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  1. #1
    ian
    ian is offline Mechanic
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    I'm confused : Chevy 350 ignition timing

    Hi folks, I think I need some help! I'm very confused.
    Last night I spent ages puzzling over the ignition timing on my Chevy engine. I am using a new dial-back timing light, complete with digital tacho and advance readout. Vac advance was disconnected throughout. At tickover (690 rpm) my timing mark is approx 40 degrees advanced - it's way off the timing tab. When I turned the dizzie to get the tdc mark to line up with the 10 degree timing mark on the tab the engine started to run really rough, revs dropped down to about 500 and it was obviously struggling. So I turned the dizzie back slowly to where the rpm was highest and the engine seemed most comfortable, again the timing appeared to be about 40 degrees advanced.
    Normally you would suspect the tdc mark on the damper. However when I rebuilt the engine I reset the tdc mark using a dial-guage on no.1 piston. Then I thought that the outer ring on my damper may have slipped (it is quite an old one.) So this morning I checked for tdc position using the positive stop technique, and guess what, my tdc mark is bang on.
    Other information : the dizzie is an HEI with MSD HEI kit ie hot coil, module. Also fitted Crane adjustable vac advance, although it was not connected. I am using the silver and blue springs for mechanical advance which were supplied with the Crane vac advance kit. The mechanical advance should start at 800rpm and be fully advanced at 2800rpm.
    Any suggestions would be most welcome. Thanks. Ian.

  2. #2
    ian
    ian is offline Mechanic
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    RE: I'm confused : Chevy 350 ignition timing

    Just found the following on a US website :

    "Small block Chevy - These engines have used three different timing pointer locations. Pre '69 engines have the TDC mark 2 to the left of the keyway centerline. The '69 to '85 dampers have the TDC mark 10 to the left of the keyway centerline. The 1986 to 1991/1992 have the TDC mark 40 to the left of the timing mark. All of our SB Chevy dampers, as well as all other aftermarket dampers, use the 10 TDC position. Our 6.75" diameter models have two TDC marks, one that is 10 and one that is 40 to the left of the timing mark. If you have a pre '69 vehicle you can use one of the aftermarket bolt-on timing pointers to align correctly with the timing marks on our dampers."
    Looks like I may have the 1986 to 91/2 damper fitted to my engine which was made in 1981. This must mean that the TDC mark is NOT the mark to be used for timing. I think there may be another mark on there somewhere, I'll check. What a silly boy I am.....

    Cheers. Ian.



  3. #3
    simon's Avatar
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    RE: I'm confused : Chevy 350 ignition timing

    Ian, surely if the damper mark is set up to align with where TDC is, it doesn't matter where the pointer is, if the damper TDC mark is where the piston TDC is and it all lines up with the pointer then you're ok (or am I missing something here?)
    Simon
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  4. #4
    ian
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    RE: I'm confused : Chevy 350 ignition timing

    Hello Simon. I don't think you're missing anything, I just think I'm going crazy. When I use the TDC mark on the damper as the timing mark with the timing light, it is showing about 40 degrees advanced at tickover. It should be about 10 degrees. But the timing in reality is obviously not that far out because that is where the engine is most comfortable, and it actually drives really well. The TDC mark is in the correct place on the damper ie when it lines up with the zero on the timing tab the no.1 piston is at TDC. I know this, I have checked and re-checked today. But what I understand from the website I saw earlier is that the timing mark is not the same as the TDC mark. It is 40 degrees 'behind' the TDC mark. If I can find this mark on my damper and use it instead I think I should be OK. Shouldn't I??
    Anybody else have any ideas?
    Ian

  5. #5
    Happy Jim is online now Technical Director
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    RE: I'm confused : Chevy 350 ignition timing

    Ian,

    Are you clipping the light to no1 ignition lead or a.n.other ?

    Jim

  6. #6
    ian
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    RE: I'm confused : Chevy 350 ignition timing

    Hello Jim, definitely no. 1.....

    Ian

  7. #7
    markn is offline Chief Bottle Washer and Tea Boy
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    RE: I'm confused : Chevy 350 ignition timing

    Ian

    Assuming you've got the timing light on number 1 lead and are as you say at TDC with the 0 on the damper pointing at 0 on the timing plate you have a good starting point - there is nothing wrong. So long as you have measured TDC with a dial gauge or similar and noted a point on your damper and related that point with a position on your timing plate you can't go wrong.

    If it helps, I started mine first time the other day following a build up and it had 40 degrees initial, it ran OK because I had increased the idle speed really high and got more air in the engine. It ran better when at 16 (mine is high lift cam with overlap) but involved adjusting mixture and idle speed.

    If I were you, I'd set the timing where you think it should be, and adjust idle screw and mixture until things are right. Don't forget to check for air leaks and unplugged vacuum ports.

    Hope this helps

    Mark

  8. #8
    ian
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    RE: I'm confused : Chevy 350 ignition timing

    Thanks Mark, I'll have another go later. What concerns me is that the engine is obviously very unhappy when I set it to 10 btdc but seems fine at 40 btdc! I was wondering if I had got the distributor installed incorrectly by one tooth but I don't think it would make any difference, I'd just have to twist it more. For the record, no. 1 terminal on the dizzy cap is almost pointing dead ahead, maybe slightly towards the near side if anything. Thanks for your help, it is appreciated. Ian.

  9. #9
    kdavies3's Avatar
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    RE: I'm confused : Chevy 350 ignition timing

    I assume you have your dial back timing light set up correctly?
    It sounds to me as already said that you may be one lead out and are timing on number two or something strange.
    That said I had some very strange readings on my original (new HEI) it was moving all over the place. I had to replace the HEI dissy for another new one, before I could get a steady reading.

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  10. #10
    markn is offline Chief Bottle Washer and Tea Boy
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    RE: I'm confused : Chevy 350 ignition timing

    I think you'll find you are 90% there. Most Chevy dizzys have no 1 at or about 11 o'clock. Just adjust it a bit at a time, a bit of timing - keep it idling, a bit of mixture adjustment - keep it idling - repeat the process and see how close you can get to your target 10 degrees.

    Sure as eggs are eggs - 40 degrees is too much

    Mark

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