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Thread: BBC tuning help needed

  1. #1
    Senior Member
    Join Date
    Feb 2009
    Posts
    254
    I just got my engine back from the rebuilder and it's running like crap. It's been over 20 years since I've messed with a "hot rod" engine and it's becoming obvious I've forgotten more than I remember.
    '69 Chevy 427 forged crank, stock rods, Sealed Power domed forged pistons, '69 427 101cc closed chamber heads, about 11:1 compression, Schnieder JB4 cam (510 gross lift, 310 gross duration), Pete Jackson gear drive, HEI distributor with a Hypertech coil, Accel wires, Autolite plugs, Holley Strip Dominator intake, Holley blue electric fuel pump and reg (6 psi at any given RPM), Holley 800 DP w/mechanical secondaries (fresh rebuild by a pro with alcohol gaskets in it), 91 octane gas.
    When I first fired up the engine it was very snappy with no hesitation. My engine's builder told me to set the timing at 36 degree's total which I did several days later. In those several days I would fire the engine up just because I liked the way it sounded but it seemed to be slowly running crappier and crappier. The total timing set at 36 degree's put my initial timing at 20 degree's which broke the nose on my starter, yesterday I replaced the starter and moved the initial timing to 10 degree's. It just keeps running worse and worse the more I mess with it/run it. A friend commented last night that he saw flames shooting out the headers. I'm thinking it's a fuel problem (too lean) but I don't have a clue (it seems) as to what I'm doing. I'm leaning towards a vacuum leak as the culprit. The guy that built my carb is well known in the area and he jetted my carb for my application.
    At this point when the pedal is stabbed the engine dies. It sounds very rough, "choppy" if you will, it refuses to idle below 800 RPM and although I know I don't want it to idle that low in a jet boat, shouldn't it be able to idle below 800 RPM out of the water and not hooked up to the jet drive? What kind of aresol spray (carb cleaner, WD40, etc.) can I use to check for vacuum leaks that won't stain my parts? I want to do a plug check, should I do it at idle or at higher RPM's, or both?
    Any advice out here? Where else should I "look" besides a vacuum leak?

  2. #2
    ck7684
    hmmm, I'm no expert but I would look towards a vacuum leak, possible around the intake gaskets. I'm just taking a guess seeing as it's a new engine that was running great when you got it. The front and rear of the intake is usually just RTV'd
    Did this thing idle at or below 800 when you first got it? I think mine idles around 1000 and I may have less duration than yours...

  3. #3
    victorfb
    first off, only use a spray to check for a vacume leak if you really have to. vacume guages are cheap at almost any autoparts store.
    the fact that this engine was running well prior leads me to believe you have something else rather than a vacume leak. cam going flat, contamination in the carb, ?,?,? start at the basics and check your plugs, take a compression check of all cylinders, check your electrical system (plugs wires cap and rotar ect), check the carb and filter, make sure no jets are clogged, check valve train and clearances (lash).

  4. #4
    thatguy
    first off, only use a spray to check for a vacume leak if you really have to. vacume guages are cheap at almost any autoparts store.
    the fact that this engine was running well prior leads me to believe you have something else rather than a vacume leak. cam going flat, contamination in the carb, ?,?,? start at the basics and check your plugs, take a compression check of all cylinders, check your electrical system (plugs wires cap and rotar ect), check the carb and filter, make sure no jets are clogged, check valve train and clearances (lash).
    Cam would be my first guess, since it just happened to me EXACTLY how you describe.
    Pull covers and see if any rockers are loose. If not, RE-CURVE distributer.
    I have 12 initial, 36 total. All in at 3200RPM (MSD)
    Tommy

  5. #5
    Mr. Crusader 83
    Timing at Idel is nothing. You only need to worry about total advanced timing. Set it back to 36degress total. Then pull you valve covers and make sure all your rockers are moving the same amount.
    Also with a fresh motor. Did you or the builder break in the cam? If not it could be going flat. When my cam went flat it back fired out the carb.

  6. #6
    Senior Member
    Join Date
    Feb 2009
    Posts
    254
    cam going flat is a possibility. I originaly built this engine myself, in the 23 minute cam break-in I got a lot of water in the oil. The engine builder said my cam looked and measured fine when he rebuilt the engine after me, but water in the oil can't be good for a new cam... I'll check the rocker arms for play after work today, as well as all of the other rec.'s. Thanks!

  7. #7
    Mr. Crusader 83
    I have a 468 with closed chamber heads and two holley 750's. My buddy tuned it all in for me lastnight. At idel it at 1100rpms, on the trailer.

  8. #8
    thatguy
    Timing at Idel is nothing. You only need to worry about total advanced timing. Set it back to 36degress total. Then pull you valve covers and make sure all your rockers are moving the same amount.
    Also with a fresh motor. Did you or the builder break in the cam? If not it could be going flat. When my cam went flat it back fired out the carb.
    Initial does matter if he is breaking starters off. I respectfully must disagree with you on this.
    Tommy

  9. #9
    Senior Member
    Join Date
    Feb 2009
    Posts
    254
    first off, only use a spray to check for a vacume leak if you really have to. vacume guages are cheap at almost any autoparts store...
    With the cam being a non-stock cam, how much vacuum should I be looking for?

  10. #10
    pw_Tony
    Try running it without the valve covers on to try and check me make sure they're all moving around the same amount, the rockers. Just cover everything up so you don't make a mess

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