Sunday, 28 October 2012

Mixing chamber

Mixing chamber

Check and clean all threads in the mixing chamber. If no choke is fitted ensure the opening for the choke cable is sealed with a bolt, (P/N 4/137A) . Replace worn screws and ensure they are correctly fitted with spring washers.

Check the manifold flange against a flat surface to determine if it has been distorted by over-tightening or tightening to an uneven surface. If there is distortion so that the throttle slide jams when the carburetter is bolted to the inlet manifold then the carburetter needs to be replaced. Replace the O ring if there is are any signs of hardening or damage.


The free length of the throttle spring should be 3”. Replace if corroded, damaged or compressed.  Inspect the air valve components for excessive wear or damage.

Inspect the throttle slide for excessive wear. If the wear pattern on the engine side of the slide is visible as a series of furrows or waves in the metal or the step or groove at the top of the slide on the engine side has been worn away then the slide requires replacement. Eventually the mixing chamber body will also wear to the point where the amount of air leaking around the throttle slide prevents accurate tuning of the carburetter and a replacement carburetter will be required. Wear to the carburetter will be much reduced by ensuring that an effective air filter is always fitted.

Check the tickler mechanism to ensure it moves smoothly.  An upgrade kit is available to convert earlier ticklers to the later version with the large button.

Sleeving the Carburetter

There are solutions available to address body and slide wear which involve boring the mixing chamber body and fitting a brass, steel or aluminium sleeve to either the body or the slide.  Sleeving is not recommended for the following reasons;

(a)     Thinning the walls of the mixing chamber makes the carburetter more prone to warping when the flange bolts are being tightened.
(b)    Sleeving the carburetter to an incorrect clearance between slide and body will make the carburetter difficult or impossible to tune correctly. 
(c)    Inadequate wall strength and tight clearances can cause the slide to stick dangerously. 
(d)    Poorly executed sleeving can dislodge or damage the spray tube.
(e)    Slides in bored mixing chambers can only be replaced with a further sleeved slide.
(f)    Poorly sleeved carburetters can have the slide locating lug masked to the point where the slide can override the locating slot and jam.

Pilot Circuit

Mark 1 Concentrics are equipped with two types of pilot jets.  2 Strokes use a removable pilot jet which should be replaced if it is damaged or shows signs of oxidisation. 4 Strokes generally use a pressed in bush pilot jet in the gallery behind the pilot air screw. 

Fuel residues and oxidisation can cause problems with the pilot circuit, particularly in bikes that have been left standing with fuel in the carburetters for several months or more. It may be sufficient to clean the pilot circuit with an aerosol carburetter cleaner, stopping the various outlets selectively to ensure the entire system is clear.  Do not forget to inspect the two pilot circuit outlet holes either side of the slide location slot in the bottom of the carburetter bore. If necessary the pilot bush can be cleaned by removing the pilot air screw and using a No 78 or 0.016”drill, held securely in a suitable extension, rotated lightly in the bush to remove encrusted deposits. 



No comments:

Post a Comment