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Slight miss at idle with popping exhaust

Discussion in 'Engine 4 cylinder' started by sdibbers, Jun 11, 2018.

  1. sdibbers

    sdibbers Well-Known Member

    This is with a 2000TC running 9:1 2200 pistons with HIF6 carbs.

    I had a great weekend up in Mass for the Nortb American Rove. Managed about 600 miles round trip with no issues.

    Got home Saturday evening and noticed that she has a slight miss every second or so at idle with a popping at the tailpipe. I know I have to repair a crack on the exhaust manifold that I’m pretty sure got worse on the trip home. Could this be related? Power seems good still, idle is smooth apart from these occasional misses. My other concern is maybe this is the beginning of a burnt valve. I plan to whip off the manifold, weld the crack between the 2nd and 3rd cylinder pipes and try and anneal somehow

    Any thoughts?
     
  2. harveyp6

    harveyp6 Well-Known Member

    Repair the exhaust before you start worrying too much.
     
    Chalky and sdibbers like this.
  3. sdibbers

    sdibbers Well-Known Member

    I guess I'm jumping the gun. Ordered new copper sealing rings and intake gaskets this morning. I'll update when I get the work done.
     
  4. Demetris

    Demetris Well-Known Member

    I always associated what you describe with a lean mixture. As you have HIF carbs, these lean out when hot.
    If you do a CO check, you will find out.
     
    sdibbers likes this.
  5. sdibbers

    sdibbers Well-Known Member

    I agree I've had that sort of experience too Demetris, although this time the idle seems steady when not missing (if that makes sense?). I have noticed that it only takes about two minutes of idling while warm to cause the bimetal strip to lean out the jet. However this time it's doing it from cold pretty quickly. I've even considered making a solid part to replace the bi-metal jet adjuster, seeing as modern fuel seems so different to the stuff available in the 70's.
     
  6. Demetris

    Demetris Well-Known Member

    Tight valve clearances also can cause the misfire that you describe.
    Equally easy to check.
     
  7. sdibbers

    sdibbers Well-Known Member

    That's true. I set them last year and I always use lead replacement additive, so I hope not!
     
  8. j_radcliffe

    j_radcliffe Member

    What lead replacement additive are you using? I use Flashlube valve saver fluid. After 9 years I checked my clearances and they were still to spec.
     
  9. sdibbers

    sdibbers Well-Known Member

    I use Lucas oil lead substitute. Not had valve seat recession in the last five years.
     
  10. sdibbers

    sdibbers Well-Known Member

    Update time. Exhaust manifold off. I’ll TIG the crack tomorrow at a friends house.

    Bad news is I did a compression test. Not a happy camper:


    Cyl 1 180psi

    Cyl 2 175psi

    Cyl 3 80psi

    Cyl 4 170psi

    Looks like the exhaust valve in cylinder 3 is the culprit. Oh well, head off later for an exam.
     
  11. harveyp6

    harveyp6 Well-Known Member

    Oh well, at least with the exhaust manifold off already you're more than half way to getting the head off.
     
  12. sdibbers

    sdibbers Well-Known Member

    Yep, that’s what I was thinking. Next job, find a reliable machine shop. I’m sure someone will know of one.
     

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