Originally Posted by t_l75
Through the intake. And a lot. Most of the time it spits a 3 foot tall fireball back up through the carb. Im using the rochester quadrajet and original HEI ignition. Carb just rebuilt. Im thinking its a valve thats off or sticking. Is this common to be happening on a brand new engine? The engine was running perfectly fine and then this randomly started happening. Could it have something to do with fuel? the engine was runnning fine then it sat for maybe 2 months without being turned over. Then this started happening.
Well there always seems to be a bug or two to work out of a new engine. The first of this thread you indicated you were trying to get a new engine running. I get this confused with the statement above that indicates it was running fine.........
I assume (correct me if I'm wrong) it was running fine before the rebuild. New engine in there now and not running or running for a few turns and backfiring out.
Before you disassemble the engine, some tests first may save you some work. Do a compression check, if you have a valve that is not closing or a flat cam lobe (as suggested earlier) it will show up here that will narrow down where you need to be checking too. Hydraulic or solid tappet cam? If all have roughly the same readings, it likely isn't a valve.
Any engine only needs 3 things to run, fuel, ignition and timing. You indicated spark plugs were wet and it's back firing, so your getting fuel. Ignition is also present as represented by the 3' plume of fire out the carb when it does back fire.
Soooooo, that leaves timing, engines have more than one type of timing. Cam timing, the cam turns at half the rate of the crank. Cam does one complete revolution for two revolutions of the crank. The timing marks on the cam sprockets need to be lined up in the number one firing position. This has been mentioned as a possible source already.
Valve timing is also a factor. It has to do with when the valves start to open/close in relation to crankshaft position. This is usually adjustable through different keys on the crank sproket. The above are mechanical settings, they are set static by the builder according to cam specifications and general assembly procedures. Once an engine is assembled these are generally not adjustable.
Ignition timing is by far the most complex of the timing family.
Ignition timing must work together with the engine timing the delivery of spark to each cylinder to make an engine run as expected.
with others here, this is where you will likely find the backfire problem.
First bring the engine up on number one firing position, you have to be sure it is at the top of the compression stroke. The way to do that...........pull the plugs. Install a compression gauge in the number one cylinder. Rotate the engine slowly until you start to see compression on the gauge. Keep slowly rotating the engine until the timing mark is at the 0 position.
Pull the cap on the distributor and look at where the rotor is pointing. It should be pointed at the #1 spark plug wire. If necessary loosen the distributor and rotate one way or the other to align the rotor position with the #1 wire connection.
Firing order on SBC. * 18436572 * Install the wires sequentially around the distributor in the direction of rotation following the firing order. This should be close enough to fire the engine.
Ignition advance........ another subject of interest. Has to do with advancing the spark signal as engine rpm's increase to keep the spark happening at the top of the compression stroke. Without the advance the timing would retard as the engine turns faster rpm.
There are a couple of types of advance. Vacuum advance and mechanical advance are the two most common. Computers also have advance curve programs for ECU controlled ignitions.
I believe the HEI has both, mechanical and vacuum. There are books on how to dial these in. I'm not going there, but do a simple check to make sure the mechanical advance is not sticking and the vacuum advance pod isn't leaking.
The vacuum connection on the distributor should be connected to a hose connection on the carburetor. It needs to be plugged into venturi vacuum. Not usually at the base of the carburetor. Consult the service manual for the correct location to plug this in. I cannot remember which one it is on the Rochester. If this is plugged into manifold vacuum it is in the wrong place and it will advance your timing at idle. This is a potential source of your current back fire problem. Make sure this is correct.
For initially timing of the engine make sure the distributor vacuum advance is unplugged and capped at the carburetor. If all is correct the engine should fire and run allowing fine adjustment in timing at the distributor.
Long winded and some is likely information you already know. Just the same, some may be helpful.