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I need help with a timing problem

Discussion in '1969-1972 K5 Blazer | Truck | Suburban' started by travis, Apr 7, 2001.

  1. travis

    travis 1/2 ton status

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    My valves rattle pretty bad (diesling) when I'm running about 2000 rpm. I checked the timing today and it's set at 10 deg with the vacuum lines disconnected. When I hook up the vacuum advance I'm running at about 50 deg at 2000 rpm. This problem seems to be getting worse. I thought the rattling was due to high compression because of my steel shim head gaskets, so I changed them out a while back but it didn't help. I have an HEI distributor and it came with another mechanical advance. I didn't install it and don't know what it's for. I have an Edelbrock 1406 carb and performer manifold. I'm running a 218 deg cam. Vacuum on the passanger side port is 0psi at idle and about 20psi at 200 rpm. On the drivers side vacuum port it has about 12psi at idle and 35psi at 2000 rpm.
    What could be causing my problem? Any help would be appreciated.

    Travis
     
  2. Steve_Chin

    Steve_Chin 1/2 ton status

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    Boy... Need more info: What is the mechanical compression ratio of the engine? What is your ignition advance curve like? What's your total mechanical advance set to now (less vacuum)?

    I forgot to add that you also should know what vacuum sensitivity and how much additional advance your vacuum advance can has. One with less sensitivity will drop its advance quickly when you open the throttle blades. In general, most guys try to run a can that has 12-15 degrees (crank degrees) of advance, rated at their cruise vacuum. My can has 20 degrees of advance and I'm running 36 mechanical crank degrees of advance in my distributor (56 total with vacuum) with no detonation problems on 87 octane. My mechanical advance comes in at 2400 RPM, starts at 800 RPM, and shows 28 degrees of timing at 1800 RPM with 10 degrees initial advance. Your requirements may be different, based on your compression ratio and the gas you're running (I've got mine tuned to run on horse-whizz gas if I have to run it).<P ID="edit"><FONT SIZE=-1>Edited by Steve_Chin on 04/08/01 07:30 AM (server time).</FONT></P>
     
  3. 4x4k20

    4x4k20 1/2 ton status

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    10 degrees is to high drop back to 6 hook vac up to ported side and try that check you weights on your dist and the bushnings for wear 50 deg is to much you need to be in the 32 to 38 range try chageing your springs also

    4x4k20
     
  4. travis

    travis 1/2 ton status

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    I've got to get hooked up with someone around here who knows about this stuff. My HEI is straight from Summit. I don't know what the mechanical advance is. I taked to an old friend yesterday who helped set my timing again and adjusted the carb. He's built several racing motors and says 50 degreees is normal with mechanical and vacuum advance kicked in. I made the mistake of putting in regular octane at my last fill up. Hopefully when I get premium back in it will help.
    Steve, my pistons are advertised 9.75:1 into 64cc heads. I'm running stock 72cc heads so I'm guessing around 8.5:1. Just a guess though.
     
  5. Steve_Chin

    Steve_Chin 1/2 ton status

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    Close guess, Travis... Should be about 8.67:1 (given a zero deck height). FWIW, my true compression ratio is a tad under 8.4:1 (70cc chambers, 10cc dished pistons, 6.7cc compressed head gasket volume, 0.040" over 350, pistons 0.040" in the hole).

    I agree that 50 egrees of total mechanical plus vacuum is more or less normal. I think that you might find that a vacuum can with a higher vacuum rating (i.e. takes more vacuum to pull it to full advance) will drop its vacuum off more quickly and may give you the edge you need to kill the detonation. You also should check how much mechanical advance you are running around 2K RPM. If it appears that you're running too much, perhaps a stiffer set of springs will help slow the curve, or you could get a center advance cam that is profiled to provide less low RPM advance and more at the higher end (making the advance curve more of a straight line than a curve that rises sharply upward and tapers off toward level near its advance limit).
     
  6. travis

    travis 1/2 ton status

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    Thanks for the help Steve. I'll have to wait til this weekend to check it out. How much mechanical advance do you recommend? I don't have a vacuum can so that could be part of the problem. Actually, I think I have one but I broke the cap trying to get the old lines off sometime last year. Who makes good aftermarket cans?
     
  7. Steve_Chin

    Steve_Chin 1/2 ton status

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    Travis, when I was talking about a "vacuum can" I was referring to the vacuum advance can that is bolted to the distributor and moves the breaker plate... I'm using a Crane adjustable unit right now in my Blazer. I am using a GM part in my Trans Am. Both work fine for me...

    Oh, the cool thing about the Crane vacuum advance can is that it comes in a kit with several different advance springs so you can also set your curve up close to how you would like it.
     

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