[dv657172]

65 GTO drag car
Holley 4150 DP 750 (4779-6)
1968 400 (461) with 16 heads

Ended last season with an idle AFR of 14.0 and an avg run AFR of 13.0

First time out this season my idle AFR was 21 and avg run AFR was 14.0
Increased jet size by 3 to get to 13.4 on a run, idle AFR unchanged of course.

Heard a small pop (lean backfire?) when I lifted at the finish line both times and my data logger shows my AFR shot up to 21-22 when I lifted after the finish line
Density Altitude was 300' higher to start this year vs. last years runs
Fuel levels look correct in both float bowls

The only thing changed vs. last year was adding jet extensions on the secondary side along with the notched float and going to a looser stall converter.

The car idles ok at 925 rpm, 6.5 lbs steady fuel pressure, 13 degrees advance, 13.5 inches steady of vacuum in park. Idle mixture screws are all 1/2 turn out.

Plugs look lean to me (see pic) although I am no expert but along with the backfire I feel pretty confident the AFR numbers I am getting are correct.

Any ideas what to check next or what the issue may be?

Turned idle mixture screws out a quarter turn and idle AFR only came down to about 18. Started going another quarter and vacuum started dropping and car ran rough.

I did a simple rebuild on the carb in Oct 2022, about 50 1/8th miles passes since then. Checked fuel filters pre & post carb before the season started and they looked fine.

[AG]

Make sure you don't have any vacuum leaks that would lean it out. I had one before from a slightly torn 4500 gasket that I couldn't see until I removed the carb. That will make it difficult to tune. I have gotten my best mph with a 12.7-12.8 AFR.

[Mike Davis]

Were you running the exact same fuel you ran before?

[dv657172]

Yes same fuel
I looked for an obvious vacuum leak given the big change in idle AFR, 14 to 21, but so far I havent found anything.

I am not a carb expert, does a Holley 4150 have a fuel passage that both the idle and main curcuit share which could have some kind of small blockage which would impact Idle AFR and AFR under load?

[johnta1]

The only thing changed vs. last year was adding jet extensions on the secondary side along with the notched float and going to a looser stall converter.

I'd look at the rear float setup?
Is the extensions keeping the float from full travel?
Still have power valves?

Try changing back to old carb combo and see if stills does it?

BTW, do you have a fuel cell?

[dv657172]

johntal,

No PV in the rear just the front (5.5) and 10 jets higher in the rear 70/80, changed to 73/83 as noted in OP.

Will switch back to old float and no extensions tomorrow and see if any change. I assumed if float level was good then travel would be good?

No fuel cell, original style (although basically new) tank with 1/2" pickup 1100 RobbMc pump and dead head regulator. All the same as last year.

[ID67goat]

Could it be the O2 sensor not reading correctly?

Seems to me that it would not even idle at that AFR…

[dv657172]

Well given the pop at the end of the run when the AFR appears to jump which hasnt happened in the past and the look of the plugs which were fine last year I tend to believe the sensor. It also registered a reasonable AFR change when we increased the jets by 3 moving the AFR during the run from 14 to 13.4

Of course its possible...

[johnta1]

Check your tank for a clogged vent? (try it with cap off maybe?)

[SRGN]

If you have an Innovate WB unit, do a free air calibration. If you use leaded race fuel, the sensor does need to be replaced occasionally. I have a second WB unit that I use to verify accuracy, I also keep spare sensors on hand just in case. Are you running at the same track, same altitude? If you got a day with a negative DA, it will affect the tune quite a bit, but not as far as the numbers you're seeing.

[steve25]

The question here is this.

When you lifted off the throttle did the backfire come out of your carb, or the exh ?

Out of the carb would be from a lean condition and the exh would be from excess fuel lighting off when the temp gets high enough.

[dv657172]

steve25 - I am not positive it came from the carb, it could have come from the exhaust. Never heard it in the past, engine temps were in line with usual - 180 to start run, 210 at finish. If from the exhaust, could it still be due to a lean run resulting in excess heat at my headers/header extensions or unrelated?

SRGN - It is an Innovate system and I did the free air calibration before going to the track. It was the same track and the DA was 300' higher which if I understand correctly should have given me a richer AFR reading all things being equal - same amount of fuel (same jet size to start) and thinner air. Car does run leaded fuel and will invest in another sensor for backup.

[dv657172]

Installed a brand new O2 sensor and calibrated it. No change.

Same results as the old sensor, around 21 at idle and in Park at 3,000 the AFR was 14.5 with 73/83 jets.

Last year in Park at 3,000 rpm the AFR was around 12 with smaller jets of 70/80 (a data point I saved from mid last year)

Will remove jet extensions and notched float (the only change to carburetor vs. the end of last year when AFR was reasonable) and replace with old float and see if that has any impact

[Half-Inch Stud]

Idle lean: Vac leak at manifold, carb, vac-hose(s).

[J.C.you]

Something simple ….trash in fuel bowl restricting jets

[dv657172]

Changed jets at the last track outing by +3 (see orig post).
While that did bring down the AFR on a run from 14 to 13.4 last year it was pretty consistent at 12.75 plus the weather should have resulted in a marginally richer reading.

Also the Idle AFR is around 21 to even over 22.4 (max reading on data system) and that would not involve the jets as I understand.

I tried a new O2 sensor with the same result and since its affecting both the Idle AFR and run AFR, the float levels are correct, and the fuel pressure is correct I am thinking its air related and not fuel related although I still havent figured out what the issue is.

Removing jet extensions had no impact on the readings either

[SRGN]

Originally Posted by J.C.you

Something simple ….trash in fuel bowl restricting jets

This is where I'd look now that the sensor is known to be good. Especially if it's ethanol pump gas.

[dv657172]

Well it would appear to just be a false lean reading at idle as there are no vacuum leaks, the fuel bowls, fuel pressure and fuel level are all fine. The car idles fine and based on vacuum is near optimal on air fuel mixture screws.

Will likely jet the car up another 2-3 potentially the next time at the track if its still lean under full load.

The looser stall converter appears to have increased my average rpm on a run by about 300 rpm but wouldnt think that has any impact right?

[SRGN]

Is it possible that there's an exhaust leak throwing the sensor reading off? Would explain what you thought was a lean pop on decel as well. Possibly a collector gasket from when the transmission was pulled for the converter swap?

[steve25]

If your fuel system was on the edge of suppling the motor as it was with enough volume, then your now added 300 rpm could have pushed it over that edge .

Also due to production jet tolerances Holley aways says that if you need to go up or down one jet size then you should do such by two sizes.