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Posted by D. Sherman on Wednesday, January 09, 2008 at 13:38:13 :
In Reply to: Re: Well truck is backfiring and runs kind of sick. posted by Todd Wilson on Wednesday, January 09, 2008 at 12:50:14 :
"runs kid of sick" could be lots of things, but backfiring through the carb requires that a spark plug fire while an intake valve is open. No dirty plugs, flaky coil, or other electrical problems can cause that. It has to be a timing issue -- a plug is firing when an intake valve is open. That can only happen if the valve is open when it shouldn't be (valve stuck or burned or cam timing jumped) or the spark is sparking when it shouldn't be (wrong wire to plug or distributor moved a lot). In my experience a lot of people try to work magic upon spark plugs when there's really a fundamental problem somewhere else.
Troubleshooting is sharpshooting -- do specific tests to identify the problem before replacing things. Shotgunning is replacing everything you think it might be, usually starting with the things that are easiest to replace -- plugs, wires, filters, cap & rotor, carburetor, fuel pump, etc... until it starts working. In troubleshooting, you "divide and conquer". For example, a compression test will tell if it's an internal engine problem or not. If it's an internal engine problem, a wet compression test will tell if it's rings or valves. If it's not an internal engine problem, turning the engine to TDC and looking at the distributor rotor will tell if it's a mechanical or an electrical problem in the distributor. You get the idea. Divide the engine into sub-systems and do tests to narrow down which sub-system is bad. Or just shotgun it and hope you get lucky.
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