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Old September 9th, 2018, 12:21 PM   #1
thumper64
ninjette.org member
 
Name: Tim
Location: Quad Cities
Join Date: Sep 2012

Motorcycle(s): Ninja 250

Posts: 102
porting the 250 head

I'm going to be taking the head off my bike, I have a tick coming from the top end and the CCT cleaning and valve adjustment didn't fix it, so I found another head and timing chain tensioner as well as guide on eBay reasonable.

I don't expect a surge of power, but from what I've been reading, if you bump the compression and timing and maybe advance the intake cam a tooth, it could liven the bike up a bit more and add a little mid range and top end. I've already stripped what weight I can, put pod filters and a 2-1 Area P exhaust on and rejetted it. The carb tuning doesn't scare me and I've built a couple car engines, so I think I can tackle this. I have a GSXR 750, I'm just tinkering with the 250 because it's less expensive if I ruin a head, which I expect, so I'm buying a couple used ones.

Where do I work the head? I've read to leave the exhaust alone, but cleaning up casting lines and sharp edges where the bolt and runner meet shouldn't hurt. It looks like the intake side gets a little work widening the floor of the short side radius and the bowl hangs over the valve seat a bit, probably from core shift, so that could be cleaned up, though I'm not sure if I should leave a little bump in the bowl, necking down just a bit, right below the seat. Do you sand down the short side radius just a bit, or at all? That should help flow at higher RPMs. It looks like the heads I've seen have all been left with just a sandpaper finish, no polishing. For a street bike, I don't want to ruin the low end, but that's part of why I want to advance the timing a bit and bump the compression a little, even if I need to run 89 or 91 octane. I just can't find anything over 91 reliably if I'm somewhere unfamiliar.

I know the engine is about 12.4:1 compression in stock form. If I deshroud the valves, how much volume will I be able to remove again by removing the base gasket? It's 62mm wide for the bore, but how thick is the gasket? Or is there any point on this head besides just eliminating hot spots? I don't have a complete engine to assemble and clay, so I'm not sure how far I can go without P2V clearance issues. I can mill the head at work and modify cam gears to be able to adjust the valve timing back in spec.
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