Backfiring.

[hevymetl]

So I have a 66 longbed 2wd c10 with 250ci 4spd. Runs very good(knocks on wood), starts right up, etc, etc. When Im driving and let off the gas, or go down hill with foot off the gas and in gear, it backfires slightly until I give it some gas to take away the slow down factor. Is there any way to get rid of this or minimize it? Its very annoying.

[Wrenchbender Ret]

Make sure there are no vacuum leaks. Try richening up the idle mixture a little.
I take it this is not a Calif. truck with a smog pump.

[hevymetl]

No smog pump.

[TJ's Chevy]

It is normal for engines with a points dizzy to back fire a little through the exhaust. Nothing really wrong. Unless there Is a vacuum leak. You could try richening up the a/f a bit, but I've found an HEI seems to do the trick. My 305 V6 back fires a bit, but you can barely hear it. But doesn't seem to now that it has an HEI...least I can't hear it.

[vince1]

I am just surmising here but for it to back fire doesn't there have to be unburnt fuel going down the exhaust? I remember this used to be much more common in the old days especially when seeing trucks coasting down hills. I think burnt exhaust valves on engines long overdue for an overhaul might also cause this.

That said, my 232 Rambler and 250 Chev C10 both do it. It is not that noticeable because the mufflers are good but I am still aware of it. Maybe the carbs are letting some fuel through.

[Tazimafied]

You only need to be concerned if it is a huge backfire that will make people run for cover when they hear it. If you don't want to hear it you would want a factory style muffler or to add a resonator to your current system.

All engines do this. And I do mean ALL.

Yes it is unburnt fuel in the exhaust and it happens even to modern fuel injected engines. You are cruising along at 2000rpm and let off the accelerator. The butterfly closes and creates heavy vacuum in the intake while fuel is still flowing. This creates an extra rich environment in the cylinder and raw fuel dumps out the exhaust. The exhaust is hot enough to ignite the raw fuel creating the popping cackle.

It is just a matter of how the exhaust system is put together. Most newer vehicles have extremely suppressive exhaust systems to the point that you won't hear it or there is enough back pressure to stifle it. Even factory mufflers in the '60's create enough sound deadening and back pressure so you wouldn't readily hear it. You only start hearing it when you free up the exhaust with headers, free flowing mufflers, larger diameter tubes and/or shortened tubes.

I have driven many new trucks and it is easier to hear due to less stringent emissions regulations and the fact that manufacturers don't waste their time trying to make them super quiet. Mash the pedal to the floor and let it free rev to 3000rpm and let off. When you let off you will hear a slight thrumming from the tail pipe almost as if it were misfiring. That is the backfire of unburnt fuel in the exhaust stifled by a cat, resonator and muffler. It was most prevalent on the 2012 F150 I drove for a while.

[TJ's Chevy]

Another reason the HEI helps with that issue. More spark at the lower RPM. You can pick HEI's up for the 6 cylinders real cheap.

[hevymetl]

I forgot to add it already has HEI. Rather, Im assuming it has HEI since it has newer distributor and no points or condenser. I can live with it. Its not ridiculous loud like shots going off, just a rumble or popping with the back pressure like was mentioned. I dont think its worn valves since the motor has recently been rebuilt. I really just need to fix this exhaust leak. Its right at the first joint after the manifold. Its a 2 bolt clamp, which Im assuming has a ball and socket type connection. Im hoping I can find something that I can do a bolt on replacment fix with rather than taking it all down to weld. Does anyone know if I can just order the whole shebang? Its pretty rusted so a total replcement wouldnt be out of line. Thanks for all the replies on this, Im lovin cruisin this truck around. I went to the lumber yard the other day to pick up railings for my deck, and like 3 different dudes commented on my "work" truck.

[TJ's Chevy]

Originally Posted by hevymetl

I forgot to add it already has HEI. Rather, Im assuming it has HEI since it has newer distributor and no points or condenser. I can live with it. Its not ridiculous loud like shots going off, just a rumble or popping with the back pressure like was mentioned. I dont think its worn valves since the motor has recently been rebuilt. I really just need to fix this exhaust leak. Its right at the first joint after the manifold. Its a 2 bolt clamp, which Im assuming has a ball and socket type connection. Im hoping I can find something that I can do a bolt on replacment fix with rather than taking it all down to weld. Does anyone know if I can just order the whole shebang? Its pretty rusted so a total replcement wouldnt be out of line. Thanks for all the replies on this, Im lovin cruisin this truck around. I went to the lumber yard the other day to pick up railings for my deck, and like 3 different dudes commented on my "work" truck.

I believe exhaust leaks can also cause back firing as it allows cold air in.

[Captmatt]

My truck also backfired on deceleration. Turned out to be a exhaust manifold leak. Good quality gasket with spray on copper coating was the fix. Also check the manifold for cracks...... Good luck

[hevymetl]

Can anyone steer me in the right direction as to where I can purchase the pipe and clamp set up directly coming off the manifold? Ive been searching but everything I seem to find are ss high perf systems, or headers, or for only v8s. No one seems to have stock type replacement stuff for a 250ci 6cyl.

[dracko]

I had an exhaust/muffler shop bend that pipe as well as replace my entire exhaust. They have the fittings to bolt to that manifold. Was pretty cheap too.

Keep in mind though, if there is a leak, it could be where the manifold seals to the block. The 'newer' inline 6's had the intake/exhaust cast as one piece and were more prone to warpage/leakage. If the engine is original 1966 it'll have the older style seperate intake/exhaust.

[jdl71]

Originally Posted by TJ's Chevy

Another reason the HEI helps with that issue. More spark at the lower RPM. You can pick HEI's up for the 6 cylinders real cheap.

Sorry for the hijack, but this might be relevant to the OP. When you changed to HEI did you also change to a hotter plug? I assume you upgraded to 8mm wires also?

[TJ's Chevy]

Originally Posted by jdl71

Sorry for the hijack, but this might be relevant to the OP. When you changed to HEI did you also change to a hotter plug? I assume you upgraded to 8mm wires also?

On my 305 V6 with a 4 barrel, I stayed with stock type NGK plugs and stock type wires and installed a pertronix. Hardly if Ever pops on deacceleration now.

On my 292 with a 600 cfm edelbrock I installed an HEI, Napa 8 mill wires and 1 step hotter on the plugs.

The 305 V6 has been burning the fuel without issue so I haven't seen the need for bigger wires or hotter plugs.

[ChrisDollins]

I noticed mine doing it worse during fall a couple years ago.. Had already installed the HEI but I didn't change plugs. Seems like I read where on bikes it happens more on a leaner mixture. Had an old car buddy say that the timing was off. Needed more advance. Actually just adjusted it by ear and the popping went away. I drive the truck everyday and it would do it down every hill. Never does it at all now. Could just be coincidence, but anyhow, that's my story lol. I sorta liked it. Reminded me of old race cars. But I'm in a 250ci truck lol.

Chris

[hevymetl]

Originally Posted by jdl71

Sorry for the hijack, but this might be relevant to the OP. When you changed to HEI did you also change to a hotter plug? I assume you upgraded to 8mm wires also?

hei was done by po. Ill have to check plugs to see which ones they are. it has 8mm wires.

[gabbin]

I actually love that sound!

[maxwoof]

I can tell you that I had heavy back firing - 66 C10 with points ignition. Only when my foot was off the pedal - would not backfire when stepping on it. Blew the muffler wide open. I pulled the carb, cleaned it and reassembled with a kit, and it ran like new again.