Author Topic: Jetting  (Read 1258 times)

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Marksan

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Jetting
« on: February 04, 2008, 05:00:57 PM »
Hello,

 l have a 1975 CB750F1 stock except for kerker 4into2 exhaust and pod air filters, i will be riding at 2500 to 6000 feet. l am going to rebuild the carbs (as soon as above arrives from fee bay  ;)}   Any idea of what size main jet do l start with? l understand its a $64000 question! so anyone been there?

Many Thanks

Offline KB02

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Re: Jetting
« Reply #1 on: February 05, 2008, 05:46:01 AM »
If you do a search, someone posted an article a little while back that gave good "starting points" for jet selection, and that would be a good place to look. It was one of those "If you've done this, then add one jet size. If you've done this add two. If you add so many, then take one back..." kind of things. I can't remember who posted it.

Anyway, once you find that, use that, like I said, as a starting point. Two identical bikes may run differently with identical set-ups. You elevation will play a bit of a factor (usually a smaller jet size, I think...). Once you have our starting point, ride it around, do some plug chops (do another search for those if you're not sure what they are) and go from there.

Yeah, it really is the $64,000 question; and that's mainly because every bike is different. There are just so many variables that there's never just a "Put this Jet in" kind of answer.

Well, Good luck. Hope this helps.

-Bob
KB02
1978 CB750K Project
2000 Ducati ST2
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Offline SteveD CB500F

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Re: Jetting
« Reply #2 on: February 05, 2008, 05:49:27 AM »
Try this dissertation on jetting by TwoTired in the Carb FAQ.

http://forums.sohc4.net/index.php?topic=5410.msg203462#msg203462

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Offline KRONUS0100

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Re: Jetting
« Reply #3 on: February 05, 2008, 06:54:06 AM »
I'm not sure of Central Indiana's elevation...but my 76 CB750F runs a Thrush 28" 2 1/2" diameter glasspack off factory 4 into 1 head pipes.  I run a modified stock airbox with K&N filter.. 120 mains.....40 pilots..airscrews 1 1/2 turns out.  Bike runs ever so slightly rich.  It has been to Kentucky, Illinois, and Ohio running like this. Averages about 35mpg carrying me and the wife with the Windjammer and hard bags.  Good Luck.
MATT
current bikes:  1976 CB750F, 1981 GS1100E
bikes owned:1981 GL1100I, 1990 GS500E, 1981 GS850, 1977 and 1979 GS750, 1974 CB750, 1975 CB750, and a 1982 GS750E