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'84 305 backfiring


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  • '84 305 backfiring

    My friend built a 305 for his truck and we can't seem to get it to straighten out and run right.

    The symptoms are sluggish revving, popping in the exhaust when you rev it up and it has popped back through the carb occasionally when you rev it up. It won't idle unless the idle screw is screwed pretty far in. It restarts easily but the idle wavers around and it sounds like 1 or 2 cylinders are misfiring occasionally.

    It has a new distributor, fuel pump and carb. New wires and plugs. The engine itself is basically stock with a slightly hotter cam. It isn't smoking or anything like that. This is the third carb it's had on it with the same results so I doubt it's a carb issue.All the vacuum lines are new.

    We've adjusted and re-adjusted the timing with no changes.

    We are going to adjust the valves tommorrow in hopes that might resolve the problem.

    Any ideas?

  • #2
    Sounds rich. Sluggish revving and poping means too much fuel. I would adjust the carb.
    1990 454ss - Prostreeted

    1990 454ss - 2"/4" Drop, Flowmaster, Edel 2162 Cam, Edel 3764 Intake, Edel Headers, Vortech Supercharged SOLD!!!!


    • #3
      We've tried that. Turned the mixture screws in until it was about to die from being lean. Plus, it isn't loping or smoking black.

      It's just very sluggish... you can hold the throttle almost all the way open and it's doing good to hit 4500 rpm. It spits through the carb and the exhaust both when you try that maneuver as well.


      • #4
        My 350 rocket did this
        CHeck timing
        And make sure your firing order is correct.
        I'm positive it is one of the two.
        I had the EXACT same symptoms
        1995 GMC Sierra
        Black Primer
        Shaved:Tailgate handle,roofseams,gasdoor,antenna,3rd brake light


        • #5
          I second low lyfe's comments. Timing and firing order. Mine was timing. I know you say you've checked timing but my 84 305 I had in my jeep did this and I had to turn the distributor a couple of teeth to get it to where I could adjust it correctly. I actually had to set the timing at about 28* for it to run correctly. I gave up on trying to set it to recommended values since it ran like he77 there.


          • #6
            Well, we've done some more testing:

            Putting the balancer mark at 0 sets the rotor almost one tooth advanced. However, when you turn the distributor to line #1 terminal up with the rotor, it will hardly run. Turn it back advanced and it runs, although still not well.

            Vacuum gauge reads about 13". The needle flutters, although there does appear to be a dead cylinder..don't know if that would cause the flutter.

            Compression test results were uniform with most cylinders about 130lbs, with one at 120 and a couple about 125lbs.The the front four plugs were sooty, and the rear were less so with more tan-brown coloration.

            The engine has about 700 miles on it. 1986 LG4 with small aftermarket cam. .040 over. Otherwise stock as can be.

            Are the timing marks in different places on different SB balancers? What about the timing tabs on the timing cover?

            Thanks again...


            • #7
              Don't go with the timing marks. At the point you're at, I just gave up on the timing light and set it to where the engine would run better. The other thing you could try is tuning your engine with a vacuum gauge. The link below describes it better than I ever could.


              Good luck

              Edit. My timing marks on the timing cover weren't even close to the timing mark on my balancer. Are you using a timing light with adjustable advance? If so you may be able to see the marks align. I just used a regular timing light and set it as described above.


              • #8
                I have an '85 with a 305. I got 273000 miles on that engine with never opening it up except for a Timing Chain. My advice; forget the Timing Light. If you set it to specs it will run Doggy. Look at the Firing order again. It's tough when you have so many variables. New Cam, New Carb, new tuneup parts.
                '75 Jeep Fiberglas Body, with Chevy 350/345 HO motor.
                '85 Chevy Stepside 300000miles
                '89 Volvo 290000miles
                '91 Ford Explorer 315000 miles ((piece of junk)) but it was $450
                Maintained by me. They've never seen a Service station



                • #9
                  Originally posted by MBaxter
                  Well, we've done some more testing:

                  Putting the balancer mark at 0 sets the rotor almost one tooth advanced. However, when you turn the distributor to line #1 terminal up with the rotor, it will hardly run. Turn it back advanced and it runs, although still not well.

                  Thanks again...
                  Kind of confused, here. Are you trying to run this car timed at 0 deg. initial timing? And when you say 0, are you certain your at TDC and not BTDC? And why are you returning the dist, at TDC, back pointing to the #1 terminal? If that is what you are doing then you are trying to run the car with 0 deg. timing at idle. I would suggest 8-12 deg. at idle.

                  Heres the EASIEST way to ensure your dist. is set right. Pull it back out, turn the motor back to TDC using the finger over the #1 cylinder method. Once you know your on the right stroke then turn the balancer to 8-10 deg. and drop the dist back in w/ the rotor pointing at the #1 terminal on the cap. Now you know for sure you have atleast 8-10 deg. initial timing (depending on where you set your balancer when dropping it in) right from the jump. The car should fire and you can adjust from there.

                  Stocker timing specs for a non stocker cammed and carb'd mill are worthless. I would start at 8 deg. and work it from there.

                  With HEI's its either in right or 180 out of phase (as in dropped in at 0 BTDC instead of 0 TDC) One tooth, etc doesnt matter as long as you can adjust the timing and get the numbers the car likes to see. Its either in right or 180 out. I would eliminate this possibility first.

                  Sorry If I misunderstand what you are trying to do. Just trying to help out.

                  Oh- who did the assembly of the mill? Who degreed the cam and did you use a degree wheel and follow manufacturer specs for advance or was it installed straight up. Cam being out of phase will cause this and lack of power as well.
                  05 Sierra 1500 ECSB -

                  5.3, 3.73's, wait4me tune, vette / billet servos, TransGo,
                  Volant CAI, Escalade gauges, GM 20's and some tint.


                  • #10
                    Okay, here's what I did pertaining to the balancer/timing marks:

                    I pulled the cap off so I could watch the rotor. I then turned the engine with a turning tool until the rotor was nearing the #1 firing position and the mark on the balancer was coming up. When I lined the mark on the balancer up exactly with 0 on the timing tab, the rotor was at almost the next terminal clockwise from #1(it would be # 8). It was nowhere near #1 terminal.

                    So, I rotated the distributor body clockwise so that with the balancer mark on 0, the rotor pointed at #1 terminal. That's where it would hardly run at all. Rotate it back to it's previous position and it runs, but sluggish.

                    I understand what everyone is saying about the timing being low, which is what it definately seems like. However, cranking the distributor around advanced DOES NOT make it run better. It still spits and sputters, and the starter drags. That's why I'm thinking it isn't just a simple distributor problem.

                    We are going to run the valves tomorrow and take a good hard look at the valve timing.

                    I *think* the cam that is in it is the Lunati LUN-00016LK.


                    It was not degreed, installed straight up(we hope) with a generic timing set.

                    Thanks again...


                    • #11
                      make sure the timing advances when you rev it up. you could try putting a vacuum gage on this is the easiest way to figure out whats going on.


                      • #12
                        When you turned to crank till it reached the 0 timing mark, did you check to see if the #1 was in the compression phase or the exhaust phase? Exhaust phase would make it 180* out from TDC. At that point, the engine would not run at all just backfire through the carb. By adjusting the distributor from #8 to #1, you advanced the timing. Maybe just enough to get the engine to start up, but I wouldn't know unless I tried it myself.

                        Someone stated earlier to check for TDC by the thumb method. Remove the plug from spark plug #1 hole and turn the crank towards the 0 timing mark. As the piston approaches TDC you should feel pressure building in the hole. If it's 180* out (exhaust) there should be no pressure when the crank reaches the 0 mark. Now you know that piston #1 is at TDC and you can drop your distributor in to align.

                        Sorry if we keep going on about the timing but to me, this is the most probable cause of your issues. Yes, it could still be other things but until youhave to rule out the timing completely. It's the easiest thing to correct.


                        • #13
                          We fixed it. It turned out to be 3 bad carbs. We finally broke down and installed a new 600cfm Holley and it runs fine now.The valves were rather tight also.

                          Thanks for all the help


                          • #14
                            3 bad carbs. That really bites! Congrats and enjoy your work.