Mick is it truly a "bog"?, I know your new to Holleys/6 packs, but Holley 6bbl transition from 2 bbl to 6bbl will have a momentary "delay" in RPM as it dumps open the end carbs and starts to adjust RPM for all that fuel/air, the 6 pack is not like the previous dual quad set up you had where mechanical primary to secondary transition is continuous between both carbs


What's the current PV?...the 8.5 is to high IMO, personally I'd go with the 6.5

Mike

Fuel mileage will likely suffer greatly with a 8.5 pv. Unless you are downshifting each time you climb a slight grade in top gear, hard to imagine not dropping occasionally below 8.5 inches of vacuum when cruising except on fairly level ground unless perhaps you are pulling excessively high vacuum at idle. Perhaps with the torque of a 540 hemi, who knows?

Nevertheless when you mash the throttle and therefore expect outboards to open, vacuum should fall close to zero, so whether a 4.5 or a 8.5, pv should open instantaneously. Doubt a 8.5 will make outboards open any sooner however.


But can't hurt to try 8.5 except for a little labor.

Point of curiosity, what is vacuum at idle?

Have you mentioned what cam you are running?

Accelerator pump dialed in properly with longest duration spray?

I might disable outboards so as they cannot open at all. Then running only on center carb, do you still get a leanout when you mash it. If so, I would be looking more askance at pump.

Sounds as though center carb is writing a check that outboards can't cash.

Where is distributor initial and total timing. I would be looking at having more initial and limiting total advance by distributor modification.

This would tend to lessen tendency of closed throttle butterflies to be improperly positioned in transition zone at idle or light cruise(once idle speed screw is set back as more initial timing tends to increase idle speed). May or may not help current complaint but if initial timing not properly advanced, would likely improve overall performance and responsiveness with an added possible improvement in fuel economy.

Are you running vacuum advance,,,,,if so disconnect and plug line and see how outboards then kick in.

With that cam if you haven't done anything with end carb jetting unless I missed it,I think you will still be lean at getting to W.O.T.

Wow... a lot of stuff. On power valves - If your idle vacuum is 12"Hg, then you should be running a 5.5 power valve. I believe you're over-thinking it. The power valve has nothing to do with the outboards opening.

At cruise, you are in the main metering circuits because your cruise vacuum is 11" and 2 x 4.5 = 9, so the power valve is open. To adjust your cruise A/F ratio, change jets. To change your WOT A/F ratio, change the "jets" in the outboard metering plates.

At cruise I'm well above 11. 11 is where the secondaries finally kick in, and I'm well past 50% throttle for that.

Unless I don't understand it correctly, the rating on the PV is where it *starts* to open. So a 6.5 PV starts to open at 6.5Hg. My impression was not that it snaps open fully and instantly at 6.5, but that the spring pressure will be partially overcome at 6.5, and then still needs lower vacuum to completely open it.

But I don't know if that is correct.

Ron's is correct.

A few other concepts may be helpful.

The idle system works on manifold vacuum. This includes when the throttle is off-idle or on 'transition'.
* The manifold vacuum is the pressure difference that drives the fuel.
* It also provides the distribution to each cylinder. Higher vacuum = stronger signal, usually better distribution.
* The idle system responds somewhat slowly to throttle opening. The accelerator pump helps cover for the slow response.

The Power Valve is there to enrich the mixture when the Load gets high near maximum.
* With a powerful engine, high gearing and so forth, it may not be at under full load until 3 gear and high speed. (Best to test at track or on dyno.)

The Secondaries (or in this case the outboards) are there to allow more airflow when the air velocity is high enough. This produces more power by providing more air and less restriction (higher pressure) as rpms climb.

Finally, find the AFRs that work best. Use the numbers as reference points rather than targets. Some engine combos can go leaner than stoich at part throttle.

You may find the AFR curves posted on RFS helpful.
AFR Characteristics Under Increasing Loads
If the text is confusing, look at the graphs and then come back to it. (The thread was edited by one of the admins when some old and new stuff was joined together.)

With an engine that big you may have to use a 50cc pump and custom grind a pump cam to give most the shot late. The only way to slow the secondaries after the stiffest springs is to make the restriction smaller in the vacuum line that feeds both carbs, use some 3/16 set screws and drill them out starting at about .030, smaller.029 will open later. Changing the power valve dont work too good on a engine that size because the pvcr the holes behind the power valve are too small.