I've got a 69 F-250 4x4 that I recently installed a offenhauser intake and 390cfm on. I had already had an exhaust off of an F-600 (2 1/2 inch). There's a lot more power, but usually it bogs in the mid range. When you step on it in second it picks up great and bogs right about when the 4 barrels should kick in. Then it takes off again. I checked my timing and point gap and both were perfect. I found out the vaccum advance wasn't working on my distributor so I swapped in an electronic ignition and distributor from a 76 f-250. The bog's not as bad but it's still there. Any suggestions?

Also, since I am gaining rpms with the carb do I need to advance or retard my timing?

 

First, try changing the accelerator pump shooter nozzle from a #25 to a #28. You may want to throw in a 8.5 hi-flow "picture-window" power valve too; the stock cam makes a bit of vacuum ("uhh, Beavis, it sucks! Change it!") even with a honking single plane, and especially with the Offy DP, so the usually 6.5 pv might not be opening soon enough. Timing curve I dial in as per specific application. To oversimplify, peak power is made on the edge of detonation (lightoff should be a few milliseconds before peak cylinder pressure rise so the big bang occurs at the moment the expanding fireball will do the most effective work, obviously inertia is a key variable here...). Every particular weight/gearing/load pulling/octane rating/altitude/combustion chamber shape etc. combination has different optimal advance curve requirements. I recommend picking up a copy of "The Doctor's Guide to Automotive Ignition" by none other than Christopher Jacobs (I don't prefer to use his hardware, but this book is a great resource...) and burying your nose in Section II (pp. 17-76, a good year, BTW!). That will tell you everything you need to know about how to dial in your particular vehicle for the best spark curve without having a physics textbook and distributor machine handy.