We know they are a little bit big to run on these motors and there is a flatspot , Just wondering what the average main jet size people are running? I have in that the moment 67.  Plugs look at little dark , I was thinking about coming down a couple of sizes and then change the pump squirter and then cam blokcks to try eliminate the flatspot.

What’s the list number on your Holley?  With there being at least a couple of dozen different 600 Holleys over the years, jetting varied in their as delivered states due to specific air bleed and PVCR sizes as well as venturi booster design.  The baseline number is always what the carburetor was supplied with when new and then adjust up or down from that point.  With alcohol laden fuels here in the U.S., increasing the jet sizes is normal just to compensate for the reduction in BTU’s of the alcohol and to keep the engines from running on the lean side.  Quite a few of the stock 600’s Holleys do come with 65 & 66 primary jets so #67’s is not out of line and especially if the quality of the fuel or the BTU content is low.  Many of the earlier 600’s came with a #25 shooter but many of the newer ones now come with a #31 shooter.

Between accelerator discharge nozzle sizing and pump cams, I’ve found the pump cams to be a much quicker solution to eliminating hesitation issues.

Any ideas on pump cams??