mcwho
04-17-2017, 09:25 PM
Trying to understand what the relationship is between Transition slot (t-slot) for primary and secondary on Holley QFT carbs. As they seem to be of similar design, not much info on QFT but lots on Holley.
I have a QF ss-650-an. It came from the factory with the primary t-slot showing about .090 and the secondary t-slot not showing at all. Was told to make the secondary t-slot show a "square" and also the primary a "square". that would make both of them about .050. Supposedly the reason is the lower manifold vacuum which causes a stumble at just off idle.
My engine vacuum is right now at 14.5 ~ 15. A normal engine (according to my vacuum guage is in the 17 ~ 22 range. Therefore I believe what is normal for a normal aspirated engine would make the factory settings work for that engine.
The theory seems to be that the lessening of the vacuum for souped up engines requires these changes to the t-slot settings. Since my Vacuum is not that far from a stock the t-slot settings need not be as radical.
Therefore if my engine vacuum is say 15 or if it is 13, would the engine with the 13 vacuum need to have twice as much t-slot derivation?
That being said, after making these adjustments, my engine would not idle below 1200 and the idle air mix screws had no effect when I turned them in and out.
After that I closed the secondary t-slot setting by turning the set screw out about 1/3 of a turn. That change now made my idle at about 900 and the ims screws to work but my front screws for best vacuum are in the 5/8 turn range and the rears are in the 1/4 ~ 1 1/8 range.
My mathematical side says that closing the secondary screw slightly more , say 1/4 to 1/3 turn should allow me to get the idle to around 800 or 850.
Is this a reasonable idle for a 331 stroker with a mild cam, edelbrock performer rpm heads and RPM air gap intake, and a cam with 224/232 duration @ .050
Before I started this I replaced the spark plugs because the previous carb, an Edelbrock always ran rich and they were black.
I marked and noted where the original settings were. I increased the set screw on the setting of the secondary
by 5/8 of a turn or.625 turn, (1/8 - .125 turn, x5 = .625)
then I backed it out by 2/6 or a turn or .333, note 1 notch on an allen wrench is 1/6 of a turn or .166
my current effective setting is (.625 - .333) or .292 of a turn
therefore that .292 of a turn lowered the idle by 300 rpm
if it is a straight line 1/3 of my last change should net me about -100 rpm or 800 which is my goal.
.292 / 3 = .097 of a turn, or just over 1/2 a hex wrench notch
The pics are:
Primary after and before
and
Secondary after and before
I have a QF ss-650-an. It came from the factory with the primary t-slot showing about .090 and the secondary t-slot not showing at all. Was told to make the secondary t-slot show a "square" and also the primary a "square". that would make both of them about .050. Supposedly the reason is the lower manifold vacuum which causes a stumble at just off idle.
My engine vacuum is right now at 14.5 ~ 15. A normal engine (according to my vacuum guage is in the 17 ~ 22 range. Therefore I believe what is normal for a normal aspirated engine would make the factory settings work for that engine.
The theory seems to be that the lessening of the vacuum for souped up engines requires these changes to the t-slot settings. Since my Vacuum is not that far from a stock the t-slot settings need not be as radical.
Therefore if my engine vacuum is say 15 or if it is 13, would the engine with the 13 vacuum need to have twice as much t-slot derivation?
That being said, after making these adjustments, my engine would not idle below 1200 and the idle air mix screws had no effect when I turned them in and out.
After that I closed the secondary t-slot setting by turning the set screw out about 1/3 of a turn. That change now made my idle at about 900 and the ims screws to work but my front screws for best vacuum are in the 5/8 turn range and the rears are in the 1/4 ~ 1 1/8 range.
My mathematical side says that closing the secondary screw slightly more , say 1/4 to 1/3 turn should allow me to get the idle to around 800 or 850.
Is this a reasonable idle for a 331 stroker with a mild cam, edelbrock performer rpm heads and RPM air gap intake, and a cam with 224/232 duration @ .050
Before I started this I replaced the spark plugs because the previous carb, an Edelbrock always ran rich and they were black.
I marked and noted where the original settings were. I increased the set screw on the setting of the secondary
by 5/8 of a turn or.625 turn, (1/8 - .125 turn, x5 = .625)
then I backed it out by 2/6 or a turn or .333, note 1 notch on an allen wrench is 1/6 of a turn or .166
my current effective setting is (.625 - .333) or .292 of a turn
therefore that .292 of a turn lowered the idle by 300 rpm
if it is a straight line 1/3 of my last change should net me about -100 rpm or 800 which is my goal.
.292 / 3 = .097 of a turn, or just over 1/2 a hex wrench notch
The pics are:
Primary after and before
and
Secondary after and before