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Old 03-26-2012, 03:00 PM   #1
Jae
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How to size a carburetor?

Ok, quick rundown of what I'm looking to do and why:

bike: 1982 Kawasaki KZ305
problem: torn diaphragm in each carb
possible solutions:
- replace diaphragms @ $90 each
- replace carbs entirely with single Mikuni flat slide (~$130-150)

Just from a cost standpoint, it seems that it would be cheaper to fabricate a 1-2 manifold for the little p-twin and use a single new carb than to buy the parts to do a full rebuild on the stock CV carbs. What I can't figure out is how exactly to size a carburetor. I've seen the Mikuni TM series offered in everything from about a 24mm up to 38mm (and higher, I'm sure), but don't really know where to start. Can someone provide some insight? This wasn't exactly a popular bike like the CB750, so there's not much tuning information available out there.
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Old 03-26-2012, 03:04 PM   #2
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I'm guessing a carb sized comparable to a 150-175cc single would work. You are only feeding one cylinder at a time so that should put you in the ballpark.

Personally I would attempt to repair the diaphragms with RTV if the holes are small. Of course, if the tears are large that might not work.
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Old 03-26-2012, 03:06 PM   #3
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One more thought...

While it might be cheaper to do the single carb idea (especially if you can find a used carb in good shape) if you factor in the time to fabricate the manifold and then dial in the jetting (plus the cost of possibly several sets of jets) repairing the stock carbs might really be the better way to go.
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Old 03-26-2012, 03:23 PM   #4
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one problem with the stock carbs is that there are hardly any aftermarket jets available for it (none that I've been able to find to date), and most of the guys on the KZ forum seem to think that if you need to rejet you almost just have to have a carb jet reamer kit and custom make jets.

when i bought the bike the guy had modified the exhaust (no real mufflers) so it's already running lean. a good cleaning of what is there helped some, but plugs are still pretty light so I'm thinking that any more modern carb would be an improvement over what's there, especially since I want to finish the bobber conversion on this little bike and run a pod filter (eventually).

I'll look at stock 4-stroke dirtbike carbs around the size you suggested and see what's out there.
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Old 03-26-2012, 03:24 PM   #5
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one problem with the stock carbs is that there are hardly any aftermarket jets available for it (none that I've been able to find to date), and most of the guys on the KZ forum seem to think that if you need to rejet you almost just have to have a carb jet reamer kit and custom make jets.

when i bought the bike the guy had modified the exhaust (no real mufflers) so it's already running lean. a good cleaning of what is there helped some, but plugs are still pretty light so I'm thinking that any more modern carb would be an improvement over what's there, especially since I want to finish the bobber conversion on this little bike and run a pod filter (eventually).

I'll look at stock 4-stroke dirtbike carbs around the size you suggested and see what's out there.
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Old 03-26-2012, 03:41 PM   #6
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Ok, I realize this isn't an apples to apples comparison, but what I've seen is this:

Sportster 883 v-twin stock carb size 40mm (CV)
Buell Blast 500 single stock carb size 40mm (CV)

so your comparison here for sizing the carb for a single cylinder closer to a 150cc seems spot-on. time to research some dirt machines i guess.

edit:
hmm... according to this website, the Honda CR125 uses a 38mm flatslide...
http://www.bikez.com/motorcycles/hon...125_r_2005.php
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Last edited by Jae; 03-26-2012 at 03:43 PM.
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Old 03-26-2012, 04:36 PM   #7
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my xr650r takes a tm40. make sure it has an accel. pump! it makes a big difference
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