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246 cam timing adjustment

Discussion in '206/246' started by 246tasman, Aug 14, 2009.

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  1. 246tasman

    246tasman Formula 3

    Jun 21, 2007
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    Will Tomkins
    #1 246tasman, Aug 14, 2009
    I need to retard one (modified/no timing marks) exhaust cam by 8.5 degrees as it has been mistimed. The engine is in the car (Stratos) and it's going to be a pain trying get the right advance by trial and error. Does anyone know the relationship between the sprocket teeth and the vernier pin adjustment so I can work out how to set it right first time?

    The car had had the heads removed and came back lacking performance at the top end. When we checked all the cams at peak lift the other three were all timed the same +/- 1 degree but this one was advanced.
     
  2. jselevan

    jselevan Formula 3

    Nov 2, 2003
    1,873
    #2 jselevan, Aug 14, 2009
    I believe that Wallace's Work Shop Manual translation speaks to this issue, and while I do not have my copy in front of me, I recall a 2 degree advance/retard with each adjustment position. Someone should check this.

    Jim S.
     
  3. 246tasman

    246tasman Formula 3

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    Will Tomkins
    #3 246tasman, Aug 15, 2009
    It's not in the manual I have though I'm not sure if it's Wallaces's transalation. I will also post this query in the Tech Q&A forum.
     
  4. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
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    Steve Magnusson
    #4 Steve Magnusson, Aug 15, 2009
    Last edited: Aug 15, 2009
    If the figure in the 246 WSM is drawn accurately, it would be a system "easier" on the mechanic, as it appears to be the (expensive) type (with many holes & patterns) that allows continuous fine adjustment while leaving the sprocket engaged to the chain -- I know that that doesn't help you with the numbers. Anyway, was just thinking to myself that you could literally eyeball/count things while making an adjustment and then, even if it's still not quite right after remeasuring, (and knowing where everything was in the "before" measurement), make a very good prediction for making the necessary re-adjustment correction (by again counting hole pairs in alignment from the orignal position). If you had the (inexpensive) fewer-holed systems, like on the 308s, where you have to disengage the belt from the sprocket to skip tooth pitches in order to make the math work out for small angle changes, that makes is much harder to fake it without having the dimensions to plan a move.

    Would like to see some dimensions on those 246 holes & patterns myself to share an understanding with the Designer. Anything available from the earlier, chain-driven V12 WSM/materials on sprocket to cam attachment/adjustment? Did Fiat feel the need to invent something "new" just to be different for the 246. or did they use the (proven) F V12 technology?
     
  5. 246tasman

    246tasman Formula 3

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    Will Tomkins
    #5 246tasman, Aug 15, 2009
    There are 16 equally spaced holes in the cam and 2 sets of 7 holes opposite each other in the sprocket. I worked out approximately that the offset is such that moving from one hole to the next makes about 1.3deg of cam rotation which is 2.6deg of valve timing with respect to the crank. This is fine but my timing was such that there wasn't an a hole in line for the adjusted timing and then I had to move the tooth location on the chain and it's back to trial and error.

    The 2 cam engine manuals give info on how to achieve a timing adjustment but the design is different.

    On the bench it's not too bad but in the car not at all easy.
     
  6. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
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    #6 Steve Magnusson, Aug 15, 2009
    Are you describing the 246 (i.e., the actual 246 parts don't look like the figures shown in the 246 OM and WSM) or some other F model/engine?
     
  7. 246tasman

    246tasman Formula 3

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    #7 246tasman, Aug 16, 2009
    I'm describing the 246 (135CS) engine. Are you saying you have a manual that shows some detail of this arrangement? Mine looks like a translation of the factory manual (uses their drawings) but doesn't have a decent drawing of the sprocket and cam holes.

    I could of course measure up the sprocket accurately and get these figures for future reference. If I do then I will post them here.
     
  8. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
    Lifetime Rossa

    Jan 11, 2001
    26,825
    30°30'40" N 97°35'41" W (Texas)
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    Steve Magnusson
    #8 Steve Magnusson, Aug 16, 2009
    That's my bad -- the images I have available are B&W scans so I've just realized that where the pin is located looks like a missing hole in the scan. Consequently, my brain said "a pattern of 3 holes in four places" rather than your correct description of a pattern of 7 holes in 2 places".

    Does seem strange that they left out a useful diagram...
     

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