Technical Backfire/Spitting through carb - troubleshoot?

OK, so having a bear trying to get the SBC running smooth. Thought I had it, but alas seems to change its mind if I look at it funny. Now getting a backfire through the carb (seems to always be the one over the 5/7 cylinders) on acceleration.

SBC 283 with 4 Holley 94. All recently rebuilt. 1 turn out on all 4 seems to give the best response.
Timing was set to 10 initial. Vacuum advance pulls another 15 or so, on the manifold. Mechanical is working and goes all in by 3000 rpm. Richening the mixtures (1.5 turns all carbs) made the car run like crap, so not sure if would have helped the backfire. Checked the accelerator squirt in the offending carb and seems to be doing its job.

Double checked the wires, all in the right spot. Made sure they were all separated.

Starts and idles OK.

Not sure where to head next other than an intake with a single 4 barrel

What would you guys do next?

 

"Timing was set to 10 initial. Vacuum advance pulls another 15 or so, on the manifold. Mechanical is working and goes all in by 3000 rpm."

All in...what? 10° initial, +15° vacuum advance (intake...not ported?)... what's the total?

Sent from my Bell Candlestick

 

Starts at the basics square 1.
Build up from there.

Doing this I always start with a compression test.
Piston stop timing marks verification.
Plug wire routing and firing order.
Funny that 5/7 is most common mix up too.

If you’re getting fire out the carb that generally means something is sparking with an intake valve open,,,

 

What intake manifold is it?

 

Assuming the ignition is correct, and the engine is mechanically sound, I would say that that particular carb is way lean.
Popping through the intake is a lean condition.
While the engine is running, pass an un-lit propane torch over that carb. If the rpm picks up, its lean.

 

I don't know if this will cure the backfiring, but just setting the idle mixture screws all the same at 1 or 1.5 is not how you do it. Each idle mixture screw on each carburetor has to be fine tuned individually. Use either a tune up tach that will show precise rpms at low speeds, or a vacuum gauge, and adjust each screw, one at a time, to achieve maximum idle speed or vacuum reading. Then turn the screw back in slightly to achieve about a 20 rpm drop, or a slight drop in vacuum. Then readjust the idle speed set screw and move to the next screw/carb. Do this one screw at a time until you've done them all, then start over & do it again. This is quality time spent with your engine, better than polishing valve covers.

 

It is an Offy

 

I figured it probably was.

Tell us about when it backfires, what are the driving conditions? you said it's under acceleration...but at what RPM range? what throttle opening range? does it always do it, or only under certain conditions?

Also, as mentioned, what is the total timing advance? you told us initial, vacuum, and that it's all in by 3000, but we dont know what the total mechanical advance is.

 

Timing was verified with piston stop tool. Marks were accurate, halfway between the stops.
Compression was good in 7 of 8 cylinders. One was a bit lower, but not much. Got better with oil added to the cylinder. This was the #3
I checked and rechecked the firing order around the cap, but will check again now that my frustration has worn off from the weekend..

 

Will have to check my notes, but if I remember it was adding 12-14.
Engine was warmed up. Have only been taking it around the block as its all in testing phase. Will do it accelerating when at cruise. Less so from a stop but still occurs. No tach in the car so would be guessing at the RPM at cruise. Tested the mechanical with the RPM gauge on the timing light.

Have not really tested all the condition because I didn't want to blow something out before I had a better handle. Majority of my wrenching to date is from book learning, so figured I would stop for some real world input before I hurt something..

 

do you have a dial or digital readout timing light, so you can see what the total timing is? If you are getting 12 degrees in the distributor, that's 24 at the crank, added to 10 initial, would put it at 34 degrees total (without vacuum), which is about right. but checking it with a light would be a place to start.

 

I get a kick out of people : "compression was good"....
Well, how good was it ?
" one was lower but not much"
Well how much is not much?
" 12-14° "
Crank degrees or can degrees ?
Seems that getting accurate , usable , specific information is REALLY difficult ... .,

 

its a Digital light, and has the ability to set (not sure what this is called) the timing reading (i.e. input the advance on the gun to 34 for example, and it will bring the timing mark back to 0 on the balancer to confirm the reading).

 

Sure, thanks for asking. I did note that I am new to all this
Compression was in the 130s in the 7. (all well within 10% which I understand is important). the other was 110s
the 12-14 degrees was measured with a timing light using the marks on the balancer.

Any thoughts?

 

If your initial timing is 10° and the mechanical is only adding 10-12,° then your timing is severely retarded at 20-22° .You should have a total of 34-38° total (initial plus mechanical) . Vacuum advance is for cruising , don't worry about it .......

 

Advance is attached to the intake manifold. The two rear carbs on the motor have vacuum ports (These are ported vacuum? Currently have plugs in them) Tried using one of them for a vacuum source, but there was no impact on performance. I then checked them with a vacuum gauge and they they barely pulled any vacuum at all, not enough to move the vacuum advance. I checked the Vacuum advance function with a mighty vac and it needed more vacuum than the carb was pulling to give its full advance)

 

disconnect the vacuum advance, leave it disconnected and plugged for now....

Set the timing so the total at 3000 RPM or so is 36 degrees, and then tell us what the initial timing is.

And take it for a drive, see how it does.

 

After verifying timing - advance - firing order, that sort of thing, check the distributor cap for carbon tracks (allowing wrong cyl. to fire at wrong time). It's also possible you have a loose or broken rocker arm or a worn camshaft lobe causing you problems.

 

Points adjustment OK?

 

Gotcha.

 

Will do

 

measured with a feeler, seem to be right based on the what the book stated. Have not done a dwell measurement.

 

Ported and Venturi vacuum operate differently.
Holley 94s normal port is Venturi.
The power valve will blow out whenever you backfire.

 

Thoughts -
That cranking compression sucks at 130
110 is not good .
Did you say this was a recent rebuild on the 283?

 

Valve adjustment OK?

 

X2

 

Check for full motion of valvetrain.

 

If this was ever rebuilt I am not aware

 

Cant speak to it other than I dont hear any noticeable clacking.

 

Was thinking tight side.