I purchased a yz 490 today for my Blaster project, from a speed shop on Long Island. I asked the same question as the engine has no carb or boost bottle. He stated that the common application of a 38 or 40 mm would be a little unsavy for a wheeler. Better choice is a 36 mm. and a 6 pedal reed, or any reed at around 68 cfm. NO boost bottle, as most aftermarket intakes prefer not. This setup will give great low and mid punch. Nothing on top. Also if a pre 87, use blue 100 octane avgas to avoid detonation and hard starts. You can lower compression by adding a base gasket but a suptle port timing change will result in carbon deposits. Hope this helps.
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