the belief that carbs were voodoo, plain and simple.
Now I can't wait to blow a weekend or six tuning my jet kit.
My fiancee hates you.
this is my...boomstick!
Excellent...my demonic carb kit relationship destroyer seems to be working just as plannned, muhahahahahahahahaha...Please sign this contract in blood.
Sure, I'll just... crap, looks like my blood pen is empty.
Sorry about that. Gotta go drain another chicken.