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Ignition timing measurement on 350 V8

Discussion in 'Chevy Trucks' started by 75gmck25, Aug 28, 2003.

  1. 75gmck25

    75gmck25 Senior Member
    Messages: 119

    I decided to tune-up my '75 GMC (350-4v) this weekend and started out by replacing the plugs, rotor, cap and wires, checking compression (cylinders ranged from 153-168 psi) and checking vacuum (a little above 20 inches at idle). While I had the distributor cap off I rotated the centrifugal advance back and forth and it appeared to rotate and spring back okay. I also verified that the vacuum advance lever moved when I pulled vacuum on it.

    Now for the problem -
    Last night I tried to check initial ignition timing and got some screwy results. With the vacuum advance disconnected and the idle down to about 600 rpm, my initial timing looks like its at about 22-24 degrees (way past the indicator). I find that a little hard to believe, since I don't get any pinging during acceleration. I also found that I couldn't get the idle any lower than about 600 rpm, after I backed the idle screw off until it wasn't even touching the linkage.

    Anybody have any idea what I'm doing wrong, or what might be wrong on the engine? The crankshaft damper might be installed wrong, but I don't think its ever been off the engine. Could the centrifugal advance springs be so weak that its stuck and kicking in at idle? Or could the vacuum advance unit plunger be stuck or binding? I didn't look at them carefully when I had the cap off, but they seemed to work okay. The vacuum advance plunger only moved about 1/2 inch when I pulled vacuum on it.


  2. wyldman

    wyldman Member
    Messages: 3,265

    Your timing may actually be that high.The more you advance the timing,the more the idle speed goes up.Maybe it is over advanced.

    You could also have a vacuum leak,or bad throttle shaft bushings,which would cause the high idle.

    It is also possible the balancer has slipped or the timing pointer is off,or incorrect.

    First thing I would do is to verify true TDC.I explained it in another thread,I'll see if I can dig it up.
  3. 84406

    84406 Junior Member
    from canada
    Messages: 17

    It is possible that the outer ring on the balancer has slipped, especially if it is original. You can check by putting #1 on TDC on the fire stroke and then check the timing mark position. Should be zero.
  4. wyldman

    wyldman Member
    Messages: 3,265

    Here is the link to the thread where I explained how to find true TDC.If it's off then the balancer has moved,or the timing pointer is wrong.


    They had several different pointer locations,like on the center of the timing cover,or on the drivers side,bolted on.Maybe someone has changed something,and put the wrong one on.Finding true TDC will tell you.
  5. 75gmck25

    75gmck25 Senior Member
    Messages: 119

    It sounds like the first step is to verify TDC on number one cylinder. I can use the rotor position to get it in the ballpark, but it will be hard to tell when the piston is at the top of the stroke. I'd appreciate any suggestions.

    Is it possible that initial timing is way advanced, but the centrifugal or vacuum advance is not working right? Otherwise it seems like I should be getting significant pinging under load if the initial advance is that high.

    The balancer didn't look like it had an outer ring that could slip, but I'll look at it again. The timing mark is a slot that goes all the way across the balancer. I can't really see the degree tab very well to tell how its bolted on, but I'll try to check if it looks like it could have moved.

  6. wyldman

    wyldman Member
    Messages: 3,265

    A small block chevy will take 20 or so degrees of initial advance without pinging,especially on low compression motors.You should be able to check centrifugal and vacuum advance with the timing light and the engine running.

    The balancer will definitely have an outer ring.

    You can use the rotor to get you close to TDC #1,and then use the procedure in the link I posted above to find true TDC.
  7. 75gmck25

    75gmck25 Senior Member
    Messages: 119

    Wyldeman,I read your procedure on how to check TDC and I'll see if I can put together the spark-plug TDC tool tonight from one of my old plugs.

    If it turns out that the engine really is at 20+ degrees BTDC initial timing, how does this work with tuning the centrifugal advance curve and vacuum advance? Most performance guides suggest about 12 degrees initial, about 20 degrees more from centrifugal advance by 2800-3000 RPM and about 10-15+ more degrees from the vacuum advance under cruise conditions. Do I want a similar profile if I start at 20 degrees initial?
    I'm planning to use one of the Mr. Gasket kits that has a set of advance springs and weights, and an adjustable vacuum advance.


  8. ratlover

    ratlover PlowSite.com Addict
    from IL
    Messages: 1,325

    Also do forget if you have an adjustable light that it can screw up your reading. Dont ask me about personal expreinece with that one :eek: You sure you are reading it right? if its adjustable then you should turn the dial to what ever it is timed at and the mark will show up in the 0 notch or the little tube for most sbcs. if you have a nan adjustable then you just see were the mark falls on the shark teeth looking stuff. If you know how to do a leak donw test I would think you know how to time something but I ran a sbc around for a couple days running 24 initial since I had a brain fart.

    Does it start easy? 20 initial seems up there for a sbc. You run regular low octane stuff right? no premium? does it ever ping? how easy does it shut down?

    I would think either you balancer slipped(can be installed wrong if really, there is a key way so it only goes on one way. Balancers are sometimes miss marked or the tab might get mangled. Light might also be fubar. I find it hard to believe that it runs fine with no pining at all with 20 initail and a vac advance and properly working mechanical. High advance will speed up the idel quite a bit.

    If it truely is running all that initial and it curently runs fine then you will be pretty unhappy with it backed off. If it does run with no ping at 20 inital I bet your mechanical/and possibly vac isint working quite right

    First I would in order of easyness re time it to make sure you didnt have a brain fart somewere and then barrow a friends light to make sure yours aint fubar. Then check for TDC
  9. 75gmck25

    75gmck25 Senior Member
    Messages: 119


    Thanks for the suggestions. I don't have an adjustable timing light, so that shouldn't be the problem. You've mentioned most of the other possible reasons I could come up with, and my best guess is a rotated balancer or the sharks teeth are bolted on wrong. The truck spent most of its life on a farm with several other GMC/Chevy trucks, and maybe they swapped the wrong parts between vehicles.

    The truck starts on the first rotation of the starter after I put in the new plugs, cap, rotor and wires. I was trying to use the starter to bump it over a little to look at the balancer, and every time I hit the key the engine caught and it was immediately running. I'm running 87 octane gas and don't get any dieseling when I shut down.