Carb tuning, larger accelerator pump nozzle?
One thing to note. I noticed on my last drive when I parked after driving (still running), when stopped, parking brake on and clutch in, rev the motor to 2k+ RPMs it would dip lean. Go from ~mid 13's to 16-17:1 then catch up. So even a static rev it leans out with no load. These were moderate throttle blips moving my foot on and off for about 1 second. Did it several times and it did it every time.
Basically this is correct.
Fuel mix should be somewhat rich at idle and lean out as the throttle is opened. Approaching wide open throttle for maximum power (approaching maximum load on the engine) a richer mixture is needed. This was well established emperically a long time ago.
See here *
The most basic test for the "Idle" circuit is to slowly increase the throttle opening with no load. For every increase in throttle there should be an increase in rpm and no misses. That check for too lean is from Urich and Fisher's
Holley Carburetors and Manifold. Urich was VP of Engineering at Holley. I actually found picking up a small miss that way is difficult. I'd say if it does miss with that check, then the idle/transition is going too lean for sure. The question isn't whether its lean, its whether its too lean.
I can post up some screenshots of datalogs if you want showing this is real, not just something done in test facilities. Although those folks made some damn good engines
go.
Now a point here of embaressment for not checking. I've been assuming this was a two corner idle carb without having looked it up.
:rolleyes:
Is your carb a four corner idle carb?
This one?
Quick Fuel SS-650 SS-Series Carburetor 650CFM
If so, then the secondary side is in play and has to be dealt with accordingly.
Basically having the secondary idle circuit in play provides a second system, which if basically the same as the primary idle system doubles the fuel flow.
This can be a big help with an engine that has poor manifold vacuum at idle. But yours has fairly good vacuum at idle.