Help with a 1982 Yz 490

[Crotster]

I have a 1982 YZ 490. Is there any way that i can get this bike to run better and to quite fouling out so many plugs? Any ideas on how i can get to start easier. I thought that the carb might need to be rebuilt. Does anybody have any tips. Drop a line.

 

[kx134]

that bike is notorious for being difficult to jet. If I were you, I would call around local shops and ask them whether or not they are experienced and willing to work on the 490 for you-the bike has a reputation that precedes it. fouling plugs AND hard to start? is it correctly jetted for where you live?

on the plus side, my neighbor had around an 85 490 and it kicked butt and never gave him problems, and he could smoke my kx250 easy.

 

[Crotster]

Well i am not really sure if it is jetted correctly or not. But my boss use to ride flat track and he gave me some good ideas on why my bike might be fouling so many plugs. Every time that i replace a plug the fouled plug is a dark black and what he told me is that i am probably jetted too rich so i am going to pull the carb and see if i can go a little bit leaner on my mix.

 

[bigpa]

Search within this site and you'll find lots of info on this bike.
Warning: You may not like what you see! :yikes: :ugg:
http://www.off-road.com/dirtbike/columns/dntask/

 

[wasted]

my brother in law has a blaster with a YZ490 motor in it, and I have a buddy who had an '82 YZ490 years ago... the fix to the way it runs involves engine work... oddly enough, I also found an article in one of my old Dirtbike magazines(nov. 1986) not long ago about how they fixed their YZ490...

this is what is printed in their article...

"there is an ugly cough off the bottom and the machine blubbers like it is jetted too rich, but when you try to lean it out, it detonates"

"the best modification you can do is to have the head recut and the squish band matched to the piston. The problems are the gap between the piston and the head, and the angle of the head in relation to the piston. Race Tech performs surgery on the head, which in turn allows you to properly jet the bike and use lower-quality gas without fear of major detonation."
"Race Tech ported our cylinder. Why port a cylinder on an open bike, you ask? Race Tech believes in a controlled and smooth powerband, especially on an open bike, where violent surges equal loss of traction and lack of control. They dont do anything radical when they're grinding, but say it's an important step in gaining the perfect flow from the bottom rpm to the upper hit. Combine the head mods with the porting, and your 490 accelerates brutally hard but has no hitches in the powerband. The last job Race Tech performs is to cut the carburetor slide to their own specifications and install a set of boyesen reeds.This is an important step in the jetting and fuel-flow process."

I'm in no way trying to talk up race tech here, its just who they used to do the work in this article... but it should still give you some ideas of what you need to do...
I'm sure there are quite a few places who know what needs to be done to the YZ490 motors, and how to do it...

 

[WillyM]

Head mod

I know that if you read a book by Grahm Belle it saya
that for optimum performance have 40-60 thousanths "squish"
at the cylinder the way I tell this measurement it to use a piece of soft solder long enugh to come out of the spark plug hole and reach the side of the cylinder,stick it in and kick it once all the way,pull the solder out and measure.

 

[Crotster]

Thanks for all of the info. But does anybody know what the proper mix is for the 490? Is it in the area of 35:1 or what?

 

[JWW0846]

Try This

First I Would Check The Flywheel Key And Make Sure It Has Not Sheared,that Could Be The Hard Starting Problem Also If Your Running A 40mm Mikuni Try This Jetting 440 Main,3.5 Slide,q8 Needle Jet,50 Pilot Jet

Hope This Helps

 

[JWW0846]

TRY THIS "2"

Almost Forgot Use A 60.1 Ratio On The Premix Of Yamalube.


Good Luck

 

[Crotster]

Thankyou everybody. I replaced the jet and am very happy with the performance. Everybody says only bad things about this bike but since i bought it, i have had nothing but fun and enjoyment. I dont seem to have all the problems that everyone else describes. The bike looks and runs great. Thanks for all the help.

 

[wasted]

I'm glad everything worked out for you, I'm a big fan of the YZ490 myself...
stock, they are a little blubbery off the bottom, and those head mods do clean it up quite a bit, but you'd never know the difference unless you rode a modified one, and they still go like hell stock, there was no stopping the one my buddy had and it had no problems at all about flipping you over backwards...

very fun bikes... :cool: