Stalling letting off from high rpm

[dodj]

My engine idles great at about 600-700 rpm. But when the engine is up around 3-4000 rpm and you let it drop off quickly to idle, it stalls, most of the time. If you let the rpm's drop slowly, it idles fine. It does it less when at full operating temp.
Is there something I can do to make the transition to idle more reliable? The carb is a QFT BD-850. Other than fiddling a little bit with idle mixture and idle speed, it is as factory set.

[cudabob496]

is engine in tune? Newer dist cap, plugs, wires, timing, etc?
carb in tune? (float levels, needle and seat assemblies, etc). No vacuum leaks anywhere?

[Chryco Psycho]

Do you have an A/F meter to see what it is doing ?
does raising the idle to 750 rpm help ?

[dodj]

is engine in tune? Newer dist cap, plugs, wires, timing, etc?
carb in tune? (float levels, needle and seat assemblies, etc). No vacuum leaks anywhere?

Initial is 12*. 36* by 2800
Plugs wires etc are < 1000km
Carb is new, no tuning other than idle mixture and speed. Float levels are good, half the sight glass.

Do you have an A/F meter to see what it is doing ?
does raising the idle to 750 rpm help ?

I have an A/F meter. Cruising 2500-35-- rpm it is about 13.8-14.2. Idle is about 13.0-13.5. When I let off the throttle quickly, the A/F very briefly jumps up to about 15.5-15.9 then settles back down to 13-13.5.
I could try bumping up the idle I guess.

[cudabob496]

or maybe opening up the idle mixture screws another 1/8 or 1/4 turn or so,
or at least making sure they are set right.
Now that its mentioned, idle seems low for a big block.

Some cams do not support a low idle very well.

[Chryco Psycho]

As I assumed it is spiking lean when the throttle drops closed , so how to fix ?  A slight increase in idle speed would be my first step , maybe a bit more fuel @ idle with the idle mixtures .

[cudabob496]

Maybe there's a vacuum leak. when you go off throttle, A/F goes high.
This is occuring when the vacuum is increasing due to going off throttle,
maybe drawing more air into carb.

[dodj]

I leaned out the idle from factory setting to try to get closer to 14.7. I thought that was the afr  you want?
I will do as you guys suggest and up the idle speed a touch and then if needed try opening up the idle mixture screws a bit.
Appreciate the help guys 

Cudabob, the carb has a heat dissipating coating on it that is supposed to reduce carb fuel temps up to 6* according to quick fuel. 

[Strawdawg]

due to bad distribution and poor atomization, it is really hard to get a carb to idle at 14.7.  Add a performance cam and it gets worse.  I usually am happy to get a solid idle at 13.5

[cudabob496]

I leaned out the idle from factory setting to try to get closer to 14.7. I thought that was the afr  you want?
I will do as you guys suggest and up the idle speed a touch and then if needed try opening up the idle mixture screws a bit.
Appreciate the help guys 

Cudabob, the carb has a heat dissipating coating on it that is supposed to reduce carb fuel temps up to 6* according to quick fuel. 

heat dissapating coating? that's good! I was thinking it would even be somewhat easy, with sheet metal, to
make some heat deflection shields under the bowls, just by attaching to the 4 carb hold-down bolts, without having
to take the carb off the intake.

[jimynick]

Somewhere in the tattered vestiges of my poor old memory, it seems to me that some of the factories used to have what I believe was called a "dashpot" that briefly softened the throttle closing to prevent that very thing happening. Anybody else recall that? 

[Chryco Psycho]

I do remember the Dashpots , now done with computer control

[cudabob496]

maybe the return spring is too strong

[dodj]

Well I haven't taken it for a test drive - too much rain lately- but opening up the idle mixture seems to make it work much better. A little hard on the eyes in the garage though. AFR reading 12.2.

Better than stalling when you come to a stop sign! That was kind of embarrassing.

Thanks for the help guys