Weird intermittent popping sound at idle?

[Jayru]

Hi guys,

So i swapped my old points distrubtor today for an electronic 280z one (I am very impressed) and i thought it might aleviate this problem i'm having with an intermittent popping sound when the car is idling, but it's still there.

The car idles at 550 rpm beautifully, then every once in a while you get this popping sound (like a hickup) somewhere within the intake, then it goes back to normal idle, until it happens again ~30 sec later. This repeats all the time while the car is idling.

It in no way affects preformance, on the gas, (The car pulls extremely cleanly and quickly to 6K, with the new electronic dist i might add)

Any ideas?

[Jayru]

Nobody? C'mon help me out

[Ricklandia]

plugs? wires? does this "hiccup" sound/feel like it is only one cylinder missing a beat? Or is it many successive cylinders missing?

[Jayru]

plugs? wires? does this "hiccup" sound/feel like it is only one cylinder missing a beat? Or is it many successive cylinders missing?

I'm changing the plugs/wires today (although they look good). It almost sounds like a hiccup from the rear carb/intake, like it's firing in the intake? It doesn't seem to really even affect the engine rpm. Is that possible?

[PrOxLaMuS©]

could be a backfire... so the fuel mixture ignites in the intake and "backfires".. sounds like a loud PoP... or like a popcorn kernal popping...

or you could have a minor mis-fire.. maybe a bad plug or improper gap...

[Jayru]

could be a backfire... so the fuel mixture ignites in the intake and "backfires".. sounds like a loud PoP... or like a popcorn kernal popping...

That's what it's like. So if that's it, what causes/how do i fix the problem?

[beandip]

If you have access to a compression tester check it just for giggles. You could have a leaking intake valve and it is burning fuel in the intake. You like the ZX ignition ? I shure saw the difference when I switched. you might kick the idle up some , that is a little slow , what is the oil pressure at Idle?

[Jayru]

I definately love the zx ignition. Before, my car sputtered/broke up above 4k and really felt poor, now it's crisp and clear to 6k

The idle has setteled around 900rpm, it only idled at 550 the first time i drove her (Weird), But is holding rock steady at 900 now.

I'll try and do a compression test today. My oil pressure seems to be on par with all the other z's i read about on the net. At idle it's barely off the low side of the gauge, as soon as you start rolling it gets to and stays at about half way on the gauge.

I let it idle yesterday for about 15 minutes and it's doing it much less frequently now, but seems to have the same pop/intake backfire with a quick inital blip of the throttle, then it goes away (super quick) and revs freely.

What do you think?

[Ricklandia]

Make sure your carbs are not running rich too. At idle, they'll be much more prone to misfire, leaving extra air/fuel hangin around. If your valves are even mildly off (worn) it could be causing it to backfire up into the intake manifold. Q: does it pop (out the exhaust) at all during deceleration?

[Jayru]

Make sure your carbs are not running rich too. At idle, they'll be much more prone to misfire, leaving extra air/fuel hangin around. If your valves are even mildly off (worn) it could be causing it to backfire up into the intake manifold. Q: does it pop (out the exhaust) at all during deceleration?

The car does run what seems/smells to be rich at idle, but the plugs looked good when pulled. How should i ajust the mixture, is their a specific way to do it (withought a wideband, ect)?

It does ever so slightly pop on decell, i mean only every once and a while.

So is it the carbs? The valvetrain dosen't make any noise.

[Jayru]

Problem fixed! I ajusted and balanced the carbs per the service manual's instructions (What a pain in the arse to get right, it took me an hour of fiddeling to get them as close to perfect as i could).

She idles at ~800 now with crisp throttle response, no popping or backfiring whatsoever. Still a little rich at idle though. I followed the manual to a tee and she seems to be spot on (raise the chamber 1/32" and the idle stays the same), So i can't figure why it's still a little rich at idle.

Maybe i'm looking for Fuel injection perfection out of a 33 year old carburator :stupid:

A side benifit is the car feels much stronger under power now. Very happy!

Thanks for the help.

[Ricklandia]

Rule of thumb: with carbs (or any fuel delivery system for that matter), better to err on the rich side than the lean side.