I understand but its actually another jet system completely separate from the pilot and main. It allows you to richen the fuel mixture without the use of a choke. The farther you pull it out the more fuel and air flow into the engine where is a choke chokes off the air causing atmospheric to push more fuel through the jets. You can see the fuel enrichment is a much better way to meter fuel into a cold engine. Very precise compared to choking the opening off.  Also because it is a fuel enrichment circuit there is nothing wrong with using it as aux fuel source as needed. Because it basicly like open the main or pilot more but only temporarily.    

Gary_in_NJ wrote on 05/10/24 at 04:33:41:


Well said Gary.  Completely agree.  

Whether you call it a “variable fuel enrichment” circuit or “variable choke”, it’s not intended or designed to modify the air/fuel mixture under normal riding conditions.

04Savage - your blueing exhaust header is a good sign your bike has been running HOT and LEAN quite some time (probably its whole life).  While some slight blueing is normal, yours is beyond slight.  A light golden color would suggest running a bit cool and rich.

While you can use the “fuel enrichment system” to address the mixture while on the pilot circuit and slightly into transition to the needle, your bike will continue to run lean through the mid range on up.  Eventually harming the motor.  But you being a carb expert, you probably already know that.

https://www.youtube.com/watch?v=hDsJH1Y9W_U please watch this the pilot got nuttin to do with it.   Although the author of this video explains things wrong it is close. Vacuum DOES NOT PULL FUEL IN, it creates a low pressure area and atmospheric pressure acts on the fuel pushes it through jets etc. Its important to understand how this works. So again the fuel enrichment circuit bypasses ALL jets etc and is a stand alone system that can run the engine. Trust me on this I never lie and I'm always right LMAO.

04Savage04 wrote on 05/10/24 at 13:26:30:


So... you say that atmospheric pressure pushes the fuel up to the choke valve? way above the fuel level in the bowl? you might want to rethink that statement.

A Bing carburetor - totally different design carb in the video.  As a carb expert, I suggest you do a little homework on the stock carb on our bikes (Mikuni BS40SS).  Lots of good information in the Tech Section.  In the area he describes as the fuel enrichment system, our Mikuni BS40SS has a Transient Enrichment Valve (TEV) that functions somewhat differently and it’s not part of the “choke system”.  Our carbs have a “variable level choke”.

If you can put your pride aside as a carb EXPERT, and always being RIGHT, we are trying to help you LEARN about the carb on our bike.

If you want to be stubborn, go ahead and keep using the choke feature to enrich the fuel mix.  If you want a properly tuned carb, properly rejet it for the correct air/fuel mixture throughout the entire power band.

Regardless of how I ride (idling at stop lights, putting around in the midrange, or WOT), I want a properly tuned carb on my bike.

Believe what you want about the blueing header, but it takes more than a little hard riding to get the color of your pipe.  I’d bet money it’s been blueing a long time (but you nor I, nor the prior owner’s story can prove it).

DragBikeMike wrote on 05/10/24 at 20:52:30:
I worked for GM for a while back in the day, they had something called CARB SCHOOL. They show you everything you need to know to be successful working with carbs. Of course I was one of their best engineers. Like I said probably repaired-rebuilt-modded well over 1000 carbs.  Back in the day I had a pretty good following with the Harley guys and of course every GM car in the area. Unfortueately messing with all the fuel over the years has left me with lots of airway lung troubles. But I can school anyone when it comes to carbs and electrical system engineering, I also do vintage tube amp repair. Please don't hate me.

Peace out, no sweat.  Ride it on the choke rather than jet it properly.  Your bike, your ride.