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My 67 has a late HO roller motor with a mallory dual point, edelbrock performer intake, holley 600 cfm vacuum sec. carb. Autolite 25s currently. Finally bumped up the timing to 10*, (should have been there all along) and now the plugs seem too white. They were a nice tannish brown before. I have a new set of autolite 24's (next colder plug) I was going to put in, but every now and then I put a lighter secondairy spring in the holley (little more pep :) and I don't want it to start fouling up the 24's. Any comments on this? or is my only choice to just give it a swing and see how it goes? Thanks!
 

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I suspect I have a bit hotter motor than you do and guess what I run in it? Plain old autolite 25's. I don't need 24's so I wouldn't think you would either. If the plugs are too white you have to figure out if its too lean in the primaries or secondaries. Increasing your timing a little has probably brought the power up some so that probably is what has made it go a touch toward lean.

The question is; is it the primaries or secondaries? If you cruise around at part throttle and not get into the secondaries and that cleans the plugs off white then its in the primaries. If it only whites out the plugs at wide open you need more secondary fuel. Also listen for any light throttle to wide open throttle stumbles or studders and feel for a fall over when you go from part to wide open throttle. That could be caused by the secondaries opening a bit late. If its a vacuum carb with a plate back there you would have to convert it to a metering block to get some jets in it.
 
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