Blap Blap Blap on decel - 79Sc ROW

[Johnb911]

Here's the deal.
dual In and Outs
NO cat.

Idles well.after initial warm up of about 2 minutes when it's dead cold.
Always starts right on first crank.
Exhaust doesn't burn my eyes or smell like fuel.

On decel or as soon as I get of the gas a little at all i get a blap blap blap.
I get a bit of a gurgle and a smaller blap f i hold it at around 3000 rpm on the hwy.

hard accel it seems to be fine.

Is this a sign of too rich?
Is this just a sign of a very free flowing exhaust?

Whats up.
Am i paranoid because nothing has gone wrong lately.

[Flieger]

Cat bypass = more open exhaust = leaner mixture and exhaust

this amplifies the natural "blaps" of CIS on the over-run which cannot be heard normally due to muffers, cats, and airboxes.

CIS= continuous injection system

It keeps pumping fuel even when the throttle is closed according to how much air is passing the metering plate. When you get on the throttle a little, the fuel burns in bursts.

[Flieger]

Its normal, though. I would not worry about it too much. Get a check out with an LM-1 air/fuel mixture tester too make sure you will not burn a piston or get detonation or bad stuff like that. This is just a precaution.

[Johnb911]

Flieger,
Thanks that makes sense.

[glewis80SC]

That it what i love about my cat and muffler bypass. My Harley does it too and it has a freeflow exhaust too.

[ramonesfreak]

my motorcycle does it on deceleration. music to my ears.
ive got a cat bypass but with a stock muffler on my 911 but cant hear the gurgling or pops. hoping when i get a new muffler that i will be able to hear it

[T77911S]

check the timing too. retard may do it too.

[jasonmx5]

Mine is almost exactly the same set up as yours - SSI heat exchangers with dual in-dual out sport exaust and no cats. Makes exactly the same sound you describe - and I love it

Tried adjusting the mixture and timing up and down, but it is optimal right where I've got it and it always has that little blat-pop decelerating if you come off the throttle quickly at around 3500 rpm.

[Paulporsche]

Can someone confirm that in a CIS car this usually means the mixture is too rich, or as Flieger says, too lean. Other posts have stated that intake backfiring is usually a sign of too lean whereas exhaust backfires come from being too rich.

JohnB911,

Have you got any other too rich symptoms, like a hunting idle or a very black tail pipe?

[Joe Bob]

Decel popping can ALSO be an exhaust leak and unfortunately head studs.....so if your fix doesn't work check for that.....

[Kemo]

If you havent had your CIS mixture adjusted since the addition of the of the exhaust system, I would encourage you to check with a gas analyzer and make sure your timing is right.

I would hate for you to burn up a piston.