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Weird Timing issue on a 86 I6 300cui

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Old 03-09-2011, 02:17 PM
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Weird Timing issue on a 86 I6 300cui

so rebuilt my spare distributor for my truck, new pickup, rotor, cap and case and vacuum advance module and have the weirdest thing happening

we set the timing to 10 degrees BDTC (originally tried 14 but then we got this problem so went back to stock adjustments) and all is well, UNTIL we connect the vacuum advance, then the rpm's drop and it sputters and wants to die out and idles really low, its got fresh wires and fairly new plugs and runs fine right until you connect the vacuum line. we ruled out weird vacuum issues since if we disconnect the hose and actuate the advance on its own it does the same thing. the truck is a bit of a hack job from the previous owner but everything looks to be hooked the same as the parts truck that i took the rebuild dist from (couldn't test with the old dist because the advance on it was broken thus the need for the new unit)

anyone experience something like this before?
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Old 03-09-2011, 02:22 PM
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Location: Copley, Ohio
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Are you sure you're using the right set of timing marks?
I'm not certain where the correct ones are on an 86 but I know that the ones cast into the timing gear cover are the wrong ones on an EFI truck. If you're using the ones on the front cover, look for another set on a metal tab at about 9:00 on the passenger side and see if those work better.
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Old 03-09-2011, 02:24 PM
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we cleaned everything up and are using the right ones on the passenger side front.
its not a EFI truck its the 1 barrel carb motor. (may be switching to a 4barrel carb and intake seutp in the not to distant future)
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Old 03-09-2011, 02:31 PM
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also should note we swapped in a spare coil and duraspark unit i had from my parts truck and same issue (though both coils could have issues, but it runs fine without the advance)
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Old 03-09-2011, 06:24 PM
glc glc is offline
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Sounds like you have the vacuum advance backwards/upside down and it's retarding rather than advancing. Either that or it's a dual advance/retard mechanism with 2 vacuum ports and you have the hoses crossed.
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Old 03-11-2011, 05:01 PM
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You have the wrong vacuum source. 86 came with a feedback carb with a tfi distributor. Sounds like someone changed it to a duraspark system. Not a big deal, I did the same with the 86 that I had. You need to find a "ported" vacuum source, one that does not have vacuum until the engine is revved up. Vacuum at idle will cause the advance to actuate. That's the reason it doesn't run right. Take a look where the distributor vacuum is hooked up to the carb on an 80 to 83 six cylinder
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