72 Chevelle BOG

[scottsfc21c]

I have modified my 350 (World S/R torque heads, Voodoo cam and lifters 213/219, Crane roller rocker arms, new timing chain, push rods and a Wieland air strike intake[/I]) and for about a month and a half it ran great, now it has a bad bog when the accelerator is hit fast, I have checked all the normal stuff on the carb (cleaned it, put a 4.5 power valve in it, went from a 25 to a 35 shooter, put new accelerator pump in 30cc, I even tried a new carb) and ignition (I put a complete new HEI distributor coil cap rotor and all) I also have tried the timing from 5 BTDC all the way up to 15 BTDC and put weights and three different springs in. I have done a compression test and it came out to 150 psi per cylinder, the vacuum is 19 with a slight rapid vibration, now I’m stumped on what it could be, any ideas would be great

Little more on the engine it has stock pistons, Holley 750 vacuum secondary and is in a 72 chevelle with a 700R4 trans and 3.37 rear end

[Henry Rifle]

What's your fuel pressure at the carb?

[scottsfc21c]

4.5 psi I was thinking that might be kinda low

[mrmustang]

Fuel pressure is just fine holley carbs do not like much more than this under 90% of most driving conditions, however, I believe your carb is too big to properly tune for your engine. I suggest a Holley 600cfm carb either in a DB or a Vac Sec configuration.

Bill S.

[scottsfc21c]

well I had the 750 on it after I put the new stuff on and it ran great. After the bog started I tried a new 600 holley V/s and it did the same thing, it still runs great after you get past the bog

[mrmustang]

I'll stand behind my suggestion above. Besides the downsizing of the carb, you need to find yourself a quality shop with both a chassis dyno, and someone who knows how to tune a carb at anything besides wide open throttle.

[nitrowarrior]

Two things come to mind. As much as I love Holleys, the one thing that has plagued them in the last 20 years is the metering block being slightly lean as the fuel expending from idle to big throttle opening is lacking. I am really pulling out some dead brain cells here because something about what you are saying about when this problem started. Valve springs. More common than you think. Every manufacturer has tried very hard to stay on top of this on the BBC engines and it has been a tough tough world to be consistant and succsseful in this area. I can recall on more than two hands and both feet where this was the problem (I had to stop because if if the number goes any higher I just doze off) give it a thought. I find myself frustrated offering my "2 cents" worth throught the internet, I like the hands on approach for diagnosis.