yz490 build

S

snocrazy

Active member
So I fell in to a heck of a deal. A friend of mine has had a basket case 1986 yamaha yz490.
He gave it to me so I can finish it and keep it here so that he can come and visit it......
It has been in his basement for close to 15 years. Every year I gave him crap for not doing any thing with it.

It has all new plastics. Brand New Pro Circuit Platinum pipe and muffler. Boysen high flow reeds and cage. The cylinder is bored 40 over and a brand new wiseco piston. I found all of the specs to mod the head and exhaust port to get rid of pinging and make more power. All mods that were done to the race bikes back in the day. Part of why I think he never finished it was because of not being sure about the machining. Also how to jet this beast with all of these mods.

My solution for jetting is a Lectron carb. https://www.lectronfuelsystems.com/index.php/off-road/yamaha/vintage-yamaha.html From what I have been reading this is a pretty amazing carb. It auto jets for altitude and temperature.

I am pretty stoked about this project. Any one who sees it or hears I got it says dear god you don't need that!
The warnings people who have had - ridden them in the past.... This is one bike that should scare you.
I can't wait! I plan to ride the **** outta this bad boy on the trails. If it kills me I will go with a smile on my face. Not interested in racing motocross. To old and fat for that. I just want to be that guy who passes everyone riding wheelies in all gears blasting down the trail - road.
I hope to have it running in a Month. Just need to wait for the carb and get the motor work completed.

These are photos of the bike I have.
https://photos.app.goo.gl/SCqbXPuRACX5XYur1

I am still running the Yamaha 2 stroke sleds so this seemed like it was meant to be.

head mod - Head_Mod-YZ490.jpg

This video cracks me up. This guy is a mad man.
 
Last edited:
H

hotwire

Member
Wow watching that video that guy is a Bad A** rider. His dog just wants to ride too.

- - - Updated - - -

Good luck on your build that will be an awesome bike. My son is building an 1986 XT600.
 
M

mrbb

Well-known member
I love them old big bore 2 stroke MX bikes, I have owned a LOT of them from the great MAICO 490, to this model you have now, even had some of the IT 465's and a BUNCH of the KX 500's which were always my fav big bore MX bikes, they just ran so well every where and would climb like a goat!
please post some pic's of this thing!
and best of luck getting it all up and running! there getting a LOT harder to find in decent shape these days!
 
S

snocrazy

Active member
I will post pics in this album. https://photos.app.goo.gl/SCqbXPuRACX5XYur1
So far just gathering all the missing parts. (not many)
Ordered the carb last night. $460 ouch! Doing the motor work this weekend.
Just need some other minor stuff here and there.
Decided on Amsoil 10w40 dirt for the clutches.
Going to run it on 91octane premium 32:1 with yamalube 2-R for break in. May go with Mystic after that. :)
The bike came with a bunch of quarts of the 2-R
 
E

elf

Well-known member
My son bought a 87 YZ250 this spring. He spent a bunch of time working out the jetting and now it runs pretty good. Thing is scary fast.
 
M

mrbb

Well-known member
snocrazy said:
All my research on the head squish mod and cylinder porting comes back to one guy in the world.... Eric Gorr
https://www.eric-gorr.com/index.php
He is located in Fond Du Lac. WI

Going to send him the top end so he can work his magic.

Added a bunch of motor pics to the photo album.
https://photos.app.goo.gl/SCqbXPuRACX5XYur1

Have a great Holiday weekend!
Click to expand...
Eric Gore is a smart cookie, he makes or used to make a great yz 300 kits
if he know's he KNOW"S something, he's straight shooter NO BS type of guy
 
S

snocrazy

Active member
Spoke to Eric Gorr. Seems he has ported the cylinder some time in the past. Small world. His shop used to be down in Arlington Heights. He hung out in some of the same joints.
We discussed some of his past with these bikes. Seems Yamaha had a deal with him where they shipped him brand new and he did his magic.

He is going to add a decompression button to cylinder. He says it makes it a lot easier to start. I have these on my bigger chain saws.
Did not know it was an option for 500cc. Makes sense. Also going to weld corners of reed block and make sure the piston-bore is perfect.

I have the cases split. Have most of the parts. Waiting on a shipment form cheapcycleparts. Other orders were from babbits and I have to say they ship fast. No messing around waiting for some dealer to meet minimum order requirements with Yamaha. Next day it was in the mail.

Updated the photo album with the case splitting. Great how to videos I found...
https://www.youtube.com/watch?v=mbT5L0GPgdo
https://www.youtube.com/watch?v=uUJo4-ISfjo

Shooting for the end of Month to brrrrrrap this bad boy.

PHOTO ALBUM https://photos.app.goo.gl/SCqbXPuRACX5XYur1
 
M

mrbb

Well-known member
again, Eric is a great guy from all my dealings with him yrs back, and a sharp cookie, I would take all advice he gives and use it!

and yes a de compression button would be a HUGE help, thats a lot of CC's to be kick starting!
I recall cursing a LOT yrs ago with the macio sandspider I had and that darn left side kick starter and slipping off and catching a shin if not wearing GOOD MX boots, that sucker took a LOT of leg to kick over at times!
 
F

fish633

New member
I have always had a love affair with the open class mx'rs.Still keep an eye out for a decent K5 or CR5 should one come my way.Anyhow the shin thing reminded me of the time I tried to start my old yamaha sc 500 after the decomp.cable broke, damn thing backfired trough the kick starter and busted my right foot.
 
S

snocrazy

Active member
Yeah I have heard the kick start stories. The kick start on this one broke and was why the thing was taken apart and never put back together.
Pretty common it seems. Common to break the case even. It was an easy sell when Gorr mentioned this can be added.

Have the lower case and parts all cleaned up. Just need to swap out the 6 bearings and get it back together. Bottom end should be done this weekend. Should have the top end back mid next week. Its all coming together........
 
E

ezra

Well-known member
true about kick start my cuz has 2 sitting both busted kick starter 1 case for sure and 1 shaft poss both have bad shafts not sure .
I say timbersled that bad boy
 
S

snocrazy

Active member
Got it running yesterday! One bad motor cycle!
Found out it has a prototype flywheel from the beginning of Moose Racing. It is wider so I had to fabricate a spacer for the cover to fit.
Need to get different throttle for the Lectron carb. It is in the mail.
Have a few tweaks to make yet. Need to fabricate a bracket for the exhaust.
Also need to fabricate a gasket seeing as yamaha no longer sells them.
So it looks like am a week away from tearing this bad boy up on the trails.
I uploaded several more pictures to the album. Also some videos of it firing up for the first time.

https://photos.app.goo.gl/SCqbXPuRACX5XYur1

The last picture is a double rainbow in wetmore the same day the yz fired up. Coincidence? I think not ;)
 
Last edited:
M

mrbb

Well-known member
you sank a bunch of time and care into this bike, glad its all coming together for you
just be safe these things can get you in trouble fast too LOL
 
S

snocrazy

Active member
Yeah I just had to save it from the basement it lived in for nearly 20 years.
I should be getting my throttle cable today.
I have the exhaust mounts all figured out along with the gasket.

Have to figure out how to mount the clutch cable to the top of the engine case. There is a notch it sits in but cant figure out what is supposed to keep it in that slot.
 
1

1fujifilm

Well-known member
fish633 said:
I have always had a love affair with the open class mx'rs.Still keep an eye out for a decent K5 or CR5 should one come my way.Anyhow the shin thing reminded me of the time I tried to start my old yamaha sc 500 after the decomp.cable broke, damn thing backfired trough the kick starter and busted my right foot.
Click to expand...

I liked em too back in the late 80's. I had a 84 CR 450R out of sigh school in 1984 and it loved to spray gravel.

Anyone remember the "Torque Center" from the racing circuit? Seems like his name was "Gaborski"?
Fun times, no responsibility and no problems; dang life was easy.

Bear
 
You must log in or register to reply here.
Top