Quick Fuel SS750AN Tuning
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Quick Fuel SS750AN Tuning
Ok, I'm a little long winded, but hear me out....I've got a brand new Quick Fuel SS750 with annular boosters installed on my 383 sbc. Carb has had the initial set up performed correctly (6 psi fuel pressure, float level in middle of the window, etc, etc). Ignition timing at 32 total all in by 3k rpm as suggested by engine builder.
Did my first full throttle runs yesterday, and after going wot in top gear and giving it 2 seconds or so for the AFR to stabilize, I'm settling into a 9.9:1 to 10.5:1 mixture on the AEM wideband gauge. This is obviously super rich, and going down equally 2 jet sizes on both the primary and secondary (to keep the jet split ratio the same) did little to help.
Judging by how far off the air fuel is, it's looking like it would take going down another 8-10 jet sizes to get a respectable 12.5:1 afr at full throttle. Is this somewhat normal? I don't like the idea of deviating that far from the out of the box settings, but if it's not unheard of I guess the engine "wants what it wants."
Now here's the strange part with these super rich mixtures.....I just watched my engine on the dyno, (with my exact carb), pull 12.5:1 afr at wot with a beautiful fuel curve. The only difference is that the engine is now installed in the vehicle, and it has cast iron exhaust manifolds with dual exhaust versus the open dyno headers. Is this enough to make it go pig rich?
Did my first full throttle runs yesterday, and after going wot in top gear and giving it 2 seconds or so for the AFR to stabilize, I'm settling into a 9.9:1 to 10.5:1 mixture on the AEM wideband gauge. This is obviously super rich, and going down equally 2 jet sizes on both the primary and secondary (to keep the jet split ratio the same) did little to help.
Judging by how far off the air fuel is, it's looking like it would take going down another 8-10 jet sizes to get a respectable 12.5:1 afr at full throttle. Is this somewhat normal? I don't like the idea of deviating that far from the out of the box settings, but if it's not unheard of I guess the engine "wants what it wants."
Now here's the strange part with these super rich mixtures.....I just watched my engine on the dyno, (with my exact carb), pull 12.5:1 afr at wot with a beautiful fuel curve. The only difference is that the engine is now installed in the vehicle, and it has cast iron exhaust manifolds with dual exhaust versus the open dyno headers. Is this enough to make it go pig rich?
Re: Quick Fuel SS750AN Tuning
Sorry, there's one more difference between the engine dyno and now. I'm running an open element 14x3" air cleaner.
Re: Quick Fuel SS750AN Tuning
Exhaust manifolds and full exhaust vs open headers can make a HUGE difference in afr’s! I think an 8-10 jet size reduction would be in the ballpark.
Re: Quick Fuel SS750AN Tuning
Are you still running the same fuel as on the dyno and what size high speed bleeds are in the carb you might try those first with larger size.
Re: Quick Fuel SS750AN Tuning
Fuel is 93 octane pump, which is same as the dyno. Carb is box stock with size 33 HSABs. The WOT fuel curve is pretty flat, staying at an afr of 9.9 to low to mid 10's through the entire rpm range, so I don't think it'd be a HSAB problem.
Re: Quick Fuel SS750AN Tuning
QF carbs are inherently fat out of the box. Everyone of them that I have messed with have been that way. Recently I bought 3 new FST carbs the seem to be a lot better for the street, so I used the spec card on the 750 I had and changed the bleeds and jets on my buddies QF 750 to match the specs of the FST and it runs a ton better. I know some Holley's are like that also fat as pigs out of the box, guess that's why we have tools, to tune stuff.
Re: Quick Fuel SS750AN Tuning
Any chance you could post the spec card for the FST750? I'd like to see the numbers.1980RS wrote: ↑Sat Apr 24, 2021 12:25 pm QF carbs are inherently fat out of the box. Everyone of them that I have messed with have been that way. Recently I bought 3 new FST carbs the seem to be a lot better for the street, so I used the spec card on the 750 I had and changed the bleeds and jets on my buddies QF 750 to match the specs of the FST and it runs a ton better. I know some Holley's are like that also fat as pigs out of the box, guess that's why we have tools, to tune stuff.
Re: Quick Fuel SS750AN Tuning
Sure I an dig it out and post it up for you.eric8 wrote: ↑Sat Apr 24, 2021 1:07 pmAny chance you could post the spec card for the FST750? I'd like to see the numbers.1980RS wrote: ↑Sat Apr 24, 2021 12:25 pm QF carbs are inherently fat out of the box. Everyone of them that I have messed with have been that way. Recently I bought 3 new FST carbs the seem to be a lot better for the street, so I used the spec card on the 750 I had and changed the bleeds and jets on my buddies QF 750 to match the specs of the FST and it runs a ton better. I know some Holley's are like that also fat as pigs out of the box, guess that's why we have tools, to tune stuff.
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Re: Quick Fuel SS750AN Tuning
As Travis said- headers and exhaust systems, as I have said before- placement of sensor in collector makes a big difference.eric8 wrote: ↑Sat Apr 24, 2021 8:15 am Ok, I'm a little long winded, but hear me out....I've got a brand new Quick Fuel SS750 with annular boosters installed on my 383 sbc. Carb has had the initial set up performed correctly (6 psi fuel pressure, float level in middle of the window, etc, etc). Ignition timing at 32 total all in by 3k rpm as suggested by engine builder.
Did my first full throttle runs yesterday, and after going wot in top gear and giving it 2 seconds or so for the AFR to stabilize, I'm settling into a 9.9:1 to 10.5:1 mixture on the AEM wideband gauge. This is obviously super rich, and going down equally 2 jet sizes on both the primary and secondary (to keep the jet split ratio the same) did little to help.
Judging by how far off the air fuel is, it's looking like it would take going down another 8-10 jet sizes to get a respectable 12.5:1 afr at full throttle. Is this somewhat normal? I don't like the idea of deviating that far from the out of the box settings, but if it's not unheard of I guess the engine "wants what it wants."
Now here's the strange part with these super rich mixtures.....I just watched my engine on the dyno, (with my exact carb), pull 12.5:1 afr at wot with a beautiful fuel curve. The only difference is that the engine is now installed in the vehicle, and it has cast iron exhaust manifolds with dual exhaust versus the open dyno headers. Is this enough to make it go pig rich?
2 seconds of pulling WOT won't tell you much in my opinion.
Carb tuning is a long lonely road so be prepared to have carb on and off a few times,make one change at a time and write it down,if you get out in the weeds you can always go back to baseline.
My experience with OOTB holley style carbs is they are fat at idle and transition circuit and lean at WOT.
Your main jet is for cruise and power valve and power valve ristrictors are for WOT .
if you are able to put up all the specs of your carb will help, I will at a guess say it has 86 secondary main jet with a 0.058" PVRC and a 65 power valve.
steve c
"Pretty don't make power"
"Pretty don't make power"
Re: Quick Fuel SS750AN Tuning
How close is the bottom of the air cleaner lid to the bowl vent tubes? What shape is the end of the vent tubes, straight or angle cut?
Is the lid also a filter? Those filter lids make carbs do crazy things.
A couple years ago I road tuned a "750" annular QF on a 455 Pontiac and ended up with a #63 primary main jet. It was in the winter about 45F and I think in summer weather it would run OK with #62. The annular booster in 1-3/8" venturi corks the hole quite a bit, so the booster signal is more like a 600 carb.
Is the lid also a filter? Those filter lids make carbs do crazy things.
A couple years ago I road tuned a "750" annular QF on a 455 Pontiac and ended up with a #63 primary main jet. It was in the winter about 45F and I think in summer weather it would run OK with #62. The annular booster in 1-3/8" venturi corks the hole quite a bit, so the booster signal is more like a 600 carb.
Re: Quick Fuel SS750AN Tuning
steve cowan wrote: ↑Sat Apr 24, 2021 2:39 pmAs Travis said- headers and exhaust systems, as I have said before- placement of sensor in collector makes a big difference.eric8 wrote: ↑Sat Apr 24, 2021 8:15 am Ok, I'm a little long winded, but hear me out....I've got a brand new Quick Fuel SS750 with annular boosters installed on my 383 sbc. Carb has had the initial set up performed correctly (6 psi fuel pressure, float level in middle of the window, etc, etc). Ignition timing at 32 total all in by 3k rpm as suggested by engine builder.
Did my first full throttle runs yesterday, and after going wot in top gear and giving it 2 seconds or so for the AFR to stabilize, I'm settling into a 9.9:1 to 10.5:1 mixture on the AEM wideband gauge. This is obviously super rich, and going down equally 2 jet sizes on both the primary and secondary (to keep the jet split ratio the same) did little to help.
Judging by how far off the air fuel is, it's looking like it would take going down another 8-10 jet sizes to get a respectable 12.5:1 afr at full throttle. Is this somewhat normal? I don't like the idea of deviating that far from the out of the box settings, but if it's not unheard of I guess the engine "wants what it wants."
Now here's the strange part with these super rich mixtures.....I just watched my engine on the dyno, (with my exact carb), pull 12.5:1 afr at wot with a beautiful fuel curve. The only difference is that the engine is now installed in the vehicle, and it has cast iron exhaust manifolds with dual exhaust versus the open dyno headers. Is this enough to make it go pig rich?
2 seconds of pulling WOT won't tell you much in my opinion.
Carb tuning is a long lonely road so be prepared to have carb on and off a few times,make one change at a time and write it down,if you get out in the weeds you can always go back to baseline.
My experience with OOTB holley style carbs is they are fat at idle and transition circuit and lean at WOT.
Your main jet is for cruise and power valve and power valve ristrictors are for WOT .
if you are able to put up all the specs of your carb will help, I will at a guess say it has 86 secondary main jet with a 0.058" PVRC and a 65 power valve.
Hi Steve. When I said 2 seconds, I didn't mean the pull was only for 2 seconds. I waited 2 seconds for the pump shot to "bleed off" before starting to log the afr all the way to redline. Steady state cruise at 55 mph on level ground is netting 12.5 afr, so there is definitely some room to lean out the mains (both primary and secondary) before dealing with the PVCR's, no?
As far as sensor placement, I put the AEM sensor in the GM manifold's OEM sensor bung location, at the rear of the collector. Not sure if this is ideal for a wideband though? Please see the attached photos for carb specs and 02 sensor location.
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Last edited by eric8 on Sat Apr 24, 2021 4:20 pm, edited 2 times in total.
Re: Quick Fuel SS750AN Tuning
Hi Tuner,Tuner wrote: ↑Sat Apr 24, 2021 2:53 pm How close is the bottom of the air cleaner lid to the bowl vent tubes? What shape is the end of the vent tubes, straight or angle cut?
Is the lid also a filter? Those filter lids make carbs do crazy things.
A couple years ago I road tuned a "750" annular QF on a 455 Pontiac and ended up with a #63 primary main jet. It was in the winter about 45F and I think in summer weather it would run OK with #62. The annular booster in 1-3/8" venturi corks the hole quite a bit, so the booster signal is more like a 600 carb.
Air cleaner is a non drop base, so it has a nice smooth transition to the carb inlet and about 3" of clearance from the bottom of the air cleaner lid to the 45 degree angle vent tubes. It's a non filter style air cleaner lid. Please see attached.
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Re: Quick Fuel SS750AN Tuning
All I've changed so far is to go down 2 sizes on both primary and secondary. Current jetting is 70/78 and stock was 72/80. I should also mention steady state, level ground, 55 mph cruise is right at 12.5:1 with current jetting. And full throttle AFRs are in the low 10s.
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Re: Quick Fuel SS750AN Tuning
Do the plugs show rich??
Kinda surprised it that rich with that main jetting size.
Afr can lead you astray,been talked about alot lately
Kinda surprised it that rich with that main jetting size.
Afr can lead you astray,been talked about alot lately
steve c
"Pretty don't make power"
"Pretty don't make power"