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Author Topic: How do I jet for a big bore?  (Read 2060 times)

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Offline Patrick

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How do I jet for a big bore?
« on: October 08, 2007, 06:38:28 PM »
When my K0 750 hits the road, hopefully in a couple of weeks, it will be equipped with an 810 big bore (64mm pistons times four), a cam that I'm told looks quite aggressive (http://i130.photobucket.com/albums/p247/wordguy11111/HPIM1843.jpg), new APE valve springs and what I believe is a stock head. It will have a K&N filter in a stock airbox, and it will have either a 4-2 MAC exhaust or a 4-1 MAC exhaust. I have both in good to excellent condition and I haven't decided which to put on. Suggestions are welcomed.

The bike has the original carbs with a cable running to each (I can't wait to try to synch that). The main jets, stock, are 120s. I suspect that to take full advantage of this setup I will have to re-jet. I further suspect that I will need to go bigger, but I'm not sure where a good starting point would lie. What size jet should I start with?

Patrick
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Offline Sam Green Racing

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Re: How do I jet for a big bore?
« Reply #1 on: October 08, 2007, 06:44:52 PM »
Don't be surprised if you have to go the other way ::) ::) ::) ::)

Sam. ;)
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Offline Patrick

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Re: How do I jet for a big bore?
« Reply #2 on: October 08, 2007, 06:50:50 PM »
That would work as well. Better, in fact, since I already have smaller jets. Bigger ones I'd have to buy.

Patrick
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Offline Sam Green Racing

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Re: How do I jet for a big bore?
« Reply #3 on: October 08, 2007, 07:13:22 PM »
Our CR750 ran 120 mains when it was a stock 736 with CR33s, open 4 into 4 megaphones and a Boyer ignition system.
When we upped it to 836, with cam and head work it was as rich as hell with the same set up.
We came down to 118s and fitted a 4 into 1 system and it was still rich. We dropped down to 115s then 112s and it was still rich.
We got down to 110s before it went weak. We went back to 112s and took it to a dyno.
Tests on the dyno showed that the rich running was down to the slow running during warm up and riding slow back down the strip.
We did a carb sync, dropped the needles down a notch and turned the air screws out quarter of a turn. It cleaned the bottom end up some but made it a little weak higher up. We went back up to 115 mains and it was as near perfect as we could get it.
The bike is now a tyre smoking monster. ;D ;D ;D ;D ;D ;D ;D ;D

Sam. ;)
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Offline Patrick

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Re: How do I jet for a big bore?
« Reply #4 on: October 10, 2007, 11:29:20 AM »
Thanks, Sam. That is instructive. So I guess I'll start with 115s and go from there.

Patrick
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Offline mlinder

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Re: How do I jet for a big bore?
« Reply #5 on: October 10, 2007, 12:22:05 PM »
Sam uses CR carbs.
Can't go by what he's running.
They mix fuel completely differently, or something.
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Offline Patrick

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Re: How do I jet for a big bore?
« Reply #6 on: October 10, 2007, 12:44:47 PM »
That being the case, I am open to suggestions....
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Offline mlinder

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Re: How do I jet for a big bore?
« Reply #7 on: October 10, 2007, 12:47:19 PM »
You'll probably need 125's, 130's at the outside, I'd imagine. And also, larger idle jets. 45's or 50's, I'd guess.

/edit: That being said, it also, obviously, depends on altitude, and your bike.
I'd start with 45 idle jets and 125 mains.
Not because it's big bore, but because of your exhaust and intake.
« Last Edit: October 10, 2007, 12:50:15 PM by mlinder »
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Offline 750essess

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Re: How do I jet for a big bore?
« Reply #8 on: October 10, 2007, 12:49:34 PM »
With a stock airbox on it, I would say 120's should be pretty close maybe a little bigger.
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Offline Patrick

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Re: How do I jet for a big bore?
« Reply #9 on: October 10, 2007, 02:30:13 PM »
OK, I guess I'll start at stock and see what my plugs tell me. I can always rejet without pulling the carbs.

Patrick
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Offline mlinder

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Re: How do I jet for a big bore?
« Reply #10 on: October 10, 2007, 02:46:17 PM »
Quote from: Patrick on October 10, 2007, 02:30:13 PM
OK, I guess I'll start at stock and see what my plugs tell me. I can always rejet without pulling the carbs.

Patrick
One of the nice things about our carbs, eh?
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Offline Sam Green Racing

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Re: How do I jet for a big bore?
« Reply #11 on: October 10, 2007, 03:05:33 PM »
Hey Mark, one of the good points of the CRs is that you can change all the main jets in just over 5 mins. ;D ;D ;D ;D ;D

Sam. ;)
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Offline mlinder

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Re: How do I jet for a big bore?
« Reply #12 on: October 10, 2007, 03:12:30 PM »
Quote from: SamCR750 on October 10, 2007, 03:05:33 PM
Hey Mark, one of the good points of the CRs is that you can change all the main jets in just over 5 mins. ;D ;D ;D ;D ;D

Sam. ;)

Can do the same with the K0 carbs :)
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Offline Bob Wessner

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Re: How do I jet for a big bore?
« Reply #13 on: October 10, 2007, 03:56:05 PM »
Quote
Can do the same with the K0 carbs


 ;D ;D ;D ;D ;D ::)
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Offline Patrick

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Re: How do I jet for a big bore?
« Reply #14 on: October 10, 2007, 04:02:00 PM »
If you don't mess up the float bowl gaskets. Not that I've ever done that....
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Offline mlinder

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Re: How do I jet for a big bore?
« Reply #15 on: October 10, 2007, 04:04:15 PM »
Quote from: Patrick on October 10, 2007, 04:02:00 PM
If you don't mess up the float bowl gaskets. Not that I've ever done that....

I've had the float bowls off, I dunno how many times. Lots.
No issues. Haven't replaced the gaskets.
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Offline Patrick

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Re: How do I jet for a big bore?
« Reply #16 on: October 10, 2007, 05:15:20 PM »
Me too. Screwed a float bowl gasket only once on a last minute adjustment when I was already late......
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