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Discussion Starter · #1 ·
I'm now on the second carburetor on my car.
Rebuilt 428 engine, TH400, long tube headers, 3.36 rear end.

The car runs like an old U-Haul truck and the exhaust pipes are loaded with black soot.

First carb was a 650cfm HP Holley dual pumper. No choke and probably 4 mpg gas mileage. Smoked like a mosquito fogger.
Second carb is now an Edelbrock 1411 750cfm. Mileage is around 10mpg but still a good bit of smoke, black soot in tail pipes, and hesitation when first hitting the accelerator.

The builder wants to try a standard 600cfm now.

My position is that if the fuel/air mixture, timing, etc are correct, then changing the max capacity of the carburetor should make little difference unless I am running at max rpm.

I can adjust the fuel idle mixture but swapping out springs and jets are not in my skill set. It's been decades since I threw out my timing light and tuning tools.

The builder says he put "milder" jets and springs in this carb but there has to be something else wrong.

I'm currently trying different accelerator pump settings and vacuum advance ports. No luck so far.

I've heard so many different things and I'm lost.
Fuel pressure too high? -the fuel is fed from an atmospheric pressure bowl, so how is that an issue?
Too many CFM for a cruiser? -isn't cfm set by throttle position? It's not always running at 750 cfm.
 

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600cfm will starve that engine, even if it is a mild tune.

I've been battling my carb on my 400 as well. Its a Holley DP 750. Previously I had an Edelbrock that I could not get tuned right (determined to have internal issues), then a stop-gap Holley 600 (just to move it around) and now this. Therse were all second hand carbs, so I get what I ask for.

But if you have a builder doing work for you and they want to detune a 600 cfm carb on a Poncho 428, it is time to find a new mechanic.

Timing is also critical on our Pontiac blocks. There are many on this forum that have educated me to this. It is not a SBC and vacuum advance and total timing are critical to get proper throttle response, idel quality, power at WOT and fuel economy. I'll let others chime in, as I am far from an expert.

But again, if your guy is trying to soften a 600 carb to get your 428 to run proper, he or she may not be the best person for Pontiac engines...
 

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600cfm will starve that engine, even if it is a mild tune.

I've been battling my carb on my 400 as well. Its a Holley DP 750. Previously I had an Edelbrock that I could not get tuned right (determined to have internal issues), then a stop-gap Holley 600 (just to move it around) and now this. Therse were all second hand carbs, so I get what I ask for.

But if you have a builder doing work for you and they want to detune a 600 cfm carb on a Poncho 428, it is time to find a new mechanic.

Timing is also critical on our Pontiac blocks. There are many on this forum that have educated me to this. It is not a SBC and vacuum advance and total timing are critical to get proper throttle response, idel quality, power at WOT and fuel economy. I'll let others chime in, as I am far from an expert.

But again, if your guy is trying to soften a 600 carb to get your 428 to run proper, he or she may not be the best person for Pontiac engines...
That 428 should be capable of handling a 800 cfm as I have run a 800 cfm Q- jet on a stock 350 Buick !
 

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Discussion Starter · #6 ·
Thanks for those ideas. I got rid of my timing light, tach/dwell meter, and vacuum gage decades ago. LOL

Still have lots of other tools though and a good place to work on it.

The builder isn't charging me extra to work the problems, and another mechanic at this time isn't in the budget.
Guess I'm going to have to bite the bullet and get more tools.

I check the timing as soon as I get a light.
 

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Somehow the idea that it's easy to have a carb that's "too big" has become gospel, but it's not 100% true. I watched an Engine Masters episode where they disproved it - on a dyno - no b.s. it might be possible if you go stupid big, like putting an 1100 Dominator on a stock VW flat 4, but anything even remotely reasonable will be fine, like was just said "if it's tuned right". Carbs work on the principle of the air flow velocity through them. That's what activates their various circuits and causes them to feed fuel. As long as your engine can generate enough flow velocity through a carb to "turn it on", it can be made to work for you.

I agree. Get your ignition and timing sorted out FIRST, then perhaps run a compression test (or much better, a leak down test) to assess the relative health in terms of how well your cylinders are sealing, once you've gotten all that lined out, THEN think about carb tuning.

I can attest that it makes a huge difference through first hand experience. On my previous build close to 5 years ago now, I went with an AED 850 on my 461. It literally took me 12 months to get that carb even close to being right (too rich at cruise) and to get there I had to modify nearly every single circuit on that carb. Then I discovered I had just one cylinder, 1, that had over 20% leakage. That and a rear main leak caused me to go through the engine again. Once it was back running, I ended up having to UNDO every single change I'd made to that carb to get it tuned right.

Bear
 

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Any recommendations for a tach/timing light?
Actron?
Innova?
Judging from the photos of them I think a lot of different brands all come from the same manufacturer. Unless you spend a ton of money on one, my thoughts are that quality control is such that you're rolling the dice as far as whether you get "a good one" that's accurate and will last, or not. You can read the product reviews on any site that has those and no matter which one you look at, some people love it and some people think it's junk.

Pick the one that appeals to you and roll the dice.

Bear
 

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I'm on the you are undercarbed bandwagon too.

I was running a 600 CFM Eddy carb on a stock block Chevy 305 and it ran great. And I mean bone stock, like maybe 200 hp on a good day bone stock. Only adjustments I made to it were going down one size on the jets and different metering rod springs. I have an 800 cfm on my 461 and it runs great. I was running a 600 on it for break in. even though I knew it was way too small, but used it anyway because I knew the carb ran good. I was pulling ~4 mpg with the small carb and somewhere between 8-10 with the bigger one that's on it now. Performance is night and day better with the larger carb.

You don't need a great timing light unless you want to get one where you can set it to run advanced for dialing in higher RPM timing. Any basic light will work.
 

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1964 GTO, Tri-power, 4 speed, manual brakes & steering, black ext. red int., dog dish caps, T.I. ign
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In regards to all the comments of timing. Don't forget, you might start by having your curve on Distributor examined and properly set up. Then, when you slap that timing light on her, she will tell the full story all away through the operable rpm selection. Good luck.
 

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Bear is one of the sharper tools in the drawer and he speaks the truth. As a retired mechanic myself, I'll say that you can educate yourself and address the issue or be at the mercy of 'mechanics' that may or may not know what they are doing. It sounds like your guy is more of a parts swapper. You need to get on line and watch some videos and crack the books and give it a go. My '67 GTO gets 21 mpg at 75-80 mpg and my '65 GTO with tripower, cam, 4 speed, and 3.36 gears gets 15 mpg.
These cars are very basic. The ignition timing needs to be correct and the carburetor needs to be set right. With your super rich condition with all carbs tried, I wonder if your fuel pump isn't putting out too much pressure? These cars need about 4psi fuel pressure to run right and many, many new 'offshore' pumps put out 12 psi or more, causing condtions like yours.
 

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Discussion Starter · #16 · (Edited)
...With your super rich condition with all carbs tried, I wonder if your fuel pump isn't putting out too much pressure? These cars need about 4psi fuel pressure to run right and many, many new 'offshore' pumps put out 12 psi or more, causing condtions like yours.
I don't understand how that works. Isn't the fuel bowl and float like the water tank on the back of a toilet? Fuel is blocked by the float, right? Water pressure doesn't make the tank overflow on the toilet.
It is a mechanical fuel pump, not a high pressure electric pump.
 

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I don't understand how that works. Isn't the fuel bowl and float like the water tank on the back of a toilet? Fuel is blocked by the float, right? Water pressure doesn't make the tank overflow on the toilet.
It is a mechanical fuel pump, not a high pressure electric pump.
@geeteeohguy may be onto something here. Too much fuel pressure to a carburetor will make it flood past the needles. Edelbrock carbs are extremely sensitive to this. I believe they specify no more than 8 psi. If you are running a high flow mechanical pump, or something of that sort, you need to run a pressure regulator to meter it down.
 

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Discussion Starter · #19 ·
@geeteeohguy may be onto something here. Too much fuel pressure to a carburetor will make it flood past the needles. Edelbrock carbs are extremely sensitive to this. I believe they specify no more than 8 psi. If you are running a high flow mechanical pump, or something of that sort, you need to run a pressure regulator to meter it down.
Found this from the manual:
Do not use more than 6.5 PSI fuel pressure. Excessive fuel pressure may cause flooding. If your fuel pressure is too high, install an adjustable pressure regulator, such as Edelbrock #8190
 
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