What would cause timing to retard then advance on acceleration from idle?

Discussion in 'Technical' started by whoozurdaddy, Jul 10, 2014.

  1. whoozurdaddy

    whoozurdaddy Member

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    I have a freshly rebuilt 357w roller motor, 10:1 static compression, trick flow stage 2 camshaft, older freshened up edelbrock heads, 2.02, 1.6 valves, weiand stealth intake, holley 770 street avenger, duraspark ignition with new distributor. BBK shorty headers, no other exhaust installed at this time. It seems idle best and have good acceleration at 10 BTDC @ 800 RPM. When I rev up the engine with no load, the timing will retard for a second, down to about 6 BTDC then climb to about 32 @ 3000 RPM. Does anyone know why the timing would retard before advancing? It sounds pretty good, but has a slight almost undetectable hesitation. The only thing I can think of is possibly the built in retard for starting, white wire in the duraspark wiring circuits. Does anyone know if or what else could possibly cause this split second retard of about 4 degrees on rev up? Any input would be welcomed. Thanks and God bless, wade
     
  2. Acornridgeman

    Acornridgeman MCCI Wisconsin State Rep Moderator Supporting Member

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    Just thinking out loud here ............. but could you have a little vacuum advance going on, even at the low 800 rpm? When you crack the throttle, vacuum drops off and the timing retards a bit. If you plug the vacuum line at idle is it still at 10 btdc ?
     
  3. simple man

    simple man Member

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    If you are using vacuum advance, do you have it connected to the intake manifold? You will have an increase in manifold pressure when you open the throttle. This will cause the vacuum to drop off, retarding the timing. You could try connecting your vacuum hose to the ported vacuum on the carb, and reset initial timing. If you are not using the vacuum advance, you may have some slack in the timing chain. I will say this is unlikely for you since I'm sure it's new as well. :)
     
  4. whoozurdaddy

    whoozurdaddy Member

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    no vacuum advance only centrifugal

    I did have a very slight bit of slack in my timing chain set. Its a cloyes true double roller and maybe had 1/4 inch of deflection with opposite side tight, so I think that is within tolerances, but not 100% sure. this vehicle is also a manual trans. As stated above I run only centrifugal and no vacuum advance.
     

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