Does running too lean causing "popping" on acceleration?

[azasadny]

Several of my friends have noted that my car makes a "popping" sound as I accelerate and they are behind the car. I can't hear it from the driver's seat and the car runs well, but I'm wondering if a "too lean" mixture can cause this. I don't have any way to measure the mixture but I have a local mechanic who can measure and adjust if necessary. Is this popping a sign that my mixture is too lean? Thanks!!

[Zeke]

Popping is a sign of too lean, but more at less than WOT. At full throttle, you should expect some hesitations and slight lack of power when lean. Sounds to me like an ignition problem.

I'd double check points gap if you have points and plug condition, not to mention timing. Then, play with the mixture. Of course, if you can have it measured, why not?

[Paulporsche]

Or you could try push/pull or richening up to just before idle hunt. If you have it measured, remember JW recommends 3.5%.

I think I would try Zeke's idea first. Sounds like points.

[Jerry S]

from my experience, if it is a random type pinging noise than that would by a lean mixture or timing issue, but an actual "popping" noise sometimes is a compression leak which is not too uncommon on air cooled cars especially older cars with some significant miles. The compression leak would be caused by a slight gap between cylinder and the head which in turn causes a spitting or popping noise as a result of worn surfaces or broken (maybe pulled headstuds). Not trying to scare anyone but maybe if you can get below the car and see of there are any large carbon areas between head and cylinder on any of the 1 trough 6.

Jerry S

[Jerry S]

Hopefully the problem is only tweaking the mixture or altering the timing.

Jerry S

[azasadny]

Thanks for thevice, guys! I have the Pertronix system instead of points and I just replaced my faulty "PermaDoom" CD unit with a known-good Bosch Cd unit from John Walker. Last summer, I found the barrel nut from a head stud on cylinder #1 lying loose and I tightened and torqued it and it held, so I hope it's not that... The popping noise is at acceleration, but at less then WOT. I don't have any sluggishness or hesitation at all and the engine runs smooth. Thanks again!

[Paulporsche]

Is the popping from the airbox? Do you have a popoff valve? Why not have someone else drive it and listen for yourself from behind.

[azasadny]

Paul,
I have a popoff valve and it's in good shape. I was told the popping sound is coming from the exhaust... Thanks!

[Wayne 962]

Hmm, popping on acceleration - typically means a pulled heat stud. Get this looked at right away - you can do damage to your cylinders and heads if you don't (but 2.7 heads & cylinders are cheap, so don't worry too much about it)

Popping and backfiring upon coasting down from higher RPM is a sign of a lean mixture (can also be from rich mixture). Basically if the mixture is way off, then it won't fire in the cylinder with the spark. Then, the air-fuel mixture ends up in the exhaust where it's really, really, really hot, and then it ignites there in a mini-backfire or popping noise. Timing problems can result in this too, although on carb / MFI cars, you can figure out if it's a timing problem, as the car will generally backfire out the muffler, not the top of the carbs. Timing problems result in backfiring out the tops of the carbs (don't look down there when the engine is running).

-Wayne

[azasadny]

Wayne,

Thanks! Based upon the fact that I found a loose barrel nut on cylinder #1 last summer, I believe the cause of the popping is the pulled head stud. I'm going to drive the car as it is (only about 2000 miles this driving season), then do a 3.0 engine swap after we finish the home renovation project. Right now, I can't spare the time or $ to get it fixed and I'm not going to rebuild the 2.7 engine as that doesn't make too much sense to me.