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How To Re-jet Your Cafe Racer Carburettor

11 Aug 2014

We all like having a loud motorcycle. K&N air filters and some loud pipes will do the job, but you should not forget to re-jet the carburettor(s). If you would like more information on cleaning and rebuilding a carburettor, check out our article on the matter! We even have an article on how to tune a carburettor!

Here is what you should think about if you want to re-jet the carburettor(s) of your Cafe Racer:

Exhaust Modifications:

  • 4-into-2 exhaust: main fuel jet size +2
  • 4-into-1 exhaust: main fuel jet size +4
  • No muffler (open headers): main fuel jet size +4

Intake Modifications:

  • Single K&N filter (inside a stock air box): main fuel jet size +2
  • If you drill holes in the air box (with a stock filter): main fuel jet size +2
  • Individual pod filters (without air box): main fuel jet size +4

Extra Modifications:

- Add up all the jet size increases and subtract 2 from that number.
- Decrease the jet size by 2  for every 2000 feet above the sea level
- Under certain conditions, such as the use of pod filters with a 100% stock exhaust, 4-into-1 header with the stock filter and air box, subtract 2 jet sizes.

Carburetor Gif

 

PILOT FUEL JET SIZES CHANGES According to the MODIFICATION:

 

Pilot fuel jet size changes are only related to the change in main fuel jet sizes according to what is described above. Note that this rule concerning the pilot jet size applies to the main fuel jet size changed BEFORE taking the altitude compensation into consideration.

It goes as follows:

Increase the pilot fuel jet size +1 for every +3 main fuel jet size.

Additional modifications:

- Decrease pilot fuel jet size by 1 for every 6000 feet above the sea level.

Jetting correction table

PRECAUTIONS:

Please make sure your carburettors are in perfect shape before making any changes to the jets. That means they need to be cleaned and rebuilt, operating properly according to their configuration, etc. Failing to do that will likely result in you wasting your time.

Check the plug colour often and make adjustment when needed. Proceed with 2 main fuel jet sizes and 1 pilot jet at a time. Bright white plug insulators indicate a lean fuel mixture condition which will cause damage to your engine.

- Synchronise the carburettors after every jet change.

- Make sure the floats are set correctly

- Consider purchasing a Colortune Plug Tuning kit.

- You may find it necessary to make changes to the size or shimming of the main jet needle. There are no guidelines on what or how to make these changes, this is true trial-and-error tuning!


EXAMPLE:

A 1982 XJ550RJ Seca using an aftermarket Supertrapp 4-into-1 exhaust and a single K&N air filter in the stock, unmodified air box. The motorcycle is primarily operating at an altitude of 2600 feet above sea level.

XJ550 Seca Stock Mikuni BS28-series Carb Jetting:

112.5 Main Fuel Jet
35 Pilot Fuel Jet
70 Main Air Jet
170 Pilot Air Jet
4GZ11 Needle


MAIN FUEL JET SIZE CALCULATIONS:

Changes made:

  • Exhaust: 4 into 1 with Supertrapp = +4 Sizes Main Fuel Jet
  • Intake: K&N Pod Filters = +4 sizes Main Fuel Jet

This gives a total of +8 main fuel jet sizes above the baseline. Then, remove two according to the criteria mentioned above.

Overall, you have a size with +6 according to your modifications. This means 112,5 (stock size) + 6 = 118,5.

Since the altitude is higher than 2000 feet, remove two from this number.

The jet size you need on that motorcycle is a 116,5!


PILOT FUEL JET SIZE CALCULATIONS:

The formula is: +1 pilot jet size increase for every +3 main jet sizes increased.

Stock pilot fuel jet size is 35 + 2 additional jet sizes (since we went up +6 on the previous step) = a #37 pilot fuel jet size.

Note that no altitude compensation is needed on the pilot fuel jet since our elevation is less than 6000 feet.

Here are some videos you can watch while you let this sink in!

Useful source

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