any yz 490 experts out there?

[POM]

A mate at work has bought an 86 yz490 which is running v. poorly,on closer inspection inside the motor it appears to have approx 3mm longer stroke than standard, thus causing allsorts of porting issues ,the most odd one is the exhaust port is being exposed at tdc by appox 3mm by the lower edge of the piston,you can see this when you look up the exhaust port???....the pison is on the correct way around before you ask, however, also we believe the skirt has been shaved to avoid hitting the crank webs at bdc.
Why would anybody stroke one of these things.....and more to the point how did they do it,and what else needs to be done to make it work???

[evo550]

Stroked or just a longer rod ?
Does it have a spacer plate on bottom of cylinder?

[Lozza]

The uncovered exhaust port will cause poor running, a 3mm stroke increase or a 3mm longer rod should not cause any real port timing issues. To stroke an engine the only way is to move the crank pin centre outboard in relation to the crankshaft centre axis. That should be easy to pick, stroke will be half this distance.

Easy fix would be to weld the exhaust port floor up by 4mm to cover the hole.
Pics would help.

[Nathan S]

That should be easy to pick, stroke will be twice this distance.

Fixed.

[noisy toy]

What was the fix? Any pics yet?

[POM]

This motor has been stroked,not longer rod,and no fix decided upon as yet,the initial direction is to put a spacer under the barrel.
From what i can see,the only permanent fix is to resleeve it with sleeve with a more suitable exhaust port position,the real bummer is its only just had a new sleeve in it.....or.... replace the crank with a standard one....this is turning into a real headache of a motor......

Also on closer inspection the head has had a "job" done on it......like ALL of the squish has been completely removed leaving a massive open domed combustion chamber......why would anybody do that??.....am i dealing with the results of a moron engine tuner or a wizard who's run this on on some fancy fuel/tune that i dont understand??

[POM]

Further inspection has revealed the main crank pin(big end pin) is pressed into brass bushes in the crank webs??? seems the the crank pin holes have been bored out larger and offset further to the edge of the crank webs (1.5mm) ..which would give the 3mm extra stroke........this crank is heading for the bin methinks....I cant see brass bushes holding the  crank pin tight enough to stop misalignment occuring.......any thoughts anyone?

[Marc.com]

Sounds like a freaking mess.... on the bright side, new crank and rod will probably resolve your piston and sleeve issues. Just watch the piston hasn't had the skirt shortened to clear the crank webs after it was 'stroked'. That might explain the exhaust port height.

Other thing on the menu would be new head, I imagine the combination of skimmed head and increased stroke would give a pretty interesting compression ratio.  YZ490 engine is not forgiving at all when it comes to higher compression ratios. I would discount this as the work of an engine wizard ....

Best set up would be stock with thick head gasket.

[POM]

"Just watch the piston hasn't had the skirt shortened to clear the crank webs after it was 'stroked'. That might explain the exhaust port height."

markfx....good guess......the piston has been shortend around the front skirt........another shopping list item.....Im beginning to regret getting involved with this engine...especially as the owner thinks this can be fixed for the cost of 2 goats and a sheep.

[Marc.com]

...especially as the owner thinks this can be fixed for the cost of 2 goats and a sheep.

Is he a New Zealander ?

Anyways your man can buy a truck load of old YZ490 parts off ebay so it shouldn't break the bank, unless it is unrealistic bank.... you can probably salvage the barrel if the piston hasn't hooked the port, the skirt is not really designed to run unsupported.

[holeshot buddy]

sounds like it has been stroked on the crank position
and the barrel has not been raised to compensate for the extra distance the piston travels
if it has no spacer under barrel i would remove head
and inspect any mods to it
also possible the sleeve has been stepped higher in barrel to
allow for stroke
you need to lift barrel 3.0mm or whatever it is, this will mask the
exhaust port then check your squish which will be wrong
generally piston will be just bellow deck height
work out exact length of stroke compared to standard
nothing wrong with a stroked eng would be better than standard once sorted
if the head has been heavily modified you may have to get a standard one

[tony27]

My guess is that the head has been modified to allow the piston to stick out past the top of the barrel

Earlier this year I was given a YZ490 head to alter, pretty simple,proven mod that is mean to help out in a big way with the jetting issues. All it involved was a secondary squish angle which I'd guess lowers the compression slightly. I don't have the info anymore though