Honda Twins banner
1 - 6 of 6 Posts

·
Registered
Joined
·
239 Posts
Discussion Starter · #1 ·
Trying to get this straight in my head. On the CB350 carbs we have a screw sometimes called a mixture screw, sometimes called a pilot screw. Fuel is drawn in through the pilot jet, air is drawn in through the slow air jet, and the two are mixed BEFORE ever encountering the screw. This mixture flows past the transition ports to the mixture screw and on to the idle port. The screw itself only regulates the mix going to the idle port, not the transition ports. Anyway, since the screw in question regulates this MIX of air and fuel, it seems to me that it is not a mixture screw at all, but really just a pilot screw, and would only regulate idle SPEED at fully closed throttle. I mean how could it really adjust mixture if it's changing the amount of air and fuel at the same rate. Seems that only by changing the pilot jet could you actually change the mixture ratio. Not like on some carbs that have an actual air screw or an actual fuel screw. Any thoughts on this?
 

·
Premium Member
Joined
·
24,208 Posts
The idle circuit is supplying an air/fuel mixture thru the idle port and transition ports. It's blended with the air flow from the partially open throttle plate at idle. The actual mixture supplied is too rich in of itself to burn.
 

·
Registered
Joined
·
239 Posts
Discussion Starter · #3 ·
I see. So since the mixture is too rich on it's own, then I guess it could be viewed as a fuel screw. In other words, turning it out would give more fuel and air, but in a too-rich ratio. I guess I wasn't thinking in terms of mixing with the little bit of incoming air under the closed throttle plate.
 

·
Premium Member
Joined
·
24,208 Posts
Don't feel bad, the terminology changes depending on what year/vintage vehicle you're working with. And at times there are more than 2 names for any given part which leads to lots of confusion. I've only been dealing with carbs for 50 years and still get screwed up mentally at times.
 

·
Registered
Joined
·
239 Posts
Discussion Starter · #5 ·
Maybe you can answer this as well - I understand that when setting the mixture screws you should attempt to keep the throttle plates as closed as possible to keep from getting into the transition ports early. I think if you have the throttle plates open too far, then fuel through the transition ports will make setting the mixture screw tough. So what is the process for getting to this sweet spot. If you start with the throttle plates closed so it will barely idle, the transition ports are covered and you can set the mixture. I get that, but THEN after setting the mixture, you will eventually have to open the plates anyway to get a decent idle. So does it really matter where you start with the throttle plates in the process? Thanks for the help.
 

·
Premium Member
Joined
·
24,208 Posts
There's quite a bit of throttle plate movement before you get to the transition ports generally speaking. Once you get an idle speed of 1100-1200 you're only dealing with the adjust port, somewhere in the 13-1400 range the 1st trans port starts to be exposed.
Biggest problem I've had/seen is that the stock tach is inaccurate, it may read high or low depending on temp, age, cable condition, etc. This throws accurate idle setting out the window. Also trying to balance the carbs by exhaust pressure.
Using an electronic tach for idle speed is best and also using some method of balancing the carbs so both of them are exactly the same throttle opening at idle. I use a Uni-Syn for that but there's folks that drill/tap a vacuum port for vacuum gauges also.
 
1 - 6 of 6 Posts
Top