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Old 06-07-2008, 01:06 PM
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Rzeppa Rzeppa is offline
Cruise Moab Chairman
Join Date: Aug 2005
Location: Kittredge CO, USA
Posts: 6,720

Troubleshooting 101 - Process of elimination and logic.

(1) ASSUMING that 1 & 2 are misfiring, and 4-6 are working correctly we can probably eliminate carb malfunction. If the carb were not working properly, then all the cylinders would be having issues.

(2) You say that you are getting good spark at 1 & 2, so we can probably eliminate wires, distributor cap/rotor, and plugs. What was the color and appearance of those two plugs, and how do they compare to the other plugs in the good cylinders? That can be a major clue. The appearance of plugs can tell you a LOT about what's going on in there.

(3) If 4-6 are firing well, then we can probably also eliminate timing, and distributor shaft wobble. Also, if there were a crack in the distributor cap, or even carbon tracking, it would not affect adjacent cylinders, due to firing order.

So we have to ask ourselves the question: What makes 1 & 2 different from 4-6? Maybe the valves in those cylinders aren't adjusted properly? BUT, it seems awfully coincidental that those 2 adjacent cylinders would be the only ones with improperly adjusted valves.

Since we know that an F/2F intake manifold has adjacent mounting at 1/2, 3/4, and at 5/6, it might be worth investigating to see if there is a vacuum leak at the intake manifold at 1/2. If the nuts were loose, or if there were a flaw in the gasket or mating surface there, it would only affect 1 & 2, and would cause misfiring in those two cylinders. It's easy enough (compared to some other jobs) to simply check the torque on the nuts that hold those manifold ears to the head. (Proper torque on the M12 2F manifold nuts is 33 foot pounds). There is also the time-honored tradition of spraying carb cleaner around suspected vacuum leak areas while the engine is running and listen for a change in idle. And of course, if those two plugs were white or very light compared to the other four, that would be a dead giveaway.
Jeff Zepp
Kittredge CO USA
1971 & 1976 FJ40s, 1978 FJ45, 1987 FJ60
Rising Sun 4WD Club, TLCA #4063
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