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  • 304 backfiring-no power

    I will try and keep this short as possible, my 77 cj-5 with original 304 jumped time. no biggy right? replaced timing gears and chain, made sure they were "dot to dot" turned engine over twice before buttoned her back up.
    still dot to dot - rotor button pointing to number 1 on cap at TDC. new distributor with all new vacuum advance guts etc, new ECM, new cap and wires, and new coil.
    she starts up and idles like it should but as soon as I give it any gas she starts spitting, sputtering and backfiring thru the carb. pulled compression and all cylinders are at 120 psi. pulled valve cover off to do a random check of push rods, none so far were bent.
    if valve/valves is/are bad how can I tell without pulling heads?
    any other suggestions would be greatly appreciated
    thanks

  • #2
    have you tried advancing your timing more? is the new distributor vacumn or mechanical advance?

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    • #3
      timing is right where it is supposed to be 7 degrees
      I have advanced - retard the tinig with not much change for the good, will only crank and idle at the correct timing degree
      man I am running out of options here????!!

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      • #4
        what about the advance on the distributor? are the weights still intact and working properly?

        also, some timing wheels have multiple dots on them. you have to make sure the right two dots are lined up.

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        • #5
          yea the new gears had multiple dots, markings etc, so I tore it back down and put old gears on and then matched new with the old dots to make sure they were dot to dot
          not sure what you mean about the dist weights etc please explain
          do you think it is out of time still and nothing wrong as far as valves/push rods etc?
          thanks for all your help so far

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          • #6
            set your timing with the advance disconnected. my 2cents, i'm no expert; that's really all i have to add.

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            • #7
              when you put the timing gears on, did you have the cam gear dot at 3 o'clock and the crank gear dot at 6 o'clock? they should go on like this, with exactly 12 pins between them. i am pretty sure this is how it goes, but you may want to double check. i just put a new timing set in my 304 a few month ago. also, the dizzy is in right, right?

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              • #8
                the dots are supposed to be lined up 6 oclock on top 12 oclock on the bottom. it doesn't matter where the dizzy is installed as long as the rotor is under the #1 plug when it's at TDC.

                have you checked compression? i'd be surprised if it's a valve because it's running fine when it's at idle. it sounds like an ignition problem to me, that's why i wanted to know what kind of distributor you have. it might be the computer spark control.

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                • #9


                  Or you have vacuum problems.

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                  • #10
                    yeas i am at dot to dot at 6 and 12
                    dist is a prestolite (I think) has vacuum advance hose out the front of 2 round balls (for lack of a better term) going into the carb
                    compression on all 8 is at 120 psi
                    computer spark control? is that the same as the electronic control module on the fender well? if so I bought a new one tonight as well as new plug wires (everything else has been replaced new, which means absolutely nothing these days with foreign parts)
                    thanks

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                    • #11
                      Do you happen to know a good mechanic to check it?I had a similar problem had my dist a couple of teeth off would idle but not run,my mechanic friend pulled it and reinstalled the dist then the engine ran perfectly.

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                      • #12
                        I had the same problem, and I .....

                        What I found out is that my 5 and 7 spark wires were tied up right next to each other and it caused the 304 of mine to cross fire. with my carb out of adjustment, it ended up fouling out my 5 and 7 spark plugs so bad that they did not have any gap left. ugh!

                        Another thing is that my edelbrock vacuum intake was on wrong. I built the engine and everything but put it into the manifold port instead of the ported vacuum. basically as you look at the carb, the vacuum input on the left is what you want.

                        Finally, because by carb is big, (4bbl) and sucks so much fuel when I press on it, i had to install a vaccuum port and hooked up a gadget that connects into the manifold vacuum. that way when the excess fuel is in the carb after a de-celeration, the vacuum sucks it out and it does not back fire.

                        I also had to replace the 9 inch air cleaner with something bigger like the 14 inch. I'm looking to get an enclosed air cleaner for the jeep, but can't figure out what type of GM truck would have it so I can pick it up at the junk yard.

                        I tell you after I made all of these adjustments, the baby runs like a champ. The ported vacuum was the trick, because the way I had it was as I increased by revs, the timing would decrease. now it's increasing with the revs, and boy does it jump!

                        This is what I had to do to get mine running GREAT! Maybe it will help you.

                        Good luck and post what you end up doing.

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                        • #13
                          I also had my advance hooked upto the wrong port on my edelbrock. I also has very bad backfiring through the carb.

                          With it hooked up correctly my 304 does better, but still starts very hard. I think it floods itself out while I am cranking it. I have played with the advance so much its driving me crazy.

                          She burns oil and I have low vacuum at the intake, I forget the number. But i thought 12-16 was a good reading and mine was like 8 or something.

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                          • #14
                            update - replaced a new ECM with another one as well as all new spark plug wires and she fired right off, more power sounds better but still have a slight "hiccup" in the carb if you hit it hard, if you ease into it she purrs nicely, will try some tweeking later on the carb.
                            I did however do a random check on the push rods and none were bent. question is do I need to bring each cylinder up to compression stroke before I torque them back down?

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                            • #15
                              Originally posted by boydog40
                              I did however do a random check on the push rods and none were bent. question is do I need to bring each cylinder up to compression stroke before I torque them back down?
                              On the AMC 304, as long as the cam and crank are in time the answer is no. As you tighten the pushrods random valves will open which is OK since the piston will be in the correct position to allow room for the valve.

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