Racing Fuel Systems

After three years I finally had the time and inclination to install my aluminum heads on my Mopar 451. I don't know what the old hand-ported iron heads flowed, but doubt they were better than 250 cfm. It took me considerable time and effort to get the street tune right (or at least close to right) with my QF/Holley 950 double pumper on a Performer RPM manifold, 1" spacer. Cruise AFR 14.5-15:1, full throttle 12.5:1, idle around 14:1. Large cam (272@.050, .652 lift). 8" vacuum at idle. 7.5" PV. I'll have to dig out the jetting sheet to see what IAB, TSR, MAB I'm running, but the PMJ are quite small I think (68?) and large SMJ (88?)

The new ones are a couple tenths higher in compression (10.5 now) and flow in the low 290's. It runs a lot leaner everywhere, around 1 full point AFR. Cruise is more like 15.5:1 but no surging, so that's ok. The idle is now much too lean, showing around 16:1 after full warmup, and continues to spiral in (gradually lean out and slow down until it dies), especially when fully hot.

So do I need to redo the entire carb tune now? At idle or cruise, aren't the throttle blades and the bypass air the primary restrictions to airflow, not the head (so the increased head flow at WOT should not affect it)? Vacuum seems about the same.

To get my idle back to the "way it was", I ended up adding just under 1/4 turn to all four corner mixture screws, and also cracked the secondary idle stop screw another 1/8 turn.
Tweaking big cams for the street is tricky It's a fine line between getting into the advance with too much rpm, or losing vacuum signal and slowly dying with too little rpm...

I found my log with the most recent settings: (pri/sec)
IFR .028/.028
IAB .078/.079
TSR .074/.074
MAB (HSB) .026/.026
MJ #71/#86
PVCR .066
PV 7.5"

Tomorrow is supposed to be a nice day, hoping to get the Dart out and get a few AFR readings at low-speed, high-speed cruise, and a very brief WOT on all four barrels

Since the head swap, it's a little too lean at the start of the transfer slot (that small flat spot is now worse), but highway cruise at 60-65 mph is good and lean (15-15.5:1). I may need to open up my TSRs a little bit.

Depending on what WOT looks like, maybe go up two sizes on the secondary jets.

I would add a little jet in the primary, 71 is lean

Yeah, that's kind of the point... leaner without surging, until the power really starts coming on (as the power valve and then the secondaries open). So the result is decent gas mileage on the highway (before the head swap, about 11 mpg at 60-65) and still a safe AFR 12.5 at WOT.

I took the car for a short low-speed drive today, and found that after my idle mixture adjustments, the low-speed cruise had also richened about back where it was and the flat spot is much better too. Not too surprising since the idle circuit is still a heavy contributor there. Next trip out I'll get on the highway and check my 60 mph cruise AFR before touching the PMJ.

Sounds like you have a good handle on things.

Getting the best idle always goes a long way with these carbs and camshafts. Many times it cures, off idle hesitation/flat spot.
8 inches vacuum isn’t much . Is that in neutral , no load? RPM?
IFR .028” seems small for that combo. I would think you would need larger.
What are the idle mixture screws set at now?
Idle timing would need to be around 30 to 36 degrees. Do you use manifold vacuum advance?
I would doubt the .074 TSR is doing anything at early throttle opening . I run a .064 TSR with a .035 IFR, different carb mind you.
When the idle AFR gets gets richer, the cruise AFR gets richer. So we use a TSR.

I’m working on a Mopar now with a tight 107 LSA cam . To get a decent idle we need to advance the idle timing to about 15 degrees , where vacuum starts working , so we can use vacuum advance to get to 30 degrees idle timing. Of course the distributor will have to be recurved then. And I have to work with the OEM distributor. 1971 Cuda. Beautiful car by the way.

My 454/500 hp BBC gets 15mpg US

Thanks for the input. To address your questions:

Vacuum is 8" at 1100 rpm idle. Everyone who's run this Mini-Express has similar figures. It's a 4-speed so converter load isn't an issue But with 272@.050 and 107 (in at 105) there's not going to be a nice slow idle no matter what... definitely not intended for street use, but I'm making it work.

I am not sure exactly where the screws are now, but I know they're not "way out" like 2 turns. It's been a while since I tuned the carb for the iron heads. I could check, but since it's working now, why mess with it

I seem to recall trying smaller TSRs but ended up worsening the lean flat spot just off-idle.

Yes, my vac advance is connected to manifold. The idle would clean up some with even more idle advance - but try and find a Mopar can that's all-in by 7-8". I could have one made by FBO - which would likely introduce a new problem, too much advance at part-throttle (since the cruise vacuum is higher than the idle vacuum, the advance would be all-in nearly all the time).

15 mpg is pretty impressive. What RPM were you turning, at what speed? My 3.91 rear gears (3000 rpm at 60) don't help the mileage either...

2400 @ 60mph . A lot of mopar vacuum cans are adjustable with 3/32 hex. They will adjust how much vacuum is required . I believe clockwise turn / less vacuum required. This is what I am doing on the Cuda. I then made a limiter to attach to vacuum canister to limit total vacuum advance from 20 degrees to around 15 degrees. Also installed an FBO mechanical advance limiter. We still need to test when weather straightens out. I did the same thing on my 454 BBC.