Was helping a friend of mines nephew yesterday, he has an 86 490. Seems like its down on power, having never rode one before, what kind of power should these things have? The pipe(FMF) is cracked, no holes, just cracked along two welds, the pipe was not really sealed to the cylinder either. Is there a gasket for this? Checked the timing and its close, about .070 btdc, supposed to be .080. Main jet is a 480, plug is black. Right now all I can think is the pipe can't work cause of the lack of seal and cracks, its a little rich, and the timing is off a little. Any suggestions?
This article called "Worst Dirt Bikes of All Time" by Rick Sieman says this about the Yamaha YZ490 That being said, fix the exhaust and the timing and give the bike a tune up according to the factory service manual and put fresh fuel in it so you know it was mixed right. If you need parts or parts diagrams go here.
The exhaust should be secured with beehive springs (and supported further downstream by a bolt through the frame) and yes there is a gasket in there to seal the exhaust port.
......and amazingly Broc Glover still took the 1985 500cc outdoor title on the turd. Hell, even Bradshaw won a couple races on the WR 500 "airhammer"
LOL, Hunky is right. Dead on the bottom, rev to the moon, shaking and vibrating, bad starting, plug fouling, yup, that's the one. Still, I raced those things for two years after Maico went out of business, yellow 4 speed 84, white five speed 86. The jetting/seizing problems went away if you modified the head, though. And I cannot say they ever lacked power, if anything, it was just the opposite. They just revved like a 125. I geared mine short and spun it up. In between, I had my favorite 80's bike after my '81 Maico 490--the '85 Yamaha YZ 250, now there was a sweet one. As good as the 490 was bad. If your buddy's 490 has no power, I would make sure that the bore is fresh, and everything is set to stock, timing, jetting, etc. and 32:1 Yamalube R and a fresh plug every day, and go ride. Power it has. Up top.
Thanks everybody, I have downloaded a pic of the head mod, just to verify(if anyone remembers) , its a 30 degree chamfer on the inner radius of the squish, 8mm wide. I will get it timed right, fix the pipe and do the head, if the pistons sloppy we can do that right away. Is the hed gaskeet reuseable (copper)? Any jetting suggestions for after the head is modded? Thanks Again. Rus
considering it is almost 25 years old, you might think about looking at the bottom end and the reeds, and giving it a set of crank seals too.
In this thread, is an 8 year old Eric Gorr post about this, with jetting, head machining, and other advice for the YZ490. http://www.dirtrider.net/forums3/showthread.php?t=41089
I just got my old 1982 yz490, its all original ive cleaned everything there is to be cleaned externally, comp is still good, still wont start, it just backfires after every 20 kicks, i pulled the ign cover off and there was a random shim like piece of metal digging into the cover, i have a feeling this shim somehow knocked the timing out of wack because it was just floating around, any ideas what it could be?? or how i could fix it?
I had an RD250 roadracer like that- either blubbered all the time or if you downjetted the main til it ran nice, it seized. Changed to a different taper on the needle, and it became reliable and responded normally to jetting chages.
