The 1947 - Present Chevrolet & GMC Truck Message Board Network - View Single Post - Help deciding between 750 mech and vac on my 454

burnin oil · 10-23-2014, 11:22 AM

If you really need the 750CFMs go mechanical. I say this for one giant reason. Vac seconds are rarely fully open and when they are it is at a high RPM. One of the vacuum springs in the tuning kit barely lets the secondaries start to open up until red line. I have a 750 vac on a 400sb and ended up putting a bolt in the secondary linkage to force to be a mech secondary. This is the wrong way to do it and causes a nice lean spot when the secondaries come in. I make more power and drivability went up while milage stayed the same. Funny how that worked out. I need a new carb but that is a $500 I don't have at the moment.

10-23-2014, 11:22 AM	#2
burnin oil Registered User Join Date: Jan 2013 Location: Eastern TN Posts: 1,919	Re: Help deciding between 750 mech and vac on my 454 If you really need the 750CFMs go mechanical. I say this for one giant reason. Vac seconds are rarely fully open and when they are it is at a high RPM. One of the vacuum springs in the tuning kit barely lets the secondaries start to open up until red line. I have a 750 vac on a 400sb and ended up putting a bolt in the secondary linkage to force to be a mech secondary. This is the wrong way to do it and causes a nice lean spot when the secondaries come in. I make more power and drivability went up while milage stayed the same. Funny how that worked out. I need a new carb but that is a $500 I don't have at the moment.