240z Backfiring at idle and any rpm HELP

[Chickenman]

Point gap should be  .018" to .022". I prefer to go with larger figure with new points, as gap will decrease with rubbing block wear. If using a dwell meter, you want 35 to 41 degrees of dwell. More point gap = less Dwell angle.

Important note: With new points always wipe a clean sheet of white paper through the closed points to remove any anti-rust coating. Wipe it a few times using just the closed point pressure until paper comes out clean. Failure to do this cleaning can result in the petroleum residue burning and result in a weak spark.  An "Old School " teaching that may have not been passed down through generations.

BTW, even with timing set correctly, a weak spark can cause backfiring through intake and exhaust.

[5 Star Rising]

6 hours ago, Chickenman said:

I kinda ruled out valve lash because his compression figure were all good.

How sharp is the spark? You should get a sharp blue SNAP with a good points ignition and HT lead from coil held about 1/8" to 1/4" from a ground. Weak spark or yellowish spark indicates a problem. One thing that is often over looked is the condenser. A weak condenser can cause sooting of plugs. Ignition will also break down at load and can cause backfiring through carbs.

Since you recently changed plug leads, triple check that you got the firing order right. It's really easy to swap a couple of plug leads. Engine will run, but will be rough and you often get backfiring through carbs. In original post, owner mentioned back firing through carbs. That is unusual and normally points to Valve issue or ignition problem. Since his compression readings are very good and consistent, that rules out a valve lash problem IMHO. Still, it wouldn't hurt to check. But I think this is more of a fine tuning issue.

 

I got this weekend off, so I will start going over everything on sat.  Im leaning towards an ignition issue too. I have not checked for powerful spark yet but i will.. I know when you Rev the engine it sounds like there is a slight  spudder through out the rpm range, it doesn't have that smooth snap to the throttle response, maybe the firing order is off, but sometimes at idle it runs super smooth but then starts backfiring through tail pipe then through carbs every 15 seconds or so.

[5 Star Rising]

On Wednesday, January 18, 2017 at 8:20 PM, 5 Star Rising said:

#1 was white almost, 2 black soot, 3 a little oily, 4 soot, 5 soot, 6 soot

OH MY GOD. I HAD CYL 3 AND 4 ON THE DIST CAP WRONG!! BACKFIRING IS GONE! CAN'T BELIEVE IT WAS SOMETHING SO EAST LIKE THAT. CAR IS RUNNING GOOD, NOW TO FINE TUNE. WHEN I CHANGED THE PLUG WIRES I CAREFULLY CHANGED ONE AT A TIME, BUY THE PRIOR OWNER HAD IT IN THE WRONG PLACE!! THANKS FOR THE FIRING ORDER DIAGRAM

[5 Star Rising]

I picked up this new old school unit at a garage sale for 10  bucks to use on my chevelle, gonna come in handle with the 240 as well..

[siteunseen]

That's great!

Is that what I've heard called an exhaust "sniffer"?  I want it. 8^)

[5 Star Rising]

Yep, old school

[5 Star Rising]

Got timing set to about 13-15 degrees. Got Air/fuel mix dialed somewhat using this tester. Carbs are almost leaned out all the way. But runs smooth and snappy. Now I got to find a set of 240z seats so I can take it for a test drive haha. 

[Chickenman]

Sometimes it's the simplest things... LOL.  Good score on the A/F tester BTW. Sweet!!

[5 Star Rising]

Got timing set to about 13-15 degrees. Got Air/fuel mix dialed somewhat using this tester. Carbs are almost leaned out all the way. But runs smooth and snappy. Now I got to find a set of 240z seats so I can take it for a test drive haha. 

 

Yeah I couldn't believe my eyes when I seen plug wire 3 swapped with 4. Had to check like 3 times as if I had a winning lottery ticket haha.  Thanks on the AF tester, pretty cool unit to have in your garage for old cars.