Old Britts – Modifying K&N Air Filters for Twin Mikuni Carburetors
Download the complete PDF version using the green download button at the top right of this page for a detailed Old Britts workshop article explaining how to shorten K&N tapered air filters so they will fit twin Mikuni carburetor installations on Norton Commando motorcycles. This useful guide also applies to similar clearance problems with twin Amal MKII carburetors and provides a practical solution where standard filter length causes fitment issues.
This article by Fred Eaton explains why twin Mikuni carburetors can create air filter clearance problems on Commandos, particularly because the wider carburetor bodies and splayed intake manifolds leave less room behind the engine. The guide shows how a standard RC-1250 K&N tapered filter can be carefully shortened, then rebuilt by cleaning the original end cap and bonding it back onto the reduced filter body.
Clear workshop photographs show the modified filter beside the original full-length version, the filter marking and cutting process, removal of old filter material from the end cap, the hand tools used and the final silicone sealing method. The article also notes that a later K&N filter became available that fits twin Mikuni carburetors without modification, while retaining the shortened-filter method as a useful option where extra clearance is needed.
Contents Include
- Twin Mikuni carburetor air filter clearance issue
- K&N tapered air filter shortening procedure
- Comparison of modified and standard filter length
- Marking and cutting the filter body accurately
- Removing old filter media from the end cap
- Cleaning and preparing the original filter cap
- Silicone sealing method for reassembly
- Clamping or weighting the filter while curing
- Application to Mikuni and Amal MKII installations
- Useful Commando intake modification reference
