Rejetting your Yamaha YFZ450

[quadboy-55]

heres a little how-to i decided to write up about jetting youre YFZ450. the model i preformed it on was a stock 2007 model. the kit i used was the Dynojet stage two kit but other kits are very similar and can be done exactly the same on your YFZ, and almost every FCR carburated machine.

first off, start with a clean quad, because you never want to drop dirt into the carburator.

turn off youre gas. its located right under the tank on youre left side of the quad as though you where sitting on it.

[quadboy-55]

remove youre gas tank plastic and left radiator shroud. then undo the bolt holding the gas tank in place. (all the bolts are 10mm)

[quadboy-55]

the shroud is held on with two 10mm bolts and one screw, also holding the tank plastic on.

[quadboy-55]

with the gas tank free to move, pull the whole tank back, to release it from two washers, then pull it up. you can either disconnect it entirely or turn it 90 degrees like i did to gain better access to the carburator.

[quadboy-55]

looking at the carburator, loosten the two hose clamps on either side of the carb, and pull off the tranny vent hose (grey with the coil around it, leading towards youre airbox) and cover the hole in the tranny with a piece of tape.

(left side of quad) now rotate the carb towards you (bottom side) and remove the 17 mm nut.

**note use a funnel and a cut open soda can to drain the gas. there should be about a half to 3/4 cup of gas from youre carb when its all drained.

[quadboy-55]

soda can.

[quadboy-55]

after removing the 17mm nut from the bottom of the carburator entirely reveals the main and pilot jet. the Keihin jets are a 6mm hex, Dynojet comes with a 7mm hex adaptor so youre mainjet can go on with a flat blade.

** the main jet is the big gold piece, the smaller one off to the side is the Pilot.

both Keihin and Dynojet pilots are removed with a flat head screw driver.

the leak jet is at the bottom of the float bowl. the float bowl must be removed to access the leak jet... more on that later.

[quadboy-55]

the main and pilot jets can easily be accessed without removing all of the plastic and gas tank. aswell as the leak jet.

the reason all of the plastic was removed was to get at the needle. the needle is responsible for a good portion of the mid-range RPM, and the most ridable area for some, so skimping on this area of the jetting may not be so smart.

to access the main jet, find the top of youre carburator. there is a cover that says "CR" in big letters it is held on by two allen bolts, remove them and the cover will slide off. there is no spring in an FCR to worry about so you dont need to worry about the cover flying off.

[quadboy-55]

this is what you will find under the cover. the silver piece is the needle retainer and can be removed with an allen key.

a trick to make the retainer easy to remove (make sure you drained the carb by removing the 17mm nut earler.) it to out youre throttle in the full open position raising the slide. use electrical tape to hold the throttle in place, then remove the retaining piece.

[quadboy-55]

Full throttle position, removing the retaining bolt.

[quadboy-55]

use a pair of needle nose pliers to remove the bolt. the needle is right underneath. use youre needle nose pliers to grab the needle and remove it.

04-06 YFZ's have an adjsutable stock needle but the 70+ YFZ's dont. so you can either adjust youre stock 04-06 NCVQ needle or replace it with youre aftermarket (adjustable) one like i did.

try placing your needle on groove 3 (from top) for starters as recomended by Dynojet. people with a different intake and exhaust set-up might want to try groove 4 or even 5.

[quadboy-55]

here you can see the whole needle and the different grooves.

since my quad was hardly modefied yet and was still running the stock pipe i tried my luck on groove three, as you can see in the picture below.

[quadboy-55]

as for the leak jet, you must remove the carburator float bowl, there are 4 screws, then the whole piece comes off rather easy. there is a small jet close by the accellerator pump. remove and replace youre leak jet as nessisary.

(i'll get pics tommorow.)

now replace everyrhing in reverse order noting the order of removal.

test and fiddle with jetting as nessisary.

** youre fuel screw is covered by a metal plug. drilling this plug while the float bowl is removed is the safest way to gain access to the screw. use a flat head driver and set it one turn out from fully seated to start off.

the plug is on the front of the float facing the engine. with the bowl removed the srew is fully visible as a gold rod with a scew tip at the end. dont mistake this rod as the one at the bottom of the float bowl. when the bowl is compleatly removed the screw remains on the carburator.