Topic: Another (peculiar) mixture/carb problem

[black-dahlia]

Hey all...
I have searched the forums for an answer to this, but haven't found anything that is exactly appropriate for this problem.  Ok, I have a '74 cb 750 w/ mac 2 into 1 exhaust and stock airbox/intake/carbs.  I know that the stock setting for the idle mixture is one turn out.  In addition, I know that you get the bike to operating temp and adjust the idle screws until the highest idle is achieved.  I have done this and everything seems fine: max idle is around a whole turn out.  However, with this setting, the bike cruises (3-4000 rpm) unevenly and at WOT it bogs down completely.  Turning the idle screws out an 1/8th of a turn at a time, I found a setting where the cruising speed problem AND the WOT problem are nearly nil.  However, idle is now uneven.  My question is this: Plugs and performance indicate a rich setting when the screws are at one turn out.  Leaning the idle circuit helps, but I wonder if it might make for a smoother idle to set the screws back to one turn and raise the needle from the 4th position (from the top--which is stock) to the third.  Please advise....Thanks.

[csendker]

I know this, or something similar is around here somewhere, but I'm too tired to look.  I attached a diagram showing the impact of various changes of the pieces-parts vs. various throttle settings.  Take a look and see if you can't decipher a solution based on your circumstances.  Idle air screw impact at WOT is nil.  Of course, so is the needle position. About the only thing that does have an impact at WOT is the main jet.  But you have to factor in your cruising speed problems too.

[kghost]

There are what you may consider as three distinct circuits to the carbs

Idle
Midrange
Wide open Throttle

Now, while idle screw adjustment plays a part between idle and midrange, the idle portion of the carburation is not actively doing alot at partial throttle. Turning out the screws in the idle circuit is only masking one problem at the cost of creating another.

If you are experiencing uneven running at mid range throttle positions your probably rich.
dropping the needle a clip position should help your midrange and possibly the transition to WOT

Course the big question is.....was the bike ever set up or rejetted for the 4-2 exhaust?

If the answer is no do ya really still think its the idle circuit giving you trouble?

[csendker]

For total clarification on the clip position, I keep this picture handy: