Y07_A66:

My carb is the QFT-850-PV. It is advertised as a race carb but has vacuum secondary, and I am using for a street(mostly)/strip vehicle. The carb is on my 400/470, with E-Heads, RPM Manifold, and a 248@.050 solid flat tappet cam. The 727 has a stock 440 converter in it at the moment, and the rear has 3.55s. The converter needs to change at some point, but as I said I was hoping to get a solid tune before deciding exactly what to get. My car is a 70 Barracuda, and given its weight, the CI of the motor, dual plane, and a vacuum secondary carb, I felt I could get the combo going decent.

I know most folks first response is that I should be using a big DP and lots of stall in the converter. However, I am just not convinced that is right for me. When I first put the car together I had a MP 175K converter in it, and it was like slush. I know folks rave about the more modern converters. The thing I miss is the ability to goose the throttle and get the instant response like with a 4-speed. When I had the MP 175K in it, I would goose the throttle and all that would happen is the RPM would jump and the exhaust note would rise. You had to push deeper into the throttle to get past the flash point before it would get fun. For example at highway speed, the motor turns around 3K. I could use the throttle and move the RPM from 3-almost 4K with little change in instantaneous speed. The thing was too loose for my taste, at least given the torque of the motor.


--Are you using a High Flow PV?

Yes, I use the ones from QF. Currently it has a 7.5" one in it. Original one was a 4.5".

--Have you tried increasing the IFR's (beyond .035") all around? You may not be able to cruise on the transition circuit very long with your setup.

The original IFR when new was .036. I have not tried above that. I have tried as low as .031 but the off idle problems were fairly bad. If I move the IFR down, I can at least get the low speed (say 30-40MPH) AFR somewhere reasonable. At times I have had this in the mid-14s. Out of the box, the low speed AFR was in the high 11 to low 12 range.

--I would not go leaner on the HSAB's in your situation. You are correct, this would just delay the mains.

Factory was .033 HSAB.

--What is smallest HSAB that you have tried?

I have not tried smaller than .035 with my power brake test. This behavior was something I recently discovered. However, it possibly explains why I felt I could not go below 72 primary jets (factory was 76) because of the lean miss when I tipped into the throttle. My smallest HSAB is .025. During the past 2 years, I have had as small as .030 HSAB in there but I would have to review my records to see what pri jet was in at that time.

--My "guess" would be to go smaller on the HSAB's and larger on the IFR's to richen up the transition circuit to main circuit lean spot.

Good thought, although I would likely start going back down in the HSABs before coming up further on the IFR, just because of AFR in the 1500-2000 range.

--How much testing have you done with the front jets? It is hard to pin point when the mains start (even while watching and charting off of an A/F meter), but have you tried going up on your main jets with the smaller HSAB's to see if this helps down low in the rpm range?

I'm sure that might help in the low RPM range, but at cruise that would be a real killer. At times, I worry about washing out the rings. I have had AFR as low as in the 10s under light throttle. The funny thing is that the motor wants LOTS of fuel at WOT. At one time I removed the PCVR and run 72 pri, and the AFR was 12.2 at WOT. With .061 PCVR and 74 jets, the WOT AFR is in the low 14s (secondaries disabled).

I still believe there is something wrong with the carb. Besides being pig rich, the AFR will have wild swings (to the rich side) during highway cruise. This does not occur in the 1500-2000 range. The motor is getting extra fuel from somewhere but I can't find it. I have spoken to QF a few times on it, and tried everything they suggested to no avail. I know this problem is carb related because my 30 year old 3310 does not do it. The 3310 is on the motor right now, but of course it is not so adjustable and it has its own AFR problems.

With respect to seeing the mains start, the power brake test is a very easy way for me to learn this, at least given the current gap between transition. I keep a vacuum gauge in the car, and you can see it go down as the RPM goes up (standing on the brakes) with the AFR going up, until the mains start and the AFR drops back down. If I get adventurous I might put the QF back on the motor, and shoot a video of it!

Sorry for the length, and thanks for your thoughts. I have a friend in the neighborhood, and he keeps trying to talk me into FI. After all the time and frustration, he is starting to sway me! I don't know if JimG is still around, but we use to discuss the difficulties of getting a good highway tune on a street driven BB, and I don't think he ever got to a satisfying point in his efforts either.