Tuning my mystery AVS troubles

[Secret Chimp]

I have a 625cfm 4966S Carter AVS on my 318 with a Performer intake and a Summit 6900 cam. It ran fine, but I was only getting 14 mpg on the highway and the ends of my tailpipes were getting sooty on the outside, so I figured I could lean it out some.

My carb had .095 secondaries but the primaries were unmarked. The original 3-step metering rods were 066/.063/.058.

I found a recommendation on Moparchat of an original AVS configuration of .089/.092 primary/secondary jets and .068/.057 rods for a relatively stock 340/360. I figured this would be a good starting point for my 318. I got some flat-top AFB metering rod covers so I could use Edelbrock parts.

Well, after removing whatever my original primaries were and installing some Edelbrock .089s and the .068/.057 rods with orange springs, I found it was waaaay too lean on cruise. It would idle but it was very unhappy accelerating unless I opened the throttle enough to get on the power step, then it was fine. It would burble along without accelerating just fine, but much more than a hint of gas and it would start lean stumbling until dropped vacuum enough to get into power.

I found an area-calculating Excel spreadsheet online, and if my unmarked primaries were also .089s, this only would have been a 7% leaner setup over the original rods, so I was confused as to why the car was behaving so badly.

I changed to rods that came with the 1404 calibration kit I bought with a .067 cruise step (only .001 off from my originals) and it still hated light throttle acceleration.

Just for fun, I tried installing my original longer three-step rods with the AFB covers. It ran just the same, maybe a little stumbly off the line. I'm not sure what this means as they might not have been getting pulled down all the way due to their extra length, the shorter Edelbrock jets and the flat covers.

Can I really have that much trouble from just a .002 drop on metering rods? Should I reinstall my original mystery jets and try them with the Edelbrock rods and covers or will they not work properly?

 

[lewtot184]

7 percent change in jet area is a bunch. i'd go back to the 3 step rods and .092 in the primaries. i remember back in the day my '68 road runner had the same size carb and it didn't like the factory .089's and drove better with .092's with stock rods on the primary side. i wouldn't get lost on pipe color. todays gas burns so goofy its just difficult to make judgements by color.

 

[Secret Chimp]

Somebody cracked a book for me and found the 4966S was set up with .089 primaries and .095 secondaries - so apparently .066 primary rods are exactly what it needs.

I've run the original rods with the shorter Edelbrock jets and flat covers once already, and it seemed fine - will it negatively effect anything economy-wise? Or should the flat covers make up for the shorter Edelbrock jet height? I'd like to be able to keep running this with Eddy jets and tops to make future tuning easier.

 

[ja20]

If you said that the carb # was a 4966S that's not a 625 cfm. That carb came off a 71 440. I would venture to say that it is a 750 cfm. Could it be to much carb
for a 318 ???

 

[lewtot184]

If you said that the carb # was a 4966S that's not a 625 cfm. That carb came off a 71 440. I would venture to say that it is a 750 cfm. Could it be to much carb
for a 318 ???

the lo-po 440's had the smaller carb like 383's.

 

[Secret Chimp]

Well I'm an idiot - I'd left the choke pull-off unplugged from when I was using that port for my vacuum gauge earlier yesterday. I only discovered it when my engine kept hopping back up on fast idle in the garage despite the temp gauge being well off the peg.

I plugged it back in and tried using the .067 rods (about 3.8% leaner than the stock .066) - the car behaved a lot better but I could still feel the slightest hint of a lean condition even when I took it on the highway and it was still bumbly around town. I know I have a separate low-speed cruise issue I need to solve by enlarging the idle feed restrictors a little but it was definitely still lean in the main circuit as I got above 40.

 

[Meep-Meep]

Be aware that the amount and location of the holes on the emulsion tubes have an effect on mixture at different speeds and loads. By adjusting these holes you can really dial in a carb - such as lean it out or richen it on the top end without changing the jets. I would say that's best done on a chassis dyno or with some fancy measuring equipment hooked up to the car. If you have a variety of primary boosters from different carbs try swapping them out and see what happens. It's also my belief that any old carb needs to be completely recalibrated for today's gas.

 

[MarPar]

Be aware that the amount and location of the holes on the emulsion tubes have an effect on mixture at different speeds and loads. By adjusting these holes you can really dial in a carb - such as lean it out or richen it on the top end without changing the jets. I would say that's best done on a chassis dyno or with some fancy measuring equipment hooked up to the car. If you have a variety of primary boosters from different carbs try swapping them out and see what happens. It's also my belief that any old carb needs to be completely recalibrated for today's gas.

Thanks for that, Alex. i've been lurking on a lot of the carb threads lately, cuz im in dire need of re-building my carb. But im not very well versed in it. been learnin' a helluva lot, but I totally didn't think of the sh*tty pump gas we all have to put in our MoPars! of course you have to calibrate for that!!! :book1: