Quick Fuel SS750AN Tuning

[eric8]

Seems a trifle odd, doesn't it. This is a bit like snake handling without knowing the details of emulsion bleed and main bleed sizes, but I would go down another 4 sizes in the secondary.

High speed air bleeds are 33's. I should have gotten the emulsion bleed sizes when I had it apart.

[Tuner]

High exhaust pressure causes a WBO2 to read further from stoichiometric, a rich A/F will report richer than it actually is. The effect is more pronounced over 10 psi backpressure, which is possible with a corked catalytic converter or muffler. However, you report the A/F progressing leaner as RPM climbs when pressure would be increasing and skewing the A/F richer, so odds are you don't have that problem. A couple years ago I rigged a pressure gauge connection with an old 18mm sparkplug shell in the O2 sensor bung and found a 318 Mopar pumping against a funky converter had 10 lbs. backpressure.

You can use an accelerometer ap in your phone to measure performance change with these jet and other changes and actually measure HP if you know vehicle weight.

[eric8]

Tuner,
Would it be beneficial for me to just make a full throttle run with the secondaries disconnected to fully isolate both sides of the carb? It almost seems like something not far off from square jetting may wind up being ideal for my application.
I know with a primary power valve and no PV in the secondary, a jet split of 6-8 is normal. Is it possible with the cast iron exhaust manifolds, the 383 simply may not want all the fuel and air that the secondaries offer? Is it unheard of for an engine wanting close to square jetting even with a primary PV?

[Tuner]

Tuner,
Would it be beneficial for me to just make a full throttle run with the secondaries disconnected to fully isolate both sides of the carb? It almost seems like something not far off from square jetting may wind up being ideal for my application.
I know with a primary power valve and no PV in the secondary, a jet split of 6-8 is normal. Is it possible with the cast iron exhaust manifolds, the 383 simply may not want all the fuel and air that the secondaries offer? Is it unheard of for an engine wanting close to square jetting even with a primary PV?

Sure good information to have, but keep in mind that the metering isn't going to act the same with 100% of airflow through only 2 bbls as with 4 bbls and 50% through each pair, so don't jump the shark, just observe and keep notes. In the end, if you are going to run it with secondaries, you tune it with secondaries.

[eric8]

Ok makes sense. I just did some interstate pulls and rolling it on from 65 mph I'm even seeing some 11.8's and 11.9's on the gauge so it's definitely progress. So I'll go down another 4 on just the secondaries per your recommendation which will put me at 64/68 jetting and we'll see how it looks.

[steve cowan]

Tuner,
Would it be beneficial for me to just make a full throttle run with the secondaries disconnected to fully isolate both sides of the carb? It almost seems like something not far off from square jetting may wind up being ideal for my application.
I know with a primary power valve and no PV in the secondary, a jet split of 6-8 is normal. Is it possible with the cast iron exhaust manifolds, the 383 simply may not want all the fuel and air that the secondaries offer? Is it unheard of for an engine wanting close to square jetting even with a primary PV?

You can always square jet and fit PV and PVRC jets in secondary side,that's how I run my carbs and I also use 1:1 linkage,
Alot of people don't like that setup and that's ok but it works for me.
Don't be shy looking at a couple of spark plugs on the side of the road after a couple of decent runs,not taking anything away from tuner as he helping alot.

[eric8]

Thanks for that. I'm definitely learning a lot here and appreciate all the input. On a related note I ordered AED fuel bowl and metering block gaskets which were different than the quick fuel ones that came on the carb. Mainly the shape of the metering block gasket behind the PV and an extra hole or two on the AED gasket. Will these be okay to use?

20210428_095421.jpg

[Tuner]

The gaskets have 3-circuit orifices, but if they don't block any passages in your 2-circuit gasket and don't cause any leaks they should be OK. I see a tech line phone number there, call them and get the official word.

Disclaimer here: no guarantee this calibration will apply to your engine ... I checked my notes on the 750QF annular on a 455 Pontiac I tuned on the road with WBO2. It ended up with #62 PMJ, #68 SMJ. Notes indicate I left the original .059" PVCR, but I'm not sure of that (Dang, need to take more and better notes. Only excuse is I tuned three Bandit looking TransAm cars, one an Olds engine, in a couple months of each other and memories overlap). No secondary PV.

The original metering block had a high idle jet but that is problematic with causing the cruising A/F to wander around, so I used .031" IJ moved to bottom of idle well position, .070" IAB, .081" T-slot jet. I recall it had a funky billet base with a too wide/too long T-slot, which is why the .081" in the face of the body in the horizontal passage to the T-slot.

The original metering block had three E-bleeds, top .028" or .029" (they varied), 2nd. .031", 3rd. 031". I plugged # 2 and 3 and used only #1 .029". Kill bleed .028", angle channel .144" Booster leg .160".

The reason for plugging the lower emulsion bleeds is the annular booster in such a small venturi has such a strong signal it does not need a lot of emulsion air to get it started, though this may not be the case with all engines. The funky billet bases tend to be too rich up the T-slot in the cruise range (hence the reason for a T-slot jet), so if the main is too early because of aggressive emulsion air the main and T-slot overlap too much, too rich, and that pesky light throttle too-rich blubber in a narrow throttle range just above idle is the result.

Also, because it had 4-corner idle mixture screws, to reduce sensitivity of the curb idle adjustment .036" through into the bore in the horizontal passage in the body between the mixture screw and the curb idle discharge in the base.

[eric8]

Very interesting info. The 62/68 jetting is almost dead on what I'll probably arrive at. Any notes on the state of tune of that 455? I'd be very curious if it had headers or not.

BTW, I just got back from a run with the vacuum gauge hooked up in the cab. Figured I'd dial in the PV. Idle vacuum was 16, and cruise around 55-60 I had a solid 18 inches. Steady state cruise is almost always 17 or higher. Start easing into the gas slowly and watching both gauges, the mixture goes lean (16:1) around 12 inches vacuum. You can tell it's not happy (surging) until around 8 inches vacuum which is where the #8 PV comes in. Holley only lists a 10.5 PV. Can you get a larger number PV elsewhere? Or just go with the 10.5?

[Tuner]

Very interesting info. The 62/68 jetting is almost dead on what I'll probably arrive at. Any notes on the state of tune of that 455? I'd be very curious if it had headers or not.

BTW, I just got back from a run with the vacuum gauge hooked up in the cab. Figured I'd dial in the PV. Idle vacuum was 16, and cruise around 55-60 I had a solid 18 inches. Steady state cruise is almost always 17 or higher. Start easing into the gas slowly and watching both gauges, the mixture goes lean (16:1) around 12 inches vacuum. You can tell it's not happy (surging) until around 8 inches vacuum which is where the #8 PV comes in. Holley only lists a 10.5 PV. Can you get a larger number PV elsewhere? Or just go with the 10.5?

The 10.5 PV would be appropriate, but also this might be a case where a smaller PMAB would reduce the change of A/F so much toward lean as the load is increased. I am surprised and disappointed it is surging at 16/1, but it is what it is. May need one size larger PMJ, but I would try smaller PMAB first.

Cruising and part-throttle surging can be caused by the high idle jet and I'll bet your metering blocks have the IJ at the top of the idle well instead of at the bottom. Carb fuel jets, idle and main, need to be submerged on both ends, inlet and outlet.

[steve cowan]

20210429_175420.jpg

I have had one issue with a QFT carb with the blue gasket,it wasn't definitive but the gasket changed helped.

20201003_175310.jpg

20201003_175257.jpg

.

20201003_172229.jpg

Idle feed restriction jet moved to the lower position

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, transfer circuit jet fitted to main body.

20201003_172034.jpg

,mods done on a 1150 QFT dominator carb this is a 2 circuit carb intermediate circuit deleted.
Application is a 800hp BBC pump gas streeter.
Runs very well after modifications.
This engine only has 6" vac at idle and 14" at cruise,changed from a 2.5" PV front and rear to a 8.5" cleaned up that hesitation you talk about.

[eric8]

20210429_175420.jpg
I have had one issue with a QFT carb with the blue gasket,it wasn't definitive but the gasket changed helped.20201003_175310.jpg20201003_175257.jpg.
20201003_172229.jpg
Idle feed restriction jet moved to the lower position 20201003_171930.jpg, transfer circuit jet fitted to main body.20201003_172034.jpg,mods done on a 1150 QFT dominator carb this is a 2 circuit carb intermediate circuit deleted.
Application is a 800hp BBC pump gas streeter.
Runs very well after modifications.
This engine only has 6" vac at idle and 14" at cruise,changed from a 2.5" PV front and rear to a 8.5" cleaned up that hesitation you talk about.

Nice setup! I'm sure an 800hp na setup on the street is a handful

[eric8]

Very interesting info. The 62/68 jetting is almost dead on what I'll probably arrive at. Any notes on the state of tune of that 455? I'd be very curious if it had headers or not.

BTW, I just got back from a run with the vacuum gauge hooked up in the cab. Figured I'd dial in the PV. Idle vacuum was 16, and cruise around 55-60 I had a solid 18 inches. Steady state cruise is almost always 17 or higher. Start easing into the gas slowly and watching both gauges, the mixture goes lean (16:1) around 12 inches vacuum. You can tell it's not happy (surging) until around 8 inches vacuum which is where the #8 PV comes in. Holley only lists a 10.5 PV. Can you get a larger number PV elsewhere? Or just go with the 10.5?

The 10.5 PV would be appropriate, but also this might be a case where a smaller PMAB would reduce the change of A/F so much toward lean as the load is increased. I am surprised and disappointed it is surging at 16/1, but it is what it is. May need one size larger PMJ, but I would try smaller PMAB first.

Cruising and part-throttle surging can be caused by the high idle jet and I'll bet your metering blocks have the IJ at the top of the idle well instead of at the bottom. Carb fuel jets, idle and main, need to be submerged on both ends, inlet and outlet.

Now don't get me wrong, it's happy at 16:1 steady state cruise. But as soon as it turns into a lower vacuum situation and load increases, such as accelerating up a hill prior to PV actuation, I can feel the surging. And in these lower vacuum situations it dips into the 17's. Idle jets are still up top.

[eric8]

With 33 HSABs all around, what would be a number to start with on the primary HSAB? And if im understanding correctly, change just the primary ones, correct?

I was thinking about the cruise situation today. At 60 mph, I assume the throttle blade is past the transition slot. What is responsible for enrichment in this "window" where the throttle blade angle is increasing (i.e. beginning to accelerate up a hill), but prior to the PV's opening? Strictly the booster sensitivity? And lowering the primary side HSAB should help increase this sensitivity?

[steve cowan]

I would test with 31 or a 28 bleed and see what it does,when you play with carbs you soon accumulate parts,part of the fun