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Thread: Timing Change +50°

  1. #1

    Default Timing Change +50°

    I recently had a very strange issue with my timing. While driving the truck started running strange, idle was high, IAC was closed, then it started to detonate above 1/4 throttle. Everything else looked normal on the touchscreen. I limped it into the parking lot at work. Shutdown, then it cranked really hard when restarting. I suspected a timing issue. I had a timing light with me, and when I checked the timing was ~50° advanced! It was past the end of my timing marks running about 75° at idle. The cap & rotor were both tight.

    Vehicle Specs:
    '71 Chevy K30
    427 BBC
    Terminator EFI upgraded to the latest firmware.
    GM small cap HEI controlled by Terminator.
    Distributor: GM Performance with ACDelco ignition module.
    Coil: ACDelco
    Cap Rotor wires: MSD
    I've been running the Terminator since 2016 and changed to the small cap with electronic timing control in 2017. I put several thousand miles on this setup with no issues.

    About a year and a half ago the truck was running poorly, down on power, missing, backfiring under load, and making the exhaust manifolds glow red. It sat for a year before I started fixing it. I attributed it to a bad intake gasket fouling the plugs. After I changed the gasket out and put everything back together I verified the timing. It was very retarded. I thought that I had made a mistake marking the distributor before removing it. So I moved the distributor to bring back timing to 25° BTDC. After this the truck was running great. Likely the issue was always the retarded timing. Now it's jumped ahead last week. Seems like it’s probably the same issue causing the timing jumps in both cases.

    Any ideas what could be causing these drastic and sudden timing swings? Cap & distributor are tight and not moving. Rotor is installed all the way, very little play. When the distributor was out a couple months ago the gear was in good shape. Didn’t check the roll pin that holds the gear to the shaft. So maybe. Could a loose cam chain allow the cam to skip and cause this? Seems like that would be making a ton of bad noise all the time so unlikely. Thanks for taking the time to read all this.
    Last edited by moto657; 08-07-2020 at 02:04 AM.

  2. #2

    Default

    Maybe too many words. Short version:
    Timing has suddenly advanced +50°. Cap, Distributor body are tight. Any ideas?

  3. #3
    Join Date
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    Default

    Quote Originally Posted by moto657 View Post
    Maybe too many words. Short version:
    Yes.

    Timing has suddenly advanced +50°. Cap, Distributor body are tight. Any ideas?
    Go through the Ignition Timing Synchronization LINK, it should give you a clue as to what's wrong. Maybe there's something broken inside the distributor, or it needs a new ignition module.
    May God's grace bless you in the Lord Jesus Christ.
    '92 Ford Mustang GT: 385" SBF, Dart SHP 8.2 block, TFS T/W 11R 205 heads, 232°-244° duration/.623" lift/114° LSA camshaft, 12:1 C/R, TFS R-Series FTI ported intake, BBK 80mm T/B, Dominator MPFI & DIS, 36-1 crank trigger/1x cam sync, 200A 3G alternator, Optima Red battery, A/C, 100HP progressive dry direct-port NOS, Spal dual 12" fans/3-core Frostbite aluminum radiator, Pypes dual 2.5" exhaust/off-road X-pipe/shorty headers, -6AN fuel system plumbing, Walbro 255 LPH pump, S&W subframe connectors, LenTech Strip Terminator wide-ratio AOD/3000 RPM converter, B&M Hammer shifter, FPP aluminum driveshaft, FPP 3.31 gears, Cobra Trac-Lok differential, Moser 31 spline axles, '04 Cobra 4-disc brakes, '93 Cobra booster & M/C, 5-lug Bullitt wheels & 245/45R17 M/T Street Comp tires.

  4. #4

    Default

    I tried a ACDelco ignition module, still same issue. I replaced the GM Performance distributor with a DUI unit. Problem solved. The DUI is a much nicer looking piece, and cheaper than the GM Performance part.

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