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Engine Timing Problems?

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by greatwhitenorth, Jul 22, 2002.

  1. greatwhitenorth

    greatwhitenorth 1/2 ton status

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    The motor is a 80's targetmaster with a MV4 q-jet, delco distributor/plugs - rebuild with 4000km on it.

    I've been getting what I think is pretty bad detonation as when I put the motor in I had no light so timed it by ear.

    Now I go to time it properly, and with the idle speed adjusted, vacuum advance plugged, air cleaner on. I started out with about 20 deg of advance, and that's where its been for the last 3 weeks that I've had the motor. Seemed to run fine, just pinged under load.

    Now I go to adjust it back down to about 8 deg advanced and when it's set at 8 it idles way down and bogs out sorta.

    Any idea what my problem is???

    As far as I know the guts of the motor are pretty stock, but would a different cam want more advance?? Idle mixture??

    I'm lost, any help is much appreciated.
     
  2. mdm365

    mdm365 1/2 ton status

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    the rule of thumb I was tought was 2 degrees for every 1000 feet of elevation.the total timing should be 32 or 36 depending on a given cam profile.when you get into full out racing it changes a little.I have the vacume advance in my 350 set at 26 degrees and crank timming is set at 6 degrees advanced.runs sweet.you can use a timming light with the dial to check total advance.I hope this may help mike
     
  3. Blazer_Boy

    Blazer_Boy 1/2 ton status

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    Alright, time to go to work. I'll keep this short because I could go on for pages (seriously). You have a mechanical advance and vacuum advance in your distributor. The mechanical advance has has two weights and springs (look under your rotor). The weights come out from centrifigul force and springs control how fast this happens. It will add a total of 20 degrees timing. Then of course you have a vacuum advance that adds a little more timing the more vacuum you have.

    Soooo, most small blocks like around 34-38 degrees total advance (initial + mechanical). Since the mechanical advance adds 20 degrees, we're looking for an initial setting from 14-18. 18 is a little fast and I doubt you'd need that much, so 14 or 16. Get some timing tape or a dial-back timing light and a tach. You want that 20 degrees of advance in by 3,000 (with vac advance disconnected). You can change the tension of the springs to accomplish this. You can hit the junkyard or parts store for a kit or springs. Use the stocks wieghts and find replacement stock weights if yours are worn. Play around with your initial settings and springs with the vac advance disconnected until you get good performance.

    Now, vacuum advance, its there for economy mainly. Dump that 20 degree can because its causing the detonation. Crane has a kit with new springs and an adjustable advance. Accel has one too. Throw out the little limiting plate crane gives you. You just stick an allen wrench in the can and can adjust how vacuum advance you get. 8-12 degrees should be a good ball park. Use as much as you can until performance decreases, then go back. The vac advance should be hooked up to a port on the carb that doesn't have vac stidle and does when you give the throttle a little shot (try sticking your finger over the line at idle and see).
     
  4. Blue85

    Blue85 Troll Premium Member

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    But most small blocks come with more advance than this stock. The can gives 20 degrees and the weights give about 20 degrees. Initial is sometimes as high as 8 from the factory, which is about 48 degrees total. Many tuners recommend 45-55 degrees total.

    Also, the allen wrench in the can adjust the RATE of the vacuum advance (i.e. degrees of advance per inch or mercury). The little stop plate is to limit the total AMOUNT of the advance. Of course, the maximum amount of vacuum that your engine produces also limits the maximum amount, but you may have to sacrifice advance in other areas of the curve.
     
  5. Blazer_Boy

    Blazer_Boy 1/2 ton status

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    I found my adjustable vacuum advance laying in old bucket of parts. It was brand new, still had the little rubber nipple on the tip. I too was told the allen screw adjusted the rate by Crane Cams. Just for fun, I put a vacuum pump (Snap-On unit with the gauge) on the can and tried things out. All the way out gave about 20 degrees and she dropped roughly 2 degrees per full turn to the left. It was all in by about the same amount of vacuum (can't remember what number as this was a few months ago). I watched this also with the can out of the distributor and it started in the same posistion but wouldn't pull in all the way based on how many turns I had on it. The stamped numbers on it appear to match that of the accel unit. Accel doesn't even include a limiter plate with their can, just an allen wrench.

    Most GMs never reach the number that you get if you added initial + mechanical + vacuum. There are several controls that limit vacuum advance with the stock hose routing, such as temp, deaccelaration, being in the lower forward gears. The springs in some units are so stiff that full advance won't come in at damn near red line. 305s and 400s came with a total of 44 degrees advance stock. It'll never reach that number because if you had the engine rapped up hard enough to get full mechanical advance, the vac would be so low that it wouldn't provide much advance. The numbers may not add up to be more than stock, but its going to get alot more advance than stock in real world operating conditions (crusing, high throttle with low vac, low RPMs).
     
  6. tipoil

    tipoil 1/2 ton status

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    ???? 40 to 55 degre total advance!!!!! sound WAY to much. /forums/images/icons/confused.gif
    Never heard this much timing ..never..never. Actualy yes,it was pigning at idle Yes at idle!!!

    As far as ur problem GREATWHITENORTH.
    U just have to reset ur idle up a bit,because u lower the timing the idle goes down .
    Just keep on giving it more advance until she ping a bit and back it up 2-3 degre just to be sure.

    One thing to remember : the hotter the engine is,the easyer it is for pigning / the less octane rating does the same thing too .
    So be carefull about pigning noise because it will KILL ur engine real bad.
     

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