Another "my 950 is too rich" thread. I've posted on RFS but not gotten much response there.

Anyhow, I recently bought one for my 451 (from a carb guy who used to be on here, and we are talking on RFS) but I'm considering sending it back and I eat $50 plus shipping. (1.45" venturi, Holley ultra baseplate, and I think a Proform body, billet metering blocks, Ultra fuel bowls with sight glass).

It seems to be untuneable for efficient street use with my combo (451, 272@.050 mushroom cam)- just too rich at cruise and part throttle, which is where a street driven car spends most of its time. Too bad, because it feels great for driveability and really hauls ass when I hit the secondaries that I haven't even tuned yet.

The problem (many threads I've found by Googling, including on here) is that it's big enough that I have to run on the t-slots, even at 60 mph, 3000 rpm. And the 15" cruise vacuum sucks harder on the slot than the 8" idle vacuum... 13.8-14.0 at idle with screws out about 1-1/4 turns. 5.5 PV, 77/86 jets.

With the 950, I can see richer than 11:1 at the very top of the t-slot at low load and below 2000 rpm where the mains start to come in! Which is exactly the point where I'm putting around at 30 mph.

I went back and read my earlier posts on RFS about my 4780-2 that I converted to replaceable brass and a 4-corner idle with QF 12-700 baseplate. Although the same problem exists (12:1 at top of t-slot), it cruises at 14:1 or better and that is ok). I can't get this carb to do better than 12.5 low speed and (maybe) 13 at 3000 rpm.

I've gone all the way down to .045 primary TSR, and .031 IFR, .082 PIAB. And it STILL runs 11.5-12:1 at the top of the t-slots below 1800 rpm, but 16:1 lean at tip-in (no flat spot yet but I probably can't go much leaner before I can feel it). Last ditch will be to take the IFR down even a little more, maybe .029 or .030.

My 4-corner (and replaceable brass) conversion on the old 800 4780-2 works better than this - I tweaked some more and can cruise at 15.5:1... although there is a slight flat spot at tip-in I can live with.

So - I think I'm trying to do the impossible with a big cam (decent street manners and fuel mileage when my foot's not in it, and lots of top end horsepower).
I want a big 4150 carb but don't want to cruise on the t-slots.

Is there some way to make the t-slots smaller (or even tapered) that won't break the bank? I don't have the talent to try that.
Thanks for any help.

You know what’s coming next, right?

seems like it shouldn't even run with a .045" transfer slot restrictor. when you say tsr you mean you tapped the body of the carb to screw brass in the transfer slot feed?

the PIAB and IFR seem pretty far off too, but ikd.

do you have a baseline you can go back to?
seems most people use a TSR of about .080", IFR around .036" and idle bleed around .070 for baseline on those carbs.
Also I don't think you should try to get accurate air fuel mix readings at idle, especially with a cam that large.

Yes, main body tapped for TSR.

As delivered it had: .070 IAB, .033 IFR, .081 TSR. Sounds reasonable but runs pig-rich (12:1 at tip-in, 10.5 on full t-slot)! Stinky idle too. No point in going back to what obviously doesn't work

First I tried going up on the IAB, that made little change as expected. Then going down on the TSR in steps which helps some. Now it's lean at tip-in and still 11.5 on full t-slot.
This problem exists on the old 4780-2 also, but less severe since it takes a bit more throttle to cruise with the smaller venturis and butterflies.

Sure, I am aware of misfires causing artificially lean readings due to unreacted oxygen in the exhaust.
Whatever the actual AFR is, I set the mixture screws for best idle which always turns out to be 13.8 on the meter and no eye-watering exhaust.

No... what?

Yep, that is a common problem with Holley type carbs.

Step up the high bleeds .002-.004.......And a .033 ifr is WAY small for that size carb.....

OK thanks, I will try that today
Why is the IFR considered way small, if the idle screws are out just over 1 turn? Doesn't it take the same amount of fuel to idle the engine, big carb or small

Probably for someone to tell you swap to efi. 😄

AndyF has a (frequently made) point, but I wanted a simple old-school setup, AND I don't feel like plowing another 2 grand into EFI (and doing all the associated fabrication). It would be nice to be able to tune from a laptop instead of: drain bowls, remove fuel line and four bolts, remove jet/bleed, drill a new one, installation is the reverse of disassembly! I've already had a "helpful" person tell me to get a smaller cam, too

I tried going up to .033 on the primary MAB. Not sure if it helped (no dramatic difference). Cruise at 3000 rpm, 15" Hg still 12.5:1, and accelerating moderately (10" Hg) still 14:1. There is a lean flat spot at tip-in that I don't usually hit anyway, but 30 mph 2000 rpm cruise is 15:1

Also I leaned out the idle screws 1/4 turn to an indicated 14.5-15:1... same vacuum and RPM, and I don't think any lumpier, so that was probably too rich also.

Might try enlarging the MAB some more, say .036 and see what that does. I really want to get the 3000 rpm cruise as lean as possible (like the 14-15 with my 800).

Wanting to have excellent street manners and race car performance is a difficult goal to accomplish.
When you start making compromises or concessions to one , the other will suffer.
My " street " toy is a 12.85 :1 , E-85 swilling solid roller and carburetor having bundle of joy.
There are some driveability issues that I live with , because when I push my right foot down with enthusiasm , I quickly forget that little stumble on tip in.
You can get them close , but I have yet to see perfect.

you know what you want, correct
Keep working on that problem, part throttle cruise to rich at 3000 RPM, until you get it the way you want it and then work on other parts
You can do this, sneak up on a little at a time until your happy

The ifr's have a big impact coming up on transition/tip in and too small will give you that lean stumble you talked about earlier as you open the blades so we have to find a balance for idle and transition with idle bleeds, t-slot exposure on BOTH front and rear blades, float level, timing, etc and the hi bleeds will definitely control the cruise all the way up to max rpm's, I do it daily and the results are eye opening.....

my 2 cents . You have quite a cam in a sort of small motor . With overlap it will be awkward .
T slot jet is too small .T slot and iab work in conjunction , increase or decrease together What size idle feed restrictor as this will affect idle and off idle . What powervalve ?
A long time ago a smart man told me - dont fixate on the afr number give it what it wants .
Example my cruise , 25/2900 is 13.4 any leaner and it surges .Peak hp is at 12.9
granted i run a 1050 now but all holley are same principal . When i used the 1000HP on my 505 i had to adjust the same to get it to run as good as it had on the 440 .

Tex

With the original .081" TSR the top of the t-slot was pig rich (10.5)...
IFR .031, PV 5.5.
Eventually I will put in smaller main jets, a 9.5 PV and enlarge the PVCR (.059") as needed.



If I ignore the AFR number then I'll have to stop at even MORE gas stations and buy stock in Champion or Autolite... with a highway cruise AFR of 14.5-15:1 I was still only getting 9.2 mpg at a steady 60 mph. (3.91 gears, no OD).
This carb felt great as-delivered but was stinky rich at idle and everywhere else.
I've never had a lean surge at cruise even as high as 15.5:1 and didn't bother pushing it any higher.

i get the fuel consumption .
just keep working at it . But think each circuit and then how it interrelates to the next .
Im not a luddite but carbs work pretty good and will rarely let you down on the side of the road .
oh i probably wouldnt be using a 9.5 power valve .
that is way high ! I run a 6 or 6.5 , factory is 4.5 . But i have a very large converter and hence not much load at cruise but high vacuum . My vacuum @ idle is 4 or something close . With a lower PV say 6 or 5 whatever you will find at cruise you are on the jet but pv is closed and will clean up

Tex

Thanks. Since my cruise vacuum is 15" and I want to use small main jets, I'll need the PV to kick in sooner than usual as I get on the throttle.
The car is already accelerating briskly from 3000 rpm at 10" (and at 40% throttle where the secondaries start to open, there is still 6").

9.5 might be a little high (I run an 8.5 in the old 800 carb). But unless I can get a sufficiently lean highway cruise by dialing in the main air bleeds, (which will require reducing the main jets too), I don't think the 5.5 PV that's in there now will open soon enough.

Anyhow I have to get the overrich 3000 rpm cruise sorted before I worry about more enrichment for WOT

Try using all three circuits that are feeding fuel to your motor at light part throttle cruise, idle fuel passages, all 4, transition fuel feed circuits on both primaries passages and the main jet jet size as well Maybe throw in the idle air bleed size on both primary barrels also by opening them up a little tiny bit, .0010 to .0045 Not a bunch

What do you mean, use all three circuits? Isn't that normal operation...

My low-speed part throttle is now sufficiently lean, as previously noted. The remaining problem is at higher rpm part throttle where the mains are also flowing... I've been reading up on emulsion bleed tuning and may eventually have to start playing with those (fewer/smaller emulsion bleeds to reduce the contribution from the mains at lower flows, which is what I need). Current setup from top to bottom is .028, plug, .028, plug.

I am not sure yet that the MABs are now too big (will also tend to lean it at high RPM), will be interesting to see how it runs with the .036. Can always go back to .033 or .030.

Dom had me plug my top emulsion hole to get the mains in quicker. It helped. I can cruise in the 14s now, but do have a slight lean hesitation at the top of the slots just as the mains come in. Lesser of 2 evils. It is only at light throttle up. A quick stab runs past it with the help of a big 50cc pump shot. Just have to drive it the way it runs best even if it is not the way I would particularly like to.

The transitions from idle to power circuits and then the cruise circuit all want to occupy the same throttle position. And all want different fuel ratios. A compromise is the best you will probably be able to achieve with some draw backs.

I always run 9.5 or 10.5 PVs. Mine have always wanted a lot of fuel quickly after being a bit lean in the transition to the power circuit.

I guess my theory is to have it a bit lean on the slot to get a decent cruise and then overcompensate a bit with a big acc shot and early PV opening. Not a perfect solution, but it works pretty good. At least for me.