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so many of you have shown your concerns at jetting your bikes i thought i would throw a load of idea's together that might make your lives a little easier.

first of all i would allocate a day for this and not do it in little bits and bobs.

check that your filter is quite clean, no it doesn't have to be brand spanking! check that the throttle opens immediately you pull the grip, check that the end of the vent tubes are clear of muck.

i would make sure that i had a few jets and needles around as well as the tools for the job.

bike set up

you can take your rad guards off, hold the tank on with just one bolt and not the two. you can also pull one of the seat bolts out.

for easy access to the carb why not put both engine mounting plates on the left and then the carb will come right outside the bike?

after about three modifications take a minute to put down your results and how it felt. you may not make up your mind at the time which way to go with the jetting-up or down, that is the question? but at home you can chew it over.

try to do your main jet and some needle work on a hard road where the vibration and speed don't confuse you like the dirt does.

try to mark your throttle so that you know closed, 1/4, 1/2 3/4 and full throttle. do this by cutting a tiny 'v' in the grip near the throttle housing and then put a mark for the aforsaid positions on the housing.

you'll need a phillips screwdriver for all mods.

#4 and #3 allen key for needle work.

14mm, and 6mm spanner for Main Jet.

#3 allen key, thin flat bladed screwdriver for Pilot Jet work.

if your really going to set your own bike up you must be able to turn the Pilot Screw easily and efficiently. preferably with the bike still running.

take a medium sized block connector and a small electricians screwdriver. chop the handle off, grind a flat on the side and slip into the block connector tightening the screws to the flatface.

take a plag cap, you know the one, the one that you throw away with a new plug. tap this soft aluminium 'tube' on the blade of the screwdriver and now you know when your mounted and screwing!

cost 50p (75 cents). the block connector is big enough that it won't fall out of your hands or your pockets and it has the advantage that you can see clearly what half a turn etc-is.

take all this stuff in a bum bag (sorry i can't use your name-surely you find it 'fanny' as well!)and make changes on a long ride.

if your adjusting the pilot screw try lot's of little pop wheelies to see which gives the best response. 1/4 turn at a time.

don't worry about plug chops or colour. they're unreadable to the newbie.

it's almost impossible to run these too lean or damage them internally from this so don't worry about trying things.

what i would do is take the mobile phone or a friend and don't make changes to a cold engine. always get your bike running on the old settings and have a ride like this for a little way.

try all kinds of tests. Standing start. 2nd gear roll-on, fifth gear from half throttle/speed.

in what order?

always work from the top revs to the bottom. so it's MJ, needle, pilot circuit, needle straight.

if you're checking your main jet it's all about the top revs and 4th/5th gear.

if you're checking the needle it's 1/2 throttle on. look at your grip to check!!!

the pilot circuit is difficult. a nightmare even.

diagnosing rich from lean is hard. if it's rich;

the engine sounds softer

it'll misfire if it's really rich every two seconds

it will rev more slowly (hard to spot)

it'll drink fuel

if it's lean you'll have to keep wringing its neck!

it'll pop and bang

hesitate and then bam!

now i haven't mentioned which jetting but by oing the above look how much easier your life can be.


please add too boys. thank you.


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You ARE the man!! :)

Now, go and try an FHP, # 5, and tell me what you think. JD keeps pushing this FHP!! :D

I am running the FHP (and 160MJ) as we speak, but have done very little time on the bike.

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-Or FHN, FHM for the more particular about rolling it on.

Definitely keep notes and make a copy of this 4-stroke needle graph. Jetting is ALL ABOUT THROTTLE POSITION AND RICH OR LEAN. Highlight the curve that matches your test run and compare to see what to expect.


James :)

[This message has been edited by James Dean (edited September 22, 2001).]

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Unless your graph is on green construction paper and uses little pictures of animals, I am lost and over my head. :D

The print made a good paper airplane, though! Arf, arf, arf!! :D

Seriously though James, I am studying your graphs. Thanks for taking out the time to help!! :)

P.S. I have not figured out my lower-than-normal rev limiter. If I do move to Wisconsin (interviewing soon), I may just roll my bike into Eric Gorr's office and have him figure it, and my jetting out. He might as well do the 420 mod to my bike as well.

If you want, I could send you a copy of the Mollier diagram I am looking at to put together a lecture on the steam plant cycle using the temperature/entropy graph.

DULL crap!

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I printed a copy myself to see if it was legible. The Last letters are difficult to read.

M=2.715 (rich)




R=2.755 (lean)

Note how the second letter-L1 adjusts the taper location in the same way as changing the clip position.

See that DT- needle and EK- needle tapers intersect near 2.715 mm diameter. Nearly all of the combinations of different needles end up being adjusted to transition through a narrow range at the taper start, whether the taper is F, E, or D. Otherwise, throttle response near 1/4 throttle ends up too rich or lean. :)

[This message has been edited by James Dean (edited September 22, 2001).]

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What Jason said

Thank You James


Rick Fuller

'01 yz426

'00 yz426 destroyed by fire in desert race 4/22/01

'81 490 Maico

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