Author Topic: shaker scoop messing with carb tuning?  (Read 1360 times)

Offline jarretts78

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shaker scoop messing with carb tuning?
« on: September 04, 2017, 08:39:17 AM »
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.

Offline Cliff Ruggles

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Re: shaker scoop messing with carb tuning?
« Reply #1 on: September 06, 2017, 02:52:44 AM »
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

Offline BanditRich

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Re: shaker scoop messing with carb tuning?
« Reply #2 on: September 02, 2019, 07:21:04 AM »
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?


« Last Edit: September 02, 2019, 07:34:26 AM by BanditRich »

Offline Cliff Ruggles

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Re: shaker scoop messing with carb tuning?
« Reply #3 on: September 03, 2019, 04:17:32 AM »
I didn't recommend a shorter element, that will make it worse.  Moving the lid further away from the carb will improve things......

Offline BanditRich

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Re: shaker scoop messing with carb tuning?
« Reply #4 on: September 03, 2019, 04:54:33 AM »
^ 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?

Offline Cliff Ruggles

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Re: shaker scoop messing with carb tuning?
« Reply #5 on: September 04, 2019, 04:13:52 AM »
That is EXACTLY what was required on my engine when I ran the RPM intake and shorter Shaker assembly.  It would NOT work at WOT, stumbled with the tallest air filter element I could fit, so I made a custom air filter element for the opening in the Shaker assembly....good to go.

I have since went back to the stock intake, taller Shaker, stock air filter and lid, it works equally as well if not better than the RPM with the modified set-up, and my engine is up near 600hp....FWIW.....Cliff

Offline BanditRich

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Re: shaker scoop messing with carb tuning?
« Reply #6 on: September 04, 2019, 11:09:41 AM »
I'm not going to get the stock cast iron intake unless or until the Edelbrock Performer one needs replacing. The original intake was gone when I bought the car so I will be trying what you did - omitting the aircleaner lid with a custom air filter element in the shaker.
How did you make the custom element?

Offline Cliff Ruggles

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Re: shaker scoop messing with carb tuning?
« Reply #7 on: September 05, 2019, 01:52:12 AM »
The Performer intake can add to the problem.  Although some say it's about as good as a stock intake it is not.

I tried using one on my engine way back when it only made 455.4hp and it wouldn't feed it.

Just like the air cleaner lid deal it induced a stumble/hesitation/bog going quickly to full throttle and some "surging" in the mid-range.  I removed it and went back to the stock intake and the problem instantly went away.

Not sure why but I think the engine was seeing it as a restrictor plate, just a guess, but in any case it would NOT effectively feed my 1hp/cid 455 engine.

I forgot to mention it but also make sure you are using a secondary divider behind the choke housing on your Q-jet.  It starts to become a possibility as you increase engine power and vehicle performance (acceleration) than it can "toss" some fuel out of the vent behind the choke as produce a stumble when you hit it hard......Cliff

Offline BanditRich

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Re: shaker scoop messing with carb tuning?
« Reply #8 on: September 05, 2019, 03:47:19 AM »
^ A secondary divider? Could you explain what that is? Just like the original poster said when I drove it up the street & back without the shaker, lid & aircleaner ( these are the stock original parts) it ran like a bat out of hell from a dead stop. Put the shaker, lid & aircleaner on and back to sluggish takeoff.
That's why I would like to know how you made the custom air filter for your shaker. I figure omitting the aircleaner lid will solve the problem.
« Last Edit: September 05, 2019, 03:53:21 AM by BanditRich »

Offline Cliff Ruggles

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Re: shaker scoop messing with carb tuning?
« Reply #9 on: September 06, 2019, 06:23:21 AM »
I made a custom element for the Shaker opening from a big piece of thick foam (free breathing material) that I had in the shop.  I engineered a mesh "cage" to hold it in place and made sure it sealed off tightly.

I wasn't overly fond of that set-up so I only ran it a year or so and removed the RPM intake and went back to a factory intake and the taller Shaker and air cleaner parts......Cliff

Offline BanditRich

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Re: shaker scoop messing with carb tuning?
« Reply #10 on: September 07, 2019, 08:24:37 AM »
I'm going to try that. I had noticed the old paper filter which has yellowed from age said CarQuest on the plastic ring so I got a new one from Advanced Auto which is supposed to be the correct one for my car but I noticed it looked a lot taller than the old one. So I measured the old one and it was 2 3/4" tall; the new one was 3 1/16". I also called Ames Performance to send back the one I ordered from them that was supposed to be correct ( it was 2 1/2" x 11" and the aircleaner lid stuck  out over the top - my car REALLY didn't like that one when I took it for a test drive) and got one that was specified for 76 Trans Ams - 3" x 14".
When I tried to put the new CarQuest one on the shaker retaining ring would not seal the scoop to the aircleaner. So I tried the 3" tall one from Ames Performance and the ring worked with that one as expected because the K&N one I have is 3" tall too.
So I took the car for a spin and my car likes that one a lot. Takeoff from a dead stop is great and if I'm doing 20 - 30 and nail it the downshift and acceleration is downright violent!
It has the slightest of lag from dead stop with the secondary air valve spring at 1/2  turn but it's so minor I'm debating whether to bother adjusting it because of how hard it was to loosen & then tighten the locking allen screw ( it isn't rusted it just took a lot of upward pressure while simultaneously turning it to get it to loosen and then tighten - it would just turn both ways without loosening or tightening back up without the upward pressure) as I stated in the other thread.
Now to try your custom foam filter in the shaker and snorkel without the aircleaner lid to see if my car likes that even better. I will post up here how that works out.
I think the reason my car got better acceleration with the old yellowed paper filter than the K&N one I have is that the old paper one was 2 3/4" and there probably was a small gap between the top of the filter and the aircleaner lid; whereas it was tight against the K&N one so it was akin to the old hotrodder trick of flipping the aircleaner lid.
But my car accelerates way faster with the new 3" paper filter than the old 2 3/4" one; or the 3" tall K&N one  ;D  Looking forward to see how the foam in the shaker and snorkel works out without the aircleaner lid!
« Last Edit: September 07, 2019, 08:30:54 AM by BanditRich »