Is pop-pop-pop after lifting gas bad?
 

I have advanced the timing on my 78 SC from 5 degrees BTDC to 15 degrees BTDC. I have disconnected the vacuum advance, too. The engine at 15 degrees BTDC is very lively, but goes pop-pop-pop when lifting off the gas at 5000 rpm or so. It doesn't go pop-pop-pop with 5 degrees BTDC.

I don't sense (feel, hear) any detonation during acceleration so I'm not too worried about that. Also, the hole in the side of my air box is just duct taped (no pop off valve) so I'm not worried about breaking the air box.

Any comments / suggestions, please?

I once had a similar poping when letting off the throttle and found that I had a bad fuel injector.

Raz..I would hear pinging at low rpm..reduced timing to about 32 deg. w/vac line disconnected to end it................Ron

At 950 rpm it's 15 degrees BTDC. At 6000 rpm it's somewhere between 30 and 35 degrees BTDC.

The 30-35 degrees @ 6000 rpm sounds just perfect. Is the 15 degrees at idle with or without the vacuum lines connected?

My 2.7 CIS has always done that... that's one of the most distinctive sounds of the the CIS cars... I recall reading somewhere that CIS keeps the fuel flow constant regardless of throttle position, so when you abruptly lift off the gas the mixture gets rich (relative to air volume) and the popping happens... frankly, it's for me one of the idiosyncracies that gives the car so much more character than the new P-cars.

Tyson & Loual,

Yeahhhh Baby!!!

Hi fellas.

Razor..I have not experienced any problems with my setting yet...about 10-13 degrees BTDC at idle w/o vacuum. The motor seems to come on a lot earlier too. Anyone running an SC (78-79 at least) w/ 5 degrees BTDC do not know what they are missing.

Oh, my motorcycle does that pop-pop-pop during hard decels sometimes, and I love it. If my 911 did it I wouldn't complain..unless it was a bad thing of course. It's a great sound when I brake from 10,000+ RPMs. Hehe :D

Most engines will pop on decel, but you can't hear it because of the mufflers, exhaust, etc.

A DME car shouldn't if your foot is completely off of the pedal because the dme is supposed to cut all fuel supply when not on the gas, but if I hold the pedal just slightly open on decel then I can hear it. CIS cars should probably pop especially if the fuel flow remains constant. Popping is due to a rich condition, but is normal for most cars. Maybe because of the new timing less fuel is burned on decel now than it was before.