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Thread: backfire on throttle when warm after idling...

  1. #1
    Registered User an_unusual_eye's Avatar
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    Default backfire on throttle when warm after idling...

    i think i'm facing over-influx of information. hoping that someone has experienced exactly what i'm facing and may intersperse some clarity into what i've "learned" with the search and print sources. z-therapy video is on the way as well... anyway:

    4 screw SU carbs... recent rebuild and "professional adjustment"... runs well cold and warm, and runs fairly efficiently.

    until. you get. stuck in traffic.

    after a few minutes of stop and go (warm), i get smallish backfiring under acceleration, which clears up on its own after a few minutes at steady speed.

    can't find exactly this issue related anywhere with a "cure".

    any and all insight appreciated.

    best ~ rob
    ~ rob.
    harrisburg, pa / raleigh, nc
    triangle z club

    1974 260z (early) ~ 302 green/black (sold)
    1971 240z (late) ~ 901 silver/black (sold)
    1978 280z 2+2 ~ 611 burgundy/black (parted out)
    1971 240z (early) ~ 907 green/black (my long term mistress)

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    Pull the choke on and see if that solves the problem. If it does, then you're more than likely lean which can show up as a spit back through the carbs.....
    Bruce Palmer
    Salem Or
    Sales@ztherapy.com
    www.ztherapy.com
    503-587-9800

  3. #3
    Registered User an_unusual_eye's Avatar
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    thank you bruce... a great place to start. it would be truly ironic if, after years of trying to get the car to stop running rich, we'd gotten her to the point of negatively opposite. typical... but ironic.
    ~ rob.
    harrisburg, pa / raleigh, nc
    triangle z club

    1974 260z (early) ~ 302 green/black (sold)
    1971 240z (late) ~ 901 silver/black (sold)
    1978 280z 2+2 ~ 611 burgundy/black (parted out)
    1971 240z (early) ~ 907 green/black (my long term mistress)

  4. #4
    Registered User an_unusual_eye's Avatar
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    sorry to come back to this so long after... but the first real use the car has had since the last post was today. after some city driving, the problem is back full force, and doesn't seem to be clearing up on its own after 20 miles or so on the highway. not terrible at steady throttle, but even the slightest pressure on the gas peddle gets spits and sputters and small backfires out the exhaust. choke seems to have no beneficial effect at all. ran fine for 100 miles or so before the stop and go traffic. any insight will be greatly appreciated... i'm poking it with a stick an an autozone parking lot as we speak. many miles to go. thanks ~ rob.
    Last edited by an_unusual_eye; 10-12-2013 at 12:37 PM.
    ~ rob.
    harrisburg, pa / raleigh, nc
    triangle z club

    1974 260z (early) ~ 302 green/black (sold)
    1971 240z (late) ~ 901 silver/black (sold)
    1978 280z 2+2 ~ 611 burgundy/black (parted out)
    1971 240z (early) ~ 907 green/black (my long term mistress)

  5. #5
    Rust Free'ish zKars's Avatar
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    Look for vacuum leaks next.
    -----------------------------------------
    Jim
    73 240Z HLS30 149331
    69 510 PL510 77603

    www.zKars.com
    www.calgaryzclub.ca
    Reference materials
    www.xenonS30.com

  6. #6
    Registered User an_unusual_eye's Avatar
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    i decided to limp it home (another 40 miles or so)... the symptoms persisted until i got back to the highway, and another couple miles at 70mph, then cleared up like somebody flipped a switch. i can almost guarantee that the car will be totally fine until the next time i get into an extended idle or stop and go situation. checking vacuum lines now... but most are fresh.
    ~ rob.
    harrisburg, pa / raleigh, nc
    triangle z club

    1974 260z (early) ~ 302 green/black (sold)
    1971 240z (late) ~ 901 silver/black (sold)
    1978 280z 2+2 ~ 611 burgundy/black (parted out)
    1971 240z (early) ~ 907 green/black (my long term mistress)

  7. #7
    Boat Anchor Repairman Captain Obvious's Avatar
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    I'm having a hard time forming a mental picture of what the symptoms are...

    You're saying that it runs fine when you first start it and it continues to run fine until you do some stop-n-go type of driving?

    And then after the stop-n-go, you can get onto the highway and the problem goes away all by itself after a couple highway miles?

  8. #8
    Registered User madkaw's Avatar
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    What year?
    What fuel pump?
    Fuel pump been checked for flow- measured?
    Fuel filters been changed?
    " professional adjusted" by who?
    Ignition type?
    Mixture screw adjusted to how far out?
    Steve
    71 240z,bw-5sp 2.4-40 over,balanced,e-88,big valves,ported&polished, stage2,header, triple Mikuni's 40's
    3.90 Subaru STI LSD

  9. #9
    Registered User olzed's Avatar
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    Only when hot.
    Fuel vaporizing maybe??

  10. #10
    Registered User an_unusual_eye's Avatar
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    thanks guys... first, it occurred to me today that i've used the term backfiring incorrectly. this is popping from the exhaust rather than backfiring.

    Captain Obvious, you have the symptoms exactly. we used the car for over 100 miles today with no issue at all. started and parked 4 times, local and highway mix, but no long back to back red lights or construction delays.

    madkaw... 1971, stock OE mechanical pump, not yet checked for flow rate, fuel filter was fresh and changed again yesterday, adjusted by a local brit-car-guru, ignition is pertronix/flame thrower, mixture screw... sadly i won't be able to answer until november, as we're only where the z lives for one weekend each month, and she's in the rear view mirror for october.

    olzed... NOT only when hot. if the car is in "problem state" when parked, the condition remains when she's restarted cold.
    Last edited by an_unusual_eye; 10-13-2013 at 03:25 PM.
    ~ rob.
    harrisburg, pa / raleigh, nc
    triangle z club

    1974 260z (early) ~ 302 green/black (sold)
    1971 240z (late) ~ 901 silver/black (sold)
    1978 280z 2+2 ~ 611 burgundy/black (parted out)
    1971 240z (early) ~ 907 green/black (my long term mistress)

  11. #11
    Registered User madkaw's Avatar
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    Check the fuel rate.
    Take the mixture screws a turn out and see if that improves anything.

    How do the plugs look? your desciption lends to a lean condition. Your Guru might get the adjustments right for sync, but not sure how close he can GUESS on the mixture. My first inclination would be to richen things up before I did anything else. Then I would suspect fuel pump-IF I knew my gas tank was clean and not the culprit(dirty clogging fuel lines)
    Steve
    71 240z,bw-5sp 2.4-40 over,balanced,e-88,big valves,ported&polished, stage2,header, triple Mikuni's 40's
    3.90 Subaru STI LSD

  12. #12
    Boat Anchor Repairman Captain Obvious's Avatar
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    Quote Originally Posted by an_unusual_eye View Post
    NOT only when hot. if the car is in "problem state" when parked, the condition remains when she's restarted cold.
    That should make it easier to find, right? Mixture screws don't do that.

    Stop-n-go driving isn't a particularly high fuel consumption situation. You've got way more fuel flowing through the carb (per unit time) on the highway than you do during stop-n-go. And the temperature things is weird for fuel supply. I'm having a hard time coming up with any sort of fuel supply issue that's temperature related. And certainly nothing that "latches" into a problem state as you've described.

    Besides, it's not mid-summer anymore. If it's got a temperature related issue at this time of year, what was it like this past summer?

    I'm thinking electrical... What's in the distributor? Is that the Pertronix?

  13. #13
    Registered User an_unusual_eye's Avatar
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    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.
    Last edited by an_unusual_eye; 10-13-2013 at 10:03 PM.
    ~ rob.
    harrisburg, pa / raleigh, nc
    triangle z club

    1974 260z (early) ~ 302 green/black (sold)
    1971 240z (late) ~ 901 silver/black (sold)
    1978 280z 2+2 ~ 611 burgundy/black (parted out)
    1971 240z (early) ~ 907 green/black (my long term mistress)

  14. #14
    Boat Anchor Repairman Captain Obvious's Avatar
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    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.

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    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.

  16. #16
    Registered User an_unusual_eye's Avatar
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    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.
    ~ rob.
    harrisburg, pa / raleigh, nc
    triangle z club

    1974 260z (early) ~ 302 green/black (sold)
    1971 240z (late) ~ 901 silver/black (sold)
    1978 280z 2+2 ~ 611 burgundy/black (parted out)
    1971 240z (early) ~ 907 green/black (my long term mistress)

  17. #17
    Boat Anchor Repairman Captain Obvious's Avatar
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    Quote Originally Posted by an_unusual_eye View Post
    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?

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    Registered User an_unusual_eye's Avatar
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    Quote Originally Posted by Captain Obvious View Post
    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.
    ~ rob.
    harrisburg, pa / raleigh, nc
    triangle z club

    1974 260z (early) ~ 302 green/black (sold)
    1971 240z (late) ~ 901 silver/black (sold)
    1978 280z 2+2 ~ 611 burgundy/black (parted out)
    1971 240z (early) ~ 907 green/black (my long term mistress)

  19. #19
    Registered User Stanley's Avatar
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    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.

  20. #20
    Registered User an_unusual_eye's Avatar
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    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.

    Quote Originally Posted by Stanley View Post
    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.
    Last edited by an_unusual_eye; 12-01-2013 at 07:12 AM.
    ~ rob.
    harrisburg, pa / raleigh, nc
    triangle z club

    1974 260z (early) ~ 302 green/black (sold)
    1971 240z (late) ~ 901 silver/black (sold)
    1978 280z 2+2 ~ 611 burgundy/black (parted out)
    1971 240z (early) ~ 907 green/black (my long term mistress)

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