Proposed Cold Start Mixture Improvement

[Ken Williams]

I have always found the performance of the EPA approved cold start circuit on my Vision annoying.  With a fresh battery, fuel in the carbs and a few squirts from the accelerator pump, my bike would always start.  Often though, it would die once or even twice, especially when the ambient temperature was low.  And until warmed up, unless the enrichener lever was given constant attention, the engine would either idle at 4000 RPM or die.  These symptoms indicated the fuel mixture provided by the cold start circuit was too lean.  From my reading of the forum, it appears that some but not all Visions share these characteristics.  If your Vision does, you may be interested in my solution.

To richen the cold engine fuel mixture, I placed a homemade jet in the air intake for the cold start circuit of each carb.  The cold start air intake is located in the wall of the carb air intake bell, visible with the air box top removed.  On the 83 model, the cold start air intake is the hole located farthest to the left.  My jet consists of a 1.0 inch long piece of flexible plastic tubing with a .25 inch outside diameter.  First I split the tube down the entire length.  Then for .75 of the length of the tube, I cut equal amounts of the circumference away from each side of the original cut until the .75 inch portion would fit inside the cold start air passage without doubling over.  When spread out, the cut tubing looks like the letter T.  With the .75 inch portion of the tube inside the cold start air passage, the diameter of the passage is approximately .08 inch.  The top of the T serves to keep the entire tube from entering the air passage.  If it reaches in too far, the tube will completely obstruct the passage.  To minimize air flow disruption, the top portion of the T lies against the carb body.  Even though the tube is soft plastic, it could easily cause harm should it fall into the engine.  For this reason, I ran a piece of .024 inch stainless steel lock wire through the center of the top section of the T and secured it to the air box. Even if a tube were expelled from the air passage, it will not fall into the engine or jam the throttle.

My cold air jets were designed with two goals in mind, ease of prototyping and making no permanent alterations to the carbs.  Tapping threads into the air intakes and installing center drilled screws, center drilled wooden dowels and dabs of epoxy are some other methods that could achieve the same result.

I installed my plastic cold air jets last spring and enjoyed improved cold idle performance all season.  My bike now starts without dieing and idles much better while warming up.  Overall, my cold idle performance now approaches but not equals my experience with pre-EPA Ducatis and Yamaha RDs.