Jets

[diegokid]

What size main jets are some of yawl running in a VS 750 sitting on a SA 406?

[Brent]

If you have a SA motor I assume you have a 3310 vac Holley
The main jet is 70 and the secondary plate is a #21 which equals a 75 jet
For street use you should never go up or down more than 3 or 4 jet sizes. Holley sets the carbs to run at 70 degrees and sea level. Where you live there is nothing more than 900ft above sea level so you do not need to compensate for that. 1 jet size for every 2000 ft is about right.

[pbrown]

Also keep in mind that the main jets don't come in until later in the RPM band. I see a lot of people fiddle with main jets when they should really be looking at the idle/cruise circuits.

I recommend investing in a good wideband AFR meter and tune from the bottom up. Start wth idle, then cruise, then mains, then wide open (power enrichment).

[diegokid]

I have an AFR meter. With #72's at 70 MPH it reads 13.5 - 13.8. I was just curious to see if anyone was having to use anything larger than a #74 or smaller than a #70. My carb came with the #72 by the way.

[pbrown]

Try going down to a 70 front and 72 rear see what you have for AFR.

[Brent]

Just curious why you would want to mess with a AFR of 13.8? That's a good #. If your looking for fuel economy you might look for 15+ but for effeciency combined with power you are right in the ballpark.

[diegokid]

I know thats good I was just curious to see if anyone had a motor that actually needed anything bigger than 72's.

[pbrown]

I would still change them to see what happens. 13.8 is no that great for cruising. You can do better. Remember that the power valve will open under load to provide more fuel. What is the vacuum reading at 13.8 and what power valve do you have?

[Brent]

Patrick,

Curious what would make you say that?

I think anything over 14 is an EFI corrected/self adjusting system. 13.2 to 13.8 is a good number for good power and reasonable economy. With a carb you usually jet the fronts so that you maintain about 13.2 on the primaries then jet the back so you maintain about 12.5-12.8 on the secondaries. This will give you a good cruise# and at WOT you can enjoy the best of both worlds without detonation or pinging. What did you have in mind?

[pbrown]

There is no reason not to tune for good economy now that we have access to good wideband testers. Gasoline has a stoic value of 14.7 and that is usually my target when tuning for mains alone.

The AFR does need to be more rich when under load and that is why we have the power enrichment circuit (power valve and PVCR). This is why I asked what the vacuum reading was. The fuel curve can and should be tuned for economy when under light engine loads and to increase the fuel delivery when engine loads go up. The power valve controls the timing for this circuit and the PVCR dictates how much fuel is added.

It IS possible to have your cake and eat it to. I have a 62 Falcon with a 402HP/418lbs engine that gets 25MPG on the highway and does a 12.5 quarter without any pinging. It is a lot of work to get a carb dialed in but it is very rewarding, especially when filling up at the pump.

[Aggressor]

I agree with Patrick. Even with the superchargers I dialed down the primary jets to run a bit leaner & meaner. To compensate for demand conditions I drilled the Power Valve Restriction Channels in .003" increments until I had sufficient enrichment.

Results: 42.3 mile test loop on two gallons of fuel = 21.15 MPG
Dual Holley 600 Double Pumpers; 66 Primaries; 73 Secondaries; 6.5" Power Valve
Timing 12 degrees initial; 20 degrees mechanical; 7 degrees vacuum

- Geary

[diegokid]

The vacum is at 14 at 70 MPH and the PV is the standard 6.5, at idle the vacum is 9. Today something happened and the AFR dropped to 12.8 - 13.1. Car Runs better now and the miss went up in the RPM range to about 4000.