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Thread: Backfiring Carb

  1. #1
    Registered User wizeguy4's Avatar
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    Backfiring Carb

    ok I thought I had fixed this but apparently not

    My carb backfires. It is a holley 600 or 650 CFM 4 barrel, it has an electric choke that no longer works if that mean anything

    anyway, before when I was giving it gas by just pushing on the throttle at the carb, I would get backfiring through the carb when the RMS reached their peak for that push of hte carb. Catching Second told me to check my timing and that it may be advanced to far. SO I did do that and I readjusted the timing and the backfiring stopped. Until I actually drove the blazer!

    if I give it gas from the carb with the car in park, all is good. If I get into it and then step on the gas aggressively, it will hesitate and then backfire. It will also backfire once it shifts itself from 1st to 2nd as I am giving it gas. Sounds almost another vacuum problem cause the harder I step on the gas the more repitious the backfirig it or the more likely it will happen. I have been playing around with the timing now for about 2 hours and I am going crazy. I have rotated that damn distibutor in both direction sso damn much and still nothing.

    any clues??

  2. #2
    Registered User primus's Avatar
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    i think your getting to much gas, if the car stumbles then clears it's self by backfiring, does it start to get up and go after that or run like shit?

    where did you get this carb? is it a quadrajet? how is the idle does it fluxuate alot?

    i'am new too hotroding just did my first v8 swap a year ago so other please be leniant if i'am wrong but it sounds like a bad carb, providing uneven flow.
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  3. #3
    Registered User wizeguy4's Avatar
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    well the carb came on the vehicle when I bought it. I have been working my ass off lately fixing it since I have this forum. I know that it is a Holley and it is either 600 or 650 CFM. I can not remember but I did look up the numbers on Holley.com once before. It has an electric choke but when I bought it, the wires on it that are to be connected were just hanging there, so I connected them and nothing, so I am guessing that the choke is not good. BUt it still is fine.

    Yeah when you step on the gas or once it shifts into another gear, it backfires. sometimes once and sometimes several times. I thought that maybe it was a vacuum thing again cause the harder I step on the gas, the greater my chances of making it backfire are.

    Also this problem just started recently, like in hte last month.

    I have tried changing the fuel mix on both sides of the carb. I even turned it all the way down and then turned it up a hair so it would not stall. I did this to see if having too much gas might be the proble. It still did. It ran crappy this way, but still did it. SO I turned up the air fuel mix a little higher.

    After it backfires it seems to then go faster and the idle is nice and smooth. although there must be an aftermarket cam in it cause you can here it when it is idle.

    But here is the sequence of events - step on gas, blazer hesitates, backfires (which makes it buck a bit) and then takes off till I either quickly give it mroe gas or the tranny changes to the next gear, which starts this process over again

    so for the legnthy posts. I am just trying to give as much info as possible

  4. #4
    Doesnt look like this supernova's Avatar
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    That was what my car was doing when the timing was off. what year is the engine. Call the auto parts store and get the correct timing for it. Just guessing at the timing might not get you on the right mark. But your problems sounds just like mine when my timing was wayy off.

  5. #5
    Registered User ZZ4Blazer's Avatar
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    Sounds like your timing to me too. Like you might be off a tooth on the distributor, or something.
    Jim 94 Stepside
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  6. #6
    Registered User Yenko's Avatar
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    Did you take the distributor out for any reason? If so, you might have put in back in a tooth off. If your a tooth off, you still can time it that way so that it will run, but keep in mind, it will only run and that's it. If thats not it, you might have a dead spark plug or wire. Start the motor up and let run for 20 seconds, put water on the header starting with cylinder #1 and work your way down the tubes(check both sides). If the water boil's off, thats a good cylinder, but if the water set's on the tub for more than 3 seconds, thats considered a dead cylinder. If the carb is bad, or flooding, you'd notice black smoke from the exhaust. Since the motor is backfiring through the carb, the power valve is probably blown by now. Probably all in all, its probably your timing. Lets us know what you find out and what you have done before this problem has occured. Like what you have touched or moved...etc.
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  7. #7
    Registered User wizeguy4's Avatar
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    well I have removed the distributor cap but it fit back on in only one spot, it just popped backed on and into place so I figured that can not be the problem, but I guess it can be.

    supernova - I am told that I have a 1975 corvette motor

    yenko - I have moved or touched about everything and have prolly caused myself more grief than I would have ever imagined. I remember when I replaced the high beam switch in the steering column. When I got it all back together, all I had to do was push in on the steering wheel and the car would start, no key needed :)

    but in this case, I have removed the distributor cap, and I also removed the wires off of the distributor cap, I have already loosened the distribuotr bolt hold down (since I have been trying to time this damn thing), idle screw and air/fuel mix on the carb. I do not think the last 2 are part of the problem and the the carb backfired before I removed the hold down bolt. That leaves me with either something that has gone bad, or the distributor cap or the wires.

    now i drew a pic of how the wires were on the distibutor before I removed them, just so that I would put them back correctly, but if I got 2 reversed, could that also cause this? does anyone have a diagram of what terminals on the distributor cap go to which cylinder? I am goona search for one also, maybe that is it

    I am also going to do the water on the header thing, I will post my results tonight

    no black smoke.

    time to get the neighbors ass back out tonight to help with the timing again. He wishes my blazer would just blow up!.

    Time to do a search for "power valves"

    I will post results tonight

    I should also add, that my blazer does not have timing marks, so that makes this whole process even more fun
    Last edited by wizeguy4; 08-10-2003 at 09:55 AM.

  8. #8
    Registered User Yenko's Avatar
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    Since you said that you took the plug wires off. You must have got some of them mixed up. And that would explain the back firing. I made up a little diagram that might help you out. After you check that the right plug wires are on the right spark plugs. Then try resetting your timing. Take the motor to about 2500 rpms and turn the distributor counterclock wise which will advance the timing. Once the motor smooths out, stop and tighten the distributor down and take it for a ride. If you hear the motor pinging, stop and pull over and and turn the distributor clock wise which will retard the timing(turn it just a little). Take another ride and go through the process again until you get the pinging out. That is the easist way to explain how to set timing without using a timing light. After you get the timing set, if there is no black smoke coming from the exhaust. The power valve might be alright. Sometimes they will blow and other times they will not. Just depends on how hard the motor backfired through the carb. If you have problems with the diagram let me know and we will try to explain it in more detail. The firing order for a small block chevy is 18436572.

    Starting from the front of the motor.
    Driver side bank=1 3 5 7
    Passenger side bank= 2 4 6 8
    The firing order going around the distributor goes clockwise. Start from the second prong were you plug the ign and tack wires into the distributor. That is if you have a HEI distributor. Which I think you have said that you have before in other post.
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  9. #9
    Senior Member billp's Avatar
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    SECONDARIES OPENING TOO SOON CAN ALSO CAUSE BACKFIRE
    YENKO , GOOD DIAGRAM!
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  10. #10
    BLAZIN THE ONLY WAY! tuneportblazer's Avatar
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    also a bad power vavle

  11. #11
    Senior Member bigjsp's Avatar
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    Check your wires,

    Pull the valve covers and check the rocker action, #2 exhaust in particular. Don't ask why but that one seems to be the culprit if the cam is wearing out.

    Random or situational back firing is seldom caused by excessive fuel, but is always a sign of being lean.

    Consistant backfiring is wiring or cam.

  12. #12
    Registered User wizeguy4's Avatar
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    ok I did the thing with the water on all of the tubes as described by Yenko and found that cylinder 5 and 7 had something going on with them. So I was going to come on here and tell of my news. But when I looged on Yenko had made another post about wire locations. well 5 and 7 were reversed. I changed them back around and everything is now fine
    Thanks

    Now if I may, is this what was going on? Since the wires were reversed, then the plugs were still firing but not in the right order, so when they did fire there was not really any compression cause the piston was no longer up and instead the intake vavle could have been open at the time that it did fire and that is what cause the backfire. It was just the combustion coming up through the intake valve and through the carb. Is this at all right? I am just trying to get a better understanding of all this. Now if that is correct, then how come it would not back fire in idle but it would when I would drive it. and it would also do it when I put the car into driver (or reverse) and put my foot on the brake and gave it gas so it could not go anywhere. That I do not undestand?

  13. #13
    Registered User Yenko's Avatar
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    The reason the motor did not backfire was cause the motor had no load against it. Plus inside the distributor is counter weights. The more rpms the motor turns, the weights will sling out werd, which will advance the timing. But at only certain rpms. Another reason is the vacuum advance on the distributor. The vacuum advance takes so many vacuum inch pounds to advance the timing. Hope this makes since. And your right about the valves and the pistons is why it was backfiring. One last thing and I will stop whorring your thread. A V8 will only spark knock(aka pinging) under a load. Not while revving the motor up or at ideal. So, you have to drive it down the road or you can put it in gear and press on the brake and give it some gas(like 1500 rpms). Almost like power breaking it. So, that is how you can find out if you got spark knock after you time it. By the way, glad you got it fixed.
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  14. #14
    Registered User Yenko's Avatar
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    I forgot to add. The reason it did not backfire was cause you re-timed it. You got the intake valve closer to being closed is why it did not backfire at ideal (but at while you had it in gear, it did). Once you gave it gas while in gear, it started backfiring cause it had a load against it. Have I got you confussed yet?
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