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1971 Mercury 800 thunderbolt 80HP spark troubles

carpshooter

New member
I have a 1971 Mercury 800 thunderbolt 80HP that has some spark or timing troubles.start it up one time and it will miss on two cylinders.shut down and restart and it will run fine.shut it down again and restart and it will miss on totally different cylinders. Any ideas on what could cause this?We have replaced the wire harness. it had alot of corroded bare wires. Any help would be appreciated, Thanks
 
Hope it's not the trigger but inside the distributor.Check the spring loaded brush and the graphite posts in there and the rotor arm,cracks in the cap.Coil?
 
Correct--that's the distributor model. Another item to check/ replace is the switch box. It might be intermitting on you, which would do that misfiring nonsense.

Jeff
 
MERCURY BATTERY-POWERED CDI TEST FOR DISTRIBUTOR MODELS
This test is for the 332-2986 switchbox used from 1967-1978 on all the inlines.
This test assumes your coil is good (problems with CDI coils are rare).
DISCONNECT BATTERY
1. Turn off ignition;
2. Disconnect all 3 distributor wires on the Port side of the switchbox (and the “mercury switch” if present);
3. Remove the High Voltage lead from the ign coil to the center of the dist. cap (remember it unplugs from the coil and unscrews from the cap);
4. Reconnect the H.V. lead to the COIL only;
5. Position the free end of the HV lead approx. 3/8" from ground (block, shrouds etc), and find a way to hold it there;
6. Jumper the brown and white terminals on the distributor side of the switchbox to each other.
RECONNECT BATTERY
7. Check that you have +12V at the red terminal (even with the ign off);
8. Turn on ignition and verify +12V at the white terminal (same side as the red terminal);
9. Ground the black terminal on the distributor side of the switchbox - this should cause a spark each time you touch ground.
If you get spark with the distributor bypassed, and it won't fire with the distributor connected, the trigger is bad and the entire distributor housing assy must be replaced.
If you get no spark using the test, the switchbox is probably bad. In that case, be sure to check for correct power on the switchbox, check all connections, and check the coil's resistance to make sure it's OK."
 
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