Backfire-Question!

[Sooner1970_K10]

I noticed today that my pickup has a small backfire: like pop, pop, pop whenever I down shift and the rpm's spike or I am accelerating and let off the gas in 2nd or 3rd gear. I drive a 1970 chevy with an originial v8 350 and a 4spd manual transmittion. It doesnt have headers and I dont really know anything about the exhaust system. I have only had the truck a couple of months and only drive it on the weekends or once and awhile in the afternoon when I get home from work. This is the first time I have noticed the backfire, is it normal or do I have a valve seat or exhaust problem? Anything I should check out to eliminate? Thanks again for any advice!

[highboy_04]

i always like the popping sound of a nice exhaust when you let off in a low gear. i think as long as its not like flames back firing out of the pipes then you are ok. what kind of exhaust do you have on there?

[MandarinSixSeven]

Mine does that too...I could never get rid of it

[Randy70C-10]

I believe that you can get rid of that by installing bigger pipes.

[thisolechevy]

You might want to check your timing, and your cap and rotar. I just had that similar problem but it was a little more than pop, pop it was more like thud, thud. But like highboy 04 said a little popping when you let of the gas does sound mean!!!!!

[jku72]

That is the sound of misfiring cylinders. It happens in those particular situations because there is a momentary change in the air/fuel ratio. This is largely considered to be unavoidable in the real world, but, can be reduced by keeping the ignition system in top shape. A hot spark will more easily fire in an excessively rich mixture than a weak one. Most of my older vehicles do it, but I have eliminated most of it on my burb by super tuning, and running a ACCEL HEI with super coil and 8.8 wires, plugs gapped to .055. I have the same setup on my longhorn, but the engine is higher mileage, and is generally weaker than the burb, even though it has more HP. The Longhorn pops like mad when engine braking down long hills with a load on.
There are a lot more reasons for the backfiring, but I think that is the most common reasons. Anything that causes misfiring can be a cause.

[JimKshortstep4x4]

Exhaust leaks can cause backfires. Also, a vacuum leak will in some situations cause backfires. My experience is that it is not normal.

Jim

[68C15]

no it is not normal. what is actually happening is there is combustion in the exhaust. advance timing to about 10-12 BTDC with vac. adv. disconnected & hose plugged. you will also need to lean the idle & main circuits a little. trust me.
Todd