I am running 800 4780-2 on my 383, no real problems I just wanted to know what typical spreads front to rear being used. Stock is 70 -85. Which seems like a large spread, running 70-86, probably going to 71-86, which is 3 drill size jump, plugs look pretty clean at stock heat range 26. Always seems to run good with 71's. I have a slight hesitation when secs are depressed slowy, under hard acceleration I dont notice. Could it be a squirter issue? What should accelerator pump linkage be gapped to? Thanks,SS.

Most Holley carbs have a 6-10 jet spread front to back, but in your case the Holley website confirms that your carb came with #70F/#85R jetting out of the box.

I don't know exactly why Holley did this on some carbs, but it probably has something to do with the size of the PVCR (power valve channel restriction). A larger PVCR would add more fuel to the primary side when vac drops below the PV set point. You could always jet up in the front, that may help especially if your plugs read clean & lean.

OR possibly your PV is opening too late. lowest steady vac reading at idle in gear minus 2 - 3 is the optimum PV size. If opens too late the engine will go lean for a second during transition.

OR it may have to do with the rear pump squirter. I adjust my linkage so that there is zero slack on both accel pumps. A smaller squirter on the rear pump might possibly help.

Hope that helps.

thanks for the reply, I am running a 4.5 PV and vac readings at idle no load (four speed) about 10hg. I am wondering if it is opening to early and remaining open at cruise, if this were the case wouldn't the plugs read rich? Up until I switched to autolite 26, plugs were somewhat sooty. I remember a rule of thumb to take 1/2 of vac at idle to determine pv size. I am running zero gap on accel pumps as well. I will tinker with it this weekend and see if it responds. thanks, any other suggestions are welcomed, SS

I'd try a PV that opens a little sooner -- like a 6.5.

You have a little more room to play with PV's on a manual trans car and opening the PV quicker might help cover your flat spot.

In your first sentence you said "no real problems" so I would leave it alone. I am a firm believer in the "if it ain't broke don't fix it" motto.

With todays fuels and additives the only way to read plugs is make an all out blast (preferably at the track, shut the motor off immediately and coast to a stop. Any idling time will alter the "read" on the plug.