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Spark knock at 3800 rpm's?????

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by 85 Shortbox, Jun 19, 2002.

  1. 85 Shortbox

    85 Shortbox 1/2 ton status

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    Spark knock at 3800 rpm\'s?????

    This has been going on for a while and I am wondering what you guys think. If I wind her out slowly up to say 4000-5000 r's it doesn't really make a noise. But if I really stand on it to get her there quick I hear what to me sounds like a spark knock. Any ideas?
     
  2. little79k5

    little79k5 1/2 ton status

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    Re: Spark knock at 3800 rpm\'s?????

    what octane gas are you running? have you messed with the timing at all? is it a stock motor?
     
  3. 85 Shortbox

    85 Shortbox 1/2 ton status

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    Re: Spark knock at 3800 rpm\'s?????

    I usually run premium 92 octane, though times are tight and gas is expensive and I have been running 87 octane. As far as the motor is concerned, it isn't stock. 350 -.030 steel crank, forged pistons, cam(.496 lift @ .282 duration) 1.94 heads with 3 way grind and milled -.030. Edelbrock 600, performer RPM air gap intake, and headers.
    I am just wondering if you think maybe my timing is too far ahead? Of course the cheap gas isn't helping, I know.
     
  4. Blazer_Boy

    Blazer_Boy 1/2 ton status

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    Re: Spark knock at 3800 rpm\'s?????

    Plug off your vac advance and tell us what happens.
     
  5. dyeager535

    dyeager535 1 ton status Premium Member

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    Re: Spark knock at 3800 rpm\'s?????

    Whats your compression? Sounds like you've got a fairly wild cam in there, and since you are running forged pistons, I imagine you are in the 10.5:1+ range...

    Cheap gas won't make any difference (actually works better than high octane) unless you are running a lot of compression, or have your timing too far advanced even with 8 or 9:1.

    Forged pistons supposedly are "noisier" than cast, but I'm fairly certain that is only when they are cold, and have not yet expanded. (forged pistons expand/contract more than cast for anyones benefit thats curious)
     
  6. 85 Shortbox

    85 Shortbox 1/2 ton status

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    Re: Spark knock at 3800 rpm\'s?????

    Ok, here is what I got. My vacum advance doesn't work. My compression is around 11 to 1. So now my question is do I need a whole distributor or can I just rebuild the vacum advance?
     
  7. dyeager535

    dyeager535 1 ton status Premium Member

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    Re: Spark knock at 3800 rpm\'s?????

    Sure you can "rebuild" it, but mainly it just consists of replacing the vac cannister lol : )

    It might not be a bad idea to take the cap/rotor off, and really inspect the mechanical advance weights and the vac advance.

    Vac advance should move smoothly when you suck on the hose, and it should hold at that position when you stop sucking on it and plug the line...if it lets up right away, starts moving, or you can't suck on it fast enough to make it move, its shot. As well, if the motion of the linkage on it is "jerky" as you suck on it, it probably needs replaced. FWIW, this isn't very common at all to fail.

    Mech advance, check that the bushings aren't shot, the springs are present, and that everything "snaps" back into place when you move the weights a bit. I had a problem with mine, but the symptom was an idle speed that varied every time I let off the pedal.

    Seems odd to me that you are having knock/pinging/whatever at that kind of RPM. If you are *cruising* at that RPM, then ok, I can see perhaps vac advance being a problem. But if you are talking about WOT, or low engine vacuum at that RPM, then vac advance isn't the culprit, as it won't be working. Moreso, mechanical advance. (which probably could use some fine tuning with your motor compared to a stock one anyway)
     
  8. TopOff

    TopOff 1/2 ton status

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    Re: Spark knock at 3800 rpm\'s?????

    Timing is everything.

    With that change of a cam, I suspect you don't have the factory computer, or computer ignition anymore?

    What initial timing do you have? What is your ignition timing at ~2500 - 3000 rpm.

    What distributor are you running? HEI, UNI-LITE, MSD?
    Does it have the mechanical advance?
    Can you turn the rotor and it SNAPS back?

    Basically, your ignition mechanical advance should be "full out" ~3000 rpm.
    Eveything above that you would rely on vacuum advance. Vacuum advance would be for cruising only, anyways.
    So if you don't have any vacuum advance, I would say you have problems eslewhere.

    Check for vacuum leaks. Lean carb / jets.

    If you are interested in getting better freeway miles you will want to re-connect your vacuum advance. I would get an adjustable vacuum advance kit from Crane.

    You should be ~8 initial, +20 mechanical (you can change the RATE at which the mechanical comes in by using different springs) maybe 5-12 degrees vacuum = 40 total ignition timing when cruising.

    If you don't care about mileage. Keep the vacuum disconnected, and just dial in your initial and mechanical ignition.

    Good luck.
     
  9. 85 Shortbox

    85 Shortbox 1/2 ton status

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    Re: Spark knock at 3800 rpm\'s?????

    Problem #1. Power booster for brakes has a small vacum leak.
    Prolbem #2 I don't know what my timing is set at specifically because a friend helped me do it "by ear"

    Solution. I am going to go out and by a new MSD distributor, and hopefully fix the vacum leak in my brakes. So now my question is, providing I set the timing right, will this fix my problem?

    Dyeager- Good job on your website. It is helping me out with a lot of my q-jet problems.
     
  10. dyeager535

    dyeager535 1 ton status Premium Member

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    Re: Spark knock at 3800 rpm\'s?????

    I don't like "new" pieces, I'll be honest. I can't see shelling that kind of money out for something that is fairly simple. Yeah, you are having problems, and the distributor is *likely* the cause, but if you aren't POSITIVE its the dist., and you pay big $$ for something that doesn't fix the problem, you are still in the same boat, with less money.

    Do you not have someone around that has a spare they can loan you for testing? I mean if you are that sure its the problem, and you have access to someones known good spare, throw it in and see what happens...I have had nothing but good luck with my stock HEI stuff. (after I fixed normal wear after 20+ years) I mean the most I can see spending to rebuild the stock one is $100, and thats most everything new, including new bushing at the bottom.
     

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