Re-jetting for carb change

  • moonwell
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Re-jetting for carb change

09 Jul 2007 22:52
#155838
I just acquired a set of clean looking '75 28mm carbs, supposedly from a recently running bike, off of ebay for pretty cheap (seller lost one of the pilot jets down a furnace grate :lol: )

I currently have '79 28mm pumpers on the bike (thought they were bored to 29mm, but I just discovered that my caliper was just inaccurate :unsure: ) As per the advice of wired george, I've decided to ditch the pumper carbs, especially since they're all missing parts from the check valve.

On to my question...
The jetting on my current carbs looks to be pretty much right on, the plugs are nice & tan. Should I be able to use the same jet sizes in the new carbs as in my current ones? I know they're both 28's, but the design is quite a bit different. My current carbs have 15 pilots and 127.5 mains. New ones came with 15 pilots and 120 mains with 5J9 needles on clip 3.

Thanks for any advice!

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  • wiredgeorge
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Re: Re-jetting for carb change

10 Jul 2007 05:03
#155874
The jetting in the earlier 28s has nothing to do with the later 28s. What airbox and pipes are on the bike and what altitude is it operated at? Stock jetting is #17.5 pilots, #112.5 mains and 5J9-2 (2nd clip for jet needle) for stock jetting which is OK for the stock airbox.
wiredgeorge Motorcycle Carburetors
Mico TX
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Too many bikes to list!

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  • moonwell
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Re: Re-jetting for carb change

10 Jul 2007 05:56
#155886
Bike has K&N pod filters & 4-1 exhaust. I've been told it's also 1015cc and 9:1. Operated at an elevation of 950'.

Stock size mains on the 28mm pumpers looks to be a 102.5, and they have 127.5's now, so thats a 10 size increase. Keep with a 10 size increase over 112.5, and try 137.5?

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  • wiredgeorge
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Re: Re-jetting for carb change

10 Jul 2007 12:15
#155975
The larger main jets on the pumper carbs were unnecessary... the bike will pull cleanly to redline with #112.5 main jets. It would actually run better if jetted correctly.

The Z1B 28s are richer to begin with. #20 pilots, 5J9-3 and #120 mains would be about right.
wiredgeorge Motorcycle Carburetors
Mico TX
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Too many bikes to list!

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  • moonwell
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Re: Re-jetting for carb change

10 Jul 2007 12:54
#155987
Thanks for the advice :)

I'll see if I can get some #20 pilots from the dealer after work & give that a try.

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  • moonwell
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Re: Re-jetting for carb change

10 Jul 2007 19:24
#156051
Picked up some #20 pilots, left in the 120 mains and installed the new carbs tonight. Bike seems to warm up quicker, idle better and pulls hard to redline.

I knew wired would have the right answer. Thanks :)

Now it looks like I'll have to re-tap the set screw to the top of the petcock so I can reverse the lever, it hits on the carbs now :(

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  • kawsakiman
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Re: Re-jetting for carb change

10 Jul 2007 22:00
#156066
wow!
i was watching that one also.
glad i did not snipe another member!
wish you luck with them.
someday i will be able to afford my kz habit.

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  • wiredgeorge
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Re: Re-jetting for carb change

11 Jul 2007 04:31
#156084
Loosen the locknut on the petcock and turn it so it doesn't hit the carburetor. The petcock can be replaced with a Drag Specialty or Pingle and they are both smaller profile although I can't swear they won't bump the carbs if mounted like your current set up. You can get either from www.z1enterprises.com .
wiredgeorge Motorcycle Carburetors
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  • JR
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Re: Re-jetting for carb change

11 Jul 2007 04:49
#156090
Hi moonwell,
Looks like you may be getting a kink in that clear fuel tube. May want to watch out for fuel restriction.

Happened to me.
1980 kz750E1, Delkevic exhaust

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