Carb too small?

[travis]

If your A/F ratio goes fat at higher speeds, and jet changes don’t seem to make a difference over 5k rpms, is that a sign that the carb is too small? Jetting leaner on the secondaries makes a difference up to about 5k, then it gently noses over and the a/f goes 12.0’s or richer...and I can’t seem to tune it out.

Or could this be a sign of some other issue?

[F-BIRD'88]

The carb is seeing lower than ambient air pressure under hood at high vehicle speed....
The exhaust is not getting extracted at high engine rpm.

Air cleaner too small or lid too close to the carb
Secondary air door tension to stiff. (Edelbrock)
What is the WOT intake manifold vacuum at the rpm point where it shifts rich on you.?
If WOT manifold vacuum at high rpm raises to high the PCV valve can switch from open pindle/low vacuum mode to closed pindle valve/idle mode.. Wrong pcv valve choice.

[MadBill]

What F-BIRD says, but also: in my somewhat limited experience, going richer at high RPM is the default calibration for many Holley carbs. Enlarging the high speed air bleeds leans out the whole fuel curve, but more so the top end. E.g if you increased the H.S.A.B.s by 0.004", you'd probably also need 2-3 # larger main jets to maintain the correct lower RPM mixture. If your carb has pressed in bleeds, make very conservative changes; it's way easier to make them bigger than smaller...
BTW, I'm a proponent of tuning just the primaries first, then connecting the secondary link and re-optimizing with just the secondary jets.

[F-BIRD'88]

Do EVERYTHING ELSE first before think of drilling on that carb.
If this is a relative mild engine with restrictive induction exhaust and heads and its all done at 5k,,,, Its all done at 5K rpm.

[F-BIRD'88]

Consider now that that carb would not be too small if it had a matching buddy. 2x4 induction .
The intake manifold vacuum at WOT can be a indicator.
Is the carb wide open at WOT?

[Tuner]

High exhaust pressure causes error in WBO2 accuracy. Near stoichiometric A/F there is little effect, but the further from 1𝛌 and higher pressure the greater the error will be. Lean reads leaner and rich reads richer. If the OP's exhaust is restrictive it is likely the increasing back-pressure as RPM increases is skewing the WBO2 sensor reading.

[travis]

If I make the exhaust any less restrictive my ears will bleed 1 5/8” long tube headers, 2 1/2” mandrel bent duals with, currently, 24” perforated core glass packs and turn downs exiting just behind the cab (temporary setup).

This thing should pull to 6k easily. The primary side of the carb is as tuned as I’m going to get it. It has great drivability and sharp throttle response.

I’m running a 14x4” flat base air cleaner on a 1” spacer. I don’t think there is any restriction there.

I wondered if a high speed miss would cause it to read rich. Or if it was running out of fuel flow at higher speeds which is very possible.

[Alaskaracer]

What size is the carb, and what size is the engine? Most carbs will flow considerably more than what they are rated for.......

[prairiehotrodder]

glasspacks don't flow like you think they do. Just because they are loud doesn't mean they flow good. I used to have a 350 in a 65 chevy pickup that i learned on as a kid. It had all the best stuff that i could afford as a 19 year old. I learned that 3" flowmasters were worth 1/2 a second in the 1/4 mile over glasspacks on a 15 second truck.

[BlitzA64]

An easy try is to lower the float level a touch. My carb is too small per class rules and float level/fuel pressure are very touchy with my engine. Seems to change the top much more than bottom/mid range. The metering blocks are custom though which likely makes a difference as well

[Alaskaracer]

glasspacks don't flow like you think they do. Just because they are loud doesn't mean they flow good. I used to have a 350 in a 65 chevy pickup that i learned on as a kid. It had all the best stuff that i could afford as a 19 year old. I learned that 3" flowmasters were worth 1/2 a second in the 1/4 mile over glasspacks on a 15 second truck.

Glass packs are about the worst "performance" muffler you can get...super restrictive....

[travis]

These glass packs have the perforated core, not louvers. Internally they look just likes any other straight through perforated core muffler...magnaflows, race bullets, ultra flows, etc. I bought these to try because the Pypes M80’s (14” straight through perforated core) are wicked loud, and the other 2 sets of mufflers I have just have no “character”. They are quieter, which is ok, but it’s hard to describe...they make it sound like a completely stock engine for lack of a better description. Sound has always been an important part of this hobby for me, especially for a driver. I don’t mean loud either, but I do want to be able to hear it.

Anyway...just for confirmation I put the 14” straight through mufflers back on and retested. Same result.

I’m going to try some secondary air valve adjustments, and some timing adjustments, and see if that makes any difference.

Or...I could leave it alone and enjoy the great drivability and great 2000-5000 rpm power, and quit trying to act like my 4900 pound brick is a hot rod. You guys are a bad influence

[PRH]

An easy test to see if the carb is on the small side for the application is put gauge on it and read the manifold vacuum at wot/high rpm.

I don’t think that motor would even pull 1.5” at 5K with a 650cfm carb on it.

If it’s not pulling over 1.5”, I wouldn’t be thinking it was “too small”....... or that the “size”(capacity) of the carb was the underlying reason it was going rich.

[MadBill]

...
I’m going to try some secondary air valve adjustments, and some timing adjustments, and see if that makes any difference...

Forget everything I said above. I was operating under the ASSumption that this was a Holley carb. With an air valve to adjust, it clearly isn't...

[Tuner]

How did this thread get this far with the carb type still unknown? Now it sounds like it might be a TQuad or QJet with the air door too tight..??