My buddy just installed a rebuilt 302. He is re-using his Edelbrock Carb and intake, as well as his electonic mallory distributer. The engine fires and runs fine, but has an intermittant back fire. About every 15-20 seconds it back fires big. We have checked the timing and the firing order. I suspect that the Mallory is going bad. My buddy is guessing that it's something wrong with the carb. Any opinions on what the final solution will be??? Jag

 

It is possible he could be off a notch or two on the timing gears. The carb won't backfire on its own without something to ignite it. He may have to pull the timing cover and make sure the cam and crank are aligned properly.

 

Just to take that explanation one step further. It will back fire when you have a valve open on a cylinder that is firing - probably the intake valve and you are igniting inside the intake manifold. If you were backfiring through the exhaust valve it would not be as loud.

Two things to look for:

First, run a compression check on each cylinder to assure all your valves are closing properly.

Second, make sure your spark plug wires are all separated by at leat 1/2 inch. They can not be touching each other at all. Spark plug wires will "cross talk" and you may be getting a spark jumping to a wire (and firing the spark plug) for a cylinder that the intake valve is still open and not yet ready to fire - that would explain why it is intermittent.

I'm not sure about the 302, but check the firing order on you truck. If #7 and #8 fire one right after the other, specifically pay attention to those two wires - but separate them all.

This is a common problem on FE engines like my 390. And there is a very specific warning about separating those 2 wires. I had spark jump on those two wires up to 3/4 inches apart. Your Mallory ignition like my MSD puts out a very high voltage spark.

 

Could it be an HO 302, uses 351 firing order. Joe

 

I was thinking along those lines too, but it would continuoously backfire.

I go with the sticking valve or a valve adjusted too tight.

 

I had a 351 Winsdor with an intermittent backfire. It was a broken valve spring that worked most of the time but not every time.

 

You didn't say backfire in the exhaust or backfire in the intake (out of the carb). If in the carb and it is only intermittent, then the other posters referencing intermittency are probably on target. If the timing chain is off, it will do it ALL the time.

If it is in the exhaust, it could be a lean condition on one cylinder (or more) that doesn't fire, but puts gas into the exhaust and then fires in the exhaust on occasion...

Let us know which it is, intake or exhaust...

 

Sorry, I meant thru the carb. They did a compression check and it was good. They checked the carb by putting it on another truck and it ran great. So, they are down to the plug wires or the dizzy. Jag

 

If its out carb thats and lean condition or valves.Exhaust is mostly a timing proplem.Double check the firing order.

 

Carb is good and valves are closing when they are supposed to. That means the plug is firing when the intake valve is open. If it was firing order or timing chain/gears it would do it every time Dist off a tooth or 180 out it would do it every time. Mallory HE ignition. The only way this can be happening is for a spark to be getting to the plug when it isn't supposed to (and the valve is open) Jumping spark.......Plug Wires - Plug wires #7 and #8 are too close. Bet the house a "round."



Here is a picture of my engine. Note the aluminum tube spark plug wire looms. Also note the extra spacing between the two top wire looms on the drivers side. The spark coming off the MSD Ignition/distributor was so strong, it was jumping out of the lower wire (past the insulation), through the loom tube, thru the loom tube above (but, it's aluminum so it's a good condictor) and past a second wire insulation to fire off the #8 cylinder. It backfired back through the carb about every 10 to 15 seconds.

And these are 9MM wires. And I had to separate those wires by over an inch in that configuration to stop the jumping. Wire separators are in use up by the Dist, but are very hard to see.

The 390 Firing order is 1-5-4-2-6-3-7-8
The 302 Firing order is 1-5-4-2-6-3-7-8 !!!!!!!!!

Start it up at night in complete dark - it will be quite a show!

 

Success! It turns out that a stray spark was intermitently jumping from one of the power wires in the bundle that feeds the coil, dizzy, etc. To compound the problem, they also found a bad vacuum leak on the intake. Thanks for all the help everybody, my buddy is feeling much happier today. Jag

 

YEAH!!!! I was close anyway!