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Big Bore 262 vs 280

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CBDADDY

There was no grinding involved? Was there much clearance between the cases and the Jug? I would think that it must be a snug fit in there. It makes me wonder about expantion of the jug vs the cases?

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Well after calling several of the shops which sell/specialize in the big bores for the yz250f I decided to give an 80mm (270) a try. Peterson is the only one proclaiming doom, two other shops claim no probs, unable to find individual report of problems with the 80mm.

I thought the only problem he has with the 270 was if the stock cylinder was bored. The Vertex I believe comes with a new cylinder. I don't think you'll have any trouble. But let us know. :lol:

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No problem. It kind of fools you at first until you hear it explained. More displacement and leaner jets didn't make a lick of sense to me at first. BTW guys, I started a tread about bolting on my JE 290 kit and hope to have my dyno runs posted for tommorow. Till then, Cheers

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CBDADDY

There was no grinding involved? Was there much clearance between the cases and the Jug? I would think that it must be a snug fit in there. It makes me wonder about expantion of the jug vs the cases?

No grinding required. It is snug, but still drops in with clearance around the cylinder spigot. I can't remember exactly, but I think the wall thickness on the spigot was around 3mm, which should be enough to keep it from flexing or breaking. It came with a brand new cylinder with a much larger spigot OD than stock, but still dropped right down in the cases.

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I finally got my 270 kit installed and rode the bike with it. It was a gorgeous global warming induced 70F January day if you can believe that!

Anyways took it to the mx track. Few easy laps without hitting the jumps to break it in then let it cool down a couple times. Then took it out and got on it. Needed some air screw adjustment. Could definetly feel an improvement.

To make a long story short, after break in, I experimented by going richer and leaner with the main jet just setting the air screw to optimal for each main. I was expecting better performace with the smaller main but in fact found that I got the best performance by going up one size to a 188 main 1 1/4 on the screw. It pulls hard all the way to the limiter and is strong and respnsive from down low. I'm really pleased. I can't really tell if it revs slower, i don't think it does, just hooks up better, more power and more useable power without losing the top end. The leaner jet let it rev freely but it had that hollow tinny sound and would'nt pull strong in the low to mid. It was unquestionably happier with the bigger main.

I think there is more power/response to be had with some more jetting work, probably a more optimal needle. I suspect there may be more power to be had on the top end also, my stock pipe or the head may be too restrictive, but there is no loss of power.

Now we'll see how it holds up. So far very, very pleased with the results and can't wait to get it off road in a hare scramble where I think it will really shine.

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I'm not going to make Sumter this year, have to go out of town for work.

It's not the Thumper kit. Picked up a cylinder that had been bored and plated 3mm over by Millenium Tech, a Vertex piston and some Cometic Gaskets. Same end result, just cost less.

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