i recently dropped a 355 carbed motor into my 86z an finally got a decent tune on the motor. one day i went out to start my car an it began to back fire out the exaust at idle so i grabbed my timing light an checked the timing an found it was setting right where it was suppost to be. so i checked all my rockers to make sure one didnt back off an everthing was fine. the thing that gets me is it acts as if it is a timing issue. im running a mallory dizzy an box, i have ckecked the system an seems to be working correctly any ideas would be much appreciated
thank Jim

 

How can that be?So, it's NOT like a timing issue.

Forget the "timing". Why does everybody want to blame EVERYTHING on that? (well, except of course for the things they blame on the EGR )

Sounds to me like you have a fouled plug; and that cyl's fuel load is passing right on through the motor without burning, and then getting lit off in the exhaust.

Change the spark plugs.

Could also possibly be an arcing plug wire.

 

hey thanks for the idea, i did pull all the plugs when i rechecked the rockers they looked ok no sign of fouling but the motor was cool so if it was fuel fouled maybe it dried up. you still think i might have a bad plug? is there a good way to make sure that the plug is passing the spark through it? i also checked all my wires for arcing they are all good an have spark to the plug.

 

Yup... put in new ones.

 

ok thanks ill try that. one other thing an believe me when i say i hope it is just a plug issue, i was talking with a local car guy an he told me after a brief look at the car that he thought it could be a problem with the box so i checked the function according to the flashing lights in the box all checked out ok but he still says thats what he thinks the problem is any thought on this im going to head down for some new plugs thanks for your thoughts.

Jim

 

Dunno.... I'd start with the plugs.

See my signature for advice on logic and troubleshooting. Works good, although like anything else, not a full 100% of the time; probably only works 99.5%. But still, I follow it all the time, and find it to be a valuable mental rule, even if not totally infallible.

 

replaced plugs and wires for good measure car still backfires at idle. any thoughts on what i might look at next.

thanks
Jim

 

It still boils down to a matter of raw fuel mixed with fresh air in the exhaust.

Meaning, for whatever reason, air/fuel mixture is still passing through the motor without burning.

Which cyl is doing it? When you did the water-on-the-header-tube test to identify the faulty cyl and inspected that one closely, did anything unusual appear about it?

 

it seems as if it is comeing from the left bank, idid not do the water test not familiar with it however i did do the screw stick test to no avail it is louder on the left bank though. then i pulled the plugs again they all seemed alittle on the moist side, but none were soaked. im thinking maybe a fuel issue stuck float blow maybe what do you guys think.

 

Water test = start the car up COLD, after it's been running about 20 seconds dribble water from a water pistol or something onto the headers, it'll be IMMEDIATELY obvious which ones are running and which ones aren't

 

ok 20 dribble test complete results all boiled engine cool to touch at start up, im thinking intermetiant miss? hmmm im going to try this one more time starting with the right bank nope same. suggestions please frustration so im looking at multiple slugs fireing sometimes help please
thanks Jim