99Hawk120

Here's something that got me... I hadn't heard about it here, but maybe that's because most people who pull the stock EQjet go with with an Edelbrock (blech) or a Holley...

Any how, I've been trying to dial in the fuel mixture on my non electronic Qjet, and have been having a heck of a time getting the cruise mixture right. Getting the jets right was pretty simple, it just took some time--I've finally settled on 76 or 77 jets (will need track time to determine which is better).

However, the cruise mixture has been extremely lean no matter what I've done. As I've gone up in jet sizes from 71 to 77, it went from surging badly at cruise to surging imperceptibly. By jetting alone I should have richened my cruise mixture about 15%. That certainly agrees with what I've seen. However, I went from 71/41 to 77/41... without much change in the mixture. I'd tried some larger rods as I went up the jet size, and at one point I went from 75/45 to the 77/41 combination I have now, and didn't get much improvement. And it took me until last night (and wandering across a paper detailing exactly the problem I'm having) to figure out what was wrong.

Later mechanical Qjets have an adjustable "lean stop" that determines exactly how far down into the jets the primary rods are allowed to go. If you adjust the stop "full lean", it actually allows the rod to insert itself far enough into the jet that it goes PAST the top of the machined part of the rod, where it goes from the stamped diameter (.041" for 41 rods) to the fixed diameter of the rest of the rod stock (approximately .061" according to my dial caliper). And of course ALL primary rods are that same .061" diameter above the machining line, so changing rods was having ZERO effect on the mixture.

So I took the carb apart and set the lean stop height properly, and put it back on. Quick BOE calculation show, that if that was really the problem, I just doubled my fuel at cruise. I'm betting I'll be going back in there shortly and putting thicker rods in.

Damon

Hawk- that lean-stop as you call it (a term which I like and will probably steal from you in the future!) should NOT be able to take the rod past the machined tapered part. Even if you take it out completely the power piston itself has it's own stop which is the stop that QJets had used for years and years before Rochester started putting in the adjustable one right next to it, for emissions and fine-tuning reasons.

There is a gotcha here that might explain what's happening..... if you try to put old style rods into a later QJet you WILL have exactly the problem you describe since the old rods are about 1/8" longer than the newer ones.

Old style rods are usually stamped with a letter like B, C or D behind the size number. They go in carbs that have a part number beginning with 70xxxxx.

New style rods are usually stamped with a letter like J, K, L or M behind the size number. They go in carbs that have a part number beginning with 170xxxxx.

If there's no letter stamped on it, they're probably old style rods. If you're still not sure then look at the taper. If it's really gradual and even from the fat part to the skinny tip it's an older rod. If the taper is fat most of the way down and then quickly drops off to the skinny tip in the last 1/8" then it's a new-style rod.

It's also possible you don't have the right power piston assembly for your carb.

F-BIRD'88

It's also quite possible that the reason changing jets and rods didn't effect the cruise AFR near as much as expected is that your actually cruising on the idle and off idle circuits.
This circuit is metered by a fixed orifice and air bleed.
The volume of this mixture is controlled by the idle screws.
Think of the idle screws as volume controls.

In order to enrichen the idle/low cruise mixture (AFR) you would have to drill the idle feed restriction larger.
Apparantly a extra long numbered drill bit is required.
The actual restriction (orifice) is down in the idle down well.
A book on Qjets like the one by Doug Roe will show which hole that is. Late emissions q-jets are calibrated quite lean. Therefore the surging. Especially if the motor is modified from its stock form. Earlier Q-jets from the 60's are much richer.
Only drill out the idle feed restriction a small amount 'cause it doesn't take much to richen it up. Like .002"-.003" bigger.
Once you've done this, you can probabily lean out your primary jetting a bit again . ( jet and or rod) A o2 sensor on the exhaust
should be of some help. Check the idle transfer slots for gum/dirt
and blow out all the air bleeds with compressed air first.

99Hawk120

Damon,

You like that eh? I wish I could claim credit for the term, but I stole it from Rochester, as they used the terms "rich stop" and "lean stop" when dealing with the MC solenoid on the E4ME carbs.

To answer your question, I am aware that there is a stop in the casting. However, I did all my measurements with a dial caliper, and verified that the rods were indeed sitting too low. I don't have the measurement numbers off hand, but I remember reading that the "step" in the rod from the non-machined part to the tapered part should never sit farther in the jet than the thickness of the lip of the jet (which measured 0.088" with the dial calipers). The length of the machined section of the rod was 0.410". By measuring the height of the rod, assembled on the power piston, in the lowest position (I cheated--I removed the spring) to the SURFACE of the jet, I came up with the non machined step being inserted approximately 0.100" BELOW the BOTTOM of the jet "lip". The measured length, minus the total length of the rod, plus the machined length (.410"), indicated that the machined "step" was ~0.190" below the "surface" of the jet. Of course I'm doing these numbers from memory, I have the exact values at home and will post them when I can.

Do you have the total rod lengths anywhere, or is it in the Roe book?

I believe all but one or two of my primary rod pairs are "newer" as I only have two or so "70 series" carbs. The rods I've tried in this carb have been 41J, 41K, 43 (J or K, can't remember) and 45K. This problem has existed with all four sets. I have ONLY done the measurements with the 41J rods--I will use the dial calipers to measure the total length of the rest of the rods this afternoon.

The carb itself is a 170 series carb, when I ran the numbers it supposed came off a 1977 Camaro. It was originally jetted 71/41, and one of the two sets of 41 rods I used (I think the 41K) were the ones that were in it when I first cracked it open.

That's possible, I am not sure how I'd check. Just find one that holds the rods up higher? I personally think it's a lot more likely than 4 or 5 sets of rods all being wrong... Although I suppose the "1" on the carb number could have been stamped there by a particular sadistic Holley owner.

99Hawk120

F-BIRD'88,

I doubt it. I am REALLY lean until I put enough of a load on it to activate the power system. Even up to ~40% in 1st or second gear I am running off the scale lean. If the carb is still running on the idle and transition slots at that point, something is seriously wrong with it.

Plus, the idle itself is actually RICH.

All testing done with an O2 sensor and diacom hooked up to my Genuine GM Air Fuel Meter.

99Hawk120

Here are the numbers from when I did all the measuring yesterday:

Final Power Piston setting: 1.917" from top of rod to surface of jet
Jet flange thickness: 0.088"
Total Jet thickness: 0.313"
Tip length: 0.410"
Total Rod Length: 2.371"
Desired Rod height: 1.961"-1.917" from top of rod to surface of jet

The desired rod height was calculated by placing the "step" on the rod no more than half the distance into the jet, and no higher than completely even with the surface of the jet.

Rod Height (piston up) - 2.075"
Min Rod Height (piston down, lean stop all the way lean) - 1.847"

I calculated at the min rod height, the machined "step" on the rod was about 0.114" below the surface of the jet. That places it about 0.026" below the jet flange.

99Hawk120

Also, I measured all of my rods, they are all 2.400" +/- .030" long. I can't find any reference to total length, but it is stated that the newer rods are 0.080" shorter.

Also measured 5 spare power pistons, they measured (total height) 1.345", 1.361", 1.356", 1.352", and 1.364". I did not measure the one in my carb because it is currently assembled. The Roe book stated that the power piston should be 1.950"-1.955" tall from the top of the hangar to the bottom of the piston--mine are not even CLOSE to that. In fact, even with the hangar removed from the piston I couldn't make it that tall.

99Hawk120

Few more notes:

Cruise is still just as lean, but the power enrichment fuel is now coming in at less throttle opening. I've noticed when the enrichment DOES come in, it goes from lean (4mv) to rich (750mv+) instantly. I'm thinking two adjustments here: Leaner rods and setting the lean stop higher. That will allow the enrichment to come in sooner, but not be quite so rich on initial opening.

The leanness while cruising is almost tolerable; I might try that adjustment and see how it goes.

Damon

Hawk- the 1.900" power piston is the older 70-series piston used with the old 70-series rods. later 170-series carbs have much shorter power pistons.

Possible that the arms on the power piston that the rods hang from are bent down a little?

Use your eyes on this- don't just measure. Take out the float, the plastic stuffer, etc. Get it all out of the way and visually confirm how the rods are sitting in the jets, up and down. I just have not run into a problem like this before if all the parts are correct and not bent/damaged. If it's that far in the jet the rod should be stacked tight in there with the piston in the down position. Being off by more than 1/10th of an inch is a lot.

Test to make sure this is even the problem at all: Take the primary rods out completely and take a spin. You should be wicked-rich under all conditions now. If not, you know the problem lies elsewhere.

Also, do you still have the "lean stop" screwed down into it's threads? If you simply remove it you'll have a big uncalibrated air bleed leading right down into the main fuel well. Idle and part throttle get super lean. I don't think that's what's happening here, but you never know.

99Hawk120

I suspected as much, but wanted to verify...

Not by only eye inspection, I will check it again when I have the piston out of the carb tonight.

That was how it was when I did the measuring. I didn't believe the numbers I was getting, either, so I visually inspected--it definitely agreed with the numbers I was getting, at least as close as my eyes are capable of seeing. As the jets are 77s, a 61 thickness rod fits in with plenty of slop. Perhaps I will temporarily try one of my smaller jets and see if it's a nice tight fit.

I hadn't thought of that, good plan. I assume one needs to leave the power piston in to prevent vacuum leakage In fact, I remember readon about some Oldsmobile carbs that didn't use a power piston at all, just main jetting...

Yep, it's still there. I didn't ever even think of taking it out, since it will screw down below the level of the stop in the casting.

99Hawk120

OK, I did some testing last night. I would have posted then, but my net was down almost until bed time...

Anyhow, I was too lazy to remove the air horn and pull the rods. What I DID do is adjust the lean stop so that the rods were at the full up position ALL THE TIME. I verified this by pushing down on the power piston through the casting and checking the travel.

Then I started the car, and here's what I got:



The car was at FULL operating temperature when that picture was taken. I don't know if you can tell, but the smoke is definitely gray and not white. It REEKED of partially burned gas. I checked the O2 sensor with my scanner and got about 870mv. As I thought, I'm getting plenty of fuel at idle. FYI, the mixture screws are 4 turns out.

Driving the car, I got some very interesting data.

At idle, I was extremely rich. If I opened the throttle just a little (being careful to do it slow enough not to activate the accel pump), and it got leaner. A little more, and a little leaner. I did this until it went off the scale lean again, which did not have the pedal down very far at all. If I went just a TAD bit more, it went to full stinking rich again. At CLOSED THROTTLE DECEL, I was also stinking rich; if I gave it enough gas to maintain speed, it went extremely lean.

I then did some studying of the Roe book on the idle system, and have come up with a theory (feel free to let me know what you think of it).

It appears to me that I have a massive lean spot in between the main system and the idle system. What system is between the two... the off-idle transfer slots. It almost appears as if those are not working right (if at all); perhaps partially clogged, or just too small? What could cause the off-idle system to malfunction when the idle system seems to be mroe than capable of running the motor too damn rich? When the gas pedal is applied quickly to 50% throttle or more, it seems as though the accel pump completely covers up the lean spot.

As a side note, this carb does have a MASSIVE accel shot compared to my other Qjets--it uses the "taller stem" pump (appox 3/32 toller), and with the inside hole that got me 23/32 of travel WITHOUT the pump bottoming or coming up above the fill slot. Using the short accel pump results in the pump rising above the fill slot (BAD!) if the inner hole is used, and not enough travel when the outer hole is used. Anyhow, this extra big pump shot may be why I can't feel the off-idle slots not working except by leaness at cruise. With the accel pump linkage disconnected, the car attempts to stall when you open the throttle quickly, even when it's sitting still in the garage.

One other thing... the "rich" mixture that came in when the main system came on line came in MUCH MUCH earlier than it did when I had the lean stop at a reasonable height, so I think I have two problems with the main system--one, the needles ARE indeed too far down into the hole, as I only get a satisfactory response when I raise them up a little; and two, the power system is coming in too late, which means I need a stronger spring.

It is entirely possible, however, that the extra fuel from the off-idle system once it is weaking properly could entirely negate the problems with the main system (as the fuel from the off idle system should flow any time the car is above idle as I understand it). In fact, if I'm correct in my understanding, that may even by why I'm having to use such rich jets to get satisfactory WOT performace--because there is a decent chunk of fuel that is supposed to be coming from the transfer slots that just ain't there.

And now, the 64 million dollar question... what exactly is wrong with the off idle system? Got any suggestion as to what I should look for?

Damon

Well, if there was no fuel going to the off-idle slots you would get nothing at the curb idle screws either. The fuel goes down the same passage past the off-idle slots that it does to get to the curb idle screws.

I wouldn't worry about a big puff on startup, especially after pulleing the carb apart on the motor- mighta just slopped some raw gas from the fuel bowl into the intake.

I think more than ever you need to remove the primary rods and redo your testing. Remember to screw the lean stop all the way down, too. I can do some weird stuff if it's lifting the power piston too high. This is getting damned peculiar.

I don't think it's your off-idle transition slots. I have always found that they are big enough if the idle circuit is rich enough overall. If you were 6 turns out on the curb idle screws and still too lean at idle then the off-idle would be too lean, too. But that's not the case. I did run into a carb that some moron has soldered them completely shut, but that's like one in a million.

Something odd going on here. How's your float level? It should be roughly level with the edge of the fule bowl, about 1/4" down from the top. A low float level can cause a "hole" between the idle and main systems.

I'm running out of ideas pretty quick.

One more- when you yank the primary rods also disconnect the linkage that opens the secondary throttles. If you're starting to tip into the secondaries it's going to render your testing meaningless. Is it getting rich becuase the power piston popped up or becuase the secondaries started to feed? It's hard to tell. Just disconnect them so we know for sure we're just working on the primaries.

99Hawk120

Well shoot, there goes that theory.

That wasn't a big puff on startup. The motor was stone cold when I started it. By the time it started to warm up, it began spewing smoke like that out the back. It stayed like that the entire time, even once reaching operating temperature. It smoked CONSTANTLY.



I made sure the power piston moved down a little right up until the last 1/8th turn. In fact, it may still have a slight bit of travel to it.

I will, however, remove the rods and drop the lean stop all the way back down again.

Quote:

I don't think it's your off-idle transition slots. I have always found that they are big enough if the idle circuit is rich enough overall. If you were 6 turns out on the curb idle screws and still too lean at idle then the off-idle would be too lean, too. But that's not the case. I did run into a carb that some moron has soldered them completely shut, but that's like one in a million.

I don't want to see how rich it would be with 6 turns. FWIW, I was able to get the idle rich enough, with the carb as is, when running 71 jets and 41K rods. It was so lean it wouldn't stay operating on the choke (would just run out of fuel and die), but once the motor warmed up it would idle smooth as you please at 650mv on the O2. They were probably 5 and a half turns out at that point.

I don't think the transition slots are soldered over. I inspected the castings pretty good anyplace I could see. I was concerned about an internal blockage or incorrect gasket somewhere; but if they are the SAME passge the idle screw uses, so much for that.

Which port comes first as the fuel travels out of the jet?

Quote:

Something odd going on here. How's your float level? It should be roughly level with the edge of the fule bowl, about 1/4" down from the top. A low float level can cause a "hole" between the idle and main systems.

Up until the last time I put the carb in, the float was that high, but I had problems with that. It's now set a little lower; about 0.4" below the top of the float bowl casting, measured at the proper place (slightly in front of the toe).

Quote:

One more- when you yank the primary rods also disconnect the linkage that opens the secondary throttles. If you're starting to tip into the secondaries it's going to render your testing meaningless. Is it getting rich becuase the power piston popped up or becuase the secondaries started to feed? It's hard to tell. Just disconnect them so we know for sure we're just working on the primaries.

Up until the last test I had the secondary air valve locked out with a spring, but this also wouldn't let the fast idle cam drop off completely (it hung on the last step before curb idle). The spring did work, however, because my car was slow as a dog above 3000rpm.

I'm pretty familiar with the way my car sounds and I've found I can easily tell when the secondaries open by sound alone (open element air cleaner and no hood insulation). It didn't sound like the secondaries were open at all. Plus, the secondaries don't open until about half throttle the way I have the linkages set up and the throttle NEVER went past about 1/4.

After saying all that, I will do as you suggest and lock out the secondaries again. I think I will find a better place to attach the spring and lock out the air valve, though--disconnecting the actual throttle linkage is kind of a pain. Unless you think it will make a difference.

I won't be able to mess with it again until Thursday night, so if you come up with any other suggestions (no matter how off the wall), post 'em here. I will be able to check the board again tomorrow morning.

99Hawk120

Alright, it's taken me forever to get back to this. As sometimes happens, life got in the way, and I wasn't able to get to it until today.

I took the carb apart and took some pictures, trying to figure out what the hell is going wrong... warning these are big so you can see all the stupid little passages.



A- I don't really know what this passage is for
B- This appears to be the fuel feed from the main jets to the idle/transition system. Goes to "G" on the throttle body.
C- I don't know what this is either. It appears to go to the "H" hole on the left side of the throttle body.





D- off idle transition slot
E- Looks like a ported vacuum source
F- Slightly different ported vacuum source?
G- Main fuel feed for the transition and idle fuel feeds. The gasket doesn't seem to match up perfectly with the gasket, but it sure looks like there's enough of a passage.
H- I'm guessing these are vacuum passages
I- I can't figure out what these are and they are blocked by the gasket anyway.
J- I have no idea what this... more ported vacuum maybe?
K- Idle fuel ports. You can see the adjustment needle in the hole
L- No clue
M - No clue.

Anyone see anything massively wrong here? I'm baffled at this point...

99Hawk120

Alright. I disconnected the secondary linkage on the throttle body before reassembling. I also pulled the primary metering rods, and reset the power piston to the lowest stop.

I fired it up and it sounded a bit ragged until it came off the choke... too much fuel i think. Drove it around with the scan tool and it doesn't have as much power as it did (again, I'm guessing too much fuel) even at part to light throttle.

However, the off idle lean spot is now gone, even though the car is driving worse. It doesn't seem to much care for the accelerator pump at the moment, guessing again too rich. The O2 only went below 850mv once the whole time, and then only to about 830mv.

So why the heck did removing the metering rods fix the problem? Someone want to clue me in here?

Also I noticed I'm having a slight problem with the throttle returning to idle. I massaged the throttle plates a bit and it's better--now sometimes it returns to 750rpm and other times it stops at 1000rpm. Before it always stopped at 1000 unless I manually pushed the throttle on the carb. It must still be hanging up a little. I don't think it's the spring--the spring is pretty strong.

Nordbert

A vacuum leak or sticking secondaries could cause some of your symptoms.

Damon

Sorry for the long reply time. Well, that's damned peculiar. Primary rods out and the problem goes away. Any chance you could throw a mic on the primary rods and see if they are what they are actually stamped as? Do the tips and the fat part of the taper. If you can run some fine drill bits into the jets on a go no-go basis and confirm they are actually what they are supposed to be, also. It sure sounds like your initial theory is proving to be true. If the jets were effectively "plugged" by the primary metering rods you would indeed run leaner and leaner as you open the throttle until the power valve lifted and then all would be right with the world again. Hmmmmm..... Have you measured the travel of the power piston relative to the length of the machined ends of the primary metering rods?

I'm really running outta steam on this, as you can probably tell. Stuff like this jsut doen't happen. Maybe try using a different primary power piston for the heck of it, and screw the lean-stop down all the way, too. I have seen them do weird things when they are set up too high- like cocking the power piston in it's bore and allowing for a massive vacuum leak past the piston.

All those vacuum passages can be confusing. That's as mny as I've seen on a QJet. That must be some sort of "super-emissions" QJet!. Heh heh. As long as they aren't plugged and they have the right gasket between the throttle body and main casting, they look OK to me. Let me help you out with a few of them:

A- Auxiliary idle air bleed. Also acts as an auxiliary idle trasition slot and feeds fuel under moderate throttle and on a cold start when the choke is closed and there is good bit of vacuum under the choke plate. This is why I say that you run around 80% of the time on the idle circuits! They still feed a LOT of the fuel through the primaries up to maybe 20% throttle.
F,J&H on the right side (of the pictures you posted) are all part of a ported vacuum supply. They often use a combination of slots and holes if different locations so the vacuum "comes in" at a specified rate relative to throttle position- since this fitting is most commonly used for an EGR valve.
C- a passage to take the ported vacuum to a vacuum fitting in the main body of the carb, would be my guess. Blow through it and see where it comes out- that's what I do.
H,L&M on the left side I would guess is full manifold vacuum source since it's below the primary throttle plates at idle. Not sure if L&M are connected but I would guess they are separate.
E&I: Low vacuum ports for charcoal canister purge

Everythign else looks properly labeled.

99Hawk120

Really. Now THAT'S odd.

Hmmm. What if I told you the float bowl doesn't even have all the vacuum ports on it that the electronic Qjets have? It's missing the canister purge port and at least one other port on the front. It seems odd that the throttle body would have so many vacuum passages (more than the throttle body off of any of my E4ME's; I checked last night) yet the float bowl is clearly not equipped with as many vacuum related ports as the E4ME.

I'm wondering what the chances are that the throttle body is actually not the one that belongs with this carb. I'm even more suspicious because the air horn and float bowl have a gold tinge to the aluminum and the throttle body does not. Possibly the original throttle body wore out the throttle shaft and the owner at the time replaced it with one that merely looked similar? I'm not sure you'd find out though, none of my sources list the P/N of the throttle body cross-referenced with the carb # itself.

Perhaps I should dig through my pile of carbs and see if I can find a different one to mess with and see what happens.

Damon

You're right, that's odd. Trace all the vacuum ports and see where they go- if they go anywhere. The throttle body being a different color than the upper parts is normal, but you might still be on to something. I can't imagine how having a different throttle body would affect anything over idle, but maybe it could, somehow.

You SHOULD have a canister purge port- it's integral to the throttle body and does not have any connection witht he upper part of the carb- the vacuum holes go to a passage that goes DOWN on both sides to the very bottom of the carb and then forward, connecting with eachother where there is a 7/32" vacuum hose port. The only reason the passage doesn't see full manifold vacuum, being open on the bottom, is becuase of the gasket between the carb and intake manifold seals it up when installed on the engine.

99Hawk120

Me, personally, I'm wondering if there might be a ported vacuum leak in one of the ported vacuum holes in the throttle body that I don't know where they go.

As far as the 7/32" fittings, I was under the impression the one on the throttle body was for the PCV valve (which I have). It is the upper one that comes off the float bowl that I do not have--that is the one I thought was the canister purge.

Damon

No, that's the bowl vent. It's connected to the charcoal canister, too, but it's just there to collect the vapors evaporating off the gas in the fuel bowl and sending them over to the cansiter. The purge port is down on the throttle body, next to the large PCV valve port. It's larger than a typical vacuum port, but smaller than the PCV port by quite a bit.

You might be on to something with the vacuum leaks and such, internal to the carb if it's the wrong throttle body.

You gotta start asking yourself at this point if it isn't worth going to a junkyard (or swap meet) and shelling out a few bucks for a fresh core, rebuild it and then swap all your parts and tuning efforts into that one.

You could have a "Frankenstein" or it could just be a bad carb from the get-go. I have run across them before (rarely). Found one recently that had no idle air bleeds. Go figure. They never put them in. Couldn't get a drill in at the correct angle and depth to drill it myself so I was screwed. Beat that one good with a big hammer before I chucked it.

99Hawk120

Actually, I'm going to check my boxes of junk Qjets and see what I come up with. I think I'm going to write this one off as a loss.

sdowney717

ok, I am dragging up an old thread because floats should be set right at 3/8 on the newer quads, and at 1/4 on the older ones. He was lean at cruise I think because float was too low.
0.4 minus 0.375 leaves a big difference and float level is critical to getting these things to run right. plus no one mentioned the spring tension on the power piston being perhaps too strong.

five7kid

iTrader: (14)

The proper settings are listed here https://www.thirdgen.org/forums/carb...e-rebuild.html , at the bottom of the page.

I don't see your numbers anywhere.