OK, 1970 W-31 ... supposed to have 7040255 carb with:

1. 0.057 (57) Primary jets
2. NO Primary rods
3. AU secondary rods

However, what I have is a 7040251 (which looks to be a 70 442 MT carb?) and it has:

1. 0.066 (66) primary jets
2. Primary rods (not sure which ... got them soaking to get them shined up so I can look for stamping)
3. Looks to be AU secondary rods

It runs pretty decent ... idles a little rich but part of that is this big cam, good throttle response, no surging at cruise, pretty snappy at part throttle. I've tweaked that a bit with the idle mixture screws and it's about as good as it's going to get. The car has headers and is mechanically healthy with all the numbers matching heads, intake, etc. and apparently the stock "big" cam.

My issue is that when you go WOT in 2nd gear at 35 MPH or so, it is really "flat" in acceleration until it seems to wake up at about 4500 RPM. Just doesn't pull as hard as it should ... it doesn't bog or hesitate, but feels lazy to rev.

I rebuilt the distributor, put a recurve kit in it with weights and springs to bring all the timing in at 3800-3900 RPM and set it at 12 degrees advanced at 850 RPM (which should be about 32-33 degrees all in from what i noodled out). This made throttle response a bit "crisper" although it was pretty decent already, but didn't really seem to affect the "flat" acceleration much.

SO ... I pulled the 0251 carb apart, did the typical rebuild (it looks pretty fresh though) stuff with a Tomco kit. BUT ...

1. Put in 0.058 primary jets (closest that I had to the 0.057)
2. Left the powervalve in but left out the primary rods
3. Left the secondaries alone and the vacuum break alone

Oh boy, that wasn't the right direction! *grin* Idle was REALLY rich (to the point of black smoke with the choke on), and then with the choke off it seemed to pick up a bit of a miss and idle was high. I drove it a couple blocks just to see and it was prone to stalling coming off the throttle and WOT in 2nd gear basically was about the same as when I started.

... off the carb comes and I have the airhorn off and a bunch of primary jets are coming my way and I'm thinking ...

1. Put it back together with a bit richer primaries (maybe 68s?)
2. Reuse the original primary rods
3. Same secondary setup ... seems all of the 70 Olds carbs use the same ones, so trying to make one change at a time

Going from the power tuning section of 442.com, it appears that WOT is affected by primary jetting a lot, so that's where I'm at with this.

At least I have a baseline of 066 primaries with rods that work fairly well to fall back on, but would like to dial this in better. My wideband I use with my other cars would be a good tool except I need to get a clamp on "saddle" for the Olds to screw it into.

Does that sound like a good plan? I was originally thinking to replicate the 0255 specs, but that didn't work so hot ... so now I'm going to basically put it back to what it was and go maybe 3% richer on the primaries and see what that does? My original thought was that it was too rich at WOT, but now I'm thinking it almost feels like it's lean and down on power?

Thoughts from the Quadrajet gurus? I've rebuilt my fair share of these, but always had a good baseline to work from, but I got this car this summer and have most of it all sorted out (and it's a hoot to drive) except this one.

 

I'm in no way an expert, but it sounds to me like the secondary air flapper or whatever it is called isn't opening up until there is enough air flow to open it. My suggestion would be to re-adjust the spring tension so they will open earlier.

 

First off, I would advance your timing. Another 2 or 3 degrees base and bring in the advance a bit quicker, around 3000 rpm. Maybe try a 67 primary jet to start for the carb.

 

Large cams sometimes require idle bypass air, some carbs had holes drilled directly in the primary throttle plates. Secondary air valve opening rate is controlled by the choke pulloff diaphragm and spring tension adjustment on the sec. air valve shaft. Check for primary nozzle drip. Also have someone sit in the car with engine off while you check for full throttle opening at carb. Another thing is to check for proper throttle blade placement at idle, this requires removing the carb and looking at it from underneath as pictured. Good luck

 

All good suggestions ...

- Full throttle is fine, so it's not linkage
- I dialed in a couple more degrees of timing (should be about 34 degrees now all in at 3500)
Idle is actually pretty decent (a little choppy, but not bad). I'll look at the blades just to double check here on the bench, but I'm pretty sure they are OK.
- New jets are supposed to be here Friday, so I'll try a bit more jet and see if that helps or hinders the problem (I think I have a 67 coming ... if not I'll do a 68 just to see if the problem changes).
- I'll double check the secondary spring tension, but it is supposedly to spec according the rebuilt sheet. I'll make sure when I get it back together with the new jets and adjust if necessary. I'll see if maybe I can set it a little on the "loose" tension side of the spec and see if that makes a difference.

THANKS! I'll report back with results!

 

Just don't get it so loose the engine will bog. That is why most people have a problem with Q-jets, they don't have the tension set properly.

 

Yup ... I just loosened a very little bit. Still will snap the plates shut, but a tad looser than it was ... not much.

Also verified the secondary throttle plates open correctly in the base plate. Nice, smooth and start to open when the primaries are about 60% open. The choke pulloff seems to be smooth and relatively quick as well ... not sticky or leaking that I can tell.

 

Get a wideband A/F gauge to help dial that thing in!

 

I've got one (actually two, but one is semipermanent in my GN) ... just have to get the "no drill" bung that I ordered to come in. Should be here Monday.

A quick update ...

Went with 0.068 primaries and car runs pretty well until the secondaries are wide open and then it "burbles" in fuel ... waaaay too rich. SO.. datapoint ... that's the wrong direction

I'll go the other direction with the jets tonight (I think I have some 0.064s) and see what that does. Double check and dial it all in when I get the WB installed.

 

You could try some CK or CV secondary rods, slightly leaner if you are close on the primary side.

 

Just put in 0.64s and will test drive ... good point on the secondary rods.

Checked the tracking on my wideband clamp on bung ... Monday *sigh*

In the meantime, the ol' trial and error method is on point! Getting good at whipping this thing apart and back together!

 

Small update ... got things pretty lined out on the primary side now. Combination of setting the idle screws and fiddling with primary jets and now it's pretty healthy! Crisp off throttle and decent idle.

SO ... ordered a few sets of secondary rods now (CV, CK, and AY (for the hell of it) to try to get the WOT smoothed out (still pretty "flat"). Wide band mount should be here Monday, so I can really get a finger on the pulse at the point. Thanks for all the suggestions folks!

On a side note, I can have the carb off the car, apart, new jets, back together in about 15 minutes now. Not a skill I choose to have but ... Secondaries will be a piece of cake after this ... just one screw and some care to not drop stuff down the carb while it's still on the car.

 

The 7040251 was the 455 auto carb in 70. It would have come with 70 jets and 49b primary rods. AU secondary rod.
I would go to a 44b primary rod, and keep 70 jet for starting point. The AU is a good secondary, but you could try DA.

 

Yup, should have ... but someone has diddled with this thing quite a bit. It seems to still have the AU secondaries, but 0.066 primaries and I can't tell the primary rods ... stamping is worn on it.

 

Having been done this path before, you can really save a lot of time if you tune WOT on the primaries first. Lock out the secondary air valve with a tie wrap and do some WOT runs on just the primaries. That will allow you to set the primary jets. Once that's done, you can tweak the secondaries.

Here's the logic: The primary rods are a fixed diameter on the ends, so at WOT, it's a constant .026" diameter. The only variable is the jet size. If you tune without the secondaries interfering with your readings, it will give you the best jet for your combo. With that finalized, it's one less variable in the complex tuning equation. (Per Cliff Ruggles)

If you decide to skip running rods, then you can run smaller jets to get the same result. The only loss is part throttle economy which isn't a concern for hot rods anyway

 

Excellent! Got the wideband mounting rig and once I get a few moments, I'll get it on the Olds and get a baseline. It runs pretty good right now on the 0.066 primary jets until WOT. Good throttle response and decent idle ... I'll see what the WB says. Then I'll double check with the above method (great idea!) to see where we are at. Then I just got my secondary rods in the mail yesterday, so I can try to fine tune from there.

Thanks!

 

UPDATE

Got the Wideband hooked up ... some interesting results:

1. Idle when warmed up and off the choke is really lean. It hovered around 1.35 lambda (about 19:1) ... surprised it idles!
2. I got it to richen up a little bit 1.20-1.28 (17.6-18:1) with the mixture screws and idle quality improves, but that's ridiculously lean. I'm again, surprised that it's not a terrible idle ... it's a little lopey but steady!
3. At light throttle cruise, it was way on the lean side 1.05-1.10 (15:1 - 16:1) ... again, surprised it doesn't surge. Pretty decent behavior and decent throttle response.
4. At WOT, it now stumbles a bit (probably my air door adjustment needs to be tightened back up)
5. WOT Lambda is 0.75-0.80 (11-11.9:1) and surges. It's pulling a bit harder, but still kind of inconsistent on the "pull"... initially pulls hard, then kind of lays down (and you see the AFR dip) then picks up in the upper revs (above about 4000).

I guess what surprised me is that I *thought* it was RICH all over (not just at WOT) ... turns out it's mostly LEAN! I wonder if the large discrepancy between primaries being lean and then WOT being rich is causing the surge.

So my plan is to richen up the primaries again ... I think I'll try the 0.068s (two steps up from the 0.066s I have in it now) and tie the secondaries closed like suggested. Get that lined out and then I'll be I need to go leaner on the secondary rods? When I did the 0.068s originally, the car ran pretty decent at part throttle but was drowning in fuel at WOT ...

 

Are you sure your O2 isn't gulping fresh air at idle? Where's it mounted?
You're going to find that getting that Qjet correct will take a looong time and lots of rod and jet changes, not to mention hangers.
Then change the fuel pressure and/or climate even just a little and it'll change your air/fuels as well.
Conversely look at the dyno sheet in my post "374 Stroker". Notice how flat the fuel curve is at wot. On top of that idle was around mid 14's by simply adjusting the four corner idle circuits. Part throttle was again around 14.0:1. Then we did the wot part, what the sheet shows was after just one jet and air bleed change, done. Those are the figures you should shoot for.
Best of luck in your project, you have your work cut out for you.

 

The O2 sensor is right at the end of the header collector (in the reducer). I'll double check it's tight and no leakage, but it should be OK.

As to the long time and effort ... unfortunately, you're on point with that! I've gotten to where I can do the whole "carb off, tear down, changes, reassemble, back on the car" in about 1/2 hour I've done one "stem to stern" rebuild and about a half a dozen jet changes so far. I've kept the hanger/secondary rods to try to reduce variables a bit ... I figure, get the primaries in line then go from there.

If this was fuel injection, I'd have this thing all lined out ... (I've been in ECU development for both the aftermarket and OEMs (Mercury Racing) for many years) so I get the principles and targets that we need to reach for the motor. I put fuel injection on the pickup for that very reason (it already had a square bore aftermarket intake on it and a swapped later model 454).

I wish I still had the engine dyno at Mercury (that was the bomb!) and all that instrumentation! ANYWAY, I'd like to keep the W31 more "original" with the Quadrajet though instead of a spreadbore to square bore adapter or aftermarket intake and all the finagling to keep the W24 OAI on top of a throttle body injection unit (although it can be done). I guess I have all winter to fiddle, and it's just one of the projects in the queue, so I'll stick with it!

Thanks to all!

 

Isn't Efi great!
I built Mercury Marine EFI'd IR big block chevys in the early 80's. The only "tuning" you could do was with a potentiameter on the back of the ecu. You could only change the fuel curve + or - 4% across all the board. That was it. The only other way was to burn a new prom. Amazing!!

 

I still have to do that with my GN which has the original C3 ECU ... you have to burn a PROM to adjust major fueling/spark parameters, although there's now a built in "back door" where you can adjust trim fuel/spark by turning the key on/off in a sequence and using the accelerator to change parameters (using a scan tool to monitor) ... called a "TurboTweak" chip. Pretty clever ...

I have the Sniper EFI on the pickup ... which we sketched out the basic idea on a napkin back in 2005 in a hotel bar! The idea was to use MotoTron (i.e. Mercury) ECU and self-learning technology. Couldn't get MSD (we were their skunk works at the time) interested because they were in turmoil with bankruptcy and our group got axed shortly after.

The ECUs and chipsets continue to improve over the years and now the self-learning stuff is everywhere (FI Tech, Holley, MSD, QFT, AEM, etc.). Everything from distributed CAN based multi-controllers (like MSD's Atomic series) to all in one air valve/ECU/EFI carb replacements (like the FI Tech and Sniper). Pretty neat and it'll be my fall back on the Olds if it comes to that I hope to get the Quadrajet dialed in though ...

 

Oh, and my current update ... took the carb off last night, tore it down did a jet change (went up a size to see what happens) and while inspecting it, saw the accelerator pump cup was torn and a piece was missing. *DOH*

Fished around and found the torn off lip piece jammed in the accelerator pump well. Got everything cleaned up and blown out and now have a handful of new ethanol tolerant pump cups on the way to replace. This was a new cup from the rebuild kit and I wonder if it was defective or if I it was my fault and I snagged it on the slot in the well and tore it or what. Makes me wonder if that is contributing factor to my tuning challenge ... anyway, looks like new parts should be here tomorrow/Sat. Went ahead and got cups, new pump (just in case) and some new gaskets.

 

Another update ... finally ended up with 0.070" primaries and starting to dial in the secondaries. According to the WB, I've got cruise and light throttle right at 14.6:1 most of the time. Sometimes a dip into 15.x:1 in throttle off transitions or sudden throttle on transitions. Doesn't seem to stumble or bog.

At WOT it lays down PIG rich (10.9:1 and maybe richer because the gauge bottoms out) and then cleans up a bit at extended WOT.

Based on that, I'm concentrating now on swapping secondary rods in to see if I can lean out WOT and hit the magic combo ...

A couple more data points:

• At idle with no load, RPM ~1000, warmed up and off choke it is 14.4-14.7:1
• In gear and idle, RPM~650-700, AFR is 15-16:1 no matter what I do with mixture screws
• Fiddling with min idle screw, fast idle cam, choke and idle mixture screws have got me to where it starts and idles with no load really nicely.
• It's lean in gear, but much better than it used to be and seems to drive pretty decent. Idle is a bit choppy, and I think a lot of it might be due to the cam.
• I've turned the choke a notch richer to help cold start and to keep idle up slightly until it's warm and that helps

 

How much is the secondary door opening? Cliff Ruggles book has the amount the door should open.

 

Love a good tuning thread. Nothing to add on the fuel side other than keep up the work and updates.

One suggestion... you mentioned the all-in timing "should" be _____. Are you checking it with a dial back light? I've seen plenty of funny stuff, so I always verify the all-in numbers. Also, don't be afraid to run more base timing (limit the mechanical some), and put it lighter springs to bring the timing in quicker.

Once you get into transition tuning, and you are messing with the air valve door, that vacuum pull off on the front passenger corner impacts the opening rate. Ruggles sells vacuum pull offs that don't have the elbow and can be drilled to open quicker, or you can use the plastic ones that open the fastest.

 

I'll have to look that up how to check ... looking for Ruggles book.

[QUOTE=83hurstguy;1051862]Love a good tuning thread. Nothing to add on the fuel side other than keep up the work and updates.

One suggestion... you mentioned the all-in timing "should" be _____. Are you checking it with a dial back light?

I was checking with an old Xenon light I have and marking the damper. I just did electronic points conversion (with the Ignitor III) and check it when I reinstalled the distributor. I also did the "light" spring kit from Mallory so the curve looks to be all in at around 3400 RPM from what I've seen with my old fixed light.

Yup, that's what I've always done with the HEI units ... and so I did it with this one too ... the brass bushing limits the mechanical and I set initial at 14 degrees advance at 850 RPM.

 

Also, I just swapped in some leaner secondary rods and we'll see what it does ... a front just blew in today and light snow and NASTY outside (it was 80 yesterday!) Should be nice this weekend though, so I'll see what the rods did and report.

 

just a reminder, olds used 2-bbl. jets in the 255s.

bill