My question in a nutshell- My Quadrajet had #77 jets stock, car has modern high-rise intake, good street heads, headers & free-flowing exhaust, etc. and is running lean under all conditions.

Jet sizes appear to top out at size 79, which I'm now using, still lean. With the biggest jets available already in this carb, it's still lean. I don't think I can physically make the WOT metering area any larger.

It's so lean that I'm getting audible detonation (at least I think that's what I'm hearing) at anything above cruise, and temperatures creep up if I retard timing to avoid detonation- and it doesn't seem to reduce the detonation anyway. I'm failing to think of words to describe the sound, except I remember it when old 70's cars would overheat it was common to hear, and it sounds a bit like run-on used to sound. Kind of like a metallic, more pronounced version of what frying bacon sounds like I guess.

Would it help to switch to another Quadrajet model that had smaller primary jets stock so that I have a little more range to fatten it up? It doesn't make logical sense since the metering area at WOT would be smaller, but I may be missing something- like a richer idle circuit might exist in some models or something, or other factors completely unknown to me.

I know so little about tuning that I'm flat out dangerous with this wideband O2 sensor. It seems impossible to get anywhere near 12-13:1 at WOT no matter what I do, so I can't even begin to tune part throttle yet.

HELP!!!!

Details:
Idle AFR: 14 (idle screws so far out that half-turns no longer register any additional flow)
Cruise AFR: 15-17
Part (light) throttle accel: 14:1
WOT AFR: 13.5-14:1
Idle quality- moderate, idles in gear at 600 RPM warm easily

Car combo:
409 SBC, 10.56:1 CR, .039" quench (zero decked block),
Quadrajet 17057204, stock primary & secondary rod sizes
Edelbrock Performer Airgap (non-RPM) intake
Edelbrock Performer RPM heads
Camshaft: Hyd. FT .471/.480 lift, 226/228 @.050, 110 LCA, 4 deg advanced, degreed.
Summit/Dynomax 1-5/8" headers
2-1/2" mandrel bent exhaust, TIG welded stainless with X pipe, made from Magnaflow kit, using Magnaflow 2-1/2" mufflers
New aftermarket HEI ignition, one gold & one silver curve springs, tried

Tuning parameters:
I started with the timing range 34-38 deg total mech. timing
16" vacuum at idle in park

 

metering rods and power valve springs are used to
set mixture along with jets.

 

There is no power valve on a Quadrajet, but there are springs to tune the piston, hangers to tune the secondary rods, and of course, rods on the primaries and secondaries.

However, except for the secondary hangers, none of these come into play under WOT, do they?

 

the spring under the main metering rods IS the power valve.
springs are color coded for in of vacuum just like a holley.
main metering rods have different tapers to enrichen the mixture.

 

Your power piston spring may be too weak. You should have a spring rated at about 14.5".

Also, remove the secondary rod hanger (and rods) and see if it goes rich, or stays lean. If it goes rich, you just need to get a richer set of secondary rods (this one is easy). If lean, you may have to have the idle circuit modified.

Try these and let me know how it goes. There are also other factors that could be causing the problems. I've seen poor commercial rebuilds where fuel passages were partially blocked by cleaning media! The bottom line is, you don't need another carb, the one you have can be made to work.

God bless, Sensei

 

OK, I'm sorry I just am learning about this carburetor. It's the first Quadrajet I've ever owned, and after years of working with AFB's I am trying to work with a better designed carburetor and there's a bit of a learning curve.

I've read through Lars' papers and absorbed a lot, probably missed much more.

As I understood it, possibly in error, the tips of all primary rods are the same diameter, and this is the portion of the rod that is positioned in the jet at WOT (regardless of which spring is in the carb- I think). Also, and I could be wrong, the secondary rods all have the same size tip, only the taper is different from one to the next.

If I understand this right, first you tune idle (for maximum vacuum), then WOT with the secondaries locked out because the only variable in play affecting WOT is primary jet size. That's the step I'm on.

I'm getting detonation at WOT (and of course at part throttle too).

Again, I might have this all wrong, so I'm just laying out what I know so I can be corrected. Please don't be offended, I am not saying you're wrong- just that I'm obviously confused!

 

Yes, primary rod tips are all the same size, but the spring being too weak can keep the fatter part of the rod in the jet causing leanness.

This is incorrect, secondary rod tips vary in size greatly

 

Hey, this carburetor was commercially rebuilt- so problems like this ARE a distinct possibility. I bought the carburetor with very few miles on it since purchased rebuilt, and I did note that the rods that came in it were not correct. This was last October. I put little effort into tuning back then because this woke it up, and I knew that after the winter hibernation the nice new exhaust system (described above) would be fabricated. I just finished all that and surprise! It needs more fuel...

Anyway, that's definitely possible. It could be clogged/goofed up/etc from either the original carb owner (before the rebuild) or by the rebuilder.

I drove the car today from my home in central Illinois to Kansas City (with my son). I drove it around town for two weeks with few issues, but during this trip today it seemed to get worse as the engine was thoroughly heat soaked. I don't know if it's temp related or what, but the hour long cruise the night before last didn't reveal the severity I saw today.

 

 

I'll call around first thing in the morning to get another tuning kit. I'd probably be smart to just keep one in the car!

 

The power tips on all rods are not the same. One rod/jet chart notes:
"A" Rods are single taper, "B" (and all others) are double taper.
All rods, except those noted below, have a .026" diameter tip.

"M" =.032" diameter tip

"50D" has a .036" diameter tip


I recall the "M" rods were for 1975 & up truck carbs but don't remember about the "D" rods. A 1705xxxx corvette q-jet should use .26 power tip for the most fuel delivery at WOT . The "M" rods would run too lean. I'm sure there was a discussion on this board concerning a method for "turning" the fat power tips down by hand. The "M" rods seem much easier to find.Good luck.

 

This deserves a bit more scrutiny - this is your INITIAL timing (at idle, vacuum unplugged)?

 

Check your secondary hanger letter & rods (2 letters). Probably you need smaller dia. rods.

 

Good catch- I typed that in a big hurry, horribly worded.

That was my starting point for tuning- not at idle. This was at 3000 RPM (mechanical timing "all in") with vacuum unplugged.

 

It's the K series primary rods that you need, and they're not manufactured anymore except in a handful of sizes by JET. I'm running M rods right now because I had no choice. They're fine for idle and part throttle, I just go lean at WOT so at some point I was going to try to turn them down the tips to .026.

 

Ah All good, just wanted to make sure the timing was square first. Smart Q-Jet people giving guidance, I'm

 

I seem to be missing something fundamental here. When thinking only about rods that are compatible with my carburetor, don't all primary rods have .026" diameter tips?

Certainly, the piston spring might be too weak to be pulling my piston up (and the rod taper out of the jet) so I'm shopping for a tuning kit this AM.

Sure, there are incompatible rods out there with different primary tip sizes (mostly larger?), but are there primary rods with smaller tips? I'm hopeful there are.

 

You might find the parts you need here http://thecarburetorshop.com PG.

 

All K series rods have .026 tips. But as I mentioned, new K rods these days are only made by JET in a few sizes, and I was unable to find the size I needed. So your choices are find someone with an old stock of K rods or get an M rod and turn it down. Or be like me and just live with not going WOT with your M rods because you know eventually you'll put in FI.

 

I am definitely not a carb expert, but I have a couple thoughts. If your detonating at part throttle, then wouldnt the lean issue be caused by the primary side since the secondaries are still closed? I was having a problem that was somewhat similiar after my rebuild using cliffs high performace kit. I called him and he told me to check out the primary rod hanger, that he sees many of the arms bent down. It doesnt allow the rods to come up out the jets enough. I marked the rod tips with a sharpie and bent the hanger arms accordingly to get the right height when the piston is all the way up.

Hopefully someone will check what I said, it may at least help you narrow down the problem.

Sean