Announcement

Collapse
No announcement yet.

knocking question

Collapse
X
 
  • Time
  • Show
Clear All
new posts
  • Guest

    knocking question

    I had the opportunity to spend a few hours driving the T-bird yesterday, and noticed what appears to be throttle-induced knocking.

    <preamble to question>
    I've got a hobby car (the T-bird is my Mom's, believe it or not!), and I just replaced its 650dp main body with a main body from Proform. When I tried to start the car, I got an impressive fireball spouting out of the carb, so I promptly called Proform tech before I burned the house or the car down.

    What I found out from him was that when a mix gets excessively lean it tends to ignite without the help of a spark. In the case of my hobby car, there was a *lot* of extra air coming in with the new main body, leading to the carb backfire on a very cold engine.
    <end preamble>

    Thinking about this when driving the Thunderbird, I noticed that there was a break-over point with the throttle that changed as RPM increased. If I floored it from a stop, I'd get horrific knocking, but if I rolled on the throttle I could keep opening it up without causing any knock.

    This makes me think that the root of the knocking is maybe a carb problem and that I could maybe address it with richening the mix. Of course, I'll have to research that one - the carb came with the motor and car (4bbl Autolite?).

    And a quick BS-meter question. The car's been worked on by a mechanic who specializes in old Fords, and while he's done great work, he's said a couple of things that don't sound right. First, he said that the motor couldn't handle conversion from downdraft to PCV, although later relented and did the conversion (my Mom has trouble breathing, and the fumes from the downdraft were overwhelming).

    So for the "real or BS" question - he also told her that this motor couldn't handle modern 93 octane gas (apparently due to all the modern additives), but that strikes me as wrong since I'm guessing the additives are mostly the same across the octane spectrum, and the only difference is the relative willingness to combust. Anyone here care to corroborate or debunk this? I'm guessing that switching to "hi test" would also help the knocking.

    Thanks,

    JLW
  • JohnG
    John
    • Jul 28 2003
    • 2341

    #2
    RE: knocking question

    To establish a baseline of sorts, one might first ask if you have done the timing to the original specs, then if your distributor is advancing correctly. From there, does the car have the correct spark plug in it (in terms of heat range)?

    have you looked at the plugs to see if they indicate you are
    running especially lean?

    Assuming it works correctly, does your temperature gauge indicate the engine is running hot?

    any of these things MIGHT play a role in detonation or knocking.

    John
    58 Hardtop
    1958 Hardtop
    #8452 TBird Registry
    http://tbird.info/registry/DataSheet...r~equals~8452)

    photo: http://www.squarebirds.org/users/joh...d_June2009.jpg
    history:
    http://www.squarebirds.org/users/johng/OCC.htm

    Comment

    • Alexander
      Webmaster
      • Oct 30 2002
      • 3321

      #3
      RE: knocking question

      I concur with John; I think your timing is over advanced. Checking it by a timing light may not work if the outer part of the harmonic balancer has shifted. Use a vacuum gauge attached to the intake manifold and advance the timing until you get the highest vacuum (~18 inches). Test-drive the car with some moderate acceleration. If it knocks, retard the timing a little. The optimal point is when you get slight pinging on hard acceleration.

      You have no problem with gradual acceleration because that calls in only the centrifugal advance. Hard acceleration also puts in additional advance from the vacuum advance.

      Motors need no special modifications for PCV valves. I have run my 1959 for nearly 15 years with a closed PCV system. No problems and the oil stays cleaner longer. Kaiser Frazier already had PCV valve systems in 1951 exactly for that purpose.

      Modern gas is not harmful to your engine. The only problem with modern gas on these engines is that it has no lead. This should not be a problem if you do not run your engine hard as the lead helps to cushion the valves. Installation of hardened valves solves the problem.



      Alexander
      1959 Hardtop
      1960 Golde Top
      Alexander
      1959 Hard Top
      1960 Golde Top
      sigpic

      Comment

      • Guest

        #4
        RE: knocking question

        Hmm - never thought about the timing, or centrifugal advance factoring in.

        I'll definitely try the vacuum gauge method of setting the timing. Does this mean running 93 octane should be OK, though?

        Also, what constitutes "hard" driving for this motor? I hadn't thought much about it, but the other night on the road from Houston to Austin I was running 70-75 mph for over an hour at a time. The temp was fine - right after we got the car, we had all the radiator hoses replaced, and added a new overflow tank. Since then, it's never run hot, either on the freeway or in stop-and-go traffic.

        Thanks,

        JLW

        Comment

        • Alexander
          Webmaster
          • Oct 30 2002
          • 3321

          #5
          RE: knocking question

          Hard driving is flooring the engine a lot and driving over 60. 93 octance is fine for these cars. The only problem is that the valves and the seats are not hardened. You only need to get hardened valves if the valves do burn out on you.

          Alexander
          1959 Hardtop
          1960 Golde Top
          Alexander
          1959 Hard Top
          1960 Golde Top
          sigpic

          Comment

          Working...
          😀
          🥰
          🤢
          😎
          😡
          👍
          👎