Vintage Dirt Bike Q & A

Bought a 82 YZ-490J off internet and restored it. Need a few opinions. Rode one back in the day and the issues are known. Issues are plug fouling, and jetting fuel mixture.
1. Manual states 24:1 oil ratio, initially I run 95 octane 24:1 Yamaha R2 in it with stock #440 main jet. Starts on third kick, smokes like mad. Once it hits pipe smoke stops and the bike goes like a scalded ape. Now if I roll off throttle fast, plug fouls. No 'air' leaks after rebuild of motor.

2. Went with 8EIX iridium plugs and it now just 'burps' on fast roll off, but continues running and 'blows out' well till next throttle roll off then repeats process, no stall out as of yet. Temp has a lot to do with this as well and been running it between 45-55* outside.

Question: With modern oil is there a leaner oil mixture avail? The old days if you ran it stock rich, fouled plugs, if you even ran a #430 main even with 24:1 could seize motor. Been told can go 34:1 with new Castrol RS TTS 2T best option out there. Plugs are cheaper than motor rebuilds....opinions?

I am never going to run this as I did back in 82-83, just reliving a bit of my youth with the local vintage group.

I have a 82 yz125 that would foul a plug in less than two hours running. I switched to Maxima castor 927 @ 40:1 and have yet to foul a plug.

You are running a lot of oil IMO. try around 40:1 like mentioned.

I have tried weening the oil incrementally. With 34:1 motor is hotter than I like, and a little 'cherry pipe' right at head pipe junction as I use 95oct and 34:1. Motor is an expensive rebuild anymore and parts are getting harder to find. Back in the day and as I did on this rebuild the main bearings were put in 'wet', at 10 hours(3 more to go) I am going to disassemble again and look. Then maybe go to a 40:1.
I do not wind motor out as I am a lot older and wiser than I was in 82/83, just want this to last for awhile.

I 'blew' 3 motors on my original 490 over it's lifetime, each seized, and really tore up motor. The 82 was notorious for seizing and fouled plugs if the mixture ratio and jetting were not 'just' right.

THX.....

There is some good info here. http://vintageyzinfo.tripod.com/
Check out the mods for the 83 490. It may apply to the 82 also.

We did our first full MX track run on Sunday. Bike on 95oct and 32:1 Castrol TTS. Started fine sat night in garage, two kicks, Sunday at track not so much, took 4 of us alternating kicking it before it finally fired. Mandatory 3min warm up, then off I went. Practice start, sat there digging in in 2nd before it shot away, runs like a scalded ape, straight you do get some torque swings to right if it breaks traction, front digs in on soft turns, rear pushes front too hard, flies great, landing not so much. Bunny hopped the real steep stuff, but on other jumps we know why this bike was the origin of the term 'Yamahoppin', as rear end bounces all over when you land. A friend who rides a modern 2stroke YZ250 and an 'A' rider, took a lap and was scared to death of it, his first comment "NO BRAKES" my response, "welcome to 1982 and drums".

Just finished reassembly of motor. Clean, no scuffs in cylinder or on piston, no real carbon, plug 'clean', guess I found the 'mix'. Still need to do some work dialing in suspention, it's first 'race' is Apr 20, on the new 'vintage' (mid-70's as model) track being built at the local MX park.