A trained ear can tell the cause of a backfire and it's usually not ignition timing-related. If this is a newly-rebuilt engine that had a valve job done, especially if an old-school repair was done called "knurling", the valve guides could be tight yet holding the intake valves open. That will let the burning fuel pop back into the intake manifold and spray fuel out of the carburetor. Often you can see that with the valve covers removed and turning the engine slowly by hand. The engine should be at the same temperature as when the backfiring most often occurs. A sticking valve will suddenly snap closed with a fairly loud "click". A clue can sometimes be found by backing off the rocker arm nuts while the engine is running. That can get messy. I'll explain the procedure if you need it. By backing the nuts off, the valves won't open so far so they'll be less likely to stick and cause that backfire.
There's a couple of different ways of adjusting those rocker arm nuts. If you just cranked them all the way down, the engine most likely won't even run. Some people run each one down until they feel a little resistance while spinning the push rod, then they go an exact additional amount, typically one turn or one and a half turns. I prefer to back one off at a time while the engine is running. You'll hear that push rod clatter. Wait a few seconds for the lifter to pump up and quiet the noise, then back the nut off some more. Keep on doing that until the noise never clears up. Now tighten the nut just until that valve becomes quiet. Remember that setting.
Now run the nut down a quarter turn. You will hear and feel the resulting misfire for as much as 15 seconds until the lifter bleeds down. When it smooths out, go another quarter turn and wait. Carefully count the number of turns you're making on that nut. You're looking for the point at which the misfire doesn't clear up. That's where the lifter is fully collapsed. Now back the nut off just enough for the misfire to go away. Once you've found those two points, back the nut off half way and leave it there. That adjustment is not exactly critical but it puts the lifter's adjustment in the center of its range. That insures it will always let its valve close fully. Also, the reason for only running it down a quarter turn at a time and waiting is because the extreme heat in the valve is drawn off when it hits the seat. This procedure holds that valve open until the lifter bleeds down. If you run the nut down all at once, you won't know when you went far enough, and the valve can overheat during that time.
I've seen people run those nuts down too far, then back them off just to the point where the misfire clears up. The lifter can't collapse any further when the push rod and valve stem warm up and expand. If an intake valve is held open, the backfire will be through the carburetor. If it's from an exhaust valve, you'll hear it under the vehicle and at the tail pipe.
The HEI ignition system is a very nice unit but a problem often develops when a spark plug wire is removed while the engine is running. These coils are able to put out WAY more voltage than any spark plug should ever need to fire. On regular ignition systems, if you pull off a plug wire, the spark just won't be able to jump anywhere and no harm occurs. With this one and its really high voltage potential, that spark IS going to go somewhere. Usually that somewhere is through the rotor to the distributor shaft and ground. That spark leaves a carbon trail behind. It is much easier for the next spark to follow that carbon trail rather than go to its spark plug. That's called "punch through" and will prevent the engine from starting. It is also possible for the spark to jump to a different terminal under the cap and fire the wrong plug at the wrong time. That can result in a severe backfire through the intake and carburetor if the cylinder fired is just starting its compression stroke. The distributor cap is really huge so those terminals can be far apart to prevent that from happening, but that assumes a spark plug wire is not removed and the spark always has a proper path to follow. There's no good fix for punch through or carbon tracking other than to replace parts.
Bad spark plug wires can cause backfiring too in a couple of ways. If many of them have unusually high resistance or the carbon-impregnated string inside is burned back from the terminals, no current will flow until the voltage has built up high enough to jump all of those gaps and the spark plug gap. That takes time as the magnetic field collapses in the coil. Since that can make the spark late by a few degrees, you adjust the distributor so the spark occurs at the correct time. Occasionally the spark will occur easier which means sooner, or when originally intended, which is now too far advanced. You can find a clue to this by switching the timing light's trigger probe to cylinder number 6. If the timing is the same, you're not sure, but if it's different, try new wires or check the spark plug gap. A gap that's too large will also make it harder for the spark to jump, so more time is needed to build more voltage.
Using the wrong spark plugs can cause preignition in severe cases. I'm not sure if I'm wording this properly, but if the heat rating is too hot for that application, they're going to get overheated and the outer electrode can get red hot. That will ignite the fuel too soon before the spark even gets a chance to do it. That overheated plug is actually what top fuel dragster engines run on during the last half of their four-second run. The spark plugs are burned away by the time they get half way there!
To clarify spark plug heat ranges, that has nothing to do with how hot a spark they create. A "hotter" plug has a longer path for heat to travel before it goes through the threads and is dissipated into the head and cooling system. That means it remains physically hotter in the engine. In a colder plug, heat can get out faster so the plug remains cooler. Different heat ranges are designed in to meet various design conditions in different engines. A plug that is too cold will tend to let carbon build up on the electrodes and it won't be as effective at burning off any oil that sneaks past the piston rings or down the valve guides. Hotter plugs burn that stuff off better but the electrodes will melt away easier, and if bad enough, an electrode can get so hot it fires the fuel as it's rushing in, even before the spark shows up.
Don't overlook the possibility of having two plug wires mixed up. We've all done that, and every experienced mechanic years ago has bailed out his very experienced coworkers for the same mistake. The order is 1, 8, 4, 3, 6, 5, 7, 2, the same as Chrysler V-8s. If you notice, 5 and 7 are next to each other in the firing order AND they're right next to each other at the back of the left side of the engine. Any time current flows through a wire it sets up a magnetic field around that wire. Any time a magnetic field passes through a wire, it "induces" a voltage into that wire. (That's how generators work). Basically that means if the number 5 and 7 plug wires are too close together, the spark from one can induce enough voltage into the other one to fire that plug too, but cylinder 7 hasn't reached top dead center yet. The fuel ignites too soon and tries to push that piston down backward. A local late model stock car champion blew up two $25,000 Chrysler engines in two weeks from this problem. What they had done was to place the spark plug wires in their holders on the valve covers in a neat orderly fashion to make them "look pretty". To prevent the same problem, all that was needed was to move the number 5 wire to the other end of the holder, an additional two inches away from the number 7 wire. That was an extreme case with real high spark voltage, real high compression, and a real rich fuel mixture that is very easy to fire with not much spark. You don't have to worry about your engine exploding, but backfiring is a definite possibility. The problem will be aggravated by a plug gap that's too big.
Old Chrysler engines all used one common system of providing vacuum advance to their distributors. Learn one and you learned them all. GM tried different ways of connecting the vacuum hose with the same goal in mind. On some models, the vacuum hose is "ported", meaning it hooks to a port above the throttle blade in the carburetor. There will be no vacuum there until the throttle is opened above idle speed. As the throttle is opened further, more vacuum appears in that hose and causes the ignition timing to advance for more fuel mileage. The fuel does not explode in the engine. It burns very quickly and smoothly, but that takes time. To develop the most power when the piston is just starting on its way down, the fuel has to be ignited sooner at higher engine speeds so it is done burning at the right time. The mechanical advance built into the distributor does that. Under light load such as when cruising at highway speed, the mixture is relatively lean. Since the fuel molecules are fewer and further apart, it's harder for the flame front to jump from one molecule to the next so the fuel burns slower. That condition also requires the mixture to be lit earlier so it's done burning in time. Ported vacuum advance does that.
Some GM engines had the vacuum hose connected to the intake manifold UNDER the throttle blade. That was done because they had so much trouble restarting hot engines. The engine started with no vacuum advance but that vacuum showed up as soon as the engine was running. With both systems that vacuum advance went away under hard acceleration to reduce spark knock.
The difference is in how you set base timing. That is normally specified on the emissions sticker under the hood. Since you don't have that, the best I can offer is to just be aware of the different methods. In general, most systems require you to disconnect the vacuum hose from the distributor and plug it, but theoretically, that shouldn't be necessary with ported vacuum systems as long as the engine stays at idle. Regardless, that's when you set base timing, then reconnect the hose. With the full vacuum systems, I can't remember if you are supposed to set base timing with the hose connected, which is how the engine will be running anyway, or if you're supposed to disconnect the hose and let it fall to its badly retarded setting, and set base timing that way. If that were the correct procedure, base timing could be expected to be close to "0" or even a little after top dead center.
The point is, if you have a regular distributor but you have the vacuum hose connected to a port on the intake manifold or on the base plate of the carburetor below the throttle blade, you'll be getting way too much spark advance. The preignition will result in those early explosions staying in the engine. You won't likely see or hear them at the carburetor.
Did you double-check the timing chain to see if it's off one tooth? Also, the "personality" of an engine is controlled by more than the profile of the camshaft, meaning lift and duration. It was common practice to advance or retard camshafts for different applications. That has nothing in common with advancing or retarding ignition timing.
If you can imagine an old stretched out, (loose) timing chain, that slack will make the camshaft run a little behind where it's supposed to be, or "late". Think of "L" for late, loose, and "low". An engine will have more torque at low speeds when the cam timing is late. That is done for the old gas mileage camshafts, which I had and really worked, and for motor homes to give them more power for pulling away from stop signs.
The opposite condition is "T" for tight, top, and, ... Uhm, ... Advanced, if that timing chain could be too tight, the cam timing will be advanced resulting in more top end torque. That was done for highway pursuit police vehicles that don't need the quick takeoff at city speeds. I have one of those too.
Anytime a nonstandard camshaft is installed, you have to consider whether its timing needs to be altered. One cam can perform one way at one setting and act entirely differently at a different setting. Said another way, a part that meets the goals in one engine might be able to be sold and packaged for a different application as long as the timing is altered. That isn't common because the manufacturer could simply take two identical camshafts and drill the dowel hole slightly differently and have two different part numbers. The advantage to having just one part number is you have half as many different part numbers to keep in stock.
The point is the manufacturer usually spells out the cam timing that is required to make it perform as expected. That is checked with a degree wheel and is much easier to do than to describe here. A two degree change will make a huge change in the response of the engine. Chrysler engines used a key between the sprocket and cam. They were available with a two, four, or six degree offset. GM engines used a dowel pin between the cam and sprocket. You had to buy a sprocket that typically had three dowel holes, one for standard timing, one advanced, and one retarded.
Even if you didn't check the cam timing, it should not be nearly off enough to cause valves to still be open when the fuel is burning.
As far as the engine cutting out, that can mean a lot of different things when we can't feel it ourselves. A backfire with a carbureted engine can blow the fresh fuel out of the intake manifold and cause a momentary cutout until the air and fuel start flowing again. That can happen at any speed but usually is more noticeable when under a load as in when accelerating or going uphill. If the cutting out only occurs when moving the gas pedal down to increase speed or power, suspect the accelerator pump in the carburetor. They are adjustable by bending a lever or relocating a linkage into a different hole in a lever. At a minimum, you should see two nice solid streams of fuel spray into the intake when you work the throttle by hand. Don't do that with the engine running because it will go away too quickly to see, and you can burn your eyebrows off if it backfires!
One last thing to consider, if the cutting out occurs over longer periods of time, as in a few seconds, see if you can make it happen while doing a mild brake stand and standing still. If it acts up, throw a timing light on to see if you're losing spark. If you are, suspect the pickup coil around the shaft of the distributor. The wires flex every time the vacuum advance moves the plate and eventually they break. The common failures are a no-start after a "hot soak" where a hot engine sits for just enough time for the heat to migrate into the distributor instead of being blown away on the highway, cutting out when the vacuum advance mechanism tries to move the plate and coil, and a broken connection on one of the wires can be caused by corrosion. Repair of those wires is usually not practical. To replace the pickup coil you have to remove the distributor, remove the drive gear, then slide the shaft out. Another way to find this is to use an ohm meter to measure the coil's resistance, then watch for a change when you apply vacuum with a hand-held vacuum pump. Normal, as I recall, is about 800 ohms. The value isn't critical. What you don't want is for it to go open circuit or very high in resistance. Try driving it with the vacuum hose disconnected and plugged. If no longer cuts out, that pickup coil is a good suspect.
A lean mixture can cause popping. Vaporizing fuel gets cold and is harder to ignite. Less fuel in the mixture means it won't get so cold so hot valve faces and spark plug electrodes can ignite it easier. I don't have a better explanation than that, but the mixture is set in part by the size of the jets and in part by the fuel level in float bowls. Holley four barrel carburetors usually have a threaded plug on each float bowl that can be removed so you can see the fuel level while the engine is running. Most commonly they say to adjust the threaded adjustment on top until the fuel just about runs out of those holes, but that's just the starting point. If you have the equipment for measuring tail pipe emissions, you can adjust them more accurately. In the name of better fuel mileage, I just lowered my floats a little each time I was under the hood, then drove it a few days to be sure no running problem developed.
Don't overlook the possibility of a vacuum leak. Pinch off various hoses or disconnect them and plug the ports to see if any cause a change.
Please consider a to help us answer more questions.
Thursday, January 12th, 2012 AT 12:46 PM