Rejetting the Choke Circuit
Remember! These are hints only.
mechanic, Stewart Carr of Mick Hone's Motorcycles of Melbourne,
Australia doing restoration work on the owner's antique FZ750.
If, after removing the airbox,
installing a more free flowing air filter or in some other way
decreasing the amount of vacuum that is drawn through the carburetor
bore, you find that the choke circuit no longer supplies adequate fuel
to start the engine from cold, follow these steps.
Remove the carbs.
Drain the fuel (like you didn't
know to do that!).
Don't smoke (anything).
Remove the float bowls.
Identify the choke jet.
Enlarge the choke jet orifice
to .025" with a #72 drill bit or .60mm on the 750. The
original size was .021"/.53mm.
Small number drills, like
the #72, can be purchased at some tool stores, most
electronics supply stores, hobby shops and some arts and
crafts shops. A complete set of drill (from #60 to #80)
currently costs $20 to $30 USD.
You will also need to
purchase a "Pin Vice". A Pin Vise is a
4"-6" / 100mm to 150mm shaft with a very small
collet that is tightened by a hand tightened threaded sleeve.
Very small drill bits are turned by hand - not with an
To drill small holes, you
should dab a bit of grease onto the tip of the drill bit, insert
and twist, remove the drill bit, wipe it off to remove the metal
filings, recoat with grease and repeat until you are
through. That will keep most of the metal from ending up in the
carburetor and your engine.
However, even tough you THINK
that you got all the metal, you need to completely clean the carbs
Check the float heights before
you reinstall the carbs.
Replace the float bowls
Do the rest of the job of carb
installation (you may choose to balance the carbs!)
Enjoy your newly happy motors
willingness to start on cold mornings!
Obviously, to start well, you need
a good battery!
If you somehow manage to break a
drill bit in the jet, we can remove the parts for a fee (not for
free). Be careful.
We can provide that service at the
San Rafael, CA Factory Pro Dyno Center #1 location.
You are responsible for whatever you
do with this information.
If you use this information and it helps your application, contact Factory Pro with
results. Thanks, Marc