Fun with Carbs

[Kivyee]

Being a carb neophyte, it's taken quite a bit of research on getting up to speed on what's needed to set-up a carb properly and then to actually tune it. (@mike.bray you probably wouldn't want to read this )

To start with, this is a MkIV with a rebuilt 302 (306), with Edelbrock Performer 1406 carb (600cfm) w electric choke, Edelbrock RPM Performer Intake, No-name Aluminum heads (**Maybe Promaxx), roller rockers, Magnum Hydraulic Roller Camshafts 35-440-8 (110 lobe separation, 281 duration, .512 lift), and finally out to BBS long tube headers and standard FFR sidepipes. I bought the car built and the PO basically handed the engine to an engine builder and asked for a rebuild to 350hp, he doesn't remember exactly what the internals are but I'm assuming they are probably stock ford stuff since 350hp isn't really "forged internals" territory.

The cams are pretty aggressive, so the car has a very distinct lope and slightly rough idle. When I first got the car in December, I had to adjust the choke to richen up quite a bit for it to run.

Fast forward to this week when the weather was finally nice (dry) enough for me to make the 45min trip to the mechanics for the mandatory out-of-province inspection prior to getting it registered and insured, the car was running mega rich on cold start and idle, then would be fine after about 5 minutes of warm up. It finally occurred to me that I needed to adjust the choke for the warmer weather and voila, cold starts no longer sting my eyes.

Then I discovered that the car would bog/hesitate momentarily if I depress the loud pedal quickly. It's a relatively new discovery thanks to wearing in of my new Nitto NT05s and warmer weather. More research and shifted the accelerator pump linkage for "faster action". Boom, instant transition to acceleration on WOT.

The car would also burble and pop something fierce on downshifts (overrun), which I kind of liked but which also told me too much fuel was making it past where it was supposed to be burned, leading me down another rabbit hole. I know in EFIs you can actually tune for that by retarding timing when the throttle plate is closed and RPMs are above idle, so I looked into that and discovered the wonderful world of "ported vs manifold vacuum for timing advance" debate. I verified that my distributor vacuum canister was hooked into ported vacuum, swapped it for manifold vacuum (more timing at idle, off idle), readjusted idle mixture/idle speed back to 800rpm (was high right after the swap), and boom, smoother idle (I do miss the crazy lope), and no more crazy popping on decel (I miss that a little too).

So now we are FINALLY ready for actually tuning the carb, which I will take slowly as the engine actually runs very well right now. Between playing with the timing curve and the carb jetting sounds like I will have tonnes of teeny tiny parts to play with and drop into the bowels of the engine bay - fun times ahead.