74 L48 Struggles Down Low
Thread Starter
3rd Gear
Joined: Sep 2021
Posts: 3
Likes: 1
74 L48 Struggles Down Low
Hey everyone! I bought a 74 with an L48 about 2 months ago and have been lurking the forum for help with miscellaneous things ever since. I finally have a question directly related to my car, so I finally pulled the trigger and made an account today!
My L48 seems to really struggle down low, and when I get up around maybe 1800-2000 rpm it absolutely rips, relatively speaking. Since purchasing I have put long tube headers, an Edelbrock 2101 Performer intake manifold, and converted to an HEI distributor.
When dialing in my carb with a vacuum gauge, I seem to have max vacuum with both mixture screws backed all the way out. I'm thinking I may need different jets/metering rod since I opened up airflow to the engine, but I'm curious to everyone else's input as this is the first older car I have owned and I'm new to all of this.
From my research, I believe the jets that came on the 74 are .075 although that could be wrong. Assuming the quadrajet is stock, do you think upgrading to 77/78s would help my situation? And what metering rod would be best if I was to up the jets a size?
Thank you for any input! The L48 is a little bit of a dog as is, I can't afford to be losing out on any power or optimization lol
My L48 seems to really struggle down low, and when I get up around maybe 1800-2000 rpm it absolutely rips, relatively speaking. Since purchasing I have put long tube headers, an Edelbrock 2101 Performer intake manifold, and converted to an HEI distributor.
When dialing in my carb with a vacuum gauge, I seem to have max vacuum with both mixture screws backed all the way out. I'm thinking I may need different jets/metering rod since I opened up airflow to the engine, but I'm curious to everyone else's input as this is the first older car I have owned and I'm new to all of this.
From my research, I believe the jets that came on the 74 are .075 although that could be wrong. Assuming the quadrajet is stock, do you think upgrading to 77/78s would help my situation? And what metering rod would be best if I was to up the jets a size?
Thank you for any input! The L48 is a little bit of a dog as is, I can't afford to be losing out on any power or optimization lol
Race Director
Joined: May 2006
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Before digging into the carburetor let's go back to that HEI.
Do you know what kind of advance curve it came with?
Did you set the timing for a total mechanical advance of 36 degrees or did you just set initial timing?
If you just set initial timing, what did you set it to?
Do you know what kind of advance curve it came with?
Did you set the timing for a total mechanical advance of 36 degrees or did you just set initial timing?
If you just set initial timing, what did you set it to?
Thread Starter
3rd Gear
Joined: Sep 2021
Posts: 3
Likes: 1
Before digging into the carburetor let's go back to that HEI.
Do you know what kind of advance curve it came with?
Did you set the timing for a total mechanical advance of 36 degrees or did you just set initial timing?
If you just set initial timing, what did you set it to?
Do you know what kind of advance curve it came with?
Did you set the timing for a total mechanical advance of 36 degrees or did you just set initial timing?
If you just set initial timing, what did you set it to?
Melting Slicks
Joined: Sep 2016
Posts: 2,215
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From: Wauconda IL
Le Mans Master
Joined: Apr 2011
Posts: 6,284
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From: Hermosa
What are the numbers for timing now. initial, all in (mechanical plus initial) and cruise. You said you have your vacuum limited to 8*. so if initial is at say 14* then you should have 22* at idle?
Using the weakest springs means it is likely mechanically advancing at idle. This is not desirable. At what RPM is your mechanical advance fully advanced? Advancing too soon slows you down. Spark knock at idle? don't see how that can be.
Do you have Lars' timing papers?
Is the accelerator pump working? Can you see two squirts in the primary side if you depress the throttle with engine off?
Using the weakest springs means it is likely mechanically advancing at idle. This is not desirable. At what RPM is your mechanical advance fully advanced? Advancing too soon slows you down. Spark knock at idle? don't see how that can be.
Do you have Lars' timing papers?
Is the accelerator pump working? Can you see two squirts in the primary side if you depress the throttle with engine off?
Last edited by REELAV8R; Sep 14, 2021 at 03:52 PM.
Pro
Joined: Jun 2021
Posts: 505
Likes: 230
It's a L48, so depending on gearing I wouldn't expect it to feel fast off the line. A modern V6 minivan will be faster in the 1/4, it's 50 year old tech after all 
But tuning and setting the timing curve will help some.
But tuning and setting the timing curve will help some. 
Race Director
Joined: Mar 2017
Posts: 11,934
Likes: 4,497
From: Virginia
Is it an automatic?
Following Lars' timing papers woke up my 79 L48 (auto). It can't do burnouts, but it will take off as the revs get higher and the secondaries open (about 3000 rpm). A better exhaust might help, but the real problem is the low stall torque converter.
I'd go back to timing again. Low hanging fruit might be to make sure that your vacuum advance (limited to 10-12 degrees total) is connected to manifold vacuum. You shouldn't be backing out the idle mixture screws all the way, either. That may be timing related, or something else.
EDIT: @Jebbysan says it perfectly below.
Following Lars' timing papers woke up my 79 L48 (auto). It can't do burnouts, but it will take off as the revs get higher and the secondaries open (about 3000 rpm). A better exhaust might help, but the real problem is the low stall torque converter.
I'd go back to timing again. Low hanging fruit might be to make sure that your vacuum advance (limited to 10-12 degrees total) is connected to manifold vacuum. You shouldn't be backing out the idle mixture screws all the way, either. That may be timing related, or something else.
EDIT: @Jebbysan says it perfectly below.
Last edited by Bikespace; Sep 15, 2021 at 09:14 AM.
Dr. Detroit
Joined: Mar 2012
Posts: 10,097
Likes: 4,027
From: New Braunfels Texas
Admittedly I only set inital, but I bought a hei recurve kit and also tried every spring configuration possible while messing with inital timing. It seems to run best with the lightest springs. I started with two medium ones, then one light and one medium, and settled with the two light ones. I have also limited the vacuum advance to every amount possible. Basically, I've tried every timing setup to every degree possible messing with inital timing, mechanical advance, and vacuum advance every step of the way. Still, no ***** down low. I don't even know what inital is set at right now, I have the vacuum advance limited all the way down to 8 degrees and the car runs best with inital timing set to the point just before spark knock. It only spark knocks at idle also, once I'm above idle the spark knock goes away.
If you need instructions as to how to do this, E-Mail our member Lars at V8fastcars@msn.com for a copy of his paper on how to do it. Vac advance on these is usually always best pulling 12 degrees.
Changing springs and playing with initial timing without knowing why you are doing it or the proper procedure will drive you nuts. Timing is the single most important aspect of tuning and must be set and recorded with numbers. Only from there can you continue to move on to the carb.......
If the engine is mechanically sound and it is the stock Q-Jet that functions well....it will respond the way you want it to.....
Jebby
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Le Mans Master
Joined: Apr 2011
Posts: 6,284
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From: Hermosa
If you don't have a digital dial back timing light, get one. It makes all this all at least 10 times easier. If your using timing tape and the old school non dial back timing light your gonna have a much harder time of it. Innova makes really nice timing lights.
Burning Brakes
Joined: Apr 2020
Posts: 788
Likes: 418
From: Western NY
Just want to add that if your Qjet is unmolested and original there is a almost 100% chance the jets and rods ARE NOT your problem. i would not mess with them. backing out the mixture screws all the way is not correct either. it may not hurt to install a quality rebuild kit in your carb but don't change anything - rods, jets etc. put a new float in if it needs it. the only thing i would check on the carb is if the secondary air valve is opening to quickly which would create a bog when you floor it. check if the spring tension is to weak on the air valve or if the choke pull off is faulty. follow the posts above in regards to your timing more than likely the majority if not all of your problem is with your ignition.
Thread Starter
3rd Gear
Joined: Sep 2021
Posts: 3
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Problem solved
I found the issue, boiling fuel in the carb when the engine was up to temp. It not only caused me to not be able to set the mixture screws properly, it caused it to stumble at WOT off the line until more fuel got pumping through. I threw on a 1" carb spacer and low rise air filter today, and problem fixed. Suddenly my mixture screws would adjust properly, and when mashing it off the line it just goes now, zero hesitation whatsoever. What I thought was spark knock at idle when turning timing up, was actually a very small backfire. Trust me guys, I've messed with the timing more then anyone on the planet, and what do you know, timing wasn't the issue. It was a hot carb issue.
Race Director
Joined: Mar 2017
Posts: 11,934
Likes: 4,497
From: Virginia
I found the issue, boiling fuel in the carb when the engine was up to temp. It not only caused me to not be able to set the mixture screws properly, it caused it to stumble at WOT off the line until more fuel got pumping through. I threw on a 1" carb spacer and low rise air filter today, and problem fixed. Suddenly my mixture screws would adjust properly, and when mashing it off the line it just goes now, zero hesitation whatsoever. What I thought was spark knock at idle when turning timing up, was actually a very small backfire. Trust me guys, I've messed with the timing more then anyone on the planet, and what do you know, timing wasn't the issue. It was a hot carb issue.
Can you post a photo, and tell us what your final timing is?
I ask because I had the same issues with my 80. Overall retarded timing caused the engine to get screaming hot, to the point of run-on (dieseling). I tweaked the timing a bit, and still had afterfire from lean misfire. I fixed that by following Lars' timing instructions to the letter. 12 degrees of vac advance on manifold (not ported) vacuum and an idle mixture adjustment got rid of the afterfire. Total mechanical advance is 36 degrees, all in at 3000 RPM.
Do you still have a stock exhaust? Is it possible that your heat riser is stuck closed? That will heat up your carb really quick. If you have an IR heat gun, you may want to take some readings in your engine compartment.
Le Mans Master
Joined: Jun 2019
Posts: 7,650
Likes: 2,466
From: Indiana
I found the issue, boiling fuel in the carb when the engine was up to temp. It not only caused me to not be able to set the mixture screws properly, it caused it to stumble at WOT off the line until more fuel got pumping through. I threw on a 1" carb spacer and low rise air filter today, and problem fixed. Suddenly my mixture screws would adjust properly, and when mashing it off the line it just goes now, zero hesitation whatsoever. What I thought was spark knock at idle when turning timing up, was actually a very small backfire. Trust me guys, I've messed with the timing more then anyone on the planet, and what do you know, timing wasn't the issue. It was a hot carb issue.
Last edited by interpon; Sep 15, 2021 at 08:45 PM.
Le Mans Master
Joined: Apr 2011
Posts: 6,284
Likes: 1,171
From: Hermosa
I found the issue, boiling fuel in the carb when the engine was up to temp. It not only caused me to not be able to set the mixture screws properly, it caused it to stumble at WOT off the line until more fuel got pumping through. I threw on a 1" carb spacer and low rise air filter today, and problem fixed. Suddenly my mixture screws would adjust properly, and when mashing it off the line it just goes now, zero hesitation whatsoever. What I thought was spark knock at idle when turning timing up, was actually a very small backfire. Trust me guys, I've messed with the timing more then anyone on the planet, and what do you know, timing wasn't the issue. It was a hot carb issue.