Have a '68 with a 383, XE274 cam, auto trans and Q-jet. I have been playing with the carb for a long time and think I almost have it down but I have one more problem to solve.

While trying to accelerate lightly/moderately from say, 45-60 (or any acceleration like this regardless of speed), the engine bucks a little and detonates. I hooked up my wideband O2 and during this time the a/f ratio goes up to 17-18.

During these runs, the power system has not kicked in. My power piston spring kicks in at about 10" vac and during this surging, I have about 11-12" vac. I would go with a heavier spring but I only have about 11" vac at idle.

If I run larger jets, that helps the prob some but creates a really rich condition during normal cruise (like 11:1). I am trying to tune for driveability, power and some fuel economy. I have already set the APT, by the way.

How can I get more fuel during this time? Does enlarging the idle tubes affect this?

What does your vacuum advance curve look like?
What is the normal cruise a/f ratio?
What are the 2 jet sizes that cause you to go from lean to rich?

what is the current main jet size?
hows the accelerator pump shot?
The enlarged Buick needle/seat helps too.
It does look like a possible timing prob though..
What is your base and total timing and by what RPM?

Once you work through the timing issue... can you find primary rods with a much sharper taper?

As has been suggested vacuum advance. It sounds like a stock HEI vacuum advance (if you are using one)or one that is simular. The engine simply will not tolerate that much advance under any load at all.

I am using a ZZ4 HEI, new from GM. It is several years old and I have never messed with an adjustable vac advance. Base timing is 10 degrees.

"What does your vacuum advance curve look like?"

I do not know, how do I tell?

"What is the normal cruise a/f ratio?"

With what I think are the correct jets, normal cruise a/f ratio is low 14s to high 14s depending on the speed. If I go up to 76 jets, the cruise ratio drops to high 11s to mid 12s.

Total timing is I guess around 34 but I have not checked it in a while. The curve is all in by about 3,000 RPM. The problem I experience (surge/detonation) occurs even at low RPMs (as low as about 1,600 or so in OD).


"can you find primary rods with a much sharper taper?"

I am using a 39B rod but this prob occurs before the rods lift out of the just at all. The power piston spring is about 10" vac. If I had more vac at idle, this would not be an issue b/c I could go with a stiffer spring.

"It sounds like a stock HEI vacuum advance (if you are using one)or one that is simular. The engine simply will not tolerate that much advance under any load at all."

This engine has somewhat low compression (for aluminum heads anyway) at 9.7:1. I did not know that the stock HEI might put too much vac advance in.

Still seems like I need more fuel since the a/f ratio goes way high (lean) then it surges and detonates.

Thanks for all of the suggestions. I may try an adjustable vac advance.

you said you adjusted the APT, how so?

the APT just sets how far the power piston will go down in its bore.

What is your vacuum at cruise? most often it is higher than at idle (mine pulls 12" @ idle and 17" at 2000 rpm cruise), then you could use a stronger spring and it will not effect(affect?) idle.

or you could play with different metering rods and APT settings.

just trying to help. good luck

you said you adjusted the APT, how so?

I found instructions and a diagram on the net. Found several that were the same so I think I am okay. Basically, I made sure that the tip of the rod was completely down in the jet and the tapered portion as well. I did not have the surge problem before I adjusted APT but had all sorts of other richness problems (I would have to look back at my notes but this seemed to help a few things and create another problem).

Vac at cruise is higher, maybe 17 or so. Problem is if I go with a spring that kicks in the power piston at say, 12", then at idle, my vac is less than that so the rods will be kicked up and thus idle will be real rich.

so why dont you put the APT back where it was originally and put a set of leaner rods to take care of the richness.

The metering rods shouldnt affect the idle if the throttle blade/transfer circuit relationship is set correctly. The metering rods should only affect the main metering circuit, which comes in soon on a qjet because of the small primaries and double booster venturi. Your main metering circuit could be activated at idle because you might have the throttle blades open enough for the verturi to get signal

your transfer circuit slot should only have about .050" exposed below the throttle blade. if more than this the transfer circuit is working at idle and possibly you main circuit.

your transfer circuit slot should only have about .050" exposed below the throttle blade. if more than this the transfer circuit is working at idle and possibly you main circuit.

It is about .040 exposed, so that should be fine.

I will have to check my notes but I think setting the APT correctly fixed another problem I had.

Originally posted by 68Malibu:

"What does your vacuum advance curve look like?"

I do not know, how do I tell?

Put a Mighty-vac on the can and measure how many degrees advance you get at increasing levels of vacuum. The ZZ4 distributor is calibrated for an engine with cam timing of 208°/221° @ 0.05" lift. Your cam has quite a bit more duration and won't run as well at the low end. Do you have the VA hooked to manifold vacuum?

"What is the normal cruise a/f ratio?"

With what I think are the correct jets, normal cruise a/f ratio is low 14s to high 14s depending on the speed. If I go up to 76 jets, the cruise ratio drops to high 11s to mid 12s.

What do you think are the correct jets?

Total timing is I guess around 34 but I have not checked it in a while. The curve is all in by about 3,000 RPM. The problem I experience (surge/detonation) occurs even at low RPMs (as low as about 1,600 or so in OD).

See the comments on the distributor above. If I had a detonation or knocking problem, I'd sure as hell know what the timing is.

"can you find primary rods with a much sharper taper?"

I am using a 39B rod but this prob occurs before the rods lift out of the just at all. The power piston spring is about 10" vac. If I had more vac at idle, this would not be an issue b/c I could go with a stiffer spring.

With a 39 rod you ought to be using a jet in the 70 range. All the rods have the same tip size so you select your jets for the power mixture and your rods for the cruise mixture. You may get more vacuum at idle with some more initial timing.

"It sounds like a stock HEI vacuum advance (if you are using one)or one that is simular. The engine simply will not tolerate that much advance under any load at all."

This engine has somewhat low compression (for aluminum heads anyway) at 9.7:1. I did not know that the stock HEI might put too much vac advance in.

With aluminum heads, 9.7:1 compression and a cam with 230° of intake duration you should have no detonation problems due to ZZ4 timing.

Still seems like I need more fuel since the a/f ratio goes way high (lean) then it surges and detonates.

Thanks for all of the suggestions. I may try an adjustable vac advance.

I am using a 74 jet but have gone up to the 76 jet which helps, but does not eliminate the problem. This should be enough fuel for this engine.

With a 39 rod you ought to be using a jet in the 70 range. All the rods have the same tip size so you select your jets for the power mixture and your rods for the cruise mixture. You may get more vacuum at idle with some more initial timing.

The vac advance it hooked up to full manifold vac. This allows a greater idle speed without cranking open the throttle screw.

I do not have a mity vac so I will have to borrow one and see what is going on there.

The deal with not knowing the full timing is not too bid of a deal (I don't think) since this happens at low RPMs as well. If it only happened at say, 2,800+, I would think maybe the mechanical advance was applying too much.

Who knows, maybe it is but considering my a/f ratio goes way lean with just a little more throttle input, it seems like I need more fuel during this mid load range.

Have you read this:

http://www.geocities.com/gtopercy/Pictures/quadrajetTech.html

The best info I have seen specifically for jetting, rejetting, and tuning Qjets is here:

http://www.corvettefaq.com/redir.asp?site=215


It's written by Lars at the Corvette forum, and he deserves all the credit. GREAT stuff.

Thank you. I will check both sites. I have seen some of this before but maybe I have missed something.

How did you adjust the APT?

This is the APT procedure I used...

http://www.corvetteclub.org.uk/files/downloads/adjusting_qjet_power_piston.pdf

If this is wrong, let me know.

Thought I would update everyone. I emailed Lars (mentioned above) and he told me the carb I was using was a 1967 327 unit and that they typically have the problem I described.

So, I installed a spare 1970 unit from a 396 and the problem is gone. I have not taken the time to jet or tune yet but the lean hole and detonation are gone. I figured all along it was a fuel delivery problem and it was. Thanks for the input folks!!!