Wierd Holley issue

[Wayne Maybury]

Hi guys. I have a really wierd issue with my 600 cfm Holley (vac secondaries, elec choke) in my '66 Mustang. Engine details: 289, mild hydralic cam, aluminum heads with roller rockers, flat top pistons, performer RPM dual plane intake, full length headers, timing set and re-set at 31 degrees full advance. The fuel level in the bowels has been checked, the power valve has been replaced and I am now running #67 primary jets.

No matter what primary jets I use (tried as low as #62 or 63 but the car ran like crap with those smaller jets) the gas mileage is really bad with about 9.5 to 10.5 mpg while most of the plugs are snow white. The timing was backed off but that didn't help so it was re-set at 31 degrees. Right now I am running #67 primaries and I just pulled all 8 plugs with these colours: cylinders 1 & 3 are off-white, 2,4,7,& 8 are snow white, and 5 & 6 are black.

With a duel plane intake I would expect 1, 4, 6, & 7 to be similar and 2, 3, 5, & 8 to be similar but that is not the case at all. Remember that I have all of these white plugs and really bad gas mileage. There are no gas leaks anywhere and the exhaust doesn't smell terrible. I have spoken to my engine builder (he put the 289 together for me) and he said that he has never seen plugs like mine. Any ideas?