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Old 07-08-2011, 01:31 PM
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motor runs rough under light load

One more time.SBC 350/auto.replaced dist wires coil plugs carb filter gas,tried evrything possible and the car still misses under a crusing load,at 1200-1400 rpm.He says stock cam.I find that hard to believe with the lumpy idle.WOT the car seems smooth but should have more balls.Checked for al possible vacuum leaks even uplugging everything and capping them off,then spraying around EVERYTHING.Adjusted the valve lash,started with zero lash + 1/4,then+ 1/2,then +3/4.still no change.We've come to the conclusion its internal,maybe valve related or cam related.Anywhere else?If its internal he could take it back

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Old 07-08-2011, 01:35 PM
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My guess would be valve adjustment but you said you already ruled that out... how do the plugs look? What the timing curve setup for on this? Initial and total mechanical?
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Old 07-08-2011, 01:37 PM
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Did you remove the adapter plate? Did you make sure the holes in the intake were plugged?
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Old 07-08-2011, 01:52 PM
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Did not remove the adaptor plate.The holes are plugged.It has a MSD dist of unknown curve.But I put in an HEI just to rule that out.Vacuum reads 12-14 inches.but it flutters with the motor which makes me belive valve train.I did not do a compression test.just a PITA when its in a Corvette.
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Old 07-08-2011, 01:53 PM
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Plugs look light tan no oil burning good
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Old 07-08-2011, 01:55 PM
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Do 300hp/350 motors in a 69 vette come with domed type pistons?When at TDC I put a small screwdriver in the spark plug hole and was def not a flat top or dished.
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Old 07-08-2011, 01:57 PM
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initial timing was bout 16,dont know what total,
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Old 07-08-2011, 03:07 PM
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I had the same problem on a trans am,i had a bent valve
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Old 07-08-2011, 04:24 PM
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your reading my mind
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Old 07-08-2011, 04:25 PM
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compression test tomorrow
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Old 07-09-2011, 09:15 AM
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compression test comlete
cyl # 1-180
2-170
3-185
4-170
5-170
6-165
7-170
8-175
no cylinders leaked down.WTF?!! I can't get a hold on this.wouldn't a bent valve show up on this?
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Old 07-09-2011, 12:13 PM
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OK.Adjusted the initial timing to the sweet spot with a vacuum gauge.Without reconnecting the vac advance I put it in gear and hold the brake.give it gas till the problem rpm and it seems fixed,Hook up the vac advance to ported vac on carb,very slight miss.hook the vac advance to full manifold vac and it runs like crap.Its a MSD distributer.How do you adjust the canister?I could fell anything in the canister with an allen wrench.Is it adjustable?How should I adjust it?
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Old 07-09-2011, 01:19 PM
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ok,I take it for a ride without vac advance.Miss is almost totally gone.starts to break up at around 3500,I'm assuming its because no vac advance.what now?
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Old 07-10-2011, 12:33 PM
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anyone?anyone?
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Old 07-12-2011, 08:40 AM
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I guess no one has any other suggestions
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