Backfire

[Rifter]

Hello all, so today on my way to work the truck started backfiring. Not real bad but definatly noticable. It only does it at above 20-30% throttle, if i keep my foot off it it runs smooth as butter. Doesnt do it at idle at all, does do it in neutral if i rev it up.

Background on truck, never backfired before ever. Owned for 9 months. Changed carb 2 months ago has been running fine since. Wires/plugs/HEI distrubutor all brand new 9 months ago when purchased. Checked/adjusted timing 2 months ago when i installed new carb.

Its a small block 350 all stock except edelbrock 650 AVS off road carb, its a 84 3/4 ton pickup.


What do you think i should look at first?

 

[GalDemSuga]

I had a similar issue while driving my 89 TBI K5, it turned out to be a loose wire at the distributor.

Hello all, so today on my way to work the truck started backfiring. Not real bad but definatly noticable. It only does it at above 20-30% throttle, if i keep my foot off it it runs smooth as butter. Doesnt do it at idle at all, does do it in neutral if i rev it up.

Background on truck, never backfired before ever. Owned for 9 months. Changed carb 2 months ago has been running fine since. Wires/plugs/HEI distrubutor all brand new 9 months ago when purchased. Checked/adjusted timing 2 months ago when i installed new carb.

Its a small block 350 all stock except edelbrock 650 AVS off road carb, its a 84 3/4 ton pickup.


What do you think i should look at first?

 

[Rifter]

yeah im going to check that for sure, at work now so wont be able to till morning.

A buddy also suggested it could be a overheating ignition coil so will proabably swap that out and see if that is it too since its cheap, even though the one in there is only 9 months old.

 

[Hogback Fabrication]

What's the timing set at?

 

[Rifter]

13 degrees at the crank

 

[DokWatson]

I had that issue once... I had two plug wires reversed That is why I like my diesel

 

[Rifter]

I had that issue once... I had two plug wires reversed That is why I like my diesel

THANK YOU!

It wasnt my problem but it found my issue.

I checked the plug wires on my break and they all looked connected properly and i was to lazy to climb into my engine compartment to verify they were snug but after reading this post i climbed in to confirm firing order and found one plug wire was ever so slightly not all the way attached to the stud on the cap. Pushed it on all the way and now no issues!

 

[diesel4me]

Your lucky--last time I had a truck with the same symptoms,it had a flat cam lobe on the # 1 exhaust valve,and the exhaust couldn't escape fast enough,pressure built up and went back out thru the intake valve next time it opened..fairly commom on Chevy V8's...
I was going to ask if it backfired thru the carb,or the exhaust--when it comes back thru the carb its usually a dead cam lobe,or a valve has burnt,valve spring broke,etc...out the exhaust,its usually a burnt exhaust valve ,or retarted timing...none of which are easy or fun to repair!..glad you found it was something simple and easy to fix..

 

[travisnj]

Your lucky--last time I had a truck with the same symptoms,it had a flat cam lobe on the # 1 exhaust valve,and the exhaust couldn't escape fast enough,pressure built up and went back out thru the intake valve next time it opened..fairly commom on Chevy V8's...
I was going to ask if it backfired thru the carb,or the exhaust--when it comes back thru the carb its usually a dead cam lobe,or a valve has burnt,valve spring broke,etc...out the exhaust,its usually a burnt exhaust valve ,or retarted timing...none of which are easy or fun to repair!..glad you found it was something simple and easy to fix..

I've got the same issue here.

Everything fine, out of the blue...

Mine is backfire out of carb.

I have checked wires/plugs and are in good condition.. Any suggestions?

Dead Cam Lobe, not sure what that is...

Anyone advise please? Here is my video: https://vidd.me/rHP

 

[paratrooper307]

I had that issue once... I had two plug wires reversed That is why I like my diesel

I had a facepalm moment like that on my head swap. All the hard stuff I did right, but in my arrogance at having installed plug wires 100 times before, I sped through the wiring and mixed up the order on plug 5 and 6. Ran just rough enough to where I thought I had a vacuum leak. Chased around that and other troubleshooting for about 45 minutes before I thought about checking the wires. Sure enough...

 

[paratrooper307]

I've got the same issue here.

Everything fine, out of the blue...

Mine is backfire out of carb.

I have checked wires/plugs and are in good condition.. Any suggestions?

Dead Cam Lobe, not sure what that is...

Anyone advise please? Here is my video: https://vidd.me/rHP

If you've already checked your timing and wires, and say the carb is good, I'd verify you don't have any vacuum leaks, since they're easy to check for.

A dead lobe generally refers to a cam lobe that has worn down to the point that it doesn't properly open the valve. An easy way to spot one is to remove the valve covers and run the engine, and look for any valves that aren't moving as much as the others. If you do this, put up a barrier to prevent oil from getting slung all over the exhaust. A loose rocker arm can cause similar behavior. It's tough to tell from the video, but it sounded like there was some rattling in the top end. Again, this will be obvious if you remove the valve covers.