86LG4Bird

iTrader: (1)

Quoting here what I posted in the Tech/General Engine section:
"Finally made it to the strip with the cobbled together (not rebuilt) L31/ZZ3 I swapped into the Firebird last fall. It's a 248k motor from my 98 pickup with new rod bearings, new timing set, .015" head gaskets and a ZZ3 cam (208/221/112). TC is an Street Edge 2800. 12-yr old Hoosier 26" slicks. Edelbrock Performer intake, stock TPI manifolds/exhaust/muffler.
Despite some carb issue where it stumbles badly after the 2-3 shift, it still ran a best of 13.06 @ 103.8. 1.72 60' times.
I'll post in the carb section, but I'm guessing fuel slosh from the shift starving the main and/or secondary jets. Maybe float level too low? Maybe not enough pump capacity to keep the float bowl filled to the correct level, and then the lurch on the 2-3 shift just enough to disrupt the supply? I have just a cheapo Airtex inline pump."

It recovers and pulls great to the stripe.

I have never opened up a Quadrajet. For all I know this one hasn't been opened since 1986! I did swap to DR rods and G hanger on the secondary side a couple of years back which helped even the LG4 engine immensely.

The things runs really well and is pulling down 23-24 mpg with the Vortec, so I'm looking for simple suggestions rather than "just rebuild the carb".
Thanks for looking

redneckjoe

kinda sounds like a fuel delivery problem. like your running out of it for a second.

on the carb, have you ever tried adjusting the secondaries? its trial and error, but i've got some q-jets to run real good once they're dialed in.
http://www.78ta.com/HTAF/index.php?topic=13167.0

skinny z

Sounds air valve related.
What I understand about Quardajets (and it pretty dated stuff) was that the tune was all in air valve adjustment (AFRs notwithstanding).
A little blurb from a Buick webiste.
"Did you see how to adjust your air valve? You might have to take an allen key and a small screwdriver (I use one of those long electrical ones with the red insulation that goes down to the tip which usually came in Stanley screwdriver sets) because you have to tune it until any flat spots disappear. Loosen the spring tension slightly on the air valve until it starts to bog down a little and bring it back up until the bog disappears and a little more and your set. Then it's set for your car and you'll have bog free performance when you floor it from there on and it shouldn't change either. It seems like a lot of work but if you can find a section of isolated road. you'll know pretty quickly if you have a bog. Then you get out and adjust it a little and shut the bonnet and floor it again."
That's from,
http://www.mako.com.au/buick/html/qjet_tips.htm

Just throwing it out there.

Fast355

iTrader: (2)

Check the inlet filter. Had one come apart on me. Would run fine for several seconds then break up and then run fine. I would actually do away with the stick thin tiny inlet filter and go bigger.

Also it could be the secondaries opening a little too early or not closing enough after the shift causing a lean spot. Try 1 turn of windup like most 350s specify rather than the 7/8s turn most 305s specify.

naf

iTrader: (7)

Fuel delivery or secondaries. If you still have some issues after listening to fast355, it may be worth it to pull the airhorn off. Over time the emulsifier and inlet tubes attached to the airhorn can fall off inside the carb.

86LG4Bird

iTrader: (1)

Thanks for the ideas. I'm pretty sure it's not the air valve adjustment. It's been running perfectly on the roads for months; never a bog on either WOT upshift. It's snappy out of the hole and on the 1-2 upshift.
That adjustment hasn't been changed from where it was on the LG4, and it was fine there.

I can't see it being a filter issue because it's doing it only after that 2-3 shift; same spot every time, nowhere else.

I'm going to read up on checking/setting float level. I'll see if biasing that toward the high side of tolerance helps. If not, I have to assume the pump isn't quite keeping up with the fuel demand, and by the time the 2-3 shift comes, the fuel level has dropped enough that the lurch uncovers a jet inlet somewhere.
I just thought of something else that makes me think float level: When going through turns aggressively, it sometimes has a lean stumble when I squeeze the accelerator slightly coming out of the turn and the TCC is still locked in 4th.

86LG4Bird

iTrader: (1)

Thanks. I guess checking for loose tubes would be step #2 after checking float level. As I said, other than adjustments I've made, I assume this carb hasn't been touched since 1986.

Damon

Give this is an automatic trans car, I seriously doubt there is a problem with the secondary air valve tension, dashpot or pull-over enrichment tubes. Those have all finished doing their job from about 1/2 a second after you buried the pedal into the carpet.

Sounds like fuel delivery to me. A fresh filter is always job 1. After that, you need to actually measure fuel pressure during a run. I have a remote-mount gague setup that I install and (temporarily!) duck tape the gague to the windshield somewhere I can actually see it while driving. If you're outrunning the fuel system it will be OBVIOUS. FP will nose-dive, followed shortly by the engine stumbling (the engine can suck the fuel bowl dry pretty quick at WOT).

Now, I will tell you that the factory fuel filter in the nose of the carb is going to be a bit stressed trying to keep up with the fuel delivery demands of a low 13 car. I would pick up something good-flowing and install it in the line just ahead of the carb inlet instead. This can be as cheap as a Fram G3 filter (about $3) patched in with some softline ahead of the carb (I still use one on my supercharged 383 Malibu and it keeps up no problem with a ~475HP motor). How much more you spend above that is your choice- the sky is the limit.

Fast355

iTrader: (2)

From prior experience I disagree on it being the air valve. The airvalve in most applications will rarely open more than 75% and following an upshift will close almost entirely. If it is too l9ose it will bog after upshifts, especially as the load increases in third gear. Might not notice an air valve problem in first or 2nd gear but you can bet your butt can definately feel it in third gear.

Fast355

iTrader: (2)

Damon

Well, given that I've run QJets with a MANUAL air valve opening setup (an old manal choke cable rigged up to open it by hand), I can tell you they run just peachy wide open right through the shifts. I've also done similar experiments on QJet equipped boats and motorhomes where you can sit right next to the engine while underway. You can hold it fully open and it won't bog or stumble on the upshift at all. Factory ones have the tension set higher than necessary and they often have a limiting tang on the air valve arm to prevent full opening.

Once you're beyond about 50% air valve opening the QJet is incredibly insensitive to further changes in opening (or closing, for instance, after an upshift). Once fuel flow through the secondaries is established the chances of "stalling" that flow and causing a bog are very remote. Manual car with a full pedal lift between shifts it's a different situation, but on an auto-trans application, that's not going to be an issue.

He's running out of fuel.

Fast355

iTrader: (2)

I guess we will agree to disagree!! My G20 Van was the same way and it was bogging badly on the 2-3 shift. Tightened the air valve slightly and the bog vanished.

Gumby

volume not PSI can be a problemon the top end.


start fresh, so you know all is 100% right. home bench top rebuild often result in nothing as there is much more to consider.
http://www.nationalcarburetors.com/

86LG4Bird

iTrader: (1)

Got back to the track today to try some things and learned more.

After tightening the air valve about a 1/4 turn a few weeks ago, I could tell the power was down. So, I backed it off quite a bit, got the power back, and had no bogs or stumbles whatsoever in daily driving.

1st pass today: Engine fairly cool after driving to the track, going through tech and then sitting for about 45 min. Spun on the launch (1.78 60'), but pulled well through all the gears, no flat spots after the shifts. 13.01 @ 103.6
2nd pass: no changes, just hotlapping it. Terrible stumble on the throttle hit at the line. this pass was a write-off WTF? is the spring in the airvalve that sensitive to temperature ??
3rd pass: airvalve tightened 1/4 turn. clean launch but still spun. 1.79 60'. 13.02 @ 104.3. The flat spot is back when going into 2nd gear and into 3rd gear. The airvalve isn't staying open, but it clearly needs to stay open on the gearshifts. Turns out Fast's suggestion was backward; loosening the tension keeps it happy after the shifts. Without the flat spots after the shifts, this would have been a 12.9X
Another thing about this car, which is opposite of my injected LT1 in the Impala and opposite of the injected L31 in my pickup: it likes to run hot; mph is better on the hotlaps than it is after a cooldown. It was also very windy with gusts today, a quartering headwind, so don't make too much of the mph on these runs.
6 more runs after this, all trying to find that happy spot with the airvalve adjustment between going flat after the shifts and bogging at the start - all totally unsuccessful.
So what do you think? Except for the starting line where I need to flash it from idle to hit the converter for the best launch, it clearly wants to run with the airvalve tension at minimum. The accelerator pump link is already in the "fast" hole of the arm. Is there a simple way on a Q-jet to get a bigger pump shot? And will that be effective in compensating for the secondary airvalve swinging open at the hit? What are my other options?

Gumby

Doug Roe qjet book

86LG4Bird

iTrader: (1)

I have it. I'll dig into it.

Gumby

a 800cc Qjet with all the right mods can deliver allot of power, beyond that you go straight to a 1000DP

86LG4Bird

iTrader: (1)

Yes, we know this has nothing to do with the power level
I'm well aware of Q-jets feeding full weight 8 and 9 second classic Pontiacs.
Can you say......... Cliff Ruggles ?

Gumby

One things used in some per apps, like guy said above about having the air valve wide open.

Ive seen several stock qjets [ had old manual one off a caddy like that] and others, where the 2nd air valve has about 1/2" holes in them right over the jet nozel, so soon as the bottom flaps open, air can rush past them and start the mix, while it opens up.

86LG4Bird

iTrader: (1)

I already have the slots cut into the front edge of the airvalve blades in an attempt to get the fuel moving.

naf

iTrader: (7)

You can increase the pump shot by moving the rod closer to the fulcrum on the pump lever (drill another hole) or shave a 1/8" or so off the top of the pump rod. There are some other mods that can be done to the secondary airflaps, which may be beneficial to you. One example on these boards is, I think, Fast355.

In many cases qjet inconsistencies have been tracked down to bad pull off valves. I'm sure you've already checked something so basic, but others may look here for solutions to similar problems.

AV issues I've had in the past included flaps that would hang open or occasionally get stuck open, partially open or closed, and a secondary rod cam that would sometimes stick open keeping the secondary rods up after the secondary throttles closed. Booger issues that you wouldn't expect... can confound you for a time.

Damon

What it tells me is that your air valve tension is not the cause of your problems, as I posted above. It's a fine-tuning aid. It won't cure flat spots or stumbles much beyond where you first bury your foot in the throttle.

Your problem is elsewhere.

86LG4Bird

iTrader: (1)

Actually I'm finding that for this engine, the airvalve tension is not a good fine-tuning aid at all. There isn't enough "fine adjustment" in it to slow it down at the hit and yet permit the required airflow for full power after the shifts. That coiled spring arrangement with the inherent friction isn't even consistent run-to-run when I don't mess with the adjustment at all.
I'm really surprised at the noticeable power penalty when the air valve doesn't stay open on the shifts. It feels like an extra 20-30 hp when it opens again. When I first ran the car a few weeks ago, I was in denial about this, so I wrongly described it as a stumble/bog.

I'm now convinced I need to leave the airvalve loose and somehow give it enough fuel at the hit.

naf, I've read the tip in Roe's book about shaving off that 1/8" or so from the pump rod. But....my brain isn't processing it. I can't see how that would accomplish anything. Are we talking about the ~1/2" of the rod that sticks up above the airhorn at rest? Help me understand that one, please

naf

iTrader: (7)

1/8" off the top of the plunger rod. I've never done it but have also read that it will work.

I'd have a problem wrapping around the volume increase if the bore were square but it does taper out slightly at the top. Still seems a miniscule increase at best.

A better fix to increase pump shot IMO would be to nab another pump lever that has the single hole and drill a second one slightly inboard of where your current lever's hole is. Shaving the 1/8" or so off of the tip of the plunger may need to be done concurrent with this to keep the plunger from bottoming out.

Are you running the stock fuel lines and do you have headers? I've had to thermo-tec my fuel lines where they pass my Hedman's. Problem symptoms included 'fuel starvation' and boiling over at the carb - but manifested itself after long (several hours) interstate cruising in Springtime as the tank got low. Happened on two separate trips down 95 to Florida and one to the beach here.

Gumby

there is that old school backyard adjustment, a washer and light spring n wing nut, far down the aircleaner stud to slow the open rate of the needles,,,,, gluing a penny or piece of one.........

skinny z

Have you given up on the Q-Jet?

86LG4Bird

iTrader: (1)

No, I won't do that.
The car runs nearly perfectly in everyday driving and has knocked down an average of 23.5 mpg over the past 3 tankfulls, even with a few full throttle blasts each day.
It's happy in all driving situations with the airvalve spring about 1/4 turn from loose. Too busy with work and other projects right now to worry about the missing .1 sec at the strip

skinny z

THAT to me is an impressive as a 12 second time slip for a daily driver.
I didn't set out to build a dedicated drag car and as such I've got a heavy mother with a suspension more in tune for cruising than racing. That said, my 20+ mpg is saying that things are dialed in. For you to get over 23 mpg (and a smaller US gallon at that) is a bonus. Long live the Q-Jet and it's tiny primaries.
I know today's EFI/computer controlled cars can pull that off but we're talking about a carburetor here and that's saying something.
Keep it up. You know you'll get that 12 second slip (although you don't sound too concerned) and not spend a fortune getting to and from the track.

86LG4Bird

iTrader: (1)

Thanks for the compliment, skinny
It was really satisfying the compliments I got at the track also. there were 4 other 3rd gen street cars there, and every single one of them came over to check out the "sleeper". It's quiet, just barely a hint of any cam lope at all, it's clean, all stock appearance, only the slicks (mounted on a spare set of factory wheels give a hint that it's more than a 15-sec car

gbeaird

iTrader: (1)

Are you using a mechanical pump only, or do you have an electrical pump? I had a 71 Trans Am I ran a Q-jet on with all the Doug Roe mods, and the thing wouldn't run on the secondaries after 1st gear unless I had the electric fuel pump on. And I was running a 421 SD 'Nascar' mechanical pump on it. You may need more volume. I also drilled out the secondary well supply ports a few thousandths to help with fuel flow to the secondary metering wells.

Chase the fuel lines from the tank to the carburetor and make sure you aren't touching anything that can get hot. You may be experiencing some vapor lock with the heat. You may even want to cover the line from the pump to the carb with some insulation to help head soak issues with heat from the fan.

Keep the float at the proper level to avoid fuel slosh on the throttle hit. And yes, cutting some off the top of the accelerator pump will net you more volume. I had mine set so that the pump bottomed out just as the throttle was going WOT. Many will bottom out long before that.

86LG4Bird

iTrader: (1)

Update:
Well, funny how some things just take care of themselves: My daughter came home from Huntsville for a visit and showed me how to drive the car!
..This IS her car by the way. I've just been keeping it until she got a place with a garage to keep it nice.
So.....all along I've been shifting the car manually at the strip thinking that with this ZZ3 cam it will surely want be shifted higher than the 4600-4800 rpm shifts that you get when running in D or OD. And I realized that this "flat spot" didn't happen when I let the car shift sooner.
Anyway.....she left the old street tires on it and was practicing easy roll-in launches as practice for the upcoming Street bracket class at the Pontiac Nationals. To concentrate on the launch only, she made all runs in OD, letting the car shift itself. Never a hiccup; the car ran perfectly like this; and she got the launch perfected to the tune of high 1.7x - low 1.8x 60' times and ran a best of 12.91 @ 105 ...... without even putting the slicks on the car!

Oh, and an update to the highway mileage with the car: I took it on a 500 mi roundtrip a couple of weekends ago, driving ~60 mph on two lane roads and 65-70 on interstate and it turned in 26.5 mpg. I'm done.....I'm welding the hood shut!

Damon

Back to what I suspected was the problem in the beginning- you're outrunning your fuel system. Not by a ton, but when you short-shift and the problem goes away, there's nothing left to point the finger at (except valve float, which is NOT the problem you're having).

Get a remote fuel pressure gague on it so you can actually read it while you're making a run. You're going to be very enlightened at what you find.

Swamprunner

iTrader: (1)

How do I find out what q jet is on my 84 z28 ho?

naf

iTrader: (7)

If it's stock it will be the ccc-qjet with a three wire connector up front and a two wire connector on top. If you're looking for a rebuild kit, any kit for a E4ME will work.

Swamprunner

iTrader: (1)

Thanks naf yeah it is stock for now,doing a reemergence so to speak after 25 years of not being on the road. I did do some more research on my own and finally did find that out.
Now the vacuume hose routing is going to be a beast in itself. Just wanted to say and I'm not sure if this is the proper place to do so being that this is only my second post but I think this site is fantastic!!! Also looking for some advice on what to do as far as an intake and carb setup, I am putting a Chevy Create 350 in the car, I want to go with the stock if I can but not sure that I can or if that will make me happy, live in jersey and need to pass emissions are there any after market choices? Again great site and thanks for the help!

naf

iTrader: (7)

No after market options to pass emissions, but the stock system will work splendidly on a mild 350. Feel free to start your new thread on the build.

You may want to read up some before tackling the carb. There are a few screw settings that should be returned to original positions, necessitating a count of 'turns out' prior to removing.

86LG4Bird

iTrader: (1)

I tend to agree with you, Damon. Back to my first suspicion - it pulls the float bowl just low enough by the 2-3 shift point that the slosh from the shift uncovers the jets for a split second.
But at this point, I'm not going to do anything about it. I can keep it WOT in 3rd gear for close to a half mile and it never feels like it goes lean, so my guess is that when the needle/seat is opened more fully to relieve some pressure, the pump is able to keep up. When that cheap Airtex fails, we'll upgrade

skinny z

The 12 second pass notwithstanding, that mileage is where it's at!
Well done.

Damon

It's also POSSIBLE you could just have too small an inlet valve. The stock ones from the factory are typically around .100". A "high performance" one is usually around .130". Some of the ones I've seen in cheap "rebuild kits" can be as small as .085". Yikes!

Smaller ones are used because they do a good job of controlling the float level in stock applications and where somebody pushes more fuel pressure against the carb than recommended (typically ~6 PSI max for a QJet).

BUT... they will run out of wheaties in a performance application where high fuel flow is required at WOT/high RPMs.

It's gotten so bad these days I often reuse the stock inlet assy. (the part that screws into the base of the fuel bowl) and just install a new needle valve to work against it, in many cases. The factory stuff was GOOD stuff. The rebuilder replacement parts.... not so much.

I'd still bet you're outrunning the fuel delivery system, though. That's VERY common, especially when using stock-ish fuel lines.