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The jetting language

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Sorry to ask, but I'm getting lost reading the jetting posts. I'm riding a '99 WR400. I'm looking at jetting specs in people's sigs and reading the dialogue in old posts concerning jetting (there's just a couple :) ) I'm having trouble understanding all of the lingo.

Here's what I can figure out:

MJ=main jet

MAJ=main air jet

EMM...#=needle type and clip position

PJ=pilot jet

PS=pilot screw

SJ/starter jet=starter jet

The ones I don't get are APJ, PAJ(S), PAS or pilot air screw, and timing the accelerator pump. For now I took my E-series pipe off and am running complete stock for stealth. I still have the lid removed and WR timing, grey wire intact. I am planning on cloning Taffy's jetting and see what happens. My main concern is with the sleepy/non-existant bottom end with this pipe. The E-series seemed to make power right from the bottom. I don't care as much about top end, but the talk of snap wheelies on the jetting posts really interests me to help lift the front end over logs and roots. Thanks for your help. In the meantime, I'll keep searching.


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I'm new to this stuff to, but I will tell you the BK mod (timing you accelerator pump) is a big improvement. The best advice I was given was Motoman393.com or net.

Here you will find all the posative mods to be made.

Better to be looking at it than for it.

Jason :)

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in the front face of the carb you have a jet to let air in at low revs and it's called the Pilot Air Jet. by unscrewing this and putting what at first appears to be a self tapping screw with the spiralled thread missing in, you can control the air with the screw.

this saves money once you need more than two PAJ's. it's called the Pilot Air Screw. but when posting how we set it we have tried to help newbies in the "crossover" so we might say that "i set the PAJ(S) to 45" this is telling people who want to buy jets and people who just want to twiddle that little screw what to do.

the APJ is your Accelerator Pump Jet (APJ). the BK mod is for 2000+ models so oddball is out on that one.

most of the stuff on snap wheelies is a bit stretched and i'm afraid whilst the jetting is a dramatic improvement it ain't going to put the petrol tank infront of your face at 70mph when you snap the throttle. BTW i'm generous with the word snap here!

the open pipe does help but it has a double whammy in that the jetting is rich from the factory and an after martket pipe will lean it off to roughly "superb jetting" this means that it not only flows better but that the mixture is far better and thus the double-whammy.

timing the pump is easy. look at the rod in the end of the black rocking pivot bar. you can see the end of it. you want it to be sat at the top of the oval groove so that as the arm drops the rod has to move IMMEDIATELY. you can do this by looking at the back of the carb and twisting a forked tab AWAY from the other end of the rocker and that will remove the slack.

you can also make a longer rod from a coat hanger like me!

then pull rocker arm off, put collar on rod and replace rocker back on. set the collar above the housing to .024". i teased people to guess how any mechanic worth his salt would measure .024" but nobody had guessed after a week. let's see if you can work out how to measure a gap of .024"? a real brain teaser hey!!!! :):D


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