Some time back I swapped on an Offy C intake, Edelbrock 500cfm carb and EFI exhaust onto the 300 in my Bronco. I used a wideband O2 sensor to kind of halfass tune it (didn't buy the whole kit, just a couple of sets of rods and some step-up springs) and it's run okay for the past few years. Since then I've added a heat plate to the bottom of the intake and had the distributor recurved, so I decided to break out the O2 sensor and see how everything looks now.

Turns out it ran a little richer than I'd like. The rods in it are one stage lean from stock at power and cruise. The other rods I bought aren't on the tuning chart but size wise should be about 1/2 leaner. It looks pretty good at cruise but still rich at WOT. The next step leaner in the manual is a jet change and I'm going to give that a go tomorrow.

It's always been a little boggy when I first crack the throttle at any rpm - the A/F meter jumps out of range, over 16:1. I bought the carb used and it had a smaller pump nozzle than stock. With the big, open plenum I think it needs a bigger pump shot. Edelbrock doesn't sell a stock size replacement nozzle. You've got to drop $30+ for a set of three. Well I finally did and plan to have that lean spot cleaned up this weekend too.

I'll write down the details and part numbers once I'm done fooling with it in case anyone's interested.

While I was rooting around looking for Edelbrock parts in the shed I found an old 2" open spacer. When I swapped on the Offy (but before I got the O2 sensor or heat plate) I tried the carb right on the manifold (didn't work because the throttle arm hit the intake), with a 1" open spacer and with a 1" 4-hole spacer. The 4-hole ran better, by far. It's been on there ever since. Well just for fun I stuck that 2" open spacer underneath it so the carb is sitting 3" above the intake. I've got a 4" tall K&N filter on the way to complete the ensemble.

No hood clearance issues with the inline six, baby. Maybe, though, if I Wile E. Coyote a 1" open spacer at the bottom.

Not surprisingly, the lean spot off idle is slightly worse but otherwise it drives really well. Not a huge difference but definitely better seat-of-the-pants. As they say, it ain't pretty but it works.

 

Which model Edelbrock carb is 500 cfm?

 

The one I've got is a 1403 - electric choke. There's a manual choke model also.

 

The Offy C has more than enough plenum volume and using an open spacer just increases that volume. Try just the 4 hole spacer. If you want a taller looking set-up, use a spacer between the air cleaner and the carb. That also might help columnar flow through the carb.

 

I've been running the 4-hole spacer for a while and other than some things that needed to be tuned out of the carb anyway it runs better with the extra space between the carb and intake floor. I probably will swap the 2" open for a 1" depending on how well I can clean up a couple of things with the carb. Or try just the 1" open spacer.

The pump nozzle on the carb was the small one - .024" and I went up to the next biggest one in the kit, .033. It's still very lean right off idle. This is probably where the extra plenum space is hurting. The biggest nozzle is a .043". I'll give it a go this weekend.

That very lean condition shows up at low rpm when I go WOT too. Once it winds up a bit it looks okay, but really slow to rev. At least from ~30 mph in 4th gear. The added plenum space probably isn't helping there either. I'm hoping the bigger pump shot helps here but I may need to go richer on the secondary jets. With a long weekend and any luck I intend to get it pretty close.

 

Dig the fancy throttle cable bracket.

 

Well I had a chance to fiddle around with the carb this past weekend. I took off the 2" spacer and just kept the 1" 4-hole. It's turning ~625 rpm and pulling 22" of vacuum at idle.

I felt like something was off other than the carb and rechecked the timing. I had a shop recurve the distributor and whatever they did wasn't right. No mechanical advance until ~2,200 rpm and very slow from there. I changed the heavier spring in the distributor for the lightest one I could find and it's coming on at ~1,000 now and all it ~3,250.

The options for mechanical advance are 30 and 36 degrees, I went with 30 and plus 12 degrees base timing for 42 total, too much for my taste. I'd prefer maybe 20 or 22 degrees mechanical so I could run the static around 16. I reckon that means a trip to the yards and scrounging around with my fingers crossed for a couple of cores to plunder.

Other than the pump nozzle the carb is at factory specs. Swapped on the biggest pump nozzle (.043") and that helped a bit, but it still goes crazy lean if I just tip in off idle. I thought it was a little rich overall, now I'm waiting on a set of rods to try one step richer in cruise mode to see if that helps. Once I get my foot into it everything looks okay, maybe a shade on the lean side.

The lean spot at WOT is still there regardless of pump size or the power stage on the metering rods. With the smaller rods it was super rich ~9:1 once they stepped up at part throttle acceleration and no real improvement at WOT. I went up one size on the secondary jets to a .098" and it helped some. I ordered a set of .104" jets for the secondaries - 3 stages rich according to the manual - to see if that helps even more.

Funny, I went in thinking the truck was running rich and I end up tuning it richer. I'm very disappointed in the shop that reworked the dizzy, too. They're close to home but won't be getting any more machine/mechanical work from me