Author Topic: 302ci. backfired when starting.  (Read 196 times)

joeydee

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302ci. backfired when starting.
« on: September 14, 2019, 01:25:03 AM »
69 Z-28 302ci. backfired through Carb. and now making metal sounding noise? Pulled covers and push rods, look good. Pulled Distrib. and looks good except for excessive play on shaft, Up and Down, aprox. 1/8 in. play ? ? Pulled Plugs and #1,#3,#2, Carboned & Fouled !!!   #6 Semi Carbonded !! #4,#8,#5,#7, Cylinders real Clean ! Will dump oil tomorrow and check for metal Filings. This is a low mileage original paint,#'s matching  RS Z with only 13,600 miles, and I haven't changed the plugs in Years! But it was running pretty good that day of the Car Show,not missing or anything noticeable.Turned it off when I got home to let it cool down for about 30 min. and when I went to start it up ,it backfired through the Carb. . Shut it down and restarted ,and noticed this Metalic sound ? I kind of wondering , maybe it's the Smog Pump or Diverter Vavle?

william

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Re: 302ci. backfired when starting.
« Reply #1 on: September 14, 2019, 02:35:28 AM »
May have lifted a rocker arm stud. Place a straight edge across them.

firstgenaddict

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Re: 302ci. backfired when starting.
« Reply #2 on: September 14, 2019, 03:36:10 AM »
1/8" on the shaft is WAY too much, I think spec is .020. it will walk up out of the cam when you back off and retard the timing.
James
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joeydee

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Re: 302ci. backfired when starting.
« Reply #3 on: September 14, 2019, 12:12:05 PM »
Rocker studs are Pinned. I'm baffled ? Tks Joe

joeydee

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Re: 302ci. backfired when starting.
« Reply #4 on: September 14, 2019, 12:14:35 PM »
Will shim above bottom gear, Tks Joe

ko-lek-tor

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Re: 302ci. backfired when starting.
« Reply #5 on: September 14, 2019, 12:25:37 PM »
Plastic Timing gear disintegrated?
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69Z28-RS

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Re: 302ci. backfired when starting.
« Reply #6 on: September 14, 2019, 01:33:00 PM »
Plastic Timing gear disintegrated?

Very possible if it's the original one - after 50 yrs (or less) plastic deteriorates...
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70z28lt1

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Re: 302ci. backfired when starting.
« Reply #7 on: September 14, 2019, 03:15:43 PM »
Might want to check the powervalves in the carb.  A backfire usually takes them out.

z28z11

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Re: 302ci. backfired when starting.
« Reply #8 on: September 14, 2019, 04:45:07 PM »
Stock exhaust ? Thermal (warm up) valve break the return spring ? Could be flopping, makes weird noises occasionally, especially when the spring breaks.

I've seen a lot of stock distributors with .080 - .100" plus play due to sloppy tolerances. If you convert to a breakerless system, you can take the axial play down to recommendations with the (usually) supplied shims. I'd convert it if it was mine - takes the chance out of point failures when you least need it to happen.

Regards,
Steve
1968 Z28 M21/U17 BRG/W
1969 Z28 X77/M20/VE3 LeMans/W
1969 L78 X66/N66 Cortez/BVT
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