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Old 06-19-2017, 09:13 AM
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Quick Fuel Tuning Help

Hello everyone,

I have gotten my S10 on the road and it drives fantastic. It has a manual vb TH400, roller 383, Dart 215 heads, and a 240ish @ 50 cam, with a Quick Fuel HR750 which has been a big learning curve for me but I think I'm getting the hang of carbs slowly. I am tuning with a wideband O2 as well.

As of now:
Primary : 70
Secondary : 80
Both squirters :35, stock cams
Primary PV : 5.5
Secondary PV blocked off
Idle air bleeds : 70
Hi speed air bleeds : 31

It idles at 14.5:1 with 3/4 turn of the idle screws. I'm pretty sure I need bigger idle air bleeds to help turn the screws out to the 1.5 range.

WOT gets around 12:5 - 13:1 on 34* of timing and the plugs look great.

My issue is part throttle. On reeaally light part throttle is fattens up to around 12:1, and about 12.5:1 on deaccel.. but if I give it say a heavier part throttle (say 25-40% throttle) it runs a healthy 13.5-14:1. No bogs or anything. What should I be doing to get that first bit of part throttle leaned out without sacrificing anything else, as the rest of the tune seems great?

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Old 06-19-2017, 12:54 PM
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Quote:
Originally Posted by bc-longshot View Post
Hello everyone,

I have gotten my S10 on the road and it drives fantastic. It has a manual vb TH400, roller 383, Dart 215 heads, and a 240ish @ 50 cam, with a Quick Fuel HR750 which has been a big learning curve for me but I think I'm getting the hang of carbs slowly. I am tuning with a wideband O2 as well.

As of now:
Primary : 70
Secondary : 80
Both squirters :35, stock cams
Primary PV : 5.5
Secondary PV blocked off
Idle air bleeds : 70
Hi speed air bleeds : 31

It idles at 14.5:1 with 3/4 turn of the idle screws. I'm pretty sure I need bigger idle air bleeds to help turn the screws out to the 1.5 range.

WOT gets around 12:5 - 13:1 on 34* of timing and the plugs look great.

My issue is part throttle. On reeaally light part throttle is fattens up to around 12:1, and about 12.5:1 on deaccel.. but if I give it say a heavier part throttle (say 25-40% throttle) it runs a healthy 13.5-14:1. No bogs or anything. What should I be doing to get that first bit of part throttle leaned out without sacrificing anything else, as the rest of the tune seems great?
First I would like to say I'm an experienced mechanic, but a novice at tuning carbs - having only one Quick Fuel carb tuning with an AFR gauge under my belt. I did study Vizards Tuning Holley Carbs in detail and the following strategy worked very well for me.

IMHO you're going to have to change the IFRs, as the light throttle fuel mix is coming from the idle circuits. Once you open the throttle more (25-40%), the blades are open too far for the idle circuit to be involved anymore. Unfortunately, change the IFRs and you change EVERYTHING else.

So change you carb back to the way it came and start again.

First make sure throttle blades are not too far open at idle by checking idle slot position and that vacuum/timing in drive are optimal when mix needles are adjusted to maybe 13.5:1. Select the IFRs (keep them all the same in both sides for 4-corner idle) for best transition mix and let the idle mix needles fall were they may. Transition is light throttle before fuel starts to flow from the main boosters maybe 2000-2500 rpm at very light throttle, depending on carb. Doubt if you will ever get mixture needles to 1.5 turns, but that is really irrelevant.

Air bleeds change the slope of the mixture curve through RPM so typically you shouldn't change those unless you have a mix that is changing over RPM - say from 3000 - 6000 (yeah, I know they are easier than changing jets and IFRs/PVRs, but resist the urge). Make sure you don't have off-idle stumbles, and that engine operation during warmup and light throttle are exactly as desired. If it stumbles, work on accelerator pump. One you have this nailed down to your satisfaction - then and only then go on to other things.

Next evaluate your "cruise at 3500 RPM" A/F ratio. Figure out the primary jets it takes to give you the desired A/F ratio at 3500 cruise. If you went up two steps from factory on the primaries, install secondary jets two steps bigger than factory. Finally figure out the PVRs you need to give you the desired full throttle mix from 3000 up to your redline. If the full throttle Mix changes significantly throughout the RPM range, then you can play with the main air bleeds.

My Quick Fuel SS830 on my 383 in my S10 with Comp Cams XE274H, Edelbrock Air Gap, AFR 195 heads, and fenderwell headers was pig rich from the factory and required significantly smaller IFRs and Jets. I did not have to change any air bleeds or modify the accelerator pump. I bought 4 small IFRs and 2 small PVRs and took them oversize stepwise with number drills to get them just right. IIRC I used a 4" vac power valve. Initial timing of 17 @ 800RPM. MSD Billet lt silver/lt blue springs - blue advance stop +18 centrifugal in by 2500. Using Autozone DC1810 (B28/VC1810) points style vac canister hooked up to full manifold vacuum. Lots of timing at idle with vac advance - about 30. Don't know if your cam is too big to use this vacuum canister in this way. Mine is 230/236 @ 0.050.

Good luck!
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Old 06-19-2017, 01:11 PM
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I'd say 2001blazer4x4 has it pretty much covered. The only thing I'll add is not to worry about the rich decel. Unlike fuel injection which either cuts injectors off or way back on decel, a car goes rich. With the closed throttle extremely high vacuum is in the manifold so it'll pull even more fuel than it would at a normal idle.

I'm running a problem 750 double pumper on my Fairlanes 390. It actually has a quick fuel center section, and all the holley variants tune the same. I had to step down 3 sizes on my ifrs to get off idle cruise where I wanted it.
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Old 06-19-2017, 03:32 PM
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If you don't have any issues such as a bog or hesitation then if all else is good I would leave it alone as I have read many folks on using 02 sensor for tuning and the numbers they always looked for were not available all the time cause many factors come into play on a carb going from the idle circuit and transition circuit and many times they got the idle mixture in the range they wanted and also the transition circuit was tuned of what they wanted as well. When they tried to get rid of that momentarily rich ratio during opening throttle and also on letting it off to be leaner it then would cause some other issues on the idle circuit.

You can't always get the magic number of 14:5 to 1ratio and that would go on the idle circuit with the idle feed restrictors as well.Some engines depending on the cam it has along with how big it is will always require a slightly richer mixture and if a cam has a pretty good amount of overlap it will make it need a richer mixture cause of lack of a carb signal from the exhaust reversion.

I am running a quick fuel 650 double pumper on my 350 build and it was way to rich for my 350 build and I had to lean it out. From my own experience with the four corner idle carbs is you will mostly be at only 1/2 to 3/4 of a turn out on all four and rarely ever get them at 1 turn out of more.When my quick fuel was to rich on the idle circuit I could only get my idle mixture screws out only half a turn and I had to go down a few sizes in the idle feed restrictors to the leanest idle fuel curve I could get and my screws are all now at 3/4 of a turn out and trying to get them at 1 turn out was just to lean.

Most carb tuning with a 02 sensor is just one part of setting your carb up for the best tune of what your engine requires and sometimes you can't get the 14: to 1 range ratio in certain areas cause performance engines in general require a richer mixture compared to something with a smaller cam but too will vary and it could just be that your pump shot is to rich as that will also effect things in the transition faze
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