Need advice on Tuning a Quad

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Velder

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Hello all,
Been a while since I was last on, but I have got my truck running now and need someone familiar with Quadrajets to give me some advice.
Some of y'all recommended that I try rebuilding it a second time, which I did, and now it actually runs pretty good. Lots of power, great throttle response, and pulls about 18in of vacuum at idle in drive. But I also installed an AFR gauge, and that would indicate that I am running lean pretty much across the rpm range. I am floating at about 13 at idle in drive, and between 14-16 when driving at about 50-55mph. When I step on the gas it will drop to 11-12 very briefly before jumping right back to 15.
So here is my real question; Is this normal for a quad? or am I running really lean?
As an aside, when I first installed the AFR, it would only read above 18, finally figured out it was the A.I.R. system pumping in fresh air into the exhaust. I have disabled that system just by removing the air pump belt. But the EGR is still fully intact.
I have set my timing to the recommended 4deg. initial with the vac. advance disconnected, so total timing comes out to about 25deg at idle.
As always any advice, suggestions, recommendations, or instructions are greatly appreciated.
 

Bextreme04

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Hello all,
Been a while since I was last on, but I have got my truck running now and need someone familiar with Quadrajets to give me some advice.
Some of y'all recommended that I try rebuilding it a second time, which I did, and now it actually runs pretty good. Lots of power, great throttle response, and pulls about 18in of vacuum at idle in drive. But I also installed an AFR gauge, and that would indicate that I am running lean pretty much across the rpm range. I am floating at about 13 at idle in drive, and between 14-16 when driving at about 50-55mph. When I step on the gas it will drop to 11-12 very briefly before jumping right back to 15.
So here is my real question; Is this normal for a quad? or am I running really lean?
As an aside, when I first installed the AFR, it would only read above 18, finally figured out it was the A.I.R. system pumping in fresh air into the exhaust. I have disabled that system just by removing the air pump belt. But the EGR is still fully intact.
I have set my timing to the recommended 4deg. initial with the vac. advance disconnected, so total timing comes out to about 25deg at idle.
As always any advice, suggestions, recommendations, or instructions are greatly appreciated.
You need to adjust the idle mixture screws on the front to get your idle AFR around 14.5-14.9:1. You are rich at idle, perfect at cruise, and lean at WOT. You can richen up WOT by bending the secondary hanger rods up ever so slightly. I recommend using the little paper measuring "L" that came with the quad rebuild kit to measure the hanger height before and after. You should start with them sitting about 23/64" and then see where you are. You want to be about 12.3-12.5 at WOT for power enrichment. You can loosen the spring slightly(like less than 1/8 turn) in order to keep it from getting that rich spike at secondary opening. It will spike lean if the air door opens too fast and rich if it opens too slow
 

fast 99

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Fuel pressure ok? Has fuel volume been checked? Low fuel level in bowl? Primary metering rods stuck down? Fuel filter?

Secondaries can be enrichened by placing a small, short wire under metering rod holder, however usually something else is wrong.

If the carb was rebuilt, did they replace fiber float with brass? If so, get a fiber float and raise the float level slightly.

Since the use of ethanol started, carbs have needed enriching. I don't think that alone will be the problem but if using E fuel it certainly isn't helping.
 

SirRobyn0

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You have gotten great advise so far, and I'm not saying that you shouldn't enriched it some in the areas indicated, but! If you didn't have an AFR gauge you'd have no idea this was going on and you'd be a happy camper with what you have because it's running good. Most of us guys setting up or rebuilding carbs these days are not installing AFR gauges or even sniffing the pipe with a tester. We are setting them up so they run good and leaving them at that. Again not saying that you shouldn't try to improve the AFR readings I'm just throwing out that bit of information.
 

fast 99

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You have gotten great advise so far, and I'm not saying that you shouldn't enriched it some in the areas indicated, but! If you didn't have an AFR gauge you'd have no idea this was going on and you'd be a happy camper with what you have because it's running good. Most of us guys setting up or rebuilding carbs these days are not installing AFR gauges or even sniffing the pipe with a tester. We are setting them up so they run good and leaving them at that. Again not saying that you shouldn't try to improve the AFR readings I'm just throwing out that bit of information.
Never had AFR meters years ago. Used to set carbs up for emissions using an infrared. Most shops don't have those any longer. Actually, many shops today won't mess with carbs at all or have the techs to work on them. I was lucky to find a Sun 1115 locally pretty cheap. Everything worked but the infrared was unstable. Took a chance on it. Only needed a tach lead and an optical sensor. Found someone in Florida that made parts, thanks internet. Have about $1,000 in it. Kind of insane considering these cost 9k in the 80's, obsolete junk today.
 

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Give the engine what it wants, not what you think it needs!!!

AFR gauges are nice, but people managed just fine for decades without them. I’ll come back to this later.

Do the idle mixture screws affect idle? Meaning you can hear a definite change in idle turning the screws in and out? If not, you need to do some work on the idle circuit.

Does this carb have the APT system? If so, you’re in luck. I have never tried to tune a Q-Jet without the APT system. It obviously can be done, but it’s more labor intensive.

On a typical nice weather day, drive the truck on a level road at highway speed. Pay close attention to how it feels, and how it accelerates. Pull over, remove the air filter, remove the APT setscrew, and using a tiny screwdriver turn the APT screw 1/2 a turn clockwise. The screw limits how far the metering rods come out of the jets. Reassemble the carb and air cleaner, take it out on another drive along the same level road, paying attention to how it drive.

The idea to all this is to find the lean limit. You will know you are there when you get a lean miss at cruising speed. The miss will kinda feel like “fish bite” the truck will just feel lazy. Once you have found the lean limit, turn the APT screw the other direction half a turn.

I think you have 4 turns either direction with the APT screw. If you need more adjustment than that, you need to change the jet accordingly. If the truck feels lazy on part throttle acceleration, you can change the power piston spring (the little spring that goes under the metering rod piston) engine vacuum holds the piston down, keeping the metering rod in the jet. As vacuum falls off, the piston rises, pulling the rods out of the jet.

Now, back to my AFR gauge story.

I went thru this process years ago with the Q- Jet on my Olds. If I remember correctly, I had to buy a power piston spring and a pair of jets to get it tuned correctly. Once done, I could easily get 16-17 mpg, not to many big block 11 second cars can make that claim!!

Fast forward a few years, I got a LM-1 AFR from a friend. Once it was installed, image my surprise to find part throttle cruise was 15.2-15.5, far leaner than anyone would expect. I guarantee I would have chickened out leaning out the engine that far if I had the gauge first. I have yet to melt a piston, burn plugs, pop head gaskets or any of the other nonsense people say when talking part throttle AFR. At part throttle, the engine isn’t under a heavy load, it’s not working very hard, it’s not going to get hot enough to hurt anything.

Now that I have the Holley sniper, once tuned the car runs EXACTLY the same quarter mile times, and gets the same fuel economy. So much for the myth EFI is more efficient!

BTW, you tune the secondary side of the carb with metering rods. The hangers are more for fine tuning once you have found the correct rods. It’s real easy bend one hanger farther than the other, affecting the balance. Changing metering rods is faster, cleaner, and far easier than changing keys on a Holley!!

The biggest hassle with tuning a Q-Jet is: #1 sorting thru all the “expert” bs, and #2 finding the parts. Ray klemm on Facebook is a well know Q-Jet expert, and Cliff Ruggles use to be THE go to guy. Unfortunately, Cliff has retired from building carbs, but still sells parts. He also wrote an excellent book, I’d suggest buying it. Cliff also has a Q-Jet forum that has a wealth of info.


How to Rebuild & Modify Rochester Quadrajet Carburetors (S-a Design) https://a.co/d/2spYwKQ
 

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Your going to get way too far into this if you try replacing the secondary rods. You are good on the cruise AFR.. don't mess with it.

You would need to do like matt says and drive it while leaning it out, but first you need to modify the air horn and get the right tool. I've never seen a squarebody quadrajet that didn't use the inverse flat style APT screw and have a well plug in the airhorn. You have to knock the plug out and tap it to fit a set screw. Part of adjusting the APT though is also adjusting the timing. You would need an adjustable vacuum advancer can and once you hit that lean limit you would start adding in vacuum advance and repeat until the lean stumble goes away and then repeat. It can take a while. 15.5:1 lean cruise is about the minimum I would expect to see people running. It's not uncommon to see people in the 16-16.5:1 range in LS applications while tuning lean cruise where you have full control of the timing and knock monitoring. Mine runs right in that 15-15.5 range and I know I can lean it even more if I wanted to.

Start with getting the idle mixture right. As you adjust the idle mixture leaner(start by lightly bottoming both screws in, then screw each one out 1-1/2 turns), the engine will run better and you will be able to screw out the idle set screw more, which will in turn get you onto the idle circuit better. Make sure the engine is fully warmed and the choke is off before adjusting anything. Once you are into the mid-14's AFR, you are good on idle. Now measure both sides of the secondary rod to the airhorn lip and make sure they are even(and right around that 23/64". That's a good starting point. To richen up full throttle, bend the end of the hanger up just a bit and remeasure. Richer would go to a 21/64" while leaner would go the other way.
 

Velder

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Thank You all for your help that is a ton of great advice.
@Bextreme04 I really appreciate the numbers to shoot for. I was thinking it needed to run richer, and I didn't know I could adjust the secondary rods.
I was going to ask about the APT, I removed it for cleaning during the first rebuild and only afterwards read to never tamper with it. So I did knock out the plug, and modified a screwdriver to fit so I could adjust it but had no idea where a good base line was. I will try that procedure to tune it. I will start with the Idle mixture screws, and go from there. I have them at 2 1/2 turns out right now I think.
I was mainly concerned about the safety of running it that lean, having read about the possibility of melting pistons.
@Matt69olds Yes when I run the screws in the idle changes, mostly as I get toward bottoming them out and the engine starts to run rough and eventually die. I do have the APT so I am going to try that.
@fast 99 I don't have a fuel pressure gauge, but I just replaced the pump, so I think it is okay, I am the one who rebuilt the carb and I put a new fiber float, I set it to exactly what the instructions said. metering rods move freely and so far I have only used ethanol free fuel.
@SirRobyn0 thanks for the input, I don't intend to obsess over the number, I mainly put in the AFR because I have no idea what I am doing or what I am listening for. Hopefully someday I will be able to tune by ear, but not now. But you are right, If it runs "good" I will be happy.

Is 4deg initial timing sufficient? it is what the vehicle sticker says, but online I have found guys recommending up to 15 deg.
Any thoughts?
 

SirRobyn0

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Thank You all for your help that is a ton of great advice.
@Bextreme04 I really appreciate the numbers to shoot for. I was thinking it needed to run richer, and I didn't know I could adjust the secondary rods.
I was going to ask about the APT, I removed it for cleaning during the first rebuild and only afterwards read to never tamper with it. So I did knock out the plug, and modified a screwdriver to fit so I could adjust it but had no idea where a good base line was. I will try that procedure to tune it. I will start with the Idle mixture screws, and go from there. I have them at 2 1/2 turns out right now I think.
I was mainly concerned about the safety of running it that lean, having read about the possibility of melting pistons.
@Matt69olds Yes when I run the screws in the idle changes, mostly as I get toward bottoming them out and the engine starts to run rough and eventually die. I do have the APT so I am going to try that.
@fast 99 I don't have a fuel pressure gauge, but I just replaced the pump, so I think it is okay, I am the one who rebuilt the carb and I put a new fiber float, I set it to exactly what the instructions said. metering rods move freely and so far I have only used ethanol free fuel.
@SirRobyn0 thanks for the input, I don't intend to obsess over the number, I mainly put in the AFR because I have no idea what I am doing or what I am listening for. Hopefully someday I will be able to tune by ear, but not now. But you are right, If it runs "good" I will be happy.

Is 4deg initial timing sufficient? it is what the vehicle sticker says, but online I have found guys recommending up to 15 deg.
Any thoughts?
Timing: You might get guys here that will tell you to advance it but don't. If the sticker calls for 4 run that. The distributor is calibrated for that base timing, also emissions plays a roll. 85' was a later emission year so little base timing was common. This was primarily done to improve emissions, but unless you want to start looking at total timing. I'm going to over simplify a bit, but base timing is where the timing is set without any advance. Total timing is a number that will produce maximum power without preignition. The advance mechanisms in the distributor of setup to do this. So in order to say you can increase the base timing by XYZ amount we'd need to know things like how much your vacuum advance gives for advance and how much the mechanical advance gives. Yes, it is true guys twist the distributor to increase timing, sometimes without even the use of a light, but if it pings long enough or bad enough you'll blow the motor up. My advice is don't do it. If you are determined you want to fiddle around with increasing base timing at least do it later down the road after you are well use to how the truck performs and sounds first. Better yet do it after you know the truck well and calculate what you are getting for advance so you can be sure to stay in the safe area.
 

75gmck25

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I also rebuilt and tuned my Quadrajet with a kit from Cliff Ruggles. Mine shows similar AFR readings as yours, and it seems to run fine. Not sure why it goes so lean at cruise.

I’ll have to disagree on 4 degrees being enough base timing. 4 degrees made my engine really lethargic. When I had iron heads I used 8 and then 12 degrees, and with aluminum heads I’m at about 15 degrees base. Aluminum heads will tolerate more timing than iron heads.

For timing
- I disconnect and plug the vacuum advance, and set the idle down to about 600, and then set base timing to 15 BTDC.
- my mechanical advance adds about 20 degrees and it comes in quickly, by about 2200 rpm
- my vacuum advance adds 17 degrees

I’ve been running this timing setup for about five years, with no issues.
 

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I also rebuilt and tuned my Quadrajet with a kit from Cliff Ruggles. Mine shows similar AFR readings as yours, and it seems to run fine. Not sure why it goes so lean at cruise.

I’ll have to disagree on 4 degrees being enough base timing. 4 degrees made my engine really lethargic. When I had iron heads I used 8 and then 12 degrees, and with aluminum heads I’m at about 15 degrees base. Aluminum heads will tolerate more timing than iron heads.

For timing
- I disconnect and plug the vacuum advance, and set the idle down to about 600, and then set base timing to 15 BTDC.
- my mechanical advance adds about 20 degrees and it comes in quickly, by about 2200 rpm
- my vacuum advance adds 17 degrees

I’ve been running this timing setup for about five years, with no issues.
Agree total should be checked. These trucks have been around the block. Who knows what has been done in the past. In my area we changed vacuum advances quite frequently to reduce total timing and NOX. Calibration is quite different between a 305 Impala and a 400 K20. Distributors appear the same. Having a replacement distributor is entirely possible, actually likely. I have found rebuilt distributors regularly have too much timing. 15 degrees and 52 total sounds like a lot of timing but each combination is different.
 

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What @fast 99 is saying goes right along with what I was saying. @75gmck25 I didn't say 4 degrees was enough per-say, it might be or might not be but I'm under the impression the OP is building his knowledge base with these truck and I don't want him to think well Bruce is running 15 degree base I'm going to as well, and maybe his distributor is calibrated totally different than yours, hence my note about checking total timing. But he's building knowledge and doesn't do that so he just bumps it up to 15 base and it pings. Either at times he can hear it or whatever and wastes the engine. I'm not saying don't do it, I'm saying look at both sides of the coin and only play with custom timing after you get to know the truck and what normal sounds like.
 

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Yeah, base timing is a crapshoot. You have no idea what has been done to the distributor.

I would suggest getting a piece of paper and do this:
1. With everything plugged in and the engine warm at idle, record the timing.
2. Unplug the vacuum advance and plug it on the carb side, record timing.
3. Rev it to about 3k-3500 RPM(or until the timing mark stops moving) and write down what your timing is.

#1-#2 will give you what your vacuum advance can is giving for timing.
#3-#2 will give you what the mechanical advance in the distributor is.

My personal preference is to then set the timing while doing #3 to 34-36 degrees BTDC.
That will usually give you ~12-15 degrees of base timing and somewhere in the 40-45 range for cruise timing. If the carb is set up right to run lean at cruise, this will give you the best mileage and power.
 

75gmck25

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Good point about every engine being different. My aluminum heads seem to tolerate my timing setup on regular gas with no issues, but I also know that my compression ratio is probably no more than 9.2-9.4.

I believe my GM 350 crate with 74 cc smog heads probably had only 8-8.2 compression when it was new. New aluminum heads with 64cc chambers and thin head gaskets only raised it up about 1-1.2 points. There is only so much you can do without swapping pistons. However, it’s been a solid, reliable engine.
 
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Velder

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Thank you all!
If I remember correctly my vacuum advance adds about 20deg. But I will re-measure it tomorrow as well as the mechanical advance and report back. I think it is the original stock distributer, but I don't know for sure.
What is a "safe" limit for total timing at ~3000rpms? I am afraid of hurting the engine, is that a concern?
@SirRobyn0 you are completely correct, since I gave up on using this truck as a trailer hauler, I mainly want to learn with it, and gain some sort of basic understanding of how to work on an older vehicle. So I really appreciate all the advice and explanations.
By the way, what exactly does a ping sound like? will I know if my engine starts pinging?
 

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