#1  
Old 11-18-2022, 11:38 AM
Rusty Bird Rusty Bird is offline
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Default Carb & Shaker

I guess I need to pick a new carb for my recently rebuilt motor. It's a 400, mostly stock/ street engine. Intake manifold was replaced with an edlebrock performer. I can get the specs for my lunati cam, if that helps.

The motor has a holley 750 on it ( i think). When the machine shop ran it on the dyno, he made a note that it was running way too rich, at all rpms - and that he felt my carb was too big. As per the calcs, even 600 cfm would be enough to run me up to 5,000 rpm, so 750 is overkill.

So a few questions. Is 750 cfm really too much for this motor? What is the symptom of an oversized carb? I have a 650 quadrajet sitting in my garage, but it's set up for a sbc marine motor. Can i make that work if the Holley is a no go?

And also wondering if my hood and shaker will work? This 1980 Trans Am originally had a 301 in it, and previous owner swapped in a 1975 400. Not sure if the shaker even fit with the oem intake on the 400, as the shaker was sitting in the trunk when I bought the car. If it's not going to fit - what do I need, a different hood, a different shaker?

thanks!

  #2  
Old 11-18-2022, 12:30 PM
Mr. G Mr. G is offline
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Hood will be fine as they were all the same. Probably will need a Pontiac 400 air cleaner and shaker from a 1978-1981 trans am that had a 400 motor originally. Also you will need a quadrajet carb to mount the air cleaner to.
Most Pontiac folks would rather have a quadrajet over a holley for street use.

  #3  
Old 11-19-2022, 10:16 AM
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Skip Fix Skip Fix is offline
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You can modify the stock base for a Holley . The places that hit are the float adjusting bolts. font one a hammer can dent enough rear one easier to cut a notch out and weld in a piece form a cheap drop base air cleaner that is higher. performer a little different height than a factory intake. 1/4' higher in front and 1/2 ' lower in rear.

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Skip Fix
1978 Trans Am original owner 10.99 @ 124 pump gas 455 E heads, NO Bird ever!
1981 Black SE Trans Am stockish 6X 400ci, turbo 301 on a stand
1965 GTO 4 barrel 3 speed project
2004 GTO Pulse Red stock motor computer tune 13.43@103.4
1964 Impala SS 409/470ci 600 HP stroker project
1979 Camaro IAII Edelbrock head 500" 695 HP 10.33@132 3595lbs
  #4  
Old 11-19-2022, 10:53 AM
78w72 78w72 is offline
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the carb isnt too big, these engines came with 750-800cfm q-jets, its likely the jetting or its one of the crappy "street avenger" holleys. should be able to tune the carb you have to run ok or change to a q-jet but it will still need some slight tuning to make it run best with your combo. there are places that can dial it in if you or someone local cant. also, the carbs cfm is dictated by the throttle, if you just cruise around town the engine isnt seeing 750cfm. your engine builder might be good at building engines but apparently doesnt understand carbs to well, should be able to change the jetting or other aspects to fix the rich issue.

the performer intake will fit a correct shaker even with some holley carbs, but a stock iron intake will make more power. i ran a performer with a holley 750 double pumper, mech choke, mechanical secondary on a 78 t/a with a 455, shaker fit & air cleaner fit fine with no cutting or modifying of the air cleaner base, maybe that was due to the manual choke & mech secondary? the shaker did fit very close to the hood so i didnt use the weatherstrip but it worked ok.

also, the 400 engine ended in 79, so you wont find a air cleaner/shaker for 80-81 cars as mentioned above, has to be a factory air cleaner from a earlier 75/76 455 fbody or 77-79 400. & the shaker also needs to be for a 400/455 pontiac, the 301 shaker is different, or at least its metal base plate is different, have heard of guys swapping out the base plate if they cant find a 400/455 shaker.

  #5  
Old 11-19-2022, 12:18 PM
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Skip Fix Skip Fix is offline
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air cleaner fit fine with no cutting or modifying of the air cleaner base,

No way the stock base will sit flat on the carb without massaging where the float bowl adjusting screws are! I have 4 of them. Not the W72 or the three "plain" bases. the Demon I used has slightly lower float bowl screws so not as much massaging compared to a Holley center hung bowls. here is as picture teetering on the bowl screws.
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Skip Fix
1978 Trans Am original owner 10.99 @ 124 pump gas 455 E heads, NO Bird ever!
1981 Black SE Trans Am stockish 6X 400ci, turbo 301 on a stand
1965 GTO 4 barrel 3 speed project
2004 GTO Pulse Red stock motor computer tune 13.43@103.4
1964 Impala SS 409/470ci 600 HP stroker project
1979 Camaro IAII Edelbrock head 500" 695 HP 10.33@132 3595lbs
  #6  
Old 11-19-2022, 02:39 PM
78w72 78w72 is offline
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ok im not doubting what youre saying. but i owned that car for 10 years as a daily driver, the W72 air cleaner base fit over the holley carb i had without any issues besides sitting a little higher than stock. my carb was a standard 750 double pumper mech choke/secondaries that didnt look like the one in that pic. maybe a prev owner massaged the base but it was very minimal if so & not visibly noticeable compared to all the other 2nd gen 400/455 bases ive had. definitely no cutting or welding.

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Old 11-19-2022, 04:26 PM
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Skip Fix Skip Fix is offline
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This one is an HP Holley vs the standard 750, only difference is no choke tower vs the standard 750. There are so many bumps on it some massaging is hard to notice esp the front one. The rear one is the big hump. the Demon I just put a BIG dimple in it, when i switch to the Holley 850 is when i sectioned in the hump in an extra base . Also the corners of the bowls hit but only takes a little massage there that is what is making that picture stick up so much the bowls.I thought I had a picture somewhere of my welded base and it is real hard to tell it is not factory once painted. Probably massaged before you got it. Two gaskets give a little more clearance also.

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Skip Fix
1978 Trans Am original owner 10.99 @ 124 pump gas 455 E heads, NO Bird ever!
1981 Black SE Trans Am stockish 6X 400ci, turbo 301 on a stand
1965 GTO 4 barrel 3 speed project
2004 GTO Pulse Red stock motor computer tune 13.43@103.4
1964 Impala SS 409/470ci 600 HP stroker project
1979 Camaro IAII Edelbrock head 500" 695 HP 10.33@132 3595lbs
  #8  
Old 11-20-2022, 06:44 PM
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Skip Fix Skip Fix is offline
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Pictures of massaged/welded base. Can barely tell front was massaged rear you can tel a little we welded in a piece from another base.
Also spotwelded holes for thermo vacuum switches.
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Skip Fix
1978 Trans Am original owner 10.99 @ 124 pump gas 455 E heads, NO Bird ever!
1981 Black SE Trans Am stockish 6X 400ci, turbo 301 on a stand
1965 GTO 4 barrel 3 speed project
2004 GTO Pulse Red stock motor computer tune 13.43@103.4
1964 Impala SS 409/470ci 600 HP stroker project
1979 Camaro IAII Edelbrock head 500" 695 HP 10.33@132 3595lbs
  #9  
Old 11-27-2022, 07:44 PM
Rusty Bird Rusty Bird is offline
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finally made it over to the carb shop not too far from me, place has a great reputation. He basically told me that my engine builder is nuts - first of all, the carb is a 770, which is not anywhere near too large for this motor. And if it was too large, last thing that it would do is cause motor to run 'too rich' as the builder stated.

so now I'm pretty worried about the rebuild itself, but at least I don't need a new carb! having this one rebuilt, and metered, jetted, or whatever he told, me - based on my engine specs. hopefully will have it back in a couple of weeks.

  #10  
Old 11-27-2022, 07:53 PM
tom s tom s is offline
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Put a Qjet on it and be done with it!JMHO,Tom

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  #11  
Old 11-28-2022, 04:58 PM
76TA462 76TA462 is offline
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No way is a 750 too big.
I started with a 1976 OEM set-up. Switched to 1971 cast iron intake and 1971 Q-Jet and my shaker base was fine. Then switched to a performer intake and same Q-Jet and no mods, then switched to a 770 Avenger on the performer and no mods needed on the shaker base. Disclaimer (LOL) I went to the Avenger because my Q-Jet (both) needed a serious rework and long story. And yes, it was only good for a short while then created massive grief. Just double checked (I still have all these combos minus the Avenger) and all true. Not part of this discussion, but will finish the story. Then went to a RPM intake and drop base and had to slightly mod the drop base. Then Sniper, so more massaging. Just a bit more for the Blocker drop base, but a lot more when I switched for the lower sitting Gen drop base out of necessity.

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Old 11-28-2022, 06:30 PM
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Maybe a stock base unmassaged could clear side hung bowls no way center hung bowls.

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Skip Fix
1978 Trans Am original owner 10.99 @ 124 pump gas 455 E heads, NO Bird ever!
1981 Black SE Trans Am stockish 6X 400ci, turbo 301 on a stand
1965 GTO 4 barrel 3 speed project
2004 GTO Pulse Red stock motor computer tune 13.43@103.4
1964 Impala SS 409/470ci 600 HP stroker project
1979 Camaro IAII Edelbrock head 500" 695 HP 10.33@132 3595lbs
  #13  
Old 12-13-2022, 11:26 PM
74Grandville 74Grandville is offline
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i'm no expert, but i think Holley sizes their carbs for best drivability, not WOT. they want you to run a smaller carb so the air flow gives a good signal on the primary side for street driving. i'm sure that is why the q-jet has such small primary bores and the double rings on the booster. A larger carb will give better performance at higher RPM.

I just watched a video on powernation of a 4.3 v6 on the dyno. it made best power when they put a 950cfm holley on it..

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