Drilling throttle plates?

[71_Duster]

I have some major driveability concerns with the Edelbrock AVS 800cfm on my 440.

First up the idle mixture screws seem to do nothing. Newer carb blew them out etc no change. I'm thinking to get a decent idle speed that the transfer slot may be exposed causeing it to pull fuel from there?

Also off the line stomping the pedal will usually cause an instant stall.

While cruiseing 3/4 to full throttle too quick will cause a major stumble/hesitiation and what sounds like a pop or backfire through the intake.

Float levels are checked and set, fuel pressure is regulated to 5.5psi, inside the carb is nice and clean, jets and rods are stock calibration numbers, pump cam arm is on the lowest hole -I tried mid and it didnt even want to rev decently in park-

Vacuum advance is on the ported side. All in timing is 34deg mech.

I'd read that some people find drilling a small hole in the throttle plate solves many issues by beign able to turn the idle speed screw out and covering the transfer slots. Is this my case?


Cam specs:

1973 Chrysler Newport 440 4 BBL cleaned & deburred.
.030” over, crank
.010 under mains & rods, ARP head & rod
bolts & windage tray pistons- .030
H143 CP hyperutetic & file fit rings R927835
performer RPM HEADS & intake, 800 CFM Edelbrock Thunder Carb,
Hughes-timing set, CAM, lifters, rocker shafts, roller rockers,
Hughes CAM-HEH 2832BL

Camshaft Technical Details
Intake Valve Lift 1.5
Exhaust Valve Lift 1.5 .524"
.540"

Intake Valve Lift 1.6
Exhaust Valve Lift 1.6 .558"
.576"

Intake Duration at .050"
Exhaust Duration at .050" 228°
232°

Lobe Separation Angle 110º

Intake Opening at .050"
Exhaust Opening at .050" 7° BTC
49° BBC

Intake Closing at .050"
Exhaust Closing at .050" 41° ABC
3° ATC

Min. Suggested Cylinder PSI 165

TTI 1 7/8” headers 2 1/2” pipes with X pipe.

 

[ArtH]

Have you contacted Edelbrock regarding this? I had a similar problem with a stock 455 Olds engine years ago. Normal driving was fine but when you stomped on it, the engine just went flat. I fiddled with timing, and adjusting the idle circuit (I don't think that has much to do with the problem) but never solved that issue. The car was wrecked so I never did figure it out. I read that some of their carbs are running a bit on the lean side when dealing with large cid engines. I've never changed jets or metering rods on an Edelbrock but I've seen some videos and it's pretty simple to do.

I think I'd give them a call before drilling.

1-800-416-8628

 

[fiorock]

I usually play with Holleys, so some of this may or may not apply...

I think you're right about exposing the transfer slot. In general, if your idle speed screw is turned in too far (i.e. opening the primary throttle plates too much), you'll be past the idle circuit ports, so the mixture screws will have no effect. The idle circuit is effectively turned off, since the low pressure area is moved up along the wall of the venturi, where the edge of the throttle plate is. The trick is to get the throttle plate back down around the idle discharge ports, while still admitting enough air to allow the engine to idle. This is why drilling holes in the plates is a potential fix. That being said, I have screwed up a carb drilling holes in it (it's a lot harder making the holes smaller again...).

Like I said, I'm not familiar w/ the Edelbrock AVS, but some of the Holley carbs have a provision to crack the secondary throttle blades open a bit (there's a stop screw in the base plate that's accessible when you remove the carb & flip it over). I've found this to be a whole lot easier & less angst producing solution. Your AVS may(?) have something similar - worth a look, anyway.

As for the cruise-to-full throttle stumble - sounds like the power enrichment circuit. At part throttle there usually isn't enough accelerator pump stroke left to add enough gas to go with the sudden volume of air when you go to full throttle, so the power circuit makes it up. Holleys have interchangeable power valves that come in different vacuum ratings, but I can't remember how it's handled on an AVS (power piston?). It may have some provision to change stroke or spring tension or something.

The other thing you could try is to change the accelerator pump stroke and/or squirter size to give you more volume or longer duration shot. It may be enough to get you through the dead spot until the vacuum comes back up to pull from the main metering circuit.

Hope this helps.

 

[eric383]

I would definately bump up the acc pump and also try bumping up the timing. If it pings back it down. Good luck.

 

[440roadrunner]

You can remove the carb and SEE where the slots are. Instead of drilling the throttle, try opening the secondary barrels a tiny bit. Somewhere under there is a stop screw, I believe

You didn't say what the advance curve is, or where initial is. Guys these days have found that you can run gobs of initial. Run as much as you can without kicking the starter on start. 15-20* is not uncommon. Of course you need a short advance curve to go with it.


AND>........................................I just wasted a whole bunch of time trying to find the Edelbrock manual for the AVS. What freekin Chineseo/ dam/ rannt rant raaaaaaaaaa alalalala nt Edelbrock Ca LAIMS is the manual is actually for a damn AFB!!!! Nice Edelbrock. Real nice you worthless _ _ _ _ _

 

[MarPar]

Just did a search in google for "edelbrock avs carb manual" and came up with this...

http://www.edelbrock.com/automotive_new/mc/carbs_acc/pdf/carb_owners_manual.pdf

 

[440roadrunner]

Just did a search in google for "edelbrock avs carb manual" and came up with this...

http://www.edelbrock.com/automotive_new/mc/carbs_acc/pdf/carb_owners_manual.pdf

My point exactly. That is for an AFB, not an AVS I realize fully the title page says otherwise, and that there are similarities.

 

[71_Duster]

I'm going to watch the springs for the rods, I've read about slideing the covers so you can watch them while running to see if they are being pulled in at idle.

It idles around 800-900rpm

I had rods in it for 2 stages richer but after a breif chat with the engine guy at the local shop and even myself noticeing how black and sooty the tail pipes where he recomended going 2% leaner hence back to the stock set up for rods.

Timeing for intial was 14 If I remember. My light took a dump so I borrowed a friends tonight I'll check again and try to push it up some more.

 

[MarPar]

My point exactly. That is for an AFB, not an AVS I realize fully the title page says otherwise, and that there are similarities.

huh. ill be damned. i have an AVS carb, and that's what it all looks like.

whats the difference between an AVS and an AFB?

 

[MarPar]

It idles around 800-900rpm

I had rods in it for 2 stages richer but after a breif chat with the engine guy at the local shop and even myself noticeing how black and sooty the tail pipes where he recomended going 2% leaner hence back to the stock set up for rods.

Timeing for intial was 14 If I remember. My light took a dump so I borrowed a friends tonight I'll check again and try to push it up some more.

ive noticed that on mine too. i may lean mine out a bit too. and bump up the timing as well; mine is at 12 initial right now, and it does smell like she's running quite rich. my idle is at 850

 

[Meep-Meep]

huh. ill be damned. i have an AVS carb, and that's what it all looks like.

whats the difference between an AVS and an AFB?

AVS (air valve secondary) has a large spring loaded air valve over the secondaries and the AFB (aluminum four barrel) has the velocity valves under the boosters. The primary side is functionally identical between the two.

 

[Meep-Meep]

I notice it says min suggested cranking compression is 165 PSI. Are you there or over? Have you tried a smaller carb just to see what happens.

 

[440roadrunner]

AVS (air valve secondary) has a large spring loaded air valve over the secondaries and the AFB (aluminum four barrel) has the velocity valves under the boosters. The primary side is functionally identical between the two.

Yup. You'd think a "company" the size of the new improved Edelf## could afford to do more than change the title page on a pdf. It's difficult enough to get "true" info on the www without this crap.

The AVS has no secondary boosters. The fuel discharges into the barrels through two tubes. I always figured that the air valve acts as a giant choke, and pulls fuel over the top so to speak like a choke would.

I did see that the installation instructions for the AVS shows how to adjust the spring on the AVS but you'd think that would be in the "manual."

 

[67 coronet]

have you tried bumping up the timming while lowering the idle on the carb to offset each other.i have seen that several times in performance cars i have tuned.people run the carb idle up too high,therby cutting off the idle circuit,without adjusting the timming at the same time.some motors wont let you just set timming by numbers,you have to do it by ear.

 

[lewtot184]

the edelbrock avs has a different secondary metering system than the mopar factory avs. edelbrock mimics the '67 small block chevy avs. the edelbrock and chevy carb are afb's with an adjustable air valve rather than weighted air valves. the mopar avs uses the the tubes for the secondary main metering, not a removable booster cluster. the primary side of both carbs are basically the same.

 

[Garys1969RR]

Adjust the sec air valve spring a little tighter. Or Would you want to trade it for a Eddy AFB WORKS GREAT, or a Holley 4150 series double pumper? PM me if interested

 

[RustyRatRod]

You say timing "all in". Do you mean it's locked out at 34*? If not, what's the initial timing?

 

[71_Duster]

Lots of replies.

I just bought the car a few weeks ago and this in the only carb thats been on it. Distributor was curved by a guy who ran a rail car at the track says the P.O who was pushing 70 and has been very truthful.

Base timing was 14, all the mechanical advance was in around 3000rpm and was showing 34.

I've previously messed around with Holleys but am no carb guru, I have a 750 Holley Vacuum secondary sitting on the bench -3310?- That I tried on my Duster last year. I put the adjustable QuickFuel secondary pot on it but didn't seem to be able to get much tning out of it they way they explained.

I need to get a throttle adapter but may try this on the Belvedere too.

Going to put some more advance in it tonight and see what that gets me. Engine shop guys are telling me to go 2 stages leaner from the stock calibration also. Haven't done that as I need a set of jets first.

I'll adjust the airdoor as well.

 

[snakeoil24]

Vacum should be on the manifold side. Ported vacuum is for smog motors 75 and later when your looking for late timing to still burn fuel in exhaust. Backfiring and popping are to different things.
Backfiring is timing issue or bad valve seat. Poping is starving for fuel but a very fine line. Get a vacuum gauge and read what it is doing.

 

[RustyRatRod]

I don't believe you have enough initial timing. 14* is pretty low for a performance engine. I would try bumping the timing up in 2* increments until it rattles under load then back off.