QFT SL-600-VS Tuning

[JedIEG]

I have a QFT SL 600 VS that I am using on top of my stock short block 318 in my 1972 Plymouth Satellite SSP. The engine is ~8.6:1 CR, 145psi cranking pressure, stock 318 heads, duel plane intake, ~220° cam (not sure what it is exactly, but I think it might be a stock 340 grind from Mopar), stock 340 springs. Initial timing is 18°, total 36° @2100rpm.

I have been fighting with this carburetor for a few years and have finally made some progress with modifying it. Idle AFR is finally around 13.8 in N 14.2 in D. 2100 RPM cruse at 55mph with AFR in the high 15s to low 16s.

Full power is still an issue. Main jets are a 66 secondaries the stock 74. AFR dropped to 9.8 @ WOT with the previous 0.050 PVR. Today I tried to drop the PVR to a #57 drill (0.043) and this helped marginally AFR was around 11.2 @ WTO. The engine is not real happy like this. It lays over and stumbles some when laying into it, which I think is from the excessive richness.

What should I do next to get the full power AFR in line? Still smaller PVR? Smaller Secondary jet? Larger PMAB or SMAB? I am hesitant to change the main jet since cruse and mid-light throttle AFRs are already about as lean as I want them.

[GTO Geoff]

Sometimes it is better to leave the AFR meter on the shelf & tune by feel, sense etc. When you open the throttle for more power, & the engine stumbles, & it is fuel system related [ not ign ], it is nearly always going to be caused by a lean condition. Fuel is about 560 times heavier than air. So there is a time lag for the fuel to flow.

If the carb has a choke, you can verify this. Close the choke, engine idling, until engine note changes. Then open the choke slightly & wire it in place in this position & test drive. If the stumble has gone or improves, it is too lean.

If you removable emulsion bleeds, make sure the lower ones are open.

[rgalajda]

"It lays over and stumbles some when laying into it"

GTO Geoff is correct." it is nearly always going to be caused by a lean condition. "

Vacuum secondary opening to soon
VACUUM SECONDARY TUNING. I believe your Slayer carb would incorporate The Quick Set Adjustable Vacuum Secondary Housing Baseline setting for the adjustable needle is 1-1/2 turns off the seat usually. gradually turn in the adjustment needle in 1/8-1/4 turn increments until the hesitation is eliminated.

"What should I do next to get the full power AFR in line? Still smaller PVR? Smaller Secondary jet? Larger PMAB or SMAB? I am hesitant to change the main jet since cruse and mid-light throttle AFRs are already about as lean as I want them."

I would go to a smaller secondary jet.

[Nick_nl]

You might want to treat this one as a 1850 deluxe and measure all orifices and compare them to the 1850 spec. Often the transfer slots are wide and long for starters. Does this one have a real metering plate?

[rgalajda]

Not all 1850 carbs were the same 1850-2....to.....1850-6. Primary metering blocks were different . IFR went from up top under plug to lower position. Primary throttle plates went from thick brass to thinner steel. Primary IAB bleeds went from .070" to .064" But they all used #9 secondary metering plate. Which had .031" IFR and .067" sec main jet.
So there are three options here for the owner with a Secondary Metering Plate instead of a metering Block

#1 Buy a new Metering Plate ( Lets say #9 in this case )
#2 Drill and tap sec main jet for a #8-32 set screw to make it adjustable.
#3 Resize sec main jet

[rgalajda]

There are two angles you must follow. #29 drill for 8-32 set screw.

[rgalajda]

You can also resize the jet by closing the hole off with a punch (basically shrinking) and then using an appropriate sized drill to resize it.

[rgalajda]

I mark my tap (you can see in photo) to depth so the set screw will bottom out when done.
Set screw has been installed in one photo.
The set screw method is best.
My explanation may not be good, but I hope this helps.

[Nick_nl]

I have used metric set screws, UNC is impossible to find over here for a reasonable price.
The IFR has also been resized, I mostly create a #39 plate. M4 for the main jet and M2.5 for the IFR.
M4 s=2mm (~0.078") max drill size
M2.5 s=1.3mm (~0.051") max drill size

So far it has worked good.

[JedIEG]

Reviving an old thread on my carb. rgalajda, Nick, thanks for the tips. Fortunately for me the QFT plates are already drilled for jets from the factory.

Back to the topic; I took the advice from the site and have had several months of trouble free driving and made some upgrades to the carb. I switched to a billet QTF 3 emulsion bleed metering block on the primaries, IFR moved to the bottom and the crab has been running pretty well until last weekend when the car died coming out of my neighborhood. It would not idle right or run at light throttle. Long story short, I found some debris in one of the IFRs and think that was the issue.

I tore the crab totally apart to clean it and am considering putting the old primary metering block on the carb in place of the metering plate. The base plate already has the secondary idle port drilled. I was wondering if anyone could suggest a calibration for the SMAB, SMJ, and emulsion bleeds for the plate? I have the power valve blocked off already and am going to JBweld the constant idle feed under the transfer slot.
Current cal on the secondary block is 0.028 Kill, Top, Mid and Bottom for the emulsion. 0.031 IFR (upper)
Current Carb cal on the secondary.
SIAB- 0.073
SMAB- #31 (0.031)
Metering plate
SIFR- 0.043
SMJ- #74

Should I try to match the primary for idle circuit cal from the factory for this carb or try 670cfm Brawler with VS and 2 mettering blocks?

https://www.holley.com/products/fuel_sy ... s/BR-67256