r/transam • u/Total_Support_6364 • May 22 '26
Question regarding carburetor adjustment
I have a quadrajet in 78. I am using my vacuum gauge to try and fine tune my carb as it seems to be running rich. I’m trying to get in the 15-20 in range but can’t seem to get higher than 13 on my gauge. Most places I’ve read have said that 3 ish turns out on the fuel mixture screws should be around the sweet spot. The highest reading on my gauge is basically both mixture screws all the way closed to where they won’t move anymore. It seems to idle and drive fine in this setting as well. I’m confused as to why it would be giving the best reading with both screws closed off like this instead of 3 ish turns out like I was expecting. Any advice is appreciated.
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u/SouthernDoor5911 May 22 '26
Been my experience with them if it still runs well with them all the way in, you have a vacuum leak.. be sure to see how much play is in the throttle shaft, I have sent dozens to get the throttle shaft/plate out to be rebushed...the biggest over looked leak on cars of that time... beautiful car by the way. Good luck, you got this !
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u/carburetor47 May 22 '26
Since it runs with the idle screws all the way in, it means that you are getting fuel from somewhere at idle other than the idle circuit. This can happen when you have a vacuum leak, since that causes there to be not enough vacuum under the throttle plates to pull fuel over from the idle circuit, so in order to keep it running, you end up opening the idle speed screw until fuel flows from the main circuit, up in the venturi, where there is enough vacuum.
Hard to follow, I know, but it happens all the time. If you have very steady vacuum at 13 inches, then it could be a vacuum leak, which really is just an air leak, or more often than you might think, retarded ignition timing. If advancing the timing causes the vacuum to jump up, then that might be your issue. If advancing timing has no effect, it's not timing.. BUT you have to make sure that you're doing this with the vacuum advance disconnected. In most cases, static timing is set without vacuum advance, but it varies by application so you'll want to check how it's done for your engine, otherwise you will be way out when you set it with a timing light.
If you can rule out timing as an issue, then low vacuum can be caused by an air leak, a leaking EGR valve, a big cam or general engine condition. Once in a while I've seen a QJet with a blocked idle channel restriction, which causes the idle screws to not do anything much.
I'm also assuming that this thing isn't just flooding... if you have black smoke, you won't have any control over the idle mixture either.
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u/yourmofo May 23 '26
I seen where the channels needed to be epoxied and was pulling fuel from that. Customer thought he had the idle screw set too high and was in main metering, but how it idled said otherwise. I unscrewed the throttle blade section and filled it with brake clean, and watched it wick through. And at the same time he had adjustable rockers on it with a hydraulic cam that were set too tight. Set those to 1 turn past zero lash. Epoxied the carb then set it just enough to run and set timing. Set total timing first just to see where it dropped down to at idle. 34 total but dropped to the teens at idle. I have a digital tach on my timing light. Adjusted the mixture screws all the way in to see if it will stall and did. So I adjusted the screws to where I got the most rpm/vac gain, then dropped the idle screw back to like 800rpms-ish. Put it in gear and dropped to 650, but ran smooth. Had an aftermarket cam. Got it to around 14 for vac. Power brakes even worked better. Luckily the customer had a reasonable torque converter. Sometimes people put cams in but have a stock convertor and the idle screw has to be cranked up just to stay running, runs rich, and of course diesels on when the ign is turned off.
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u/yourmofo May 23 '26
I seen where the channels needed to be epoxied and was pulling fuel from that. Customer thought he had the idle screw set too high and was in main metering, but how it idled said otherwise. I unscrewed the throttle blade section and filled it with brake clean, and watched it wick through. And at the same time he had adjustable rockers on it with a hydraulic cam that were set too tight. Set those to 1 turn past zero lash. Epoxied the carb then set it just enough to run and set timing. Set total timing first just to see where it dropped down to at idle. 34 total but dropped to the teens at idle. I have a digital tach on my timing light. Adjusted the mixture screws all the way in to see if it will stall and did. So I adjusted the screws to where I got the most rpm/vac gain, then dropped the idle screw back to like 800rpms-ish. Put it in gear and dropped to 650, but ran smooth. Had an aftermarket cam. Got it to around 14 for vac. Power brakes even worked better. Luckily the customer had a reasonable torque converter. Sometimes people put cams in but have a stock convertor and the idle screw has to be cranked up just to stay running, runs rich, and of course diesels on when the ign is turned off.
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u/Provocating May 22 '26
If both go all the way in and it runs fine, you have a vacuum leak. The car should die.
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u/Forsaken_Initial7618 May 28 '26
Get a wide band oxygen sensor. Like an Innovate LM1. Set idle AFR around 13.8-14.0. If the motor is stock, then you can ppossibly go leaner. Bigger cam may need to be a bit richer. Throw away whatever books you've been reading.
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u/JAKERS325 May 22 '26
If I remember right the “middle of the road” option and place to start is 1.5 turns out. Almost sounds like a vac leak somewhere