SBC chevy 305 pops on overrun

[AC427]

My SBC chevy 305 pops on overrun when warm. it has a holley 4 barrel 650cfm carb. does anyone know if this is a common problem?

thanks for any help


...ac427

[V8 BJC]

Originally Posted by AC427

My SBC chevy 305 pops on overrun when warm. it has a holley 4 barrel 650cfm carb. does anyone know if this is a common problem?

thanks for any help


...ac427

Check that you have a good seal on your exhaust manifold gaskets.
That is a common cause. Mine used to make the most fantastic noises on overun, then I replaced the gaskets and it does not do it anymore!

[clive a brown]

Mixture - timing - air leaks & temp will all have a effect on this

I find a small amount to be just right.

popping that is

Clive

[Clarkson]

Only an exhaust leak will cause popping on overrun.

Hey mine is so bad I have even got 4ft flames coming out of mine.

[Sidecarbod]

My RV8 used to sound like bongo drums on overrun, it usually is a mixture thing.
It's possibly a bit too rich on the idle circuit, even through the engine maybe on overrun and therefore not idling its still pulling mixture from the idle circuit, not the main circuits.
However this excess fuel cannot burn and hence make the popping sound unless there is oxygen in the exhaust, dodgy exhaust joints and gaskets can therefore provide the oxygen required.

I fitted slightly softer dizzy bob weight springs whilst messing about with the ignition and then noticed that the bongo drums had vertually disappeared. I guess the extra advance that I now had was allowing all the mixture to burn in the cylinders on overrun therefore there was no fuel in the exhaust. Shame as I quite liked the noise!

I sometimes get quite a loud pop out of each exhaust maybe 5 seconds after I've killed the motor completely, this is due to the oxygen working its way up from the end of the exhaust and comming across some unburn fuel vapour.

Of course all of the above could be a load of old bo11ox but its my theory and I'm sticking to it!.

Regards,

Pete

[alfiebeard]

I enjoyed that nice bit of old Bo11ox Pete, and agree with all of it as mine sounds like a gatling gun when on a long overun, and its probably a lot to do with the air leaks in the sidepipe joints,and my Holley is rich on idle but I reckon its sounds ok, and being a lazy get I will be leaving it.

[Sidecarbod]

Originally Posted by alfiebeard

I enjoyed that nice bit of old Bo11ox Pete, and agree with all of it as mine sounds like a gatling gun when on a long overun, and its probably a lot to do with the air leaks in the sidepipe joints,and my Holley is rich on idle but I reckon its sounds ok, and being a lazy get I will be leaving it.

Hi Alfie,

Glad that you agree with me, people certainly know that you've arrived at a set of red lights when your exhausts announce the fact with a nice drum roll!

Pete

[alfiebeard]

Originally Posted by Sidecarbod

Hi Alfie,

Glad that you agree with me, people certainly know that you've arrived at a set of red lights when your exhausts announce the fact with a nice drum roll!

Pete

Hi Pete,
Yeh it does,nt half make the grannies jump eh

[Andy S]

My Chevy does the same, even more so as V8BJC says when the exhaust manifold is leaking a bit !!!!!!!!!!!!
Mind you, what spec are you running as a 650 carb does seem a bit OTT for your engine a 600 Holley or Edelbrock is much better suited IMHO.

[AC427]

after a bit of searching it turns out it is a 600cfm 4 barrel job. sorry i didn't build the car. wish i had though so it wasn't such a mystery tour!

will check the gaskets tonight