I have a blown 392 hemi in my 34 Ford and it was always running lean at highway speeds....Tried replacing jets and metering rods but never seemed to work right,,, Running two 750 CFM carbs...Removed the metering rod covers while the engine was running and the metering rods where always in the lean position due to the vacuum made by the blower....Found an old Hotrod Quarterly magazine and it has a good article on building and tuning a blown hemi....I removed the springs from the metering rods and blocked them to the rich position....Worked great for me...Just thought I would pass this along...
A blower make's boost, not vacuum, and boost doesn't happen until W.O.T. The fact that you had to block the needle's means that the spring's were wrong or the needle and jet combo were wrong, or a combination of the two. Are these REAL AFB's or Edelbrock's?
I used 2 Holly 750's on my 671, and made sure that they were boost referenced. Lots of info. on how to do it yourself so that everything works like a regular intake. Just a thought....
Correct. But the effect of the blower doesn't come into full play until W.O.T, or close to it. At street speed's, the effect of a blower is minimal at cruising speed's. There is no more vacuum with a blower than there is naturally aspirated.
That's an interesting modification. How did you block the rods? Do you have a link to that article? There are different springs available for the vacuum pistons in the AFB carbs for the express purpose of tuning to the vacuum signal, but you probably already know that. The late great Pat Ganahl published a book called "Street Supercharging" which has a carburetor section in the back and covers various modifications for running the AFB's as well as the pro's and con's of the Carter's, Holley's, and Rochester's.
I removed the springs which were the stiffest that the tuning package had...I took a vacuum hose and cut it into about 3/16 long and put it under the pistons that control the metering rods....This kept the vacuum signal going to the pistons but they don't move keeping it in the rich position..I also drilled the main jets to 1/8 and left the secondary's alone....The article is in a old Hotrod Magazine's Street Rod Quarterly type of magazine dated July of 1971.... It has a good article on building a 392 by a guy named Cotton Werksman....Now that I know it works well going to make some spacers out of aluminum tubes...
You might want to put an O2 sensor on one of the headers, and an AFR gauge in the car, and drive it for a while. Might be surprised what you see..... I did this when setting up the 750 AFBs on my blown big block Chevy. I started with stock jetting, and the only change I made was to one size larger cruise portion of the metering rod. The carbs have been on the engine for 30k miles now, it's been in the high 9s, gets 11 mpg on the road, and doesn't foul plugs or burn pistons or anything undesirable. But you'll also see the mixture go wonky under some conditions, rich when you think it should be lean, go lean for a moment when you think it should be rich, and generally run a bit richer than you'd like much of the time. The carbs are not really made to be "perfect", and it turns out that "good enough" works. Boost referencing might be something you want to try, it could let you run it a bit closer to optimum under some conditions. Just don't expect miracles. If you haven't tried stock jetting/rods on the carbs, you might want to do that before you get too far out of range. An aftermarket carb of about the right size, with it's designed jetting, usually is driveable under most conditions, although it will tend to be a little richer than optimum.
I built a similar engine to Squirrel's (Roots BBC w/ Edelbrock 750's) that's been in 3 cars. I found the same results when tuning with an A/F monitor. Carburetors are incredibly adaptable, but highly inaccurate at maintaining proper ratio through varying conditions. However, close is generally good enough.
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