Slight Misfire Mystery

Engine, Trans & Driveline

  1. 66 Sat

    66 Sat Well-Known Member

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    Hi all, I have a confusing problem with my 318 Poly. It was fully rebuilt a year ago and has done about 2,000 miles since, long trips, short trips, driven at least once a week. Car runs faultlessly, or did until I noticed something last week. I'd gone behind the car while it was idling and noticed the passenger side tailpipe sounded very different to the driver's side. Not a nice burble but a farty misfire sound. When the engine is revved the sound is more noticeable - that side is quieter and the miss seems more obvious.
    A local expert in all things V8 had a quick listen and thought it was a very slight miss, probably an exhaust valve not seating fully.
    I've pulled the plugs - they all look good. Swapped them bank to bank - no change.
    Checked each HT lead, swapping in a new one I bought, one at a time. No change.
    Checked the distributor cap - all looks good inside.
    Compression check - on the side with the odd noise 170, 160, 170, 160. On the good side 170, 170, 160, 170. All cold readings.
    The tailpipe end on the good side is warmer to touch than the dodgy side.
    It has electronic ignition, new 600 Holley, and runs perfectly from the driver's seat. Performance seems unchanged from a year ago. If I hadn't have walked behind the car I would've had no clue there was a potential issue.
    Any ideas? I was thinking maybe a dodgy muffler, or a partially stuck manifold heat control valve - that's all I can come up with.
    Interestingly, when I told my son about the issue he said "yeah it's done that for ages - about 2 years". I reminded him the car's only been on the road for a year, and he reckons it's always sounded like that (but hadn't thought to mention it!).
    Anyway it's booked in for the guy to go through it properly next week, but I'd be interested if anyone has experienced something similar and what it was.
    Cheers
     
  2. BeatersRus

    BeatersRus Well-Known Member

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    id lean towards the heat control valve or muffler as youve already noted.
    i once had a 383 car that ran right up to a 100 and would flatline there..
    turned out the heat control valve was stuck,after fixing it the car would run 120 all day long.

    can you feel a major difference in heat from side to side on the exhaust manifolds?
    or,do you have one of these really handy harbor freight temperature guns?

    one thing tho,have you checked the cap and rotor Very carefully because if its cracked it can misfire just like you are describing.
    also do the plug wire test,ie,pull each one off while running and see what it sounds like.
     
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    • Ron H

      Ron H FBBO Gold Member FBBO Gold Member

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      Maybe no relation; but I just started having problems after succumbing to the idea of lighter springs in my dist and now have a slight miss under heady throttle. My poly engine was rebuilt and had a nonfunctioning vac advance with about a 23 degree advance so installed a new one. I had installed electronic ig years back. Timing adjustments impact the extent of miss. Runs great w/o the advance hooked up, so now likely taking those springs out to put the original ones back and see what happens next.
       
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      • R413

        R413 Well-Known Member

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        Does it still have adjustable rocker arms? Have they been adjusrpted since the rebuild? Try there.
         
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        • PurpleBeeper

          PurpleBeeper FBBO Gold Member FBBO Gold Member

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          All good advice above...my first guess would be a plug wire burnt or coming partially loose on one end, but all the idea above are good things to check
           
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          • Johnpat

            Johnpat Well-Known Member

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            Heat riser on passenger exhaust manifold
            probably the reason for sound difference.
            Check to see if it is free to move by rotating
            it by hand,with engine cold or you'll burn yourself.
             
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            • 66 Sat

              66 Sat Well-Known Member

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              Thanks for the replies. It can't be a plug lead - as per my original post I've already checked that. I bought one new long lead and with the engine running, swapped out the leads on the bad side one at a time. There was no change. The car idles nicely, apart from the sound. It does have a bit of rock at idle but I think that's from the upgraded camshaft. The valvetrain clearances were re-adjusted a few months back. I'll have another look at the distributor cap today and inspect for cracks.
               
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              • BeatersRus

                BeatersRus Well-Known Member

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                i was just thinking about this,
                occurred to me it might be as simple as a cracked exhaust manifold,bad heat riser vlave seating or cracked or worse baffles gone and no bolt thru the manifold.
                or somewheres thats letting enough air into the system to make it pop at the rear...?
                random thoughts....
                 
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                • 66 Sat

                  66 Sat Well-Known Member

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                  The heat control valve is where I need a kick up the ass anyway. It spins freely by hand from the outside, 360 degrees - I'm not sure it's even inside there. The thing that's annoying me is why I didn't check it properly when I had the manifold off. The nuts where the manifold flange connects to the down pipe were welded up at some point over the years so I'd cut the pipe off further down when I removed the engine and 20171218_161105.jpg couldn't see the valve from the inside l. I put some bent wire down there to check if the valve was open and it seemed there was no valve in there at all, but I'm not 100% sure, and it would have been 100 times easier to cut the manifold open to check and then weld back up while it was off the car, but I was keen to get the car running, so I painted the manifolds and put them back on...
                  To check it now will be a royal pain in the ass, but I will if we can't find anything else.
                   
                • BeatersRus

                  BeatersRus Well-Known Member

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                  if it was me,id take that manifold off again and have at it.
                  grab the riser wheel and try to move it up and down instead of back n forth.
                  if Any movement thats your air leak.
                  the flapper sits on a rod that wears constantly on the manifold so they can go bad.
                  think of why you have to put bushings in a carb and youll get my drift.
                  if theres no flapper in there,id weld it all up nice solid shut and call it a day.
                  then at a glance it looks stock but theres nothing there to stick or leak.
                   
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                  • 66 Sat

                    66 Sat Well-Known Member

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                    Well I rechecked the plug leads by swapping the whole lot bank to bank, and carefully inspected the distributor cap for cracks and burns. No change whatsoever.
                     
                  • 66 Sat

                    66 Sat Well-Known Member

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                    Well it's definitely not the manifold heat control valve. I pulled the manifold off and cut the damn thing open and there's no flap in there - just the rod.
                    So if the plugs are all good, the leads are all good, the distributor cap is good, the valves are closing properly, the exhaust gases are escaping, why does it sound like there's a miss on one side of the engine? I'm stumped.
                     
                  • Don Frelier

                    Don Frelier FBBO Gold Member FBBO Gold Member

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                    Exhaust leak?
                     
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                    • BeatersRus

                      BeatersRus Well-Known Member

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                      where the rod goes Thru the manifold is it welded shut or does the rod move freely.
                      if its moving freely and theres Any play air can be sucked in.
                      this is why i said if theres no flapper you should weld the rod solid on both ends in manifold.

                      at this point..i went back and re read your post.
                      this makes me wonder about the Valvesprings.
                      is it possible one is weak and not fully seating when running?
                      im guessing it wouldnt have shown up under cold compression check.
                      this is a stretch,of course,i also noted that you said the car runs great otherwise.
                       
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                      • Moparnocar

                        Moparnocar Well-Known Member

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                        Maybe a exhaust valve leaking warm? Maybe redo the compression check warm, or do a leak down test warm
                         
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                        • miller

                          miller Well-Known Member

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                          On my exhaust manifold, that valve was screwed up. Knocked everything out, tapped the shaft holes, and put pipe plugs in 'em.
                           
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                          • 66 Sat

                            66 Sat Well-Known Member

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                            Thanks for the continued suggestions. The rod that held the flapper is solid, doesn't turn at all. The housing over the spring rotates freely around it, but there's no leaks on the rod I'm sure.
                            The sound is the same with the engine hot or cold.
                            I've been thinking about this all night (as you do) trying to work it out. Is it possible that the camshaft could have one lobe slightly off, timing wise, which could cause this?
                             
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                            • 74gtx

                              74gtx Well-Known Member

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                              My first guess would be an intake leak or a vacuum line. I’ve had the same symptoms and mine was a vacuum leak. Good luck! Mike
                               
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                              • Mopar4Speed

                                Mopar4Speed Active Member

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                                First thing i would check would be the temperature on each of the runners on the manifold using a laser temp reader. See if you have cooler cyl not burning gas properly. ( if yes pull the plug again take a look). Most poly's still have solid cams so the next i would check would be the valve lash. If not i would consider, checking base timing, dwell (if using old distributor, may have worn bushings). Not saying it is but check the firing order on the wires, mixing up a pair of wires can cause what you describing, runs fine on idle but causes a worse miss when revving (I've seen this more times than I can admit). Carb jetting/fouling or cleaning may be another issue cause one bank to run leaner or richer than the other. What kind of electric distributor do you have? Most recommend not using solid core wires with them. Sometimes wires can have very small cracks as well, easy way to check is wait till it's dark and start the engine carefully look around the engine bay to see if there are any arching from the wires. Hope this helps
                                 
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                                • 66 Sat

                                  66 Sat Well-Known Member

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                                  Update: After checking just about everything, the conclusion was...there's nothing wrong. The car runs perfect, just sounded different from side to side. Today I had a H-pipe fitted which has evened out the sound, so time to move on and forget about it. Who knows if there will be an issue down the track, for now I'm not worrying. Thanks for all the advice, it was appreciated.
                                   
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