Hello,

Is there a reason why around 1.5 turns out on the idle mixture screws is recommended or indicated to be something to aim for?

On my quickfuel ss-680-vs I have it at 0.6 turns out and that seems to work for my carb. (IAB's are 0.066)

I decided to do some experimenting today and dropped my IFR's down from 0.031 to 0.025 in both primary and secondary and I managed to get my idle mixture screws out to 0.9 of a turn to achieve the same idle as before (AFR of 14.1). I don't think there is any way I can get it to 1.5 turns out, even if I drop it to the smallest size of 0.020 available (other than drilling my own).

Any thoughts on this?

Thank You

 

Just to add to this; my carb setup has always run on the rich side (low 13's on the AFR) so I am starting to do some research to lean it out a bit; starting with the idle circuit.

 

Take a coupla small pieces of copper wire (like, one strand of some stranded wire); bend em into a 90°; put one in each IFR, such that the gasket captures the bent "tang".

That's how we've been tuning carbs since ... well, I'd rather not say, since when.

 

Enlarging the size of the idle air bleed has the same effect as reducing the size of the IFR, BTW. Just thought I'd mention that. I've not had a ton of experience with the QuickFuel carbs, but in the few cases I've had a chance to play around with them, we've ended up with much larger idle air bleeds than stock to get them into the realm of "sanity". I think they're calibrated way-rich at idle to handle engines with really big cams. Put them on a milder engine with a small cam and they just go dead-rich. Just my opinion based on limited experience.

 

Thank You for the replies Damon and Sofakingdom.
I did hear of the ol welding wire in the IFR trick I will play around with it, great idea.
I read one article where a pig rich at idle carb was managed by having a 0.020 IFR and a 0.032 IAB. The small IFR was enough to get it leaned out to the point where he went to 2.5 turns and then lowering the IAB brought the circuit in sooner and allowed him to go down to 1.6 turns on the mix screws.

More experimenting on the books...

Cheers

 

Ok, did some more researching and experimenting.

1. Found articles that confirm what Damon stated, that these carbs are really designed for racing engines so they are created to be rich, very rich, out of the box.
2. Having said that, I am sure I can dial it down; I first increased my IAB's to 74's from the 66's and, as expected, my transition RPM (1300-1600) went way lean.

So I ordered some 0.020 IFR's as I believe that will bring me into the realm of a sane idle with about 1 to 1.5 turns out. If not I will play with mig wire to see if I need to go lower.
I will then bring down the IAB's to, I'm currently thinking, the 0.030's and fine tune from there. I think lowering the IAB's that low will then bring in the transition fuel sooner and get rid of that lean spot.

Will update when I get the parts in and can experiment more.

Cheers