How To Re-jet Your Cafe Racer Carburetor?
Simple guide and calculations
We all like having a loud motorcycle. K&N or DNA air filters and some loud pipes will do the job, but you should not forget to re-jet the carburettor(s). 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 / DNA 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 DNA/K&N 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 / 600 meters 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.
PILOT/IDLE FUEL JET SIZES CHANGES According to the MODIFICATION:
Pilot/Idle 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 /idle 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 6000feet/900meters above the sea level.
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:
BMW R100 with Bing carburators 94/40/123&124 using straight exhaust pipes without muffler and original airbox is deleted and replaced with 2 pcs of DNA open airpod filters. And the motorcycle is primarily used around sealevel.
Bing constant depression carburator: #94/40/123&124
160 Main Fuel
45 Pilot/idle Fuel Jet
2.66 NeedleJet
2 NeedleJet clip position
MAIN FUEL JET SIZE CALCULATIONS:
Changes made:
Exhaust: straight exhaust pipes without mufflers = +4 Sizes Main Fuel Jet
Intake: DNA open Airpod 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 = 6.
Overall, you have a size with +6 according to your modifications. This means 160 (stock size) + 6 = 166.
Since the altitude is at sealevel, no need to remove anything else
The jet size you need on that motorcycle would be a 166 size main jet. But as the closest one is sized 165, you can can start with that, but most likely you need to go up to the next ten, meaning a size 170 main jet, as example used for R100RS engines basic configuration.
PILOT/IDLE FUEL JET SIZE CALCULATIONS:
The formula is: +1 pilot/idle jet size increase for every +3 main jet sizes increased.
Stock pilot fuel jet size is 45 + 2 additional jet sizes (since we went up +6 on the previous step) = a #47 pilot fuel jet size. As there is no size 47 for bings, closest idle jet size would be 48.
Note that no altitude compensation is needed on the pilot fuel jet since our elevation is less than 6000 feet.
Extra notes:
In case you feel that after the above mentioned changes your engine feels not quite right between 25%-75% throttle, that is the level where needle jet is mostly providing the needed fuel. Then before changing your needle jet from 2.66 to 2.68, you can try to raise the needle clip position upwards. That is to raise it from original clip position 2 to 3. This change will influence the mid range throttle and has impact on transition both from idle jet to midrange, as midrange to main jet. Check syncronizing your carburator article for more detailed info on throttle ranges and reading spark plugs.
Parts of content courtesy of CRW