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Discussion Starter #1
What’s up everyone?

Recently after watching some YouTube videos I learned that too big of a carb can make a car actually run lean bc a carb operates off of the venturi effect. I do not have a qjet, the previous owner put on a 750 cfm Holley with dual side bowls and fuel inlets on.

Maybe it was an “ah ha “ moment but I now have reasons to believe my gto has been running lean this whole time I had it and that is why it’s kinda slow.

Here’s why:

1. It’s out of breath about 4800 rpm. The tach increases like a snail after that and to be efficient I shift at maybe 5k at most.

2. Weird smell. My car makes my garage smell and it’s very apparent when you walk in. It’s a very strong odor, it smells like gas but not like a rich type of smell.

3. I thought the carb was too big but when I pulled the plugs they were very very clean. Not white, they were brown but they were very clean which I thought was weird ( 3rd Pontiac I owned)

4. Exhaust pops on deceleration. My car is a four speed and has headers and 2.5 stainless magnaflow exhaust. I assumed this was just bc they were straight through Mufflers but now that I think of it maybe it’s bc it’s lean.

5. Backfires through carb ( rare) if I’m at the highway cruising at 2900 and I downshift and floor it the car will hesitate and backfire through the carb then take off.

6. Sluggish throttle response when reving the motor while the car is parked. If I rev the motor suddenly it will bog then rev. To correct this I have to slowly rev it to 1500 and then rev it. Basically the throttle response sucks.

7. Terrible gas mileage. I read that a lean carb can hurt gas mileage just like an overly rich one. Right now I got 5 to 6mpg with a basically stock 400.

What do you all think?



4,656 Posts
I am not a Holley guy, but know enough. The 750 CFM should be fine size wise. Is it a double pumper or vacuum secondary.

What intake?

You may be having several issues. The Holley has a number of means to adjust them and in my eyes, too many and you can upset one adjustment with another. I also am not a fan of a double pumper and/or a single plane intake. This is more race car stuff and you really need high stall converters and race gearing out back. Vacuum secondaries are better, and a dual plane intake is better for the street.

Do you know what your engine vacuum is? This can contribute to any carb if you have a big cam and low vacuum.

I understand what you have noted on the bigger carb getting a lazy signal, ie low velocity of air going past the venturi making the carb sluggish. But if that was the fact, it may be soft at lower RPM's, but as the RPM's climb, I would think that is when the carb would come into its own and really pull rather than fall flat.

So my first adjustment might be a look at my ignition versus the carb. Having this dialed in will make a world of difference with a Pontiac and can give the same symptoms as you are experiencing.

We need to know what you Initial timing setting is at the balancer. Then, what is your total timing - Initial plus the mechanical weights and at what RPM does the total advance stop - 2,800, 3,000, 3,200, 3,500 RPM etc.. These numbers are with the vacuum advance disconnected and port capped off. Then you can figure how much your vacuum advance adds by watching your Initial with the vacuum disconnected, let's say 10 degrees, then connecet your vacuum advance and it will jump up to let's say 21 degrees. This is important for crusing as this is where you increase your gas mileage and reduce engine water temps.

Start with the ignition and once you have those numbers, we can dial it in so the engine runs it's best. This may really wake up the engine and change how the car runs and then get into the carb.

With regards to the carb, often the stock 30CC accelerator pump needs to be upped to the 50CC pump to give more gas which can eliminate stumbling/flat spots when you accelerate.

A larger "squirter" would be my next thing because if you are getting off idle stumble, it seems to be either you need the 50CC pump or if still a stumble, step up to a little larger size on the squirter. You may as you read, be experiencing a lean condition when you open up the carb hard, but the additional gas dumped into the engine through the larger accel pump and/or squirter may eliminate that and balance out a low vacuum signal.

The use of headers, and depending on the cam, can over scavenge the engine and literally suck out some of the good fuel mixture cause a lean condition. Making the carb "fatter" may be needed IF this is the problem.

My recommendation is to work on the ignition first and get that dialed in. Get the 50cc accelerator pump if you do not have this as the engine most likely can use a better shot of gas to eliminate a stumble on hard accelerations.


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5,966 Posts
Good advice from PJ. It's unlikely that the carb is too big, even base QJets were capable of 750 cfm. It is possible that it's not calibrated/set up correctly. Dave Vizard has a pretty decent book out about how to dial one in. It helped me quite a bit when I was struggling with mine, but still I needed some additional advice and some of my own "noggin output" before I finally got it reasonably right. I would have been completely lost on it without a good reliable air/fuel meter, so you might consider investing in one yourself. That's really the only way to know for certain how it's doing. Reading plugs these days with the additives in unleaded fuel can be pretty difficult to get right.
Popping on sudden acceleration is more likely to be ignition timing than it is carb. Before you even think about messing with the carb please do make sure your ignition timing is right - and I don't mean just the initial setting. You want to shoot for "about" 35 degrees total all in by "about" 2500-3000 rpm, and that's with the vacuum advance can disconnected, hose plugged. Not having enough advance will also make it run like a dog "up high" like you described. If it's a 67 and it has the closed chamber 670 heads, it'll probably want even more than 35 to be happy.
That's just a ballpark starting point though. The specifics of cylinder head and other things can make the "happy point" vary in either direction.

Holley's are weird. They have a reputation for being tunable but that's just because of all the different parts that are available for them. In several ways they aren't nearly as adaptable as a Qjet, one of the big ones being the load-sensitive enrichment system. Holley's just have a power valve on the primary side that's either open or shut, depending on engine vacuum so it's an all or nothing proposition. They aren't like the continuously variable system on the Qjet that employs metering rods. They're also weird in that the same feed restriction feeds fuel to both the idle mixture screws and the off-idle transition slots and in some situations that can make getting the fuel mixtures right both at idle and at low speed cruise (before the main jets come online) dang near impossible.

Good luck
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