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Old 10-13-2018, 10:13 PM   #24
Marshall V. Daut
Senior Member
 
Join Date: Apr 2011
Location: Davenport, Iowa
Posts: 1,366
Default Re: Terriable backfiring

'Sounds as if you are about ready to consider the weird causes of misfires since you've just about exhausted the list of usual culprits. So, odd though it may sound, try this: Take off the terminal box cover and wiggle the bolts sticking out. If either is loose or can be pushed in and out, it's possible that the heads are touching or arcing against the metal firewall and creating a misfire. This will be especially likely when the firewall gets warm and the metal expands to close the gap between itself and the stud heads. In the 1970's, a couple inferior and somewhat shady parts vendors (old-timers know whom I mean!) imported lousy terminal boxes that had bolts with the heads too big to be completely sunk into the plastic body. The holes were not countersunk deep enough, either. In any event, after some use and vibration, the bolts loosened up and moved ever so slightly (but enough!) towards the firewall, causing occasional arcing and misfiring. This happened to me personally in Germany in 1974 in my 1928 roadster. I swapped out EVERYTHING while sitting along side the Autobahn as you have done, but couldn't fix the misfiring. 'Drove me nuts!!! Cutting to the chase, it turned out to be one of those poorly-made reproduction off-shore terminal boxes with the too large head bolts and shallow countersunk holes. I have since diagnosed a couple other instances locally that perplexed the owners because they couldn't find the source of their Model A's misfire. They also had older terminal boxes that had been bought "on the cheap" from the same suppliers. There are probably still hundreds of them out there waiting to ruin some Model A owner's day.
I'm guessing if you have a car that was rebuilt or "restored" in the 1970's or 1980's, one of these timebomb terminal boxes may be at fault. If the two studs are not loose, back off the four mounting screws inside on the firewall, pull the terminal box out a little ways from the firewall and keep it separated by inserting a couple pieces of cardboard. Then start the engine and drive the car. If there is no miss, you can bet that one or both stud slot-head bolts is/are arcing against the firewall. It may be a long shot, but you've tried just about everything else.
Did I miss that you performed a compression check to ensure that a valve isn't stuck or burned out?
Marshall

Last edited by Marshall V. Daut; 10-14-2018 at 08:34 AM.
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