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.

[Jim1855]

Good luck with your tuning adventures.

I'll be starting over with a new carb in a week or so. Similar motor but probably less aggressive cam. That might change.

And big fun will be had by all.

Jim

[cv2065]

Just made a few of those changes myself. Playing with moving from timed to manifold, but not there yet. The lope on this engine is amazing.

[F500guy]

I have been setting up my Holley 750 DP, no choke for my 427. I was lucky to have a data system with wideband hooked up so I have been enjoying my play time. Car is running really well, but not optimal. I am now at the point that I need to modify my Carb. I found that I am still too lean at moderate throttle before the secondaries come into play, a bit to rich for just cruising on the main primary jet. So I am planning to drill and tap the carb for power valve restricter removable jets so I can fix that acceleration issue, and still lean out the main jet a bit more. David Vizard book is a good reference.

[CraigS]

OP you may want to check how much advance you get from the vac on the distributer. I found that mine had 20deg running from ported vac source. I was getting engine knock while driving around town and giving it a little gas at 1600 or >. Turned out that much advance simply couldn't be pulled off quickly enough by the dist. F500guy, "a bit to rich for just cruising on the main primary jet." I had to opposite at one point, too lean. I had a lean misfire cruising at 55 in 5th which was maybe 1300 rpm. I found that the idle system can affect mixture to as high as 1800-2000rpm. A quickfuel factory tech line guy said experiment w/ the idle mix screws. If your idle mix is perfect now you will mess that up some but this is just a test to see if the idle system is causing your problem or it's the main system. Change all mix screws in 1/4 turn increments and drive after each adjustment.

[F500guy]

OP you may want to check how much advance you get from the vac on the distributer. I found that mine had 20deg running from ported vac source. I was getting engine knock while driving around town and giving it a little gas at 1600 or >. Turned out that much advance simply couldn't be pulled off quickly enough by the dist. F500guy, "a bit to rich for just cruising on the main primary jet." I had to opposite at one point, too lean. I had a lean misfire cruising at 55 in 5th which was maybe 1300 rpm. I found that the idle system can affect mixture to as high as 1800-2000rpm. A quickfuel factory tech line guy said experiment w/ the idle mix screws. If your idle mix is perfect now you will mess that up some but this is just a test to see if the idle system is causing your problem or it's the main system. Change all mix screws in 1/4 turn increments and drive after each adjustment.

Thanks Craig, I did make some adjustments there since my carb does not have adjustable Idle feed restrictions. I made some pump cam and nozzle changes last week and was really happy with that to reduce the lean conditions on throttle tip in but now that I have a proper throttle position sensor it is really obvious between 20-45% throttle AFR around drives up to about 15. After about 45% throttle it settles back down to about 12.5-13. By a bit rich it is about 13.8 ish at gentle cruise at 10 % throttle not to bad, just for grins I wanted to get it up over 14. (14.1 with 10% ethanol is the target)

[CraigS]

Glad to see you have an AFR meter. That is a huge help since it is often hard to distinguish just by feel if it is lean or rich. BTW I found that since tuning sessions were normally separated by days, keeping careful notes of changes and the results was very helpful.