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"fixed" mu ignition timing, now car runs very hot

Discussion in 'Engine' started by RaceDiagnostics, Oct 14, 2010.

  1. RaceDiagnostics

    RaceDiagnostics Hardcore MB Enthusiast

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    I checked the timing at idle 750rpm to day and the ignition was firing at 25 degrees before tdc [​IMG]

    The distributor was at its adjustment end stop.

    I left both the vacumm pipes on and set it to 0 degrees. This is what is shown on the engine sticker.

    30 degrees @ 3000rpm with both pipes removed
    0 degrees @750rpm with both pipes connected

    Now the engine is running very hot, so I am not sure I have set it correctly. Can anyone explain why this would make the engine run hot and slow to start.

    I find this section of the workshop manual confusing.

    Can someone tell me a simplified proceedure I should follow.

    Thanks.
     
  2. Tiff

    Tiff Hardcore MB Enthusiast

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    Hot running and poor starting point to the timing being too far advanced- Does the car pink badly under load? You say you used the engine sticker? Is it on the car or the engine? The engine isn't original if I remember and the settings may be different? I'd back it off a bit to see if it cures the problems, and then check if the correct pulley/marks/distributor is in place.
     
  3. OP
    OP
    RaceDiagnostics

    RaceDiagnostics Hardcore MB Enthusiast

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    Hi Tiff, I retarded it by 25 degrees from where it was, previously I had to use super to stop it pinking before. The engine sticker came from someone here, I think it is the correctt one for the engine.

    I had to adjust the idle speed screw a lot to keep it at 750 rpm as I retarded the timing, could this have impacted the mixture, making it run lean, and hot?

    Temp gauge and sticker.

    [​IMG]

    [​IMG]
     
  4. grober

    grober Hardcore MB Enthusiast

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    Running with the ignition retarded will also result in an engine running hot. The fuel is ignited later which can burn oil off the cylinder walls increasing increased friction as the fuel/air mix burns in the cylinder volume rather than the combustion chamber but the main effect is that the exhaust gas is still burning when the exhaust valve opens causing the exhaust valve/manifold and cylinder head to heat up. I would start with a initial static timing of 10degree BTDC --- first set the engine to TDC with cylinder no 1 on the compression stroke [check both valves are shut] then remove the distributor from the block- align the rotor arm such that its pointing at the number one timing slot on the distributor body then and this is the bit to watch orient the distributor body in the middle of its travel range of adjustment before reinserting the distributor shaft to mesh with its drive gears. Once you have done that set the static timing to 10 BTDC with a test light between the distributor/points low tension supply and earth- it will light as the points start to open up. This is on the assumption that the points gap is set correctly of course!! The only way to effectively check this on an old distributor is with a dwell angle meter since shaft wear can effect points clearances and thus ignition timing. E.g too wide gap due to wear means the points start to open more quickly effectively advancing the ignition. You will probably have 2 sets of points on the V8 but think of them a bit like 2 separate 4 cylinder engines to make sense of what is going on.

    ps posting at this time after watching the last of the Chilean miners being brought to the surface!:thumb:
     
    Last edited: Oct 14, 2010
  5. OP
    OP
    RaceDiagnostics

    RaceDiagnostics Hardcore MB Enthusiast

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    The ignition is transistorised with a magnetic pickup, so no points.

    I'm now thinking that I shoud be setting the timing with no vacumm lines at 3000rpm and then checking at idle to see that it is 0 degrees with the advance/retard tubes connected, rather than setting at 0degrees.

    If the advance/ retard mechanism is not working then this could mean that setting at idle gives the wrone point.
     
  6. grober

    grober Hardcore MB Enthusiast

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    No points---- OK then 30 degrees @ 3,000 with no vacuum seems as good a starting point as any. Wonder if any of the original ignition equipment was transferred to the new engine if it was supplied in "bare" form?
     
  7. OP
    OP
    RaceDiagnostics

    RaceDiagnostics Hardcore MB Enthusiast

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    I am pretty sure it is a 1976 and I'm pretty sure that that label above if for that engine.

    Anyway after a bit of tinkering today I have a bit more understanding.

    First thing I did was warm up the engine and then put the timing back to the way it was yesterday, the engine temp then dropped back down to where it was before. Next I set the advance to ~30 degrees before at 3000 rpm with no vacuum tubes. Idle then showed 10degres btdc.

    I also had a think about what the table in the workshop manual means and it became clear to me what it all meant.

    Replacing the vacuum tubes advanced the timing at 3000 rpm from ~30 to ~45, this is what the manual indicates,so all good there, however at idle, the timing was still at 10degrees btdc indicating that the retard vacuum control was not working.

    This means that when I set the timing yesterday at TDC at idle it was actually retarted by 10 more than the correct setup, not too sure whay that would lead to the engine running hot though.

    One possibility is that as the idle speed control was adjusted (to keep the correct speed as the timing was changed) the mixture went far out of spec.

    I am not sure what effect the idle speed adjuster has on the mixture but the manual does state that the mix needs to be corrected after changing the idle speed so I guess there is some link.

    So previously I was nunning 10 degrees ahead of the recommended setting and yesterday I was 10 degrees behind and today I am now set at the correct timing, but with no idle retard occuring. The engine temp today is closer to what it was previously but still a bit hotter.

    So now I think I need to adjust the mixture, however this is what my mixture scew looks like, what tool is supposed to fit it, is this a cap over the screw, looks like someone has had a wrench on it.

    [​IMG]
     
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  8. Druk

    Druk Administrator Staff Member

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    Has this car got the CIS injection? If so the mixture screw is down a hole on the top of the fuel distributor and utilises a T handle allen key to adjust.
     
  9. Bellow

    Bellow Hardcore MB Enthusiast

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    I'm with Grober on overheating being caused by retarded timing. Firing too late, the piston doesn't get the best 'work' from the burning mixture so the heat remains and as more cylinder is exposed, more heat passes into the engine.
    Main reason for this post is to clarify the vacuum and timing correlation. It is an advance mechanism.
    At full throttle the vacuum has no effect as there is no vacuum to operate it and the timing is a combination of the static setting and the degree of advance according to rpm courtesy of the centrifugal mechanism. In this running regime the cylinder pressure is high and detonation (pinking) would occur if the timing were too far advanced. Also, the mixture strength will be at its richest for full torque and power.

    When the throttle is then relaxed for cruise, the mixture will be made weaker for economy, and the cylinder pressure will be lower. Both these factors require that the timing be advanced and this is achieved by the increased vacuum created by throttling acting on the advance capsule.

    With that in mind, your task should, hopefully, be more easily understood.
     
  10. Bellow

    Bellow Hardcore MB Enthusiast

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    And typically, for idle, the mixture and idle speed are juggled in unison with the correct mixture taking priority and the throttle stop being tweaked to then get the idle speed correct.
     
  11. OP
    OP
    RaceDiagnostics

    RaceDiagnostics Hardcore MB Enthusiast

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    Yes, it is CIS, I think this must be a bolt sover over the adjuster, I'll take it out tomorrow with a wrench and check the mixture.
     
  12. Druk

    Druk Administrator Staff Member

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    Do you not have the Haynes manual for 350 and 450 cars? Page 36.
     
  13. OP
    OP
    RaceDiagnostics

    RaceDiagnostics Hardcore MB Enthusiast

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    Yes, I have the manual but it describes the hole cover as a "cap", from the picture it looks like a bolt rather than a cap.
     
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  14. Bellow

    Bellow Hardcore MB Enthusiast

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    They are supposed to be tamper proof. Can't remember if its a plastic cap or pipped allen head. Definitely remember my mate tweaking the same system on his Mk1 GTi Golf with an T-bar allen key.
     
  15. ray_hennig

    ray_hennig Hardcore MB Enthusiast

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    That doesn't look unduly hot (around 200 F). Or does it get much hotter than this?

    RayH
     
  16. OP
    OP
    RaceDiagnostics

    RaceDiagnostics Hardcore MB Enthusiast

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    The temp above was after driving the car with maximum crare and a very light right foot. I am sure it would have gone much higher with normal driving.

    Anyway after retarding by 10 degrees it is now running great. I did about 150 mles over the weekend with no increased temperatures.

    Also I moticed that Lidl or Aldi are doing T handled allen keys on Thursday.
     

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