Cliff's Quadrajet Parts and Rebuild Kits
Quadrajet Problem Solving => Dialing in your rebuilt Quadrajet carburetor => Topic started by: modrummer on May 23, 2018, 10:28:30 AM
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Hey everyone,
I have a quadrajet # 17059219 on my 79 Corvette.
Running #72 jets, .041k rods, and CH secondary rods.
Car has Brodix IK200 heads and a Lunati 268 Cam with longtube headers and side pipes.
I'm running an AEM Wideband 02 gauge for tuning.
Currently, at WOT I have about 11.5:1 A/f (too rich)
However at idle and cruise (once warmed up and choke off) I am getting 16:1 or even higher sometimes A/F (too lean).
What should I be doing to even this out? If I change to bigger primary jets to richen the idle and cruise, my WOT will be WAY to rich.
Thanks
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How far do you have the idle screws out? Do they have any
affect in or out?
When you use a larger than stock cam it lowers the vacuum & usually need some tweaking to the idle circuit.
16-1 at cruise is good if it runs without stumbles or surging.
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Right to start with 72 jets are really lean in one of those carburetors, and not surprised it's way off the mark on the primary side.
We can supply a much better jet/rod combo for the primary side. Get it dialed in there first, then go after the huge secondaries to tune full throttle.
I'd add here that running 11 something to 1 at WOT isn't really a big deal, better than too lean as EGT's shoot pretty high as does potential for detonation and engine damage.
There really isn't a lot of power gained by playing around with A/F at full throttle anyhow. I've tuned my carb at the track from "pig" rich to so lean the engine was surging just a tad in high gear and the difference in ET and MPH really isn't all that much, seldom much over a tenth and less than 2mph.......FWIW......Cliff
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Right to start with 72 jets are really lean in one of those carburetors, and not surprised it's way off the mark on the primary side.
We can supply a much better jet/rod combo for the primary side. Get it dialed in there first, then go after the huge secondaries to tune full throttle.
I'd add here that running 11 something to 1 at WOT isn't really a big deal, better than too lean as EGT's shoot pretty high as does potential for detonation and engine damage.
There really isn't a lot of power gained by playing around with A/F at full throttle anyhow. I've tuned my carb at the track from "pig" rich to so lean the engine was surging just a tad in high gear and the difference in ET and MPH really isn't all that much, seldom much over a tenth and less than 2mph.......FWIW......Cliff
Ok so I had some other jets sitting around (74,75. And 78) so I put the 74 jets in.
Measured my float height and its .300" . Checked my piston height and the screw is 4 turns out from bottom.
Went out and drove it to test. Once fully warmed up, idle A/f is about 16:1/17:1.
Cruise and normal driving in gear is right around 12.5:1_13:1.
WOT is the biggest problem. It bogs HARD under Wot And the A/F gauge pegs to 10:1 until it gets higher in the RPM range and then it flattens out around 11:1.
Thoughts?
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Do you have any different secondary rods? Idle could use some work too.
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Do you have any different secondary rods? Idle could use some work too.
I dont have any other secondary rods, but I didnt think a secondary rod change could make enough of a difference to fix a 10:1 rich condition.
For the idle being so lean, should I be drilling the mixture screw holes a little bigger ? Or possibly the idle tubes ?
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May have to do both. Could try the idle mixture holes first.
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May have to do both. Could try the idle mixture holes first.
Ok I'll start with that. What should I do on the WOT rich bog? It didnt start till I upped the jet size to a 74. Should I try jetting back down to 72 and then raise the power piston height and drill out idle mixture screws and hope that's enough to make the idle and cruise rich enough ?
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What do you have for rods & hanger? Windup on the air valve spring?
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The idle system is a BIG player here and you need to start there. Idle fuel feeds the main system right off idle and thru transition via the transfer slots, so it is part of the primary side calibration.
Never use used jets and metering rods for custom tuning, they will cause lessons in humility. Always use new parts, if you want accurate results and very, and I mean very fuel jets will pass a pin gauge test and not be worn considerably on the inside from the metering rods being in them for tens of thousands of miles.
Also, K series rods provide very little range of adjustment, we have full tapered rods for that part of the tuning process.
Start with idle tuning, then primary side jet size, then fine tune part throttle A/F with the APT system.
We have the right parts to get you a much better baseline to take a lot of the guesswork out of the equation. This will include jets, metering rods, power piston spring, idle tubes and correct sizes for the DCR's and bypass air.........Cliff
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What do you have for rods & hanger? Windup on the air valve spring?
Rods are CH rods. I'll measure the hanger height, but I believe it is stock height, so if anything I would assume it would need to be bent down a bit to lean out the secondary side.
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The idle system is a BIG player here and you need to start there. Idle fuel feeds the main system right off idle and thru transition via the transfer slots, so it is part of the primary side calibration.
Never use used jets and metering rods for custom tuning, they will cause lessons in humility. Always use new parts, if you want accurate results and very, and I mean very fuel jets will pass a pin gauge test and not be worn considerably on the inside from the metering rods being in them for tens of thousands of miles.
Also, K series rods provide very little range of adjustment, we have full tapered rods for that part of the tuning process.
Start with idle tuning, then primary side jet size, then fine tune part throttle A/F with the APT system.
We have the right parts to get you a much better baseline to take a lot of the guesswork out of the equation. This will include jets, metering rods, power piston spring, idle tubes and correct sizes for the DCR's and bypass air.........Cliff
Ok I'll work on getting the idle dialed in first.
Both the 41k rods and the 74 jets are pretty much brand new (bought them last year when I was doing some tuning) so that shouldn't be the issue.
So my cruise is currently a bit rich with the 74 jets, but I think that will be dialed in by lowering the primary piston height a little bit. Hopefully drilling out the idle mix screws (or going to bigger idle tubes) will richen up the idle. So the issue I'm left with is the absurdly rich WOT mix (again, it's like 10:1 or richer since the gauge pegs at 10:1). Is a rod change enough to fix that? It seems like the secondary rods couldn't possibly lean out the mix that much, but then again I am not the expert haha.
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Do you have one of our books?
The idle mixture is calibrated by matching idle tube size, DCR's, bypass air, and idle airbleed sizes for the engine combination.
I would NOT open up the holes under the mixture screw very much on that model as it uses fine pitch metric idle mixture screws and they may not seat if the holes are much larger than about .090".
As mentioned, the primary side calibration is lean for one of those carburetors, assuming it is a large MAB model, which it should be.
With he .118" MAB's most of those carburetors had much larger jets in them than what you are using and they were still extremely lean on the emission calibrated engines they showed up on.
Being too rich with small jets, and rich at WOT with the larger CH metering rods may indicate a fundamental issue someplace........Cliff
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Do you have one of our books?
The idle mixture is calibrated by matching idle tube size, DCR's, bypass air, and idle airbleed sizes for the engine combination.
I would NOT open up the holes under the mixture screw very much on that model as it uses fine pitch metric idle mixture screws and they may not seat if the holes are much larger than about .090".
As mentioned, the primary side calibration is lean for one of those carburetors, assuming it is a large MAB model, which it should be.
With he .118" MAB's most of those carburetors had much larger jets in them than what you are using and they were still extremely lean on the emission calibrated engines they showed up on.
Being too rich with small jets, and rich at WOT with the larger CH metering rods may indicate a fundamental issue someplace........Cliff
Cliff, interesting point.
I can measure my MABs and see what they check out as.
I'll order one of your books, been meaning to do that.
I'll try to find time to dig into the carb again. It seems like the main issue is on the secondary side, I'm wondering if when I go to WOT the secondaries aren't opening for some reason... the air door opens easily, and the lower butterfly valves open up when the peddle is all the way down so it's not a linkage issue, but maybe a vacuum issue? If the secondary air door doesn't open that would definitely make it go crazy rich at WOT... I may attach a little go pro camera under the hood and run it with the air cleaner off to see if I can see the door opening.
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How long does it take for the pulloff to release from fully pulled in to all the way out?
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How long does it take for the pulloff to release from fully pulled in to all the way out?
I'll test this tonight. I'm assuming I can just apply a vacuum to the hose to test this?
I'm wondering if I could fix the WOT issue with M series rods (maybe 50m). The bigger .036 tip might be what i need to lean out that mixture a bit more.
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It's already lean on the primary side so that will only make things worse.
The order for tuning is to get the idle system up to par first.
Then determine the best main jet size with the .026" tipped metering rods.
Then dial in the part throttle A/F with the correct primary metering rods using the APT system.
Then leave all that alone and tune the secondaries for best power at WOT throttle......Cliff
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It's already lean on the primary side so that will only make things worse.
The order for tuning is to get the idle system up to par first.
Then determine the best main jet size with the .026" tipped metering rods.
Then dial in the part throttle A/F with the correct primary metering rods using the APT system.
Then leave all that alone and tune the secondaries for best power at WOT throttle......Cliff
I was thinking more along the lines of that I have 10:1 A/F at WOT, so a fatter power tip of the M rods would lean out that part. My cruise is sitting at 12.5:1 so I think it would be fine with the fatter rods leaning it out as well.
But I see what you are saying about getting the idle dialed in first. I was more thinking ahead that with my WOT mixture SO rich I may end up needing to try some of the fat tip rods to lean it out.
Your book should be at my house today or tomorrow so I will read up this weekend.
Plan of action is to drill out idle mix screws a small amount (.090) and see if that gives me enough adjustable range. If not, I will look into changing the idle tubes.
Thanks for the help, I'll report back when I make some progress.
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It will need bigger idle tubes, DCR's and probably some bypass air for the idle system.
Keep in mind that the idle system feeds the main system at light throttle thru the transfer slots so very important to start there first.
I'd also check the MAB size in that carb, it should be quite lean on the primary side with small jets in it.
Dial in the primary side first, idle, then heavy throttle, then cruise with the APT.
Forget using the primary side for WOT, do all that with the secondaries after you get the primary side up to par for what you are doing........Cliff
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It will need bigger idle tubes, DCR's and probably some bypass air for the idle system.
Keep in mind that the idle system feeds the main system at light throttle thru the transfer slots so very important to start there first.
I'd also check the MAB size in that carb, it should be quite lean on the primary side with small jets in it.
Dial in the primary side first, idle, then heavy throttle, then cruise with the APT.
Forget using the primary side for WOT, do all that with the secondaries after you get the primary side up to par for what you are doing........Cliff
Ok so your book came in end of last week and I spent some time reading it over the weekend while I worked on the car. Nice work! Wish I would have bought it sooner! I kinda picked and chose what sections to read for now, but I'll go back and read the rest when I have more time.
I made some progress on the carb. Took it fully apart and measured what passages I could with a digital caliper and some drill bits.
I also drilled out the idle mixture screw holes to .093".
The Upper and Lower MAB passages were both about .120" according to my measurements (does that sound right?)
I lowered the APT screw another turn since I was running rich at cruise.
I also loosened the secondary vacuum doors spring a half turn and bent down my secondary hanger to .650" to lean the mixture out a little (running CH rods currently, as I understand it these rods should be pretty lean already?)
MADE A BIG DIFFERENCE! With idle mix screws 8 turns out, I have 13.5:1 at idle (is this about where I should be once warmed up, or should I make it a little leaner?)
My cruise at partial throttle is around 13-14:1, off throttle coasting or going downhill it is running rich at about 12:1. So I'm thinking I need to lower my APT screw a bit more to lean up the cruise.
WOT is better, but not perfect. Not much hesitation as it transitions to the secondaries, but it does peg 10:1 A/F for a split second right as I go WOT before settling around 11.5:1.
One thing I noticed, there is a small tear in the accelerator pump rubber seal. Any idea what kind of issues this causes? I plan to order a new one from you, but I wanted to do a few more tests this week first to see if I need anything else from you before I order so I don't have to pay shipping twice.
Thanks again for the help!
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13.5 for idle is good but if you can get a better idle by adjusting the mixture screws don't pay attention to AFR at idle. Lower the primary needles a bit more to lean the cruise.
Try loosening the air spring just a bit more & that rich transition should go away.
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I would get one of our SR rebuild kits, it will have everything required for a complete/correct rebuild including a new secondary cam/spring, complete accl pump assembly with new springs, float, and a high float N/S assembly.
That carb should have the large MAB's and should be showing much leaner at part throttle or at least more control with the APT system.
Assuming you are running vacuum advance light throttle A/F should be leaner at very light load.
8 turns out with the mixture screws and .093" holes under them indicates you need to open up the idle tubes and DCR's for what you are doing. Keeping in mind that idle fuel feeds the main system as well and is the first thing to nail down when tuning, then move on to the jets, metering rods, and APT adjustment........Cliff
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13.5 for idle is good but if you can get a better idle by adjusting the mixture screws don't pay attention to AFR at idle. Lower the primary needles a bit more to lean the cruise.
Try loosening the air spring just a bit more & that rich transition should go away.
Lowered the primary needles with the APT screw and loosened the air door spring tension another quarter turn. Some slight improvement for sure. I"m getting closer, it's pretty much fine tuning at this point.
One issue I'd like to figure out is the smell I get at idle. Part of it is that I'm running a carbureted vehicle with no catylytic converter, but I've run vehicles like that before and they didn't smell this bad. So I'm wondering if I have the PCV hoses hooked to the wrong ports, or some other vent port plugged that shouldn't be or something.
See attached pictures. The big hose circled in red goes to a T fitting with one end going to the PCV valve in the valve cover and the other end going to the charcoal cannister. The hose circled in green goes straight to the charcoal cannister. Is this correct?
And then in the rest of the pictures you can see the various ports that are plugged off. I am running full manifold vacuum for the vacuum advance.
Also, what is the port circled in blue on the back of the carb near the secondary air doors? Just another vacuum port?
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I would get one of our SR rebuild kits, it will have everything required for a complete/correct rebuild including a new secondary cam/spring, complete accl pump assembly with new springs, float, and a high float N/S assembly.
That carb should have the large MAB's and should be showing much leaner at part throttle or at least more control with the APT system.
Assuming you are running vacuum advance light throttle A/F should be leaner at very light load.
8 turns out with the mixture screws and .093" holes under them indicates you need to open up the idle tubes and DCR's for what you are doing. Keeping in mind that idle fuel feeds the main system as well and is the first thing to nail down when tuning, then move on to the jets, metering rods, and APT adjustment........Cliff
Cliff,
I just ordered one of your SR Rebuild kits. I am running full manifold vacuum for my vac advance since my engine runs better with it. Tried hooking it up to ported advance and it ran very rich at idle and the mix screws did nothing (could screw them all the way in without killing the engine).
I have 18 degrees initial timing, 18 mechanical (36 total) and 20 degrees of vac advance. So I get about 38 degrees of timing at idle with the vac advance hooked to manifold vacuum. Is this too much even though my engine seems to like the extra timing?
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Open up the idle tubes and DCR's and go back to ported vacuum for the advance. It's simply WAY too lean at idle if you need to run 18 initial plus 15-20 more degrees from the vacuum advance.
I see this a lot with aftermarket camshafts as they have more overlap and degrade vacuum at idle (signal) to the carburetor and the carb was originally calibrated for a "stock" engine making at least 20" vacuum at idle speed.
Everything will get better once you modify the idle system......Cliff
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Open up the idle tubes and DCR's and go back to ported vacuum for the advance. It's simply WAY too lean at idle if you need to run 18 initial plus 15-20 more degrees from the vacuum advance.
I see this a lot with aftermarket camshafts as they have more overlap and degrade vacuum at idle (signal) to the carburetor and the carb was originally calibrated for a "stock" engine making at least 20" vacuum at idle speed.
Everything will get better once you modify the idle system......Cliff
When you say "open up" I"m assuming you mean "replace" the idle tubes with bigger ones? What size would you reccommend? My MAB are ~.120", and my camshaft (Lunati 268) is currently pulling 12" vacuum at idle.
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No need to modify the MAB's, but remove and enlarge the idle tubes and DCR's to get enough idle fuel to the mixture screws for what you are doing.
That cam is mild enough that you shouldn't have any problem making plenty of vacuum at idle with 18 degrees initial timing, which is pretty high for what you are doing, IMHO.
We typically like to have the initial timing around 11-12 degrees for most of the engines we build and/or tune here. This will prevent difficult hot restarts when the engine is well heat soaked and is plenty of timing provided the CID, compression ratio and camshaft are well chosen......Cliff
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No need to modify the MAB's, but remove and enlarge the idle tubes and DCR's to get enough idle fuel to the mixture screws for what you are doing.
That cam is mild enough that you shouldn't have any problem making plenty of vacuum at idle with 18 degrees initial timing, which is pretty high for what you are doing, IMHO.
We typically like to have the initial timing around 11-12 degrees for most of the engines we build and/or tune here. This will prevent difficult hot restarts when the engine is well heat soaked and is plenty of timing provided the CID, compression ratio and camshaft are well chosen......Cliff
Ok sounds good. I don't have your book in front of me (it's at home), but do I need to replace the idle tubes and restrictions with bigger ones? Or can I just drill them out to the size I need?
And what sizes would you recommend for them?
EDIT: Also just FYI if it helps, motor is a 350, 9.5:1 compression, Brodix IK200 aluminum heads, ported vic jr. intake, Lunati 268 cam, 4-speed with 3.55 rear gear.
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You can drill them out if you have a numbered drill set. It will only take .002-.003 more to get what you need I think. One other thing your engine might be happier with a RPM instead of the Vic.
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You can drill them out if you have a numbered drill set. It will only take .002-.003 more to get what you need I think. One other thing your engine might be happier with a RPM instead of the Vic.
Ok cool. I did a little googling and saw that I could drill out the Idle Restriction. Didn't know if I could drill out the idle tubes. Guessing I have to remove them carefully and then drill them.
So I have a performer RPM intake that I was running on the motor. The problem is that the intake port to head fitment on the Brodix IK200 heads isn't great. The performer RPM intake is meant to use a 1204 gasket, and I could MAYBE port it up to a 1205 gasket (Fel pro part #s). The Brodix heads call for a 1206 gasket. So with the Performer RPM, part of the intake runner ports gets blocked. Not much, but enough that it was bothering me knowing that.
The Vic had enough material to gasket match to a 1206 gasket and mates up well with those heads. I would definitely rather run the RPM intake for the street, but I'm not sure how to make it work without having like a shop add material to the runners or something.
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Quick update. Carb is all back together after installing one of Cliff's rebuild kits.
Idle tubes are now .036. DCR is now .050.
New accelerator pump, float, needle and seat, gaskets all around, etc...
Installed Cliff's Primary metering rods.
Drilled and tapped APT plug hole.
Drilled out idle mix screw holes to .093.
I dug out my old Performer RPM intake (7104, spreadbore bolt pattern) and decided "what the heck" and spent some time port matching it to a 1206 gasket so it could match up to my Brodix heads. This will allow me to finally latch the hood on my C3 (the vic jr. with carb adapter was a little too tall) and I have a feeling the dual plane will be more street friendly.
I've had almost ZERO free time the last few days to bolt things back together, got the intake on and torqued, but the distributor absolutely did NOT want to go in with the rotor pointing at the #1 cylinder at TDC. And yes, I remembered to use a long screw driver to align the oil pump shaft with the distributor. But it just hit me a few minutes ago, I wonder how accurate the timing mark is on my crank pulley. I found TDC by turning the motor over until the intake rocker on #1 had fully opened and closed and then just turned the motor a little farther until the "0" mark lined up with the timing mark on the crank pulley. But if that crank pulley mark is off, that would explain the difficulty. Going to put something like a straw in the #1 spark plug hole tonight to find actual TDC of the piston stroke and see if it matches the pulley mark. I keep having only 30 minutes at a time to spend working on the car so it's hard to make progress. Hopefully running by the weekend and can update with where the carb is at after all the fixes.
Thanks for the help so far!
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Lowered the primary needles with the APT screw and loosened the air door spring tension another quarter turn. Some slight improvement for sure. I"m getting closer, it's pretty much fine tuning at this point.
One issue I'd like to figure out is the smell I get at idle. Part of it is that I'm running a carbureted vehicle with no catylytic converter, but I've run vehicles like that before and they didn't smell this bad. So I'm wondering if I have the PCV hoses hooked to the wrong ports, or some other vent port plugged that shouldn't be or something.
See attached pictures. The big hose circled in red goes to a T fitting with one end going to the PCV valve in the valve cover and the other end going to the charcoal cannister. The hose circled in green goes straight to the charcoal cannister. Is this correct?
And then in the rest of the pictures you can see the various ports that are plugged off. I am running full manifold vacuum for the vacuum advance.
Also, what is the port circled in blue on the back of the carb near the secondary air doors? Just another vacuum port?
Can anyone comment on the PCV/charcoal cannister connections I posted a few posts back in this thread? I am having some oil seep from my PCV in my valve cover and I'm wondering if I have the vacuum lines hooked up wrong...
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I wouldn't put any vacuum to the charcoal canister, only to the valve cover located PCV valve instead. This will put all the pull on the engine to remove crankcase pressure and vapor plus controlled vacuum from the PCV valve......Cliff