backfire on throttle when warm after idling...

[an_unusual_eye]

last season's plugs came out clean at about 1,500 miles... slightly carboned up, but not significant.. i can start by pulling the new ones next month, but they have less than 1,000 miles on them (NGK). it was pretty warm (high 80s ambient temp) last month when i had the first occurrence, but this weekend was high 60s to low 70s. engine temp never hits mid gauge... though i understand engine temp and fuel temp are not the same. pertronix is in the distributor... less than 2 years old, but not new enough to have not been seasoned. might have 2,500 miles on the pertronix distributor insert and matching coil set.

Edited October 14, 2013 by an_unusual_eye

[Captain Obvious]

I know it's kinda shotgun approach, but I'd pick up a can of that spray cooler stuff and take it with you next time you get to the car. If the problem recurs, I'd pop the distributor cap and give everything inside a quick shot of the cooler and then snap the cap back on and see what happens.

[comhosa]

Touch the fuel bowls with your hand when you have the problem. If it is hot, you most likely have cavitation in stop and go. Pop the hood to see problem goes away.

[an_unusual_eye]

i'm not turned off by the shotgun approach. i'll have a can on hand. saturday when this was at it's worst i stopped at an auto parts store, and one of the things i double checked was that the hoses under the carbs weren't kinked. i didn't specifically check for float bowl temp, but didn't notice any excessive heat when i was fondling the carbs to check the hoses. i even used the downtime as an excuse to top off the oil in the carbs, which i seldom have to do. no immediate improvement.

[Captain Obvious]

i'm not turned off by the shotgun approach.

Shouldn't always be your first inclination, but there are times when it's appropriate, right?

You do know that driving around with that can of freeze spray in the glove box will probably be your insurance that it won't happen again, don't you? :laugh:

[an_unusual_eye]

You do know that driving around with that can of freeze spray in the glove box will probably be your insurance that it won't happen again, don't you?

like the condom my best friend stashed in his glove box back in high school.

i'd like to figure it out, but warding it off isn't a bad 2nd choice.

researching the symptoms often leads to answers that relate to a lean running situation... at whim... but in general car seems to run rich if anything. especially when this is happening, i'm getting chimney soot specks on the back bumper.

[Stanley]

Plugs can foul up fast if it's running too rich, especially in stop and go driving. After a highway run maybe some of the gunk burns off so it runs better. Nice thing about a Z (as opposed to the new cars) is it only takes a few minutes to check. If they're sooty looking you can give each plug tip about 10 seconds brushing with a small wire brush and see if it helps. Worked for me a few times. Also, don't understand the physics behind it, but my Chevy truck started popping through the exhaust when it got a vacuum leak. Vac hose only leaks on the side you can't see.

[an_unusual_eye]

thanks everyone... this seems to have been the issue.

pulled plugs - found more carbonization than i would have thought.

cleaned them - half turn lean - ran 100+ miles including stop and go... two small "seizures".

pulled - cleaned (all six still black) - another 3/4 turn.

85 miles.

pulled - cleaned - front and rear cylinders showing slightly lean... other 4 normal enough.

changed the plugs, filled the tank, and shut the garage door until after the new year.

running better than "ever". smooth... good even pull... not smelly.

i'm willing to say she's cured. hopefully time won't prove me wrong.

i always like to see closure on threads like this so that they can be useful to others, and wanted to say thanks to all for the suggestions...

best ~ rob.

Plugs can foul up fast if it's running too rich, especially in stop and go driving. After a highway run maybe some of the gunk burns off so it runs better.

Edited December 1, 2013 by an_unusual_eye