Jets

I am new to motorcycles and I need to know how you figure out what jetsI need in my bike. I know I have after market exhaust, but I dont know what it is!? It is a 4 into 2. I bought a set of rebuilt carbs off ebay. The guy does great work but it has stock jets in it. I checked the old carbs but dont know if the jets in it are stock or not. It looks like they say 102.5 but I cant tell at this point with out cleaning them up more.

how does it run with those carbs and pipe set-up.that would be my first question. if it run's decent don't worry about jetting.

If your pipes have baffles and you're running stock airbox you likely don't need to rejet and stock jetting will be perfect for you. Stock jetting is 102.5 main, 15 pilot. That's likely what in the rebuilt carbs you bought. If you have no baffles in your pipes you'll likely want to bump up a jet size or two on your main, to either 105 or 107.5. If you're running pods you'll need to go up a few more sizes on the main, plus a size on the pilot, to 17.5 pilot and probably 112.5 or 115 main (depending on if the pipe is baffled and what elevation you're at).

Hope that helps!

It's not running right. It runs OK, but I have a very slight surge when at a steady throtle

what kind of rpm is the surge happening in.

7879KZ650S wrote:

It's not running right. It runs OK, but I have a very slight surge when at a steady throtle

that would be a lean mixture
try 107.5 mains,17.5 pilots and raise the needle 1 groove

Thanks, I knew it was lean, just wasnt sure what to do about it,as in what size jet it should have. Now, where can you purchace jets? And if I change the cam will it also affect the jetting? I have a st of Andrews #10 grind I plan on installing soon

You can order the jets you need from z1eterprises.com. And yeah, I'd imagine you'll need to rejet again after cams, not sure to what point though.

At that point would it be a matter of experimenting? or might there a formula for figuring this out. Or just keep asking questions til I find some one who has done one simular?

Nebr_Rex wrote:

7879KZ650S wrote:

It's not running right. It runs OK, but I have a very slight surge when at a steady throtle

that would be a lean mixture
try 107.5 mains,17.5 pilots and raise the needle 1 groove

Whoa! If its lean at one point in the throttle circuit, don't change the entire fuel map.

Start with one circuit at a time. Fuel delivery/circuit/jet used is entirely dependant on throttle position.
If the bike is rideable, start by tuning the Main Jets.
Get the bike up to 4th gear early, then full throttle it till a couple thousand before redline.
Do it a couple times to really know how it "feels". then change out your jets for a leaner set (smaller number).
If the bike improves, continue to install smaller jets till there is no mor improvement. Then go back one set of jets.
It works the same in reverse... go larger jets if it improves, then back off one set when no more improvement.

Next, do the needle clip position. Same process, except at half throttle. Move clip up or down, and feel for improvement. Keep position that feels best.

Next, is Pilot Jets. Adjust Mixture screws to stock positions, approx 1.5 turns out form lightly seated.
Same process with selecting jets, except you choose the one that gives you the best launch/off idle snap up to 4000rpm, from stand still. Only using about 1/8th to 1/4 throttle!

Then adjust your mixture screws with a really good tach or vacuum guage.

YES, its going to take some time and cash, and many jets to get it right. When you do, your bike will be a rocket!

Make sure float/fuel levels are correct and NO AIR LEAKS.

Good Luck!

LOL YEah, its a lot cheaper and easier rejetting a holley 4 barrel, maybe I'll design a manifold to use one of them!LOL

7879KZ650S wrote:

LOL YEah, its a lot cheaper and easier rejetting a holley 4 barrel, maybe I'll design a manifold to use one of them!LOL

It cost's nothing to adjust the needle position (just time), and I think that's where your problem is...

It'll be worth it... trust me!