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Blue smoke and burning oil

839 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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hang in there, we got some pretty smart sport atv members around here !. I am not one of them ?!..lmao. i'm mostly utility models, and old school models. anyway, hang tight, someone will be along soon to point you in the right direction !, oh, and please do not make/post more than one thread asking all the same stuff as you have here ^^^, that won't get you help any faster ! :).
I wasn't planning on it haha. I've been around other forums, this is really my first time messing arounf with 4 wheelers though. I did find a service manual with tolerances, so I will probably pull the motor apart and start checking specs to see if the cylinder is reusable
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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I already purchased an aftermarket full exhasut. I'm unsure of the brand but it was a good ebay deal, and it looked a lot newer and nicer than the factory exhaust. I planned on jetting to match that as well as running a custom intake that will hopefully supply more air to the motor. I'm familiar with chevy motors, and I know a lot of them like gasket matched porting. Is that something this ex motor will like? or would I be better off leaving it alone or having it professionally done? Are the hot cams worth it? I was looking at maybe getting a box to raise the rev limiter, but I probably would only do that if I throw an aftermarket cam in. I doubt the factory cam makes power past the rev limiter.
I bought an aftermarket exhaust, unsure on brand but it's definitely larger diameter than stock. I also went with a 75mm wiesco piston, and I'm going to have the cylinder bored to match. I've already got the airbox removed, I think I'll stick with the stock cam but I plan on getting one of those boxes to raise the rev limiter. From what I"ve been able to find out in stock form these engines peak around 7 but make good power all the way up until 8,000 rpms. I figure with an open filter and larger exhaust I should be peaking a bit higher and making usable power past 8000. I'm still not sure what jets to go with.
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