Oh yeah... the mighty big bore 2-stroke yamahas.
I completed my '83 IT490 in the early fall. My bike was also initially hard to start. I had a couple of problems, first problem was a clogged pilot jet. The other problem was a leaking base gasket.
These bikes are noisy and I was unable to hear the base gasket leak while the bike was running. I noticed it when I was running my fingers around the base gasket area. With the bike running, I could feel leaking around the transfer port area on the left side. This is a fairly common problem. Consider using a factory Yamaha base gasket as it has a sealent round it that the aftermarket gaskets do not have. I checked the base of my cylinder for flatness and it was fine.
Once I fixed these problems I then spent some time jetting it. They come rich from the factory so blubber in the low to mid range area. The fix for me was to change the slide, the jet needle and the needle jet. Do not try and solve this by going too low on the main jet or you will ping. The main jet is ineffective in tuning the low end and mid range.
The stock slide is a 2.0. I replaced this with a yz slide which is a 3.0. The trick is then to file the slide to a 3.5 (no longer available). The slide modification can be found on the yahoo yz490 group. The needle I used is fatter towards the point, this makes it leaner on the bottom 1/3rd throttle. I tried pilot jets of 35, 37.5 and 40. It starts well with the 37.5 and 40. I have the 40 in it now. I think the yz465 came with pj=50.
I will post the jetting that worked for me tonight as I do not have that information now.
These are nice bikes once fully sorted. You get the jetting right and this bike will haul the mail.
I raced it in a hare scrambles (after 30 years away). I was kind of worried as it was a dead engine start. It fired 1st kick. Went from 5th or 6th at the start to 2nd within the 1st lap. And I started with a flat front tire!
There was a nice section coming out of the woods where you could really wack the throttle. Whoa momma!
But unfortunately I am an old man now and my conditioning is just not up to snuff (yet?). I wore myself out and was unable to finish! Figured I better stop before I hurt myself, didnt want to give my wife a reason to shut me down!
So yeah, keep working on it, these bikes can really kick some a$$
. Or at least it kicked my a$$!
Here is a link to some photos of mine as I worked to get it in shape. Have fun but dont hurt yourself!