Quadrajet Problem Solving > Dialing in your rebuilt Quadrajet carburetor

shaker scoop messing with carb tuning?

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jarretts78:
I have a 78 Trans Am, just finishing up a long term restoration. Basket case I brought home in pieces. It was born with the W72 220hp 400. That engine was long gone although it did come with the original carb, distributor, and other bits. I've been driving it about 3 weeks and trying to get the Quadra jet tuned.

The short block is a 428 (69 Grand Prix) with a fairly stock-ish rebuild. I bought it as an assembled long block for a different project a long time ago. The entire top end is stock but rebuilt T/A parts. 6x-8 heads (swapped out the 62's that were on the 428), stock iron intake. I rebuilt the stock distributor. I run 2nd generation A-body ram air III exhaust manifolds, proper dual exhausts. I calculate it at around 9.1 comp ratio, maybe a little less. Runs great on 91 octane pump gas.

One issue I have: at some point in time I lost the papers for that engine, including the cam card. So the cam is a bit of a mystery. From memory it was around 220 degrees @ .050. I set up a dial indicator & measured .305" of lift at the lobe, & I run stock type valvetrain. I mocked everything up during assembly and had no issues with piston to valve clearance, & the contact pattern on the valve tip was within reason so I called the geometry good.

15 degrees of initial advance. 35 degrees total, all in by 2800 rpm. Original vacuum advance can, hooked to ported vacuum.

The carb has been rebuilt, including new throttle shaft bushings. I made many tweaks to the idle circuit as described by Cliff in his book. Everything in the choke & idle circuits works great. It starts quickly, and will hold idle at 750 all day long. In gear while stopped (like at a red light) it holds idle around 600 rpm, and pulls cleanly under light to moderate throttle. Cruises on the highway like a dream. The issue I've been fighting is a bog when going to WOT; I've been making adjustments to the secondary air door spring tension.

The WOT bog/stutter has been a battle to figure out. On a test drive it will run great, but when actually driving it around the WOT bog/stutter comes back. This weekend it finally dawned on me: it's the shaker scoop. I have the stock closed shaker. NOTE - I never run the car without the air cleaner on place, but when messing with the car & going on test drives I don't bother to install the shaker itself. Without the shaker in place the car is in effect running an open element air cleaner. Under that condition the thing is great. You can go to WOT from a dead stop and it's damn near perfect. But of course when driving it around I put the shaker back on. With the shaker on it has to breathe thru the stock underhood snorkel. The bog/stutter comes back. I tried it a bunch of times yesterday & it's always the same. No shaker =good Shaker = bad.

So I believe my issue is airflow. I'm not above cutting the shaker open, but before I do I'd like to see what the experts say. Note that other than the idle circuit work and a bit of adjustments to the secondary air valve the carb calibration is still stock.

Cliff Ruggles:
I've ran into issues going to full throttle with my Shaker set-up when testing shorter air filters and different lids which ended up being closer to the carburetor.

Even tried the K & N Xtreme lid and it induced a horrible stumble/hesitation/bog that could not be tuned out. 

I also tried a 1" taller Edelbrock RPM intake a few years back which required a 1" shorter air filter.  It wouldn't run at all when you went quickly to full throttle, the engine just puked all over itself.  I removed the air cleaner lid and all was fine. 

I ended up making a custom air filter element for the opening in the Shaker which worked fine.  The RPM intake is no longer in place and I'm back to the factory intake and stock air filter, lid, etc, and no issues.  My Shaker is not closed off however and all the incoming air to the engine comes thru the opening in it.......Cliff

BanditRich:
Cliff I think my acceleration problem from a dead stop is caused by this! I just did a trans flush fluid & filter change and I took it around the block before topping it off. I too never drive my TA without the shaker & aircleaner assembly on it but did this time because the filler tube isn't accessible with them on there. It took off like a bat out of hell! I thought to myself awesome the trans service fixed my slow acceleration from a dead stop issue. But as soon as I put the shaker & aircleaner back on it was back to sluggish acceleration from a dead stop.
I used to run a K&N air filter but after reading a discussion in one of the many TA forums I am a member of where someone had asked if it really did improve acceleration a guy posted he tested it at the track and got better quarter mile times with a stock paper element vs a K&N.
So I pulled the K&N and put in the AC Delco stock replacement and it was a definite noticeable improvement in the set of the pants if I was doing 20 mph or faster and hit it; but no difference in nailing it from a dead stop.
I then tried a shorter paper filter like you did but no difference from a dead stop and acceleration from 20 mph or faster if I hit it was like the K&N filter was in there. So I put back the 3" tall filter; back to great acceleration hitting it from 20 mph & up and the sluggish acceleration from a dead stop remains.
My TA has the Edelbrock Performer intake on it but that shouldn't be an issue should it? Isn't it the same height as the stock cast iron unit?
My shaker is open and it still has the snorkel setup on it as well.
Like the OP mine is the 78 W72 400 so same aircleaner; same issue.
So how did you make your custom element for the shaker; and should I omit the aircleaner lid just mount the shaker to the base?

Cliff Ruggles:
I didn't recommend a shorter element, that will make it worse.  Moving the lid further away from the carb will improve things......

BanditRich:
^ And there's the problem. I already have the biggest air filter that will fit under the aircleaner lid & shaker & be able to close the hood. The only solution i see is to omit the aircleaner lid and put a custom air filter in the shaker and over the snorkel hole in the base. How did you make your custom filter for the shaker?

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