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102mm Big Bore XR650L Jetting

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My 650L used to have a 101mm Weisco 8.7:1 piston that ran fine with Dave's Mods with a 158 Main, 55 Slow, DJ Needle on the 4th clip, pilot roughly 2.5 turns out. UNI filter, snorkel removed, FMF silencer.

 

It's now getting a 102mm JE XR600R piston at 10.8:1 compression (in the L it'll be somewhere from 9.5-10:1 I'd assume) so I'm wondering what jets to use? I've also got a new Mikuni TM40 pumper, but that hasn't been dialed in yet so I think it'd be easier to use the stock carb for break in and then mess with jetting the TM40 once the engine is fine.

 

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Leave it as is.

 

I have a XR's Only 103mm, 10.1 cr piston in my bike with 55/158 or 160 main jet. Runs like a top.

Edited by mcma111
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The jetting should be similar. Usually a bump in compression makes a motor go slightly rich , but the bigger bore changes nothing.

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What they all said.  I didn't change a thing when a similar modification was made to my engine and it runs great with the jetting that worked before.  I was going to drop the main jet from a 158 to a 155, but on the recommendation of mcma111 who posted above, left it a bit on the rich side to protect the engine.  As I mentioned, mine runs really well in all conditions I've ridden in from 35 degrees to 100 degrees and sea level up to 6000 feet or so.

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It only takes a couple of hundred miles to break in a new piston.  I did most of my riding in the mountains on tight twisty roads so that I was constantly shifting gears and accelerating and decelerating.  Just don't get it hot or spend significant amounts of time at full throttle.  A little bit of full throttle is a good thing, though.

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Running it to ''redline'' going through the gears is fine , you just don't want to hold it at ''high'' RPM for the first couple hundred miles. 

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2 hours ago, ScottRNelson said:

It only takes a couple of hundred miles to break in a new piston.  I did most of my riding in the mountains on tight twisty roads so that I was constantly shifting gears and accelerating and decelerating.  Just don't get it hot or spend significant amounts of time at full throttle.  A little bit of full throttle is a good thing, though.

I agree with your process sir

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