Rejetting the Choke Circuit


Exchange mechanic">

Rejetting the Choke Circuit


Exchange 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.

  1. Remove the carbs.

  2. Drain the fuel (like you didn't know to do that!).

  3. Don't smoke (anything).

  4. Remove the float bowls.

  5. Identify the choke jet.

  6. Enlarge the choke jet orifice to .025" with a #72 drill bit or .60mm  on the 750. The original size was .021"/.53mm.

    1. 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.

    2. 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 electric drill!

  7. 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.

  8. However, even tough you THINK that you got all the metal, you need to completely clean the carbs before running.

  9. Check the float heights before you reinstall the carbs.

  10. Replace the float bowls

  11. 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.




  Remember! These are hints only.
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