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Here are a few thoughts:
- Always verify ignition is working properly before diving into the CIS.
- You've mentioned plugs, wires, cap, rotor, and idle timing. Have you verified that the distribution is adding advance as it should with increasing RPM? These motors really run poorly without appropriate advance when revved; they don't seem to care much about idle timing.
- Oil sputtering out the exhaust - Have you checked the intake air box for excessive oil? Oil tank over-filled and ingesting oil through the breather hose?
- Definitely check for vacuum leaks. Pop-off valve fitted in the air box? Is it sealing? Got a hand-held vac pump? If so, check the line from the engine to the brake booster and verify that seals. Check vacuum retard on the distributor.
- Which WUR is fitted to the engine? Need that to help with CIS diagnostics.
- Leak-down test is a good idea if you don't find anything else. You described the problem as suddenly appearing; a leak-down test will pinpoint issues that took a while to get there...
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