Weber Backfire question

[serIIalpine]

When I let of the gas at speed while still in gear my car backfires through the exhaust. Kind of a blattity blat blat blat blat until I release the clutch or take it out of gear. It also does it if I run up the revs while stationary and abruptly let off the gas.

I'm running a pair of dcoe40s on my 1600cc and I think I've finally found the sweet spot for the timing as well as the jetting for driving at speed. Plugs are beautiful tan after a run at highway speed.

What I'm wondering is could this be that Im running a rich idle circuit? It sure smells it when the car sits in my garage idling ( don't worry I wasn't trying to do anything drastic)

By the way the car also intermitantly backfires through the what I believe to be the #1 choke while idling.

Thanks

Eric
62 SerII

'62 SerII

 

[Nickodell]

Blat blat in the exhaust may be nothing more than resonance, i.e. at a certain exhaust flow rate it equals the resonant frequency of the system. Ther is one brand of large truck (a rigid body, possibly a dump truck, not a tractor-trailer) that always bellows through the exhaust on the overrun, or coast in gear. I'm sure everyone on the forum knows which one I'm referring to, the one with two tall vertical chrome-plated stacks.

Backfiring through the carb can have many causes, including: too lean mixture; over-advanced ign timing; a bad valve; an air leak. Perform a vacuum analysis and compression test.

 

[RootesRacer]

Is it a "spit" or an actual backfire?

Its common for DCOEs to spit at idle, the reason is that the idle signal is so weak due to only one runner consuming the carbs idle fuel.

You might also make sure that your throttle airflow is the same from #1 to #2.
If not, the plates may be in a slightly different section of the progression circuit, and the mixtures will be different and wont track.
BTW if #1 and #2 flows are not the same, your throttle shaft is twisted, and it wouldnt be the first DCOE Ive seen that way.

 

[64beam]

Hi Eric,

I also get a small amount of backfire through the carbies as well, but mine is mainly when it's cold and once warmed up is fine ( I still have to check my timing and get my valves adjusted ). I also get like a popping when I take my foot off the accelerator as you described, but I think mine is due to a slight exhaust leak. Check the simple things first and see how you go.

Regards, Robin.

 

[husky drvr]

Blat blat in the exhaust may be nothing more than resonance, i.e. at a certain exhaust flow rate it equals the resonant frequency of the system. There is one brand of large truck (a rigid body, possibly a dump truck, not a tractor-trailer) that always bellows through the exhaust on the overrun, or coast in gear. I'm sure everyone on the forum knows which one I'm referring to, the one with two tall vertical chrome-plated stacks.

Nick,

Is it possible that you are referring to the sound made by an engine brake?

Wikipedia to the rescue -

The Jake Brake or Jacob Brake is a particular brand of engine brake manufactured and sold by Jacobs Vehicle Systems, Inc. While the term Jake Brake technically only describes Jake Brake brand engine brakes, it has become a genericized trademark and is often used to refer to engine brakes or compression release engine brakes in general, especially on large vehicles or heavy equipment

An engine brake is a braking system used primarily on semi-trucks or other large vehicles that modifies engine valve operation to use engine compression to slow the vehicle. They are also known as compression release engine brakes.

When the accelerator is released on a truck its forward momentum continues to turn the crankshaft and compress air inside the engine's cylinders. When the crankshaft passes the top-dead-center position the compressed air in the cylinder acts as a spring and pushes the piston back down the cylinder, returning the energy to the crankshaft and pushing the truck forward. Little of the energy absorbed by compressing the air is lost, so the engine does not effectively aid in slowing the truck. In a gasoline engine, some engine braking is provided during closed-throttle operation due to the work required to maintain intake manifold vacuum. Diesel engines, however, are unthrottled and hence do not provide engine braking from throttling losses. A compression release engine brake uses an extra lobe on the camshaft to open a second exhaust valve at the top of the compression stroke. The stem of this valve telescopes during normal operation so the valve remains closed, but is locked at full length by a solenoid when the engine brake is engaged so that the valve opens as directed by the cam. This releases the compressed air in the cylinder preventing it from returning its energy back to the piston and accordingly, the vehicle.

The driver controls consist of an on/off switch and, sometimes, a multi-position switch that controls the number of cylinders on which the brake is active. When the compression release engine brake is turned on, it will activate when the driver releases the accelerator. There are also switches on the clutch and accelerator pedals that will deactivate the compression brake when the clutch is disengaged or the accelerator is pressed.

Compression release engine brakes may make a loud chattering or machine gun noise while being used, ...