Heya,
My new old quadrajet backfired through the carb and sputtered when getting on the pedal. Going gently it doesn't backfire or sputter. In general it feels sluggish. The crate engine has about 1000 miles on it. Manifold vac for advance.
The local 'specialists' are of no help, so I'm diving in myself and learn as I go along. Its's a slow process.

Before I start to blame the quadrajet (which I love, especially the sound) I bought a advance timing light this weekend and started to check timing. It was set at 10' BTDC (vac advance disconnected) which is correct according the installation pages. The engine was pulling 11-12 vacuum which I thought is too low for this cam (350ho 330hp, mild cam 212/222 ). I then advanced the timing and the vacuum pulled rose to 22-23*. Idle got better but the engine is still shaky.

To get this highest vac reading I had the initial timing at 26' !
So I think I have a issue somewhere.... For now I assume the balancer hasn't slipped seen the low milage

I did keep this 26' BTDC for the drive home. It drove much better, no more backfire through carb but still a ever so slight hesitation. Before fiddling with the idle mixture screws I would like to understand my timing.
The distributor has been out twice.

Is it correct to say it might be installed wrong and is a couple of teeth off, or could this also be a issue with the mechanical advance (sticking) or perhaps the HEI module?

Just preparing my action plan for next weekend

Thanks in advance,
Nick

 

Hi,

I received these this past weekend on another members recommendations. I will attach two files. Hope these help. I'm in the process of having a new motor installed myself so I look forward to hopefully having everything working properly. Good luck

 

Nick-

There is no such thing as 'a couple of teeth off'- this concept was created by somebody that doesn't understand timing and basic engine mechanics. If you can rotate the distributor housing enough to achieve the desired timing, the distributor is installed correctly for that purpose.

Setting timing to achieve maximum vacuum has a lot of drawbacks. Put it back to the previous setting for now or adjust it in accordance with Lar's paper on the subject.

If the car is falling flat or backfiring under sudden application of full throttle, it might be the air door on the secondary venturis.

 

My first thought is check TDC on your harmonic balancer. If you used your old balancer on the rebuilt motor, they're known to slip the outer ring throwing the timing mark off!
Second thought, just start turning the distributor counter clockwise and test drive after every move. If it starts pinging, back off

 

If the timing is set correctly for best combustion efficiency the vacuum will be at its highest at that throttle opening. What would be all those drawbacks?

 

The OP has set his initial timing at 26*, at idle presumably. We really don't know what the mechanical advance curve is but let's ***-u-me it gives at least 15* by 3000ish RPM. That'll give him a total of 41* at highway speeds when detonation might/might not be heard.

 

This 26* is with vac can connected or not?. If it is with vac can disconnected and line plugged then you are too far advanced.

With the vac can connected that number is likely not a problem.

What you need to figure out is if the vac can is fully advancing at your idle vacuum.
This means that if you are pulling 16" of vacuum at 800 rpm does 16" fully retract (advance) the arm on your vacuum can? If it does your in good shape.
Best if it is fully retracted by 14" in that case, just a little margin for lower vacuum situations.
If not then you need to get a vac can that will be fully retracted at your lowest idle rpm vacuum level.

So if in gear with A/C on it idles at 600 rpm then that is the vacuum level your vac can will need to be fully advanced by, with a 1" or 2" margin.
ie. Vacuum at 600 rpm = 14". Vacuum can needed is fully retracted by 12" of vacuum.

Get that ironed out. Then start on the carb issues that you have.

 

Thank you for the answers so far guys.
According the install manual, 10' at idle will give 32' on WOT so i assume I have 22' mechanical advance.
The 26' was with the can disconnected (unfortunately). I do agree that it would be nearly impossible to hear denotation at higher speeds.
Although it solved my backfire and hesitation it seems I'm going about this the wrong way by advancing that much.
Maybe I described it wrong (a few teeth off). If I installed my HEI, let's say pointing to nr 1, approx. oriented as it should be (pointing to cylinder 1), but nr1 isn't really at true TDC. If I then set my advance to 10' and turn the distributor so the balancer mark lines up with the 0 on the tab but the crankshaft still needs to turn x' before the piston reaches TDC. Couldn't that explain why I'm currently advancing more then I should for it to run decent? I'm pretty sure the balancer is ok (it's nearly new) but I'm far from sure about the distributor install.
I think I'll check that first because if this is not correct nothing will be right? If ok or corrected I'll get into Lars's how to,set timing paper
Sry for the stupid questions, I truly appreciate the help! Still got a long way to go before I tackle the vac adv can
Nick

 

make sure your accelerator pump is working, i had the lip roll up on one i had and it caused that issue, when i would mash the gas it would backfire, then haul A$$

 

Check, I get two solid squirts, the arm is on the inner hole! I had the air horn off to solve a flooding issue i bought the quadrajet from a reputable builder on the Buick forums as Lars doesn't have cores. Everything in there is new but I guess the needle seat wasn't thoroughly cleaned. Air horn is tightened to specs as well as the carb to manifold so I assume (lots of them assumes in this thread the backfire/sputter isn't caused by a air horn leak. Float etc was set acc. to info I got from Lars
Nick

 

Not knowing how the Q-jet has been modified you may want to try the outer hole on your accelerator pump. This is where I run mine.

 

You're missing the point. The OP initially set his timing per the "installation page". He said it was sluggish. He then advanced the timing and found that the vacuum improved. He proved with this simple and scientifically correct test that the engine wanted 26* BTDC at idle for best/most efficient combustion, not 10* BTDC. He now has information to determine his low speed/low load timing requirements, and if he goes with the usual 32-36* BTDC timing at high RPM WOT he then has the timing info for both ends of the speed/load range. With this he now has a good handle on what timing curve (the mechanical and vacuum combination) to run in his distributor. If he disregards what the vacuum readings are telling him he's stuck with a sluggish, fuel inefficient engine.

The point: There's no drawback to knowing what the engine actually wants.

 

Backfiring through the carb is some sort of timing issue or perhaps a valve spring issue.

Some thoughts (I assume you've double-checked your firing order):

1) There were two different timing tab locations for Gen-1 SBC's. If the tab is mismatched to the balancer, all bets are off when attempting to set the timing.

2) Forget initial timing. With the vacuum advance disconnected, set her to 36 degrees at 3000 RPM. If your balancer has no markings for 36 degress, just remember a circle is 360 degrees, so 1/10 of the circumference is 36 degrees. Use a tape measure used for sewing then mark 36 degrees on the balancer.

Or use Pi,..(3.1416 x diameter of the balancer = circumference x 10% from the TDC mark on the balancer).

Then let the initial fall where it falls (probably somewhere between 14 to 18 degrees depending on how much mechanical advance is at work.

3) If you're still have backfiring through the carb, make sure you have a good distributor cap. A cracked cap, or a cap full of carbon or moisture will have plugs firing at the wrong time.

4) Remove the plug boots, one at a time, apply some dielectric grease to the insulators then carefully reinstall each boot making sure each 'clicks' into place. This won't do much for the backfiring but it could smooth out the missing.

Try all that then report back.