Need Help Jetting YZ490J

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shane.ballard.nf

Member
Jun 4, 2006
82
0
Hello,
I live in Newfoundland, Canada
i recently bought a second hand YZ490J 1982
The motor was done with a new piston and rings from the previous owner before me and it's bored .020 over or 20thou but he got fed up trying to jet the bike i guess.

There is a new bronze DG exhaust and silencer on the bike.
i'm gunna put a new UNI Two Stage Competition Air Filter
on the bike before attempting to jet it.

I need the assistance of Eric Gorr to provide me with some good jetting numbers so i can order the correct parts for a carb rebuild etc.

Right now the carb has an oversized needle jet (450)
stock is (440)

And the jet needle is oversized aswell, stock is supposed to be (6F16)

The bike runs but according to its poor performance in acceleration and speed i think it's running to rich and the bike needs to be leaner but not to lean.

The plug is not wet, but its a lil black and sooty after a ride

i have the clymer repair manual for my bike.

I would greatly appreciate it if someone could help me get this bike jetted good and running good so i can have some fun during the summer :D
 

oldfrt613

Feeble Sponsoring Member
Member
Jun 29, 2005
443
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So, you're not having problems fouling plugs ? What are you basing your performance expectations on ? Is the '82 a water cooled model ? Does it have a power valve ?
 
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Rcannon

~SPONSOR~
Nov 17, 2001
1,886
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I remember buying a Q4 needle jet for my old 82. Unless you mod the head you'll never get clean jetting. The stock head is fairly messed up.
 
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shane.ballard.nf

Member
Jun 4, 2006
82
0
I think my bike has a power valve
The bike is making the plugs a lil sooty dunno if this is considered fouling the plug, like i said the plug is not wet with gas or dirty black.

The bike is air cooled and i'm basing my performance on how the bike used to run, previous owners had the bike working fine with tons of power and speed
 
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Rcannon

~SPONSOR~
Nov 17, 2001
1,886
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If it is an 82, there is no powervalve.

Your going to toss a lot of money away trying to jet it in stock condition. It is virtually impossible without working over the head.
 
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shane.ballard.nf

Member
Jun 4, 2006
82
0
I got all this stuff figured out in my head now people, i'm gunna put the carb back on stock specs, since these specs are in my clymer service manual.

Then if this is not good enough i can pick it up in the needle clip adjustment.

If this is still not enough i will get an even smaller needle/seat and then put needle clip on the richest setting and try that.

Thanks for your input and stuff but i'm sure now with some trial and error testings and sparkplug inspections after some short rides i will get this bike working top notch, just goto wait on a couple sets of needle/seats to be sent in for testing :laugh:

Since i have the main logic figured out for jetting a carb
I need someone to explain now, the best way to inspect a sparkplug after a jetting test.

I heard of something called "plug chopping" but i can't quite understand it.

At the moment i clean the plug off with a grinder with wire bush thing on it, and then test ride the bike come back and inspect the sparkplug is this good enough?

And mr Rcannon, nothing is impossible, i will personally fine tune this bike so close that it will have no choice but to run right it's just going to require some patience, trial and error testing and a good supply of beer
 
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Rcannon

~SPONSOR~
Nov 17, 2001
1,886
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I believe the jetting, stock, ended up being close after the head was slightly worked on.


Beer helped. We were working right in a motorcycle shop at the time. The different sized jets were no trouble.

Good luck to you
 
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shane.ballard.nf

Member
Jun 4, 2006
82
0
Heh even the guy who sold me the bike said good luck.

I'll let you guys know in the future when all my parts are in and i'm rdy to go thru the trial and error process of jetting if i was successful or not.

Luck and Beer may just be the key components that are not supplied with the YZ490 from the factory hehe

This month or early next month i will be riding like this happy lil guy :ride:
 
J

j32

Uhhh...
Jun 8, 2006
202
0
hi - what kind of oil are you using?
what premix ratio?
second clip on the needle,(rule of thumb),or it prob needs jetted
either rich or lean
good oil makes a ferry big difference!,i use klotz ,it burns clean!!,
81 kx420 and its mixed rich
whats your air screw set at?
(plug chop)--open the bike up -around 3rd shut it off,check plug
should be golden brown,after you clean it out it should stay tan ,on midrange too
pilot stock?
check the reeds,for warpage or breakage. you dent need luck just proper knowledge
and jetting lol :laugh:
 
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shane.ballard.nf

Member
Jun 4, 2006
82
0
I'm using Lucas high performance 2stroke oil at 24:1
My needle is on leanest clip, i'm gunna order smaller jet/jetneedle and try that at stock specs and check to see if the floats are level to specs.
The bike's running to rich.
Not sure what the airscrews at, gunna put it to stock specs tho

I don't think i can plug chop because my killswitch is like hooked up wrong i got one of those ski doo things where if you falloff the wire unplugs and the machine shuts off, but for me i goto put the plug in for it to shutoff and leave it out for the bike to start.

I checked the reeds, they are the stock ones but they look fine, they all laying flat and there are no signs of damage.
I might get some Boyesen power reeds when i get the bike workin good and get some more money.

I think the rich jetting may be because the tranny oil is gettin thru a bad crankseal.

I read this on Eric Gorrs website that has carb information and stuff.
He says to check out this before attempting to jet the bike, also i got a new performance air filter coming for the bike to help with the jetting.
 
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Rcannon

~SPONSOR~
Nov 17, 2001
1,886
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At my elevation, 4500ft, I remember running a Q4 needle jet and a 400 mj. The bike did pretty well, that is for a stock 490.

The Yamalube R at that time was mixed at 24:1.
 
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shane.ballard.nf

Member
Jun 4, 2006
82
0
Is there a way to know what carb parts i need without wasting a bunch of money?

Not sure if puttin the main jet and needle jet back to default will make enough difference

Where i live i'm pretty close to sea level, so what effect will this have on my carb?
 
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Rcannon

~SPONSOR~
Nov 17, 2001
1,886
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There is not a real good way to guess exactly right the first time.
You almost have to go back to stock in order to have a starting point. I find it much easier to work on one area at a time ie. LOW, MID, or TOP end. It is much easier than trying to change all 3.
 
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shane.ballard.nf

Member
Jun 4, 2006
82
0
Ya, true.

Well i'm gunna trackdown the parts i need through my local yamaha dealer tomorrow.
I'll set all the adjustments to the book and tweak it from there.
If this still don't show improvement chances are it would be a good idea to check the crank seals.
I read somewhere that to replace the crankseals the cases don't have to be split, hope that's true.
 
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shane.ballard.nf

Member
Jun 4, 2006
82
0
well, i got the stock #440 seat but the needle is no longer made by yamaha, hopefully a bigger needle will not make alot of difference
 
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shane.ballard.nf

Member
Jun 4, 2006
82
0
looks like i goto track down the stock needle from somewhere
goto find another 490J or another bike with the 6F16 needle
but i am making some progress with the main jet, i know it needs to go down at least one size from stock #440
 
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