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Carb tuning help/Question on QFT Slayer 750 CFM Carb

547 Views 13 Replies 2 Participants Last post by  ArizonaGT, 
njp914
I just rebuilt my carburetor for my ford 302 and found out it was a 750 com which I know is definitely too big for my car. this was put on from the old owners who used to race it, I know the engine has had work done to it but im not sure what all was done. It definitely has a bigger cam in it and I know it has high compression, I have to run very high octane gas. The carb has 75 main jets and 82 secondary jets, also the primary air bleeds go 70 31 31 70 and the secondary air bleeds are 39 31 31 39. I have not tuned the air bleeds and I did change the main jet earlier to get it to idle better since the carb has so much more air flow and lower air speed then my car needs, the original owner had the jets even richer. any other jetting advice would be greatly appreciated as I plan to jet tune once I get the carb on. It also has a 65 power valve in it, not sure if I should lower that given that my cam effects my overall vacuum so im not sure if I can reach the 19-20 vacuum at idle.

Ive begun setting up my carb for install, however I was wondering if anyone knows what this Allen key screw pictured is for.

I also have set my primaries so a small square is visible of the transfer slot, however, the spring that my accelerator pump cam sits in front of prevents my throttle from closing all the way so the amount of transfer slot exposed is more then I would like right now. Any advice on how to fix this? I also adjusted the Allen key for my secondaries to expose some of the secondary transfer slot so I can do more fine tuning on idle speed with that, since I was having an issue where my primary transfer slots were too exposed making my idle mixture screws non functioning.

If any one has anymore advice on things to do before I go bolting her back down and tuning, please post here. Also if there is any advice for when I begin tuning, im all ears.
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ArizonaGT
That 6-32 Allen set screw (just forward of the primary idle speed screw) is a throttle stop. At WOT the linkage arm will bottom out on it, you can raise or lower it to get the butterflies vertical.

75/82 is one helluva lot of jet for 302 cubic inches. What altitude are you at?
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ArizonaGT
It’s missing the nut that retains the linkage head to the primary shaft.

njp914 said:
I also have set my primaries so a small square is visible of the transfer slot, however, the spring that my accelerator pump cam sits in front of prevents my throttle from closing all the way so the amount of transfer slot exposed is more then I would like right now
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Are you talking about this?

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ArizonaGT
On setting the secondary throttle plates: 1/8-1/4 turn from being completely shut. Hopefully there will be little-to-no secondary transition slot exposed, which is what you’ll want. But you don’t want the plates bound in the bores either, so you have to open them a little bit.
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njp914
ArizonaGT said:
It’s missing the nut that retains the linkage head to the primary shaft.



Are you talking about this?

View attachment 863470
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no im talking about under the carb base plate, I have a small Allen key screw that pushes on the metal piece that the vacuum secondary arm is connected to, ill send a picture when im back at my house. also ya I know I lowered the jet to 75 for some reason the original owner had it jetted super rich, 82 secondary is stock for this carb though. I believe 72/82 is stock for slayer 750. im not sure what size to change the secondaries to, but I definitely think I should jet down the primary again. do you think jetting it down to below the stock 72 would be ideal given the carb to engine size difference?
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ArizonaGT
njp914 said:
ill send a picture when im back at my house
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Excellent 👍
I‘ll look up Slayer calibration specs, BRB

edit: is this it?

Quick Fuel SL-750-VS Slayer Series Carburetor 750CFM VS

Slayer Series street carburetors are designed with many of the same innovations and features that are built into all QFT™ race-winning carburetors, including changeable air bleeds and power valve channel restrictions. It's designed with a secondary metering plate with changeable jets, and...
www.holley.com www.holley.com
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njp914
ArizonaGT said:
On setting the secondary throttle plates: 1/8-1/4 turn from being completely shut. Hopefully there will be little-to-no secondary transition slot exposed, which is what you’ll want. But you don’t want the plates bound in the bores either, so you have to open them a little bit.
Click to expand...
ok thank you, I was following thunderhead289 tips on setting the plates to where the primary has a small square of transfer slot showing and then tuning off of the secondary plate. after taking the carb off and rebuilding it, the old owner had a ton of transfer slot showing and that makes a lot more sense as to why tuning idle mixture was so difficult.

So far I should jet down the primary further and leave a small opening of the secondaries? and as for the primary plates, that spring on the throttle linkage by the accelerator cam is leaving my primary blades open a little more then I would like, making my idle speed screw to have less adjustment
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njp914
ArizonaGT said:
Excellent 👍
I‘ll look up Slayer calibration specs, BRB

edit: is this it?

Quick Fuel SL-750-VS Slayer Series Carburetor 750CFM VS

Slayer Series street carburetors are designed with many of the same innovations and features that are built into all QFT™ race-winning carburetors, including changeable air bleeds and power valve channel restrictions. It's designed with a secondary metering plate with changeable jets, and...
www.holley.com www.holley.com
Click to expand...
I was talking bout the screw in this hole, it allows me to open the secondaries by pushing the vacuum arm up, also I appreciate your help
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ArizonaGT
On transfer slot exposure: primary side, roughly a ‘square‘ or short/stubby rectangle is ideal. If you have little-to-no exposure on the secondary side, that’s good. Provided that the secondary plates are not shut completely and bound up in the bores.

Your pic with the red circle is how you ‘set’ the secondary side. That’s the screw you’ll want to back out (down) until the secondary plates ~just~ close completely, then give it 1/8-1/4 turn so there is no bind.
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ArizonaGT
Primary jets 68-70, put a stiff-ish vacuum spring in it, and it’ll run like a scaled dog on a 302 😉
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ArizonaGT
edited… I don’t need to make this any more complex than it already is 🤪 We’ll cross that bridge if we come to it
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njp914
ArizonaGT said:
Primary jets 68-70, put a stiff-ish vacuum spring in it, and it’ll run like a scaled dog on a 302 😉
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Ok thank you so much, I’ll lower it 70 first, also I have a purple spring in my vac right now but there is also an adjustment screw on the canister that allows me to screw it out to make it come on faster and in to come on slower, do I want it on slower since my carb is too big
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ArizonaGT
Purple is a good spring tension for that carb/motor combo. I’d run it as-is for a baseline and go from there. But yes, it might want to be slowed down a little but you won’t know until you test. Resist the urge to change too much all at once… don’t ask me how I know 🤪
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ArizonaGT
Just a comment on YouTube videos: there is some good information out there, but there’s also a lot of crap. You may have to sift through the crap.

I‘d browse a few different channels and compare information… if you get the same or similar info from multiple sources, chances are it’s reasonably solid. But if one guy claims to have solved the mystery of carburetor tuning all by himself, I’d be speculative.
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