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High speed misfire

Discussion in 'Performance' started by kzooman83, Dec 11, 2014.

  1. kzooman83

    kzooman83 Active Member

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    I am having an issue with a misfire around 4000-5000 rpm under moderate to heavy throttle application in my big block Duster. Some of you might be familiar with my setup by now, but here it is anyway:

    '77 400 big block bored .030 over
    9:1 compression
    Summit 282/292 hydraulic cam
    edelbrock RPM intake manifold
    Holley 3310 (750 cfm, vaccuum secondaries, dual feed)
    #74 jets on the primary side
    6.5" power valve

    This problem was absent or not noticeable during the warmer months, but is very pronounced with the lower temps lately (it was 35 degrees today). I suspect it is leaning out slightly on the top end and this is exacerbated by the cold weather. Should I consider going up another jet size, or should I look at the power valve? The engine only generates 9-10 in/Hg at idle in gear, so the 6.5 in/Hg power valve should be adequate, right? Thanks!
     
  2. dana44

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    Yes, that is the right power valve. How is the tune-up on her? I know you take good care of the car, but it could also be sparkplug gap, dirty cap or old wires and heavy humidity is affecting it at higher rpm (spark scatter, electricity passes the shortest distances between two points thing). You can also check the gap for the squirter to make sure the arm is adjusted properly, touching without any pressure at no throttle pressure so it reacts quickly. Best way to verify the power valve is working is to start and idle warmed up, then turn the two needles in. A blown power valve will allow this, a good one won't. Gas mileage usually suffers when it is blown as raw fuel dumps more (needles set at 1.25-1.5 is correct jetting, as you know).
     
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  3. ImperialCrown

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    What color are the spark plug tips? Any fouling? Is the problem there with cold or warm engine coolant temperature (choke on or off)? A low-restriction air intake will generally lean things out (maybe too much).
    If the misfire is more engine speed dependent rather than load dependent, the valve springs may be getting soft.
     
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  4. kzooman83

    kzooman83 Active Member

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    Thank you for the replies. I have not had time to pull and read the spark plugs and read them this week; I will do that as soon as I can. The plugs were replaced about a year and a half ago and I probably don't put more than 1,000 miles a year on the car. Wires and distributor cap and rotor are new within the last 3 or 4 years, can't remember exactly when I changed them. The engine dies if I try to screw the idle mix screws in too far, so the power valve works.

    I should have mentioned that the air cleaner is a home-made sealed cold air induction setup I put together and I am running an edelbrock filter. I also just changed the mufflers: got rid of the cheapo louvered glasspacks and put on Dynomax Ultra flos. These flow better than the glasspacks and might be part of the problem also, I am not sure.

    The misfire is more pronounced in the first 5 minutes of driving, but it never completely goes away with the weather as cold as it is.
     
  5. dana44

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    That's a long time for wires, and cap and rotor, ozone produced inside from the electricity does not stop corroding with less driving, over time it just continues to form greater corrosion. And yes, I have old wires on my Dakota, about 4 years old at least, and yes, it does a little bit of misfire for the first two minutes of driving, but at least I know what it is, warm, no problem, so you may be leaking electricity with colder and greater humidity that does go away when warmer and lower humidity.

    It does go away when warmer, right?
     
  6. kzooman83

    kzooman83 Active Member

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    Typically it does go away as the engine comes up to temp. However, I don't usually drive the car during the winter months. I have had it out a little lately because we haven't had any snow this month, but the temperature is a lot lower than it usually is when I have the car out. These last few times, the misfire has not gone away even after the engine comes up to temp.

    Also, there is no misfire when cruising or holding engine speed steady, only while under load at higher rpm.
     
  7. Scrounge

    Scrounge Well-Known Member

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    Have you checked the choke?
     
  8. kzooman83

    kzooman83 Active Member

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    Had the car out for a drive again a few days ago; haven't had a lot of time to mess with it because the kids are both sick right now. The ambient temp was probably 48 degrees. The misfire was present the first time I wound it up, but after I drove it a ways to the gas station it warmed up enough that the misfire went away. After that it pulled hard and smooth through redline. As a bonus, I picked up a handful of horsepower by switching to the new mufflers, and they sound better. I have the choke set to open fairly quickly because after maybe 20-30 seconds of running it starts to run badly (very rough) with the choke fully closed. When the choke opens quicker, it does not have this problem, but then the choke is fully open before the engine is fully warm. When this misfire occurs, the choke is fully open. The idle mix screws are about 1 1/4 turn out, and I set this with the engine warm.
     
  9. dana44

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    Choke open when the engine is fine, it is more to ensure fuel pulls into the cylnders and even a cold engine is capable of that within about a minute of running. On all my engines, ported, of course, after the first slight stumble within the first minute of running, choke isn't needed at all. In fact, a choke on a well ported engine almost will not work at all. Get the fuel flowing and suction going and the carbide burr surface keeps the fuel well suspended, whereas a set of polished ported heads have a little more difficulty getting warmed up because the fuel puddles on the walls at lower rpm, and cold doesn't help keep the fuel suspended at all.
     
  10. kzooman83

    kzooman83 Active Member

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    I haven't ported these heads yet, although I would like to at some point in time. I started working on the 318 heads that we were talking about a while back; I have one head almost done, but progress has stalled because I don't have a vehicle to put the engine in right now. I have to get another burr and then I will probably finish at least that one head and have it inspected and then flow tested. Keeping my fingers crossed that I didn't ruin it.

    Off topic: I have read that bronze valve guides are strongly recommended with a performance cam, but the stock guides on the 318 heads show very little wear and stem to guide clearance is well within spec. How long would stock guides live with a 268/280 duration, .477"/.480" lift flat tappet cam?
     
  11. dana44

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    You'll have to post some pictures of the port work, see how you are doing. As far as bronze guides go, this isn't a giant cam and I myself have found cast guides to be just fine, it is usually the valve stem seals themselves that cause the leakage of oil vice the guides, so if they are good, save the money at this point.
     
  12. kzooman83

    kzooman83 Active Member

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    Just was perusing the forums and realized I never posted what I found out with this situation. As it turns out, the problem was two-fold. I had a somewhat large vacuum leak at the intake manifold gasket at the #2 runner, causing cylinder #2 and maybe #4 to run lean. In addition, the charging system was on its way out; the alternator got bad enough that even with the engine revving to the moon it would not maintain enough voltage to charge the battery. When I got out the multimeter I found no continuity through the field windings.

    New alternator, voltage regulator and intake manifold gaskets (I am using paper gaskets in addition to the tin bathtub gasket because I a running and aluminum intake manifold) and the issue was resolved.
     
  13. dana44

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    Belatedly, thanks, good to hear, weak spark and air leaks suck (LOL), and I myself have never found the paper gaskets to fit (too tight), so possibly the intake side of the head may have been shaved at some point?
     
  14. kzooman83

    kzooman83 Active Member

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    It's possible that the gasket surface was shaved slightly when I had the heads rebuilt years back. I did not specify this, but maybe they took a little material off while cleaning up the surface. The intake bolts up fine with them on there and it doesn't seem to affect the port alignment.
     
  15. kzooman83

    kzooman83 Active Member

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    I realize I am resurrecting an old thread here, but I have still been dealing with this problem on and off. I narrowed it down to power valve closure. With a vacuum gauge installed temporarily, the misfire occurs under moderate acceleration as soon as vacuum climbs above 6.5" (the power valve closing point). Either flooring the pedal or lifting off the pedal causes the misfire to go away. I switched out the power valve for a 10.5" and the misfire is still there, only it moved from 6.5" to around 11, again, under moderate acceleration. I also stepped up to 76 jets, since it seems that the carb is leaning out when the power valve closes, but that did not totally solve the problem. What did completely solve the problem was swapping on a 600 cfm edelbrock performer that I had laying around.

    So the problem is definitely with the carburetor, and it definitely seems to be going lean when the power valve closes. It cruises smoothly, however, so I have to wonder if the problem is something other than just jetting. I have already increased jet sizes from 72 up to 76.

    For now, I am running the edelbrock because I need a new main body for the holley carb anyway; the threads in the main body for the pump squirter screw are stripped. I would like to swap the holley back on eventually since i am giving up some power on the top end with the smaller carb, but I want to make sure this problem is fixed when the carb goes back on.
     
  16. dana44

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    Hey Guy, long time no hear. From your diagnosis, and the fact the problem went away when the carb was changed, there has to be either something plugged up inside the carb, like an emulsifier tube or an air or fuel bleed tube/passage, or, possibly the secondaries are cracking open just a hair and leaning that way, lastly, given secondaries are vacuum operated, the metering plate for the secondary is too small. Vacuum indicates something is actually/physically happening, now it's a matter of what the fuel/air ratio is doing when the vacuum changes occur. Is it going lean, or is it going rich, as in extra fuel droplets making the miss, and that is rather difficult to check when driving down the road and/or not being able to read a fuel/air mixture. I do know you can now get that as a sensor and a gauge so you can keep better tabs on them, so the next step, given you have proven it to be the carb itself, the direction you want to go. Sounds like the power valve and jets were fine where they were, you proved it is in the fuel or air bleeds of the carb. To go way off in left field, other than talked about, could be a gasket blocking a fuel or air passage accidentally.

    How does she run with the Edelbrock otherwise?
     
  17. kzooman83

    kzooman83 Active Member

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    Life's been keeping me pretty busy as of late; it's good to be on the forum again though.

    The edelbrock carb runs really well on my engine; the only thing I have done as far as tuning was to adjust the idle mix and move the accelerator pump linkage to the slot furthest from the carb body to reduce the size of the shot. The amazing thing is that this carb has an unknown history: a friend had it on his bronco years ago and gave me the carb when he junked the truck. That was 5 or 6 years ago and it's been sitting in my garage getting covered in dust and dirt since then. I was trying to meet a deadline when I put the carb on, so all I had time to do was spray it off with carb cleaner and bolt it on; I didn't even open the carb up. When it actually ran well I was extremely relieved. Like I said before, it seems to be down on power at the top end (4500+), but other than that it runs perfectly.

    The Holley carb was a used carb when I got it also; I did rebuild it, but I don't know what was done to it prior to when I got it. Is there any way to tell if the air bleeds have been messed with (drilled oversize, etc)?
     
  18. dana44

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    Only visually to see if there are any drill marks, things like that. There could be a tiny bit of debris in one of them that is offsetting the balance of the flow of air or fuel. The only way to really check is to ensure you can stick a wire through the holes, sometimes a torch tip cleaning wire set can get in there to check for obstructions. What have you done with the secondary spring kit, a stiffer spring lets the engine pull more fuel to prevent leaning, larger metering plate on the secondary plate allows more fuel to enter as it opens.
     
  19. kzooman83

    kzooman83 Active Member

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    The secondary metering plate is the stock one that originally came with the carb; verified this based on the number stamped on it. The carb is an 80508, and according to the book I have the stock plate is a 134-21. This plate has a .081 main hole, a .040 idle hole and should be stamped "21."

    If I recall correctly, I have the purple secondary spring in place. I have to check to be sure, but I believe that spring fully opens the secondaries by 5500 or 6000 on a 400 inch engine. I selected that spring based on trial and error; I went with progressively lighter springs until there was a slight flat spot when the secondaries opened and then backed up one spring heavier.

    As for the air bleeds, I have to replace the main body anyway so I guess if the air bleeds have been messed with it won't matter. It would just be nice to know for sure if that is part of the problem.
     
  20. dana44

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    Yeah, sounds like you did pretty good procedure to determine your secondary spring strength and all that, and from there, the fine tuning of the secondary plate, just because it is proper does not mean your cam profile and compression is the ultimate setup. Again, the last step is to determine, under load, what the fuel/air ratio is doing when the flat spot hits. Although we assume it is a lean thing, it could also be a rich thing, just don't know positively without the fuel/air ratio to properly diagnose. They range from $60 to $300 and plug into an O2 sensor, but it would give you the ability to determine what is going on without being on a dyno and a probe up the tailpipe.
     

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