1983 yz 490 tuning

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Apr 17, 2000
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#1
This bike has been a bear to keep a plug in. It needs to be short shifted to perform well so I am always in the bottom of the power range. I have tried many things to get this bike to clean up off of the bottom. I tried a 45 pilot, leaned out the needle, made sure the ignition was correct, and changed reeds. My last effort was to use a B7es plug, a bit hotter than recommened. The bore is good and the bottom end is new. Does anyone know of a way to get this bike to clean up off the bottom?
 

SFO

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#2
I used to race 490's and the problem I would have is if was jetted to be crisp on the bottom it would ping.
I mean PING!
DGY did my head and barrell but I believe all they did was detune it.
People have had good results running the champion plug in these things, I forget what the # is.
I ran a projected tip in mine, BP8ES. I found the projected tip worked better.
 

HONDA65

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#3
SFO, you're right, they did de-tune it. They had way to much compression, so you had to jet it rich to keep from pinging, but then it would burble BAD!! Once I had my head done, I could drop my jetting quite a bit and it wouldn't ping. The shop at the time also sanded my needle a hair to richen it through the middle. The shop used to be called Kearny Mesa Yamaha, in San Diego. The shop is no longer there, but the owner has a hop-up shop called KMY Motorsports, in El Cajon. I'm willing to bet he still knows alot about the old 490 motors, and could help you out. If you can't find him, contact Precision Concepts, also in El Cajon, Ca. They do alot of work together, since the owner of that shop used to work for Kearny Mesa. I don't know their number either, but he runs an add in most of the mags, and he has a web sight, so I'm sure you can find it.
 

Pete Payne

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#4
The Yz490 needs to have the squish band narrowed to lower the velocity. This will also add volume to the head and lower the compression , so it will be less likely to ping or break pistons.
 
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#5
ya i have the same problem my 490 is always going through plugs and the performance is off because of it going to try a BR7ES and hope that it clears up
 

SpDyKen

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#6
As you have heard, you'll need to modify your head (bike's; not your own) if you want it to run correctly. Most reputable and experienced tuners can do this for you.

Eric Gore & Pete Payne come to mind.
 
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#7
can you run a yz465 head on a yz490 cylinder?
wouldn't that fix the problem with jetting and pinging
 
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#8
still no answer on this... can u run the 465 combustion chamber on the yz490 cylinder? or do you have to get the 490's combustion chamber modified
 
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#9
Using a 465 head would not do any good. From what I've heard Yamaha bored the 465 out to get to 490 and didn't modify the head for the new bore size.
 

zoommx

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#10
Here some suggestions I found:

Use this jetting as a starting point. It has been
tested with a stock, DG, Retro Rocket, and ProForm pipe.

440 Main Jet (stock)
R-0 Needle Jet-series 247 (richer than the stock Q-8)
6F8 Needle, clip in position 3 (Leaner on bottom than Stock needle)
40 Pilot (leaner than the stock 50)
Air Screw 1 3/4 turns out (will vary with weather conditions)
Carb Slide 3.5 (modify your stock 3.0 slide)
Fuel Mix 40:1 Yamalube 2R
Premium fuel (100-110 octane leaded is what I run)
B8EG Plug gapped at .6mm

To help with pinging some:
This head modification will lower compression slightly. You can have a machinist do it or send it to Eric Gorr.
This is probably the most prevalent head mod that was done on YZ490s.
http://tinyurl.com/yqmdyp if you know a machinist.
http://www.forwardmotion.com.mx/index.html to send it to Eric Gorr.

helped my '82 490.
good lukck, Roger MOVMX #491