The carb on mine is my first carb'd V8 since the '80s so take my lack of experience for what it's worth but I would add that a really nice feature of the QuickFuel vacuum secondary carbs is the ability able to adjust (with a screw) how slow or fast (late or early?) the secondaries come in. Adjusting this, along with a bigger discharge nozzle and hollow screw, allowed me to cure my lean peaks when going from steady to full throttle. No matter what you go with, a wideband O2 is another invaluable tuning tool as well.
Does anyone have any feedback with one of George's recommendations: 4150 Holley style carburetor, 750 cfm, annular boosters, electric choke, street calibration.
The QF Ss series is a Holley 4150 but better. George made several recommended carbs some smaller than 750.
@Sharkey posted:Vacuum secondaries are easily set up to come in as early (or late) as you like, simply buy the secondary spring kit as shown below, & go lighter until you get the secondaries to open where you like 'em. Set your timing to be 32-34 degrees TOTAL, then install the yellow spring. If the distributors advance(s) centrifugal (& vacuum if also used) are set up right to pull in advance, you'll love the vacuum secondaries. View the chart below:
Do you recommend the short or the long yellow spring? Thank you.
...On All My Vacuum Secondaries 600-750, I had the best results using the SHORT BLACK Spring (NOT the LONGER Black spring), shown in the Photo. Regardless of the Chart...It seems to be in the mid-range of that Spring Set, Average. Not too Weak, Not too Strong. The Secs will open-up right along with the Engine Revs. Quickly and Balanced. NO Bog, No Stutter, No Hesitation. Strong!! FWIW
A good place to start your testing. And, Be Sure the Accelerator Pump is Adjusted Precisely. As in the AP Cam is on the correct hole, and more.
MJ
I have the carb on the car with the yellow spring installed and made a video showing that the secondaries are opening. The yellow spring is 3 of 7 choices and there are two more lighter springs that I can test.
What is the downside to the vacuum secondaries opening too quickly?
Attachments
Videos (1)
A bog due to momentary lean mixture.
Some of you may remember POCA articles from a decade ago- a bright yellow '73 GT5-s clone that I got in my shop to 'straighten out'. It had so many issues that I did a couple of articles on the 58 problems I found. On that engine, a vac-sec carb simply Would. Not. Work. It came in with a brand new Holley Avenger (vac-sec) carb. I could NOT get rid of a mid-range bog during acceleration.
Tried swapping springs, jet blocks and even float bowls (center feed vs end feed). I tried SIX more vac-sec carbs of various sizes and List Numbers, with nearly identical results. Finally I took the car to Mallory's local shop for some expert advice. The dyno tech took one look at it and said, "Holley doesn't mention it but all Avengers are CA compliant emission carbs. You'll NEVER get that thing to function like you want on a performance 351-C".
Based on his advice, I bought a 'tuner' 700 double-pumper from Bob Oliver at Competition Carbs, and bolted it on. Instantly, no more bogs, more power and better fuel mileage. His carbs have some 24 separate changes to the new carb as-received. It worked so well, I bought a second tuner-carb for our Pantera, and it worked better than what I had (I'd tried my old carb on the yellow car; no help). A week late we drove to 'Vegas for a Fun Rally with the new carb and got 22 mpg at an avg mph well above 70! Then Judy ran the track event and was all smiles at the power increase and drivability (900 miles trouble free round trip).
Over 40 yrs of ownership and two engines (first was OEM; the last of which I built) with the same Pantera, I have learned to defer to real experts in certain areas. My advice is to either spend every evening in your garage fiddling with your setup & going to bed smelling of 91-octane until you get lucky. Or buy a tuner carb and immediately enjoy the results. A third possibility is a late throttle-body-EFI setup. I have no direct experience but they are supposedly marginally better for power AND mileage than tuner carbs, for 3X the carb's price & complication. And are still not fool-proof on street or track. Good luck.
@forestg posted:A bog due to momentary lean mixture.
Thanks Forest.
I just drove the car across town and there was no bog. Thus, I plan to try the next lighter spring. If it does not bog then I will try the lightest.
I now have the lightest spring (white) installed. The carb has good manners when driving with others around. I took the car to a more isolated place for some testing and I am impressed with how the vacuum secondaries operate. Unlike a mechanical secondary carb the vacuum secondaries begin opening predictably at part throttle. The car chirped the tires going into second gear. There needs to be plenty of Venturi velocity for the secondaries to open. The car needs to be in a higher RPM range and a lower gear. For example, the secondaries will not open when going 30 MPH in third gear even if you floor it.
Thanks all for your guidance with this.
Marlinjack,
any pictures from the Pontiac by hand ?
like to see them.
Simon