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Iginition Timing with vac. guage

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by joker89blazer, Jan 1, 2003.

  1. joker89blazer

    joker89blazer 1/2 ton status

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    On my truck if I set the timing to zero degree with the computer advance unplugged, it runs bad, like the timing is retarded.

    If I set it where the motor sounds "right" and runs normal, it is advanced beyond all the marks on the scale.

    Some ideas.
    1.Engine dampner has some how managed to slip Between the inner and outer rim so that timing marks are no longer true.
    2.Timing chain has jumped one tooth
    3.I know engine was rebuilt and/or replaced
    A.Someone installed parts from a differant
    model are year that does not align properly
    B.A three way cam gear was installed in motor
    and was installed retarded are advanced.

    Now for the question....
    Can the timing be set with a vac. guage by adjusting timing for the highest vac. reading with a smooth needle [as in no "bouncing" of needle] ???

    Should I be looking for why stock setting does not work? If so where should I start?
    /forums/images/graemlins/1zhelp.gif /forums/images/graemlins/1zhelp.gif
     
  2. joker89blazer

    joker89blazer 1/2 ton status

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    Anyone???????
     
  3. Blazer_Boy

    Blazer_Boy 1/2 ton status

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    Dumb question, but have you left it 0, turned the engine off, and then plugged the wire back in? We can rule out cam issues. A incorrectly postioned cam will only affect valve timing. The timing mark is only concerned where the crankshaft is in its stroke. We need to know a little more here to get a clearer picture. It could be a number of things. Could have a vacuum leak, which means break out the WD-40 and find that leak. IAC valve could be bad, or EGR comming on at the wrong time. The damper may be a problem if the wrong one was used. GM located the timing marks in two different locations over the years. One will have the mark 2 degrees before the keyway and another 10 degrees. This would put in in the wrong spot and make the mark invalid. The only way to know if to get a piston stop and find the correct TDC.
     
  4. joker89blazer

    joker89blazer 1/2 ton status

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    I set the timing as you said.

    Am I missing something??

    The distrubitor runs off the cam, if you change the cam timing you also change the iginition timing, If say the cam degree was changed by four degrees the iginition timing should change by four degrees also.

    Wouldn't the iginition timing have to be changed if a cam is advanced are retarded with a adjustable cam gear?
     
  5. Blazer_Boy

    Blazer_Boy 1/2 ton status

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    It kinda took me a while to wrap my mind around it to. It'll click and make sense if you think about. Its kinda like its impossible to have the distributor "one tooth off". You may have to rotate it alot and the wires won't reach, but a distributor techinically can be timed properly no matter where you drop it. We don't do that though because things like vac advance would hit the firewall.

    Think about how a distributor is timed. Its based on how many degrees before top dead center the #1 piston is. The damper is connected to crank. You can unhook the timing chain if you want and it won't change where the timing marks one bit. Also think about how the cam manufactures don't mention anything about ignition timing adjustments for retaraded or advanced cams. Say you took complete motor and changed the cam timing while leaving the distributor in, then yes it would change things. No biggie, just undo the hold down and move the distributor a little. I know I'm making abosouletly no sense and sound like an idiot, but the cam is fine. I've been there before (had the wrong damper) and figured out that it can never be the cam. Unless you've got cam walk from in improperly installed roller cam, but thats a whole other can or worms.
     
  6. schmuck

    schmuck 1/2 ton status

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    tried timing by vacume before put me so far advanced it was pinging at 800rpm
     
  7. wrathORC

    wrathORC 1/2 ton status

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    It'd go with the wrong dampener. I know the one on my 77 was way different than my 84.

    Also, are you sure the timing tab is in the right spot? I know those tend to be in the wrong spot also.

    I'd find TDC and see what happens. I'd find #1 on the compression stroke and made sure the rotor was pointed at #1. I'd then see how the timing marks line up.

    The problem with turning the distributor a lot is that you can start starving the motor for oil (ever notice the weird cut in the distributor body).

    I set timing based on RPM, smoothness, throttle response, and vacuum. The most vacuum I can get out of my truck is 14" at like 13°. But it runs smoother, the exhaust smells better, and will idle lower with better throttle response at 9° and 11" of vacuum.
     
  8. Blue85

    Blue85 Troll Premium Member

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    Aha! I had the exact same problem with my truck. It is a carb engine though. The engine was replaced and my belief now is that the old dampener was reused even though it wasn't the right one for the new engine. It showed up as firing like 100 degrees BTDC. Turning it to what showed 8 degrees or produced a barely running vehicle that puffed tons of black smoke. You can call it retarded for more than one reason.

    I finally got some timing tape and put it on the damper cover. I didn't think the finger over the spark plug hole would be accurate enough, so I dropped the oil pan and found the center point between where #1 piston started to move in either direction. After that I was able to tune my mechanical and vacuum advances the way I wanted them. Plus with the timing tape, you can use a real basic timing gun and don't need the adjustable timing one. It's like $5 from Summit.
     
  9. joker89blazer

    joker89blazer 1/2 ton status

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    I have it now set at what it looks like 8*

    Tried setting with vac.guage for highest vac.,timing was to for advanced that away.

    Thinking about now getting a scanner and advanceing the timing a little at a time until knock sensor starts counting knocks

    Truck also has a idle problem with speeding up and slowing down repeatedly. Maybe a related problem,going to try adjusting idle air control valve.
     

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