My '77 L48 is a common mild upgrade - headers, 2.1/2" true duals, rebuilt Q-jet and distributor based on Lars' papers, Edelbrock Performer. On the chassis dyno it went from 147 hp @ 3,700 rpm to 182 HP @ 4,200 rpm with these mods. But the guy running the dyno stuck his sensor up my tailpipe and said the AFR was close to 15:1 through the whole run, leaving HP on the table. Since I'm getting ready to install aluminum heads and a cam kit, I wanted to get the carb sorted out so it wasn't loo lean during the cam break-in period. So, I installed an Innovate AFR meter, took it for a drive and made a couple 300-ft baseline runs.

The carb is the original (17057203) with 77 jets and what I think are 52 rods. The rods measure 0.052" at the 'top' of the taper, but closer to 0.047" at the 'bottom' of the taper, just before it steps down to 0.026". I had the secondaries locked out, but those rods are CH.

Also, I installed the oxygen sensor at the end and on top of the 3" header collector, just before the first flanged connection.

So, at idle it ran around 12 or 13:1. I ran the idle screws in a 1/4 turn twice with not much effect. On the road at 40 mph and light cruising throttle the reading was way up around 18 or 19:1. Give it part throttle to speed up, and it would drop to 17 or so, but not to 15. During the 300-ft runs at WOT, it dropped to 11.8 or 12. Also curious is that it did not hunt or surge at cruising, as I would expect from such a lean condition. It actually runs great across the board. BTW, 300-ft times were ~5.5 seconds. Pitiful, I know, but it was just primaries.

Blipping the throttle in the garage it seems pretty responsive, but I tried Lars' test that disconnects the accelerator pump arm while running, and it stumbled noticeably, backing up what the meter says about the lean condition.

So, having read and re-read the papers, the 77's seem ok for WOT, but for the lean cruising I can go to a skinnier rod, or play with the power piston height. My question is, does raising the stop on the power piston get the cruising location of the rod up to a point where the jet is seeing the narrow (0.047") part of the rod? And is that enough to get the AFR down around 15:1?

Appreciate any feedback.

 

A split of 30 between the jets and rods usually gives pretty good results. I would find some .047 M4M rods and mess with the APT height to dial it in.

 

Never mind

 

Hey Scott,
Good to see you’re still around!

 

Thanks, Piersonpie, I found some used rods online and also found Lars' post on setting the power piston height, will try those this weekend.

 

Hi Scott your power piston adjustment must be almost bottomed out to have those kind of AFR readings. Yes, raising the stop on the power piston adjustment will limit the movement of the rods, and get it closer to the skinner end of the rod, this works for light throttle operation.
It sounds like your jetting is correct on the primary side for WOT operation, and on idle side as well (mine runs best at 12:1 = less stinky.) Just getting that mid range richer will help throughout the power band.
It will be interesting to see how the AFR readings change once the secondaries open up.

 

So, I went through the exercise of measuring the rods and a jet on the bench and then the installed rod height with the power piston bottomed out, and my power piston was way low, like six turns on the screw low. I adjusted per Lars' instructions and now it runs around 12:1 at idle, around 15.0 or 15.5 at steady light cruise and moderate acceleration, and at WOT with the secondaries locked out it starts around 12:1 but climbs to almost 16:1 at 5,000 rpm. With the secondary air rod wired closed the air horn plates still crack open, I'm assuming that's where the extra air is getting in at higher rpms.

Anyway, it's close enough for now, since I'll have to do this again after the next step - tear it apart and install aluminum heads and a cam!

 

OldCarBum - yup, still kicking. My car projects take longer since I retired. I know it's popular to say I'm busier in retirement than when I was working, but the truth is I earned the right to be lazy, and I'm making full use of it!

 

Glad to see you got it sorted out. Your final results (except for the 16:1) are pretty much how mine runs and It's really fun and responsive on the street. I'm also able to get 24 MPG at 65 MPH, and still get 21.5 @ 80 MPH.....I have a 5 speed though, so that really helps out with those mileage figures.

Once you open the secondaries, the numbers should come in line.

 

I hear you!
I sleep in until 07:00, then ease my way through the mornings.
Summer is right around the corner, so I’ll need to change my routine to get things done outside before the heat settles in.
My days of working outside in the sun when it’s in the upper 90’s are over.
Retirement is great! 😊