flat spot or lag at 3,000RPM
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  1. #1
    1000+ Posts n b j's Avatar
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    flat spot or lag at 3,000RPM

    This post is not in relating to my Peugeot, but instead to my PorscheRS which flat spots badly at 3,000RPM if you are not driving it hard and the revs slowly rise to 3,000 it will flat spot or lag, you can't get the power on quickly and have to wait until the revs climb (surging briefly through the rev range ) to about 4,000 before you have the power back. Once over 4,000 rpm the problem is gone and you can wind the engine out to over 8k

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    The problem is not there if you floor it and drive it hard through 1st, then into 2nd, 3rd, etc... no flat spot, no lag.

    My dad and I have our own opinions on why it does it and so does our mechanic...

    What I was wondering is what are peoples opinions out there as to what the issue is and/or what causes it...? since this is a techinically minded forum and all

    Oh yeah, this is a 1973 car which still has it's original engine, the engine only has 70,000 miles on the clock.
    "Do my eyes deceive me, or is Senna's Lotus sounding rough ?" - Murray Walker
    206XR 1.6ltr - SOLD
    BMW E36 325i Coupe
    73 Porsche 911RS

  2. #2
    1000+ Posts n b j's Avatar
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    ooooops, was meant to post to the "GENERAL"
    sorry.

    feel free to remove this post if you are a moderator...
    "Do my eyes deceive me, or is Senna's Lotus sounding rough ?" - Murray Walker
    206XR 1.6ltr - SOLD
    BMW E36 325i Coupe
    73 Porsche 911RS

  3. #3
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    mistareno's Avatar
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    no vacuum advance??

    This would cause the engine to behave normaly under high load situations, but flatspot under light throttle/partial load circumstances.

    It is the most obvious/cheapest thing to check.

  4. #4
    1000+ Posts tekkie's Avatar
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    Huh?

    I thought that vacum advance, ADVANCES the timing few degress under heavy load via the vacum valve on the side of the distributor bu pulling the points plate forward. Under normal/slow acceleration the revs will advance the timing via the centrifugal (spelling?) weights pulling the points forward negating the needs for vacum advance. So his problem would appear under hard acceleration not when accelerating slowly.

    Guessing game, my turn.
    air in the fuel injection system. Mechanical injection wasnt self bleeding in olden days.
    .
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  5. #5
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    tekkie:
    Huh?

    I thought that vacum advance, ADVANCES the timing few degress under heavy load via the vacum valve on the side of the distributor bu pulling the points plate forward. Under normal/slow acceleration the revs will advance the timing via the centrifugal (spelling?) weights pulling the points forward negating the needs for vacum advance. So his problem would appear under hard acceleration not when accelerating slowly.

    Guessing game, my turn.
    air in the fuel injection system. Mechanical injection wasnt self bleeding in olden days.
    No....Vacuum advance increases total vacuum at times of light load / high vaccum situations.

    An engine runs alot more efficiantly when further advanced, but under high load it will pink.

    The main purpose of vacuum advance is to aid economy by advancing the timing when at cruise or cruise like situations.

    The other purpose is to provide a transition between the cruise advance setting and the loaded advance setting.

    In EFI terms, think of the vac advance mechanism as a MAP sensor (for load sensing) and the centrifugal advance as the engine speed sensor..

    If you disconect the vac advance, you will find the car requires noticably more throttle opening to maintain the same cruise speed (try it with a vacuum gauge to test throttle opening) It will also provide a smoother more responsive transition from cruise to power when you plant the foot

    At WOT there is 0 degrees of vacuum advance (usually nil vacuum). All vac advance is always in addition to the centrifical advance.

    This is why you often hear late 80s early ULP jap cars (toyota corollas are a good example) cruising along at 60 kph, pinking their arses of. Tramp the throttle and the pinking stops.

    Why, Well to combat the pissy OZ ULP octane rating, toyota OZ simply wound back the static timing position by 2-3 degrees so they wouldn't detonate under load. Problem was that the vac advance remained the same as before, and as fuel octane ratings are measured on an exponential scale, they pink under light load.

    That's my load of useless shit for today

    <small>[ 29 July 2003, 01:57 AM: Message edited by: mistareno ]</small>

  6. #6
    1000+ Posts tekkie's Avatar
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    damn you Mistreano,,, was was going to sleep and your reply popped in, so off I go to the wonderful world of www and google. Curiosity had to be satisfied.

    I found this:
    And when rpm is high we need more advance, and when rpm is low we need less advance.

    Fortunately, the vacuum distributor (and the combined centrifugal/advance of 74+ distributors) does a reasonable job of following these two conflicting needs. The vacuum port on solex carburettors is placed just UNDER the main venturi, close to where the throttle plate passes as it opens. So imagine the engine idling. The throttle plate is nearly closed, so there is a low airspeed through the main venturi - not much vacuum there or just under it (above the throttle plate). The vacuum port sees very little vacuum, and the idle advance setting prevails (7.5-10BTDC on most models).

    Now open the throttle a little, so the edge of the throttle plate passes by the vacuum port. This creates a mini-venturi with very high air speed, which creates a lot of vacuum, so you get a shot of advance to help speed up the engine (this effect is entirely missing with the Bosch 009 distributor, which is what causes the "009 flat spot"). Since in a part-throttle condition you still have a high proportion of burned gases for a low flame speed, this high advance also meets the advance condition needed to deal with that too.

    Now open the throttle right up. The throttle plate moves away from the vacuum port (no mini venturi) and so the MAIN venturi is providing the vacuum effect, but since the airspeed hasn't yet increased much yet (engine hasn't yet increased rpm), the vacuum signal is lower than part throttle, so the advance is reduced a little. Perfect for a fresher mixture (lower proportion of burned gases with an open throttle remember?). Now the engine rpm starts to catch up with the open throttle, so the airspeed through the main venturi increases, vacuum increases, and the advance increases progressively, which is just what you want for the increasing rpm, since the crankshaft is rotating in less time so you need more advance to get that fresh charge completely burned at the right moment.
    <a href="http://www.geocities.com/vwresource/cent_vs_vac" target="_blank">source of the above</a>

    So I guess you are right in that the vacum will advance the timing upto a point after which the centrifugal (mechanical) advance takes over. When I said "heavy load" I meant initial acceleration high speed airflow around the venturi and where additional few degrees is needed to help speed up the engine (but not far enough to cause ping/pink). Once the revs build up the advance is matched to RPM via the mechanical advance.

    So the vacuum (or lack of air flow around the vacum tube or blocked tube) - as Mistreano pointed out - may be to blame. Initially I thought the vacum advance worked ONLY when the hard acceleration was required. spanner

    With regards to Jap cars. The ECU takes over the timing mapping which assumes a certain quality of fuel used. If you import a jap car (grey imports for eg) pinging is virtually assured if the insert-name-here perolem crappy fuel is used. anti-detonation sensors help a tad but not all cars have them.

    Now I can sleep.

    <small>[ 29 July 2003, 12:46 AM: Message edited by: tekkie ]</small>
    .
    1300cc's of jap buzzbox delivered the times below.

    EC 1:54.6 , Wakefield 1:13.15 , OP (short) 52.00 , OP GP 1:24.40


  7. #7
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    Ummm...yeah... I think you will find the below link is a better description and relevant to (nearly) all cars....
    <a href="http://www.stoveboltengineco.com/howto/vac.htm" target="_blank">vacuum advance and why it is needed</a>

    I would be thinking more along the lines of a hole in the advance diagphram or vacuum line would be the obvious choice...start at the easiest and cheapest to diagnose problem and work your way up...

    BTW I was talking mainly carby/early EFI (the first of the ULP cars (I did say late 80's, I meant mid/late 80's ie: 86-88)

    P.S...I never sleep

    <small>[ 29 July 2003, 02:06 AM: Message edited by: mistareno ]</small>

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