Burble on Over-run

[opus10583]

The Burble or popping on Decel is caused by a lean condition in one or even all of the cylinders, the mixtures are too lean to burn in the cylinder and it's pumped in to the exhaust where it is ignited by another cylinders hot gas or even the exhaust valve. This condition is acerbated by adding high flow headers and free flow exhaust systems as this helps lean out the mixtures. So if you want this to stop or reduce the popping Burble you need to richen your idle circuit. On a carbureted car you can only go so far before idle quality start to suffer.

The same goes for adding burble, lean the idle mixture and /or add a flow thru exhaust system or better yet add adjustable cam pulleys and increase the valve overlap.

Hi Ted,

'I'd kill a small animal' for POPPING on deceleration!

Thanks.

Mark

PS Why can't I just skip a tooth or two to increase overlap?

[baltobernie]

You could always do what BMW does; play engine sounds thru the stereo What great cars they used to make ...

[124JOE]

PS Why can't I just skip a tooth or two to increase overlap?

too far and it will bend valves

i asume

[TronDD]

Wouldn't that effect power over the entire rpm range?

Tim.

[124cso]

How do I get my newly DFEV-carbed 1800 to burble melodically on over-run?

Mark

FWIW, I have owned several Fiat Twin cams and found that it appears to be valve timing that creates that exhaust noise.
The 1608cc of 1968 thru about 1972 sound great but the 1596, 1800 & 1996/2000 have different valve laps and simply do not sound the same.
I had a 125 with 1608cc, 105Hp, single twin throat Solex, stock ex manifold and straight thru s/s exhausts. Under power up a slight rise (therefore working slightly) it cackled beautifully up to about 3000Rpm. Looking back, I should never have sold it but these things we do.

[baltobernie]

... it appears to be valve timing that creates that exhaust noise.

Yep. It is incomplete combustion that causes this sound. What you are hearing is burning fuel in the exhaust manifold. Wilder cams have increased overlap, allowing some of the fresh charge to escape the combustion chamber and mix with the exhaust, which is why we associate the chuffing, popping and burbling with high-performance motors.

One modern exotic is duplicating this sound by introducing a tiny amount of raw fuel into the exhaust. Probably more idiotic that playing the sound thru the stereo, but what do I know

[124cso]

... Probably more idiotic that playing the sound thru the stereo,

Off the topic, but where I live, a lot of town councils put up signs "No Engine Brakes" meaning "Exhaust" brakes. Unenforcible and just plain stupid, but I have thought that I should get a PA system fitted to my pickup, with an I-pod/MP3 of old type Jacobs exhaust brakes working hard. Record it into a loop and fit a push button on the gearshift somewhere.
Then at night, I can BUURRRRRRRRRGGGGGGHHHHH my way around town !

[124JOE]

install a train horn
nothing else says wake up!!!!like a train

[georgeramos]

my 71 with 1608 (all stock) burbles all over town. Sometimes more then others. Sounds great but i assumed i needed to correct it at some point...

George

[opus10583]

my 71 with 1608 (all stock) burbles all over town. Sometimes more then others. Sounds great but i assumed i needed to correct it at some point...
George

Hi George,

I came back to the FIAT-fold fully because of a 1608 that the seller had posted starting and driving videos of ...The sound man, the sound.

All of the intake and exhaust mods I've made have been primarily sound driven.

Thanks,
MArk