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Blue smoke and burning oil

841 Views 5 Replies 3 Participants Last post by  herpesfreesince93
Hey guys, I've got a 2003 Honda 300ex that I've had for a little while now. I bought it knowing it needed a rebuild, but I've been riding for two years so far without major issues. The time has come to rebuild, but I don't know exactly what I should be looking at. I was hoping if I listed my goals and budget, you guys could point me in the right direction. I'm not sure if the cylinder is trashed, but I only had 80 psi when I compression tested the motor so it's either the cylinder or the rings. I did a wet test and the compression spiked which tells me it's not a valve issue. I was looking into the big bore kits since I'm pretty sure I will need a new cylinder anyway, but I see it looks like they need some machining to fit the crank case. I'm looking for more acceleration without sacrificing too much top end. Would it be possible to get my current cylinder bored if needed? Or are the walls too thing to accept any boring? Also, what combo of sprockets would be best to help out acceleration without killing the top end? I'd like the quad to still hit at least 45mph (I weigh 210lbs).
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I wouldn't bother with a big bore kit. There's just not a lot of bang for your buck going all out modding one of these machines (ask me how I know).

In my opinion you should bore it to the next largest size where the cylinder cleans up and rebuild it with quality parts (OEM in most cases),work the exhaust and intake ports over a little bit and install a new cam chain.

You can always throw in a hot cam or something but it's not a huge difference over stock. A more open exhaust system with proper jetting does help some also but it comes with a price and extra noise.

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