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FE Technical Forum / Re: Need help with Holley carburetor tune.
« on: June 29, 2020, 10:12:25 AM »List: 14936Lance, 780’s have different venturi size for the secondaries, so calculating area doesn’t really work for this particular carb.
Certainly can calculate the area versus the original specs though, given a LIST number and before and after PVCRs
That being said, agree with all above, no need for 50cc and sewer pipe squirters. There are a few 4150 780s, so if you post your LIST number we can give you original specs to start from. My gut says dual 30cc pumps on the street, blue cam on the front if you need some oomph and 25s for squrters, 5.5 or 6.5 PVsr
I will say this, although I think the carb needs some un-loving. It doesn't seem that crazy that the engine would go backfire and popping lean. Are you 100% sure no crossfire, crossed wires, vacuum leak, tight intake valve, etc?
Also, is it a backfire through the carb (lean), or bangs in the primary pipe (rich)
I think I would run the valves, look at all the wires and plug insulators, double and triple check firing order, and be very sure you don't have a big vacuum leak somewhere, because although the ports are big on a SOHC, your build shouldn't need a crutch like that.
Another option would be to see if some buddies good running carb does the same thing without messing with anything other than idle speed
The guy I bought the car from said it’s a 780. That may be incorrect. I thought List numbers started with an R, this one doesn’t.
1. No vacuum leaks unless the one vacuum port hooked to the PCV valve counts.
2. Firing order is correct. It runs great otherwise.
3. Timing is at 29 total. Maybe it’s coming in too early?
4. I know the 50cc pumps and 40 squirters are big but watching my wide band the AFR stays perfectly steady at approx 13 to 1 when quickly smashing the throttle. It spiked 18 to 1 for 1/2 a second with the 30cc pump.
5. The snapping/popping noise occurs when AFR hits 18 or 20 to 1, not when rich.
6. Carb has been disassembled, cleaned, holes blown out etc. I’ve done this 2x.
7. Motor runs good at WOT and Cruising and smash throttle hard acceleration. It runs super lean when slowly easing into the throttle under light load (when the accelerator pumps aren’t a factor).
8. I haven’t checked valve lash. I need to do that anyway. I’ll do that next.