Jet size and A/F ratio

[texas69bee]

I just installed a QuickFuel SS series carb with VS. I fired it up and my A/F ratio is at 10. So it looks like I am changing out the primary jets. They recommend 2 sizes at a time. Knowing that I am that far off, how big of a jump would you make to minimize the changing process? Do you think 4 sizes is to much?

 

[moes]

I would try the 2 sizes and check the A/F ratio. The good thing about having a A/F gauge is that any changes you make can be seen in real time. If the 2 sizes are not correct, you can changing them till you get the A/F ratio you want. A/F gauges are worth there weight in gold!! They are the best way to tune a carb.

 

[texas69bee]

I would try the 2 sizes and check the A/F ratio. The good thing about having a A/F gauge is that any changes you make can be seen in real time. If the 2 sizes are not correct, you can changing them till you get the A/F ratio you want. A/F gauges are worth there weight in gold!! They are the best way to tune a carb.

Thanks, I do like the A/F gauge. I'll take your advice with the 2 sizes.

 

[STREETREBEL]

Mine idled at 11-1. I went 8 sizes. Holley 950 HP.

 

[Jcharger]

Just to be sure, you have A/F of 10 at WOT (3500-5500 RPM under load)? Rather than an A/F of 10 at idle. The former is the jets, the latter is your idle circuit and addressed in other ways.

 

[texas69bee]

Just to be sure, you have A/F of 10 at WOT (3500-5500 RPM under load)? Rather than an A/F of 10 at idle. The former is the jets, the latter is your idle circuit and addressed in other ways.

I have 10 at idle and 12 at WOT

 

[Jcharger]

12 at WOT could be a little rich depending on everything else. If it were me, after making sure the timing were optimized for idle and total advance, I would first adjust the idle mixture screws 1/8 turn out (richer) from the maximum vacuum reading. I expect the A/F at idle will be higher (I found 12.5 or so with lower timing and mid 13's to 14 when I have my idle timing advanced to where the engine runs smoothest and pulls the highest vacuum. Then I would check the A/F at cruise (2000-3000) and then at WOT and adjust from there. Even though the jets provide tons more fuel than the idle circuit, the 12 might change a little. I would also want to see whether it was 12 at all parts of the WOT RPM or if it leaned out a little. If ends at 12 and starts lower or the same then I would work with it. If it starts at 12, but ends in the high 12s or low 13's then I think you have about what you can get short of testing in the 1/4 mile.

 

[68 HEMI GTS]

I have 10 at idle and 12 at WOT

Leave jetting alone for now. You'll have to work on your idle and transition circuit. Will it idle if you turn the screws in a bit? What idle air bleeds does it have? How much transfer slot is showing on primary's and secondaries?

My QFT/pro form carb needed a lot of work down low including reducing the size of the ifr, putting transfer slot restrictions in, and air bleed changes. It now idles at 13.5, goes down the road in the high 13's/low 14's and is low 12's at wot (that's what it wants). Granted I built my carb from a 850 into a 1050 with a main body, but them things being really fat down low sounds like a very common problem.

 

[texas69bee]

Leave jetting alone for now. You'll have to work on your idle and transition circuit. Will it idle if you turn the screws in a bit? What idle air bleeds does it have? How much transfer slot is showing on primary's and secondaries?

My QFT/pro form carb needed a lot of work down low including reducing the size of the ifr, putting transfer slot restrictions in, and air bleed changes. It now idles at 13.5, goes down the road in the high 13's/low 14's and is low 12's at wot (that's what it wants). Granted I built my carb from a 850 into a 1050 with a main body, but them things being really fat down low sounds like a very common problem.

Thanks for the info, I'm not sure why I didn't remember there are 2 circuits.
I'll work with the idle air bleeds first before I do any thing else.

 

[Gpuller]

Playing with the float level will affect idle ratio also.