Bogging and Backfiring

[Tonner]

I'm trying to get this thing running right. It idles smoothly, but as soon as I give it gas, it bogs and backfires or dies. I have a freshly rebuilt 350, rebuilt stock Quadrajet, and new HEI distributor. I have the timing at 8 degrees, I can see the accelerator pump squirt, I checked the vacuum lines and they are all clear, Fuel is flowing from the pump, I triple checked the plug wires. Any other ideas? Thanks.

[Mbeef61]

I would adjust timing. I run mine closer to 13. Check for vacuum leaks.

[Mbeef61]

Also are you timing without vacuum advance ?

[68 P.O.S.]

Correct, timing needs to be bumped up with vac advance disconnected. The new HEI needs to be curved as well, then set total timing to 36 degrees.

[paul mora]

If you took the distributor out be sure you have in there right

[garyd1961]

The distributor may be 1 tooth off

[Livingstonm]

Try timing I run mine closer to 12.. if I'm not mistaken being a tooth of on the dist. Wouldn't make a difference just makes your degree range change so as long as you can hit 8-12 degrees your close enough on the distributor

[AussieinNC]

This has all the signs of a Quadrajet that's not setup correctly...any chance you have another carb you could swap out....?

I have seen quadrajets with warped flanges that exhibit similar issues to yours...

Also...check the vacuum advance feed is from a ported vacuum source, meaning no vacuum at idle....

Timing should be 12 BTDC...and all in at 32 by 3000...

You mentioned a new HEI dist...why was the old one replaced?
Did you replace the resistor wire that feeds voltage to the HEI dist?
Coils based systems need 8 volts and use a resistor wire to drop the voltage...HEI needs 12 volts....

[dmjlambert]

That big hose in the carburetor front is supposed to go to a PCV valve on the valve cover, instead of to brake booster. For the brake booster there should be an inlet in the intake manifold behind the carburetor. I wonder if this is part of the problem.

This is what worked for me:
With vacuum connected to distributor, turn distributor counterclockwise to advance timing until it runs smooth with throttle open some, perhaps 1500 RPM or so. Advance timing until you have slight pinging and then turn it back clockwise to retard timing so it no longer pings. Adjust idle speed screw if needed to achieve a reasonable idle speed, 600-700 RPM. Take for test drive. Further retard the timing if you detect any pinging during test drive.

Then, test drive truck with distributor vacuum connected to ported vacuum (connection in carburetor front) and with distributor vacuum connected to manifold vacuum (connection on carburetor side like you have it now). You will need to adjust the idle speed screw depending on which type of vacuum you have it connected to. Find out what works best. If both ported vacuum and manifold vacuum are equally good, as it is in my truck, it is best to connect distributor to manifold vacuum so you get more advance and more fully burn the gas at idle.

[Easttennceeten]

Where is your PCV valve? It looks like you have a fill cap where your PCV should be. The breathers are enough to vent at idle, but when you start giving it throttle without a PCV hooked to ported vacuum your engine is building crankcase pressure and bogging and blowing back through the carb.

[rpmerf]

http://www.capitalcorvetteclub.ca/at...ng%20Paper.pdf

75% of all carburetor problems are ignition problems. Make sure that is 100% first. It's possible it is strong enough to produce spark at idle, but too weak to produce spark under load. I've seen numerous threads about harmonic balancer issues causing the timing to be set incorrectly. 8* should be fine for it to run. 12-16 would be better, but it should not cause issues running at 8*.

Check for vacuum leaks. Remove and plug anything that is not necessary to run and see how it does. Check your idle vacuum.

[BigBird05]

Good advice rpmerf, I just replaced EVERY vac hose on my 1990 Chevy van. Seems that ethenal fuel vapors made them all disintegrate.

[prodjay10]

8 degrees is for leaded fuel. You should be running around 16 degrees at idle and total 30 degrees mechanical or vacuum advance. Basically as much advance as possible without pinging. Also check your tank for rust. If it's the original tank, more than likely it's dirty and clogging your carb.

I dealt with exactly these issues for some time even after cleaning the tank at home. It wasn't after everything cleared out the tank that it ran right. Now I just replace the tank.

A clogged carb pours too much fuel down the cylinders. Since it doesn't burn it winds up in your oil and causes extreme engine wear.

[2 Stovebolts]

Had this with a Rochester 'B' on a 250...Be sure the internal passages in the carb are spotless...Lots of carb cleaner & compressed air...a tiny spec of crud will do it......

[Livingstonm]

Easiest fix is bail the quadrajet throw a Edelbrock on it

[68 P.O.S.]

First off, you always start with timing. This is your trucks baseline and if it’s not correct, you’ll be chasing your tail. Read these following links. They were put together by a GM engineer who helped develop the systems. Its probably more information than you wanted to know, but it is explained very well.

https://67-72chevytrucks.com/vboard/...d.php?t=274806
http://www.camaros.org/pdf/timing101.pdf
http://www.stl-vettes.com/65Vette/co...ng_Advance.pdf

Key takeaways:
-SBCs run best advanced at 36 degrees total timing (32 degrees with Vortec heads)
-You want 20-25 degrees mechanical advance (most aftermarket HEIs provide 22 degrees)
-You need mechanical advance springs in the HEI that allow full advance of 36 degrees between 2500-3000 rpm
-Vacuum advance must be connected to manifold vacuum, not ported or timed, to operate correctly
-Vacuum advance must be restricted to no more than 16 degrees

Which HEI are you running and do you still have the directions that give the specs? Did you replace the power wire with one compatible with HEI? How did you install the HEI? What is your manifold vacuum level at idle? Was the carb rebuilt only, or was it tuned some as well, and how long ago? How is your fuel system?

[2 Stovebolts]

Aussie...Elaborate a little more on the 8/12 volts on the resistor wire? Where does this wire run? from what point to what point....My 67 250 has a GM HEI dist. which was installed by the previous owner, unknown reason, not sure if he replaced the wire you mention....Thanks

[68 P.O.S.]

Here's an excellent thread by dmjlambert that shows everything you need to know about the HEI wire.

http://67-72chevytrucks.com/vboard/s...d.php?t=708975

[AussieinNC]

dmj"s post has all the details and some good pics....great job dmj!

[2 Stovebolts]

Oh...What a wonderful tutorial.....and to get a harness from 'Year one' ...This looks like a heavy gauge wire - What gauge should it be?....Excellent...Thanks for passing this along......

[dmjlambert]

20 gauge is big enough, but I believe the HEI wire I installed is 12 gauge. Overkill. The distributor only draws about 2 amps.