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I seem to be having a similar backfire problem. I have a 1966 122 with the B18, Twin SU's that will backfire after I turn the ignition off if the timing is too advanced or will diesel for a second or two and not backfire if I retard the timing just a bit but this causes overheating and white smoke to leak out of the oil filler cap (I assume from the overheating).
Some Background on the car: I've been using the car as a daily driver since February of this year (it was previously collecting dust in my garage for a year or two after I bought it off ebay) and while I live in Phoenix AZ, the car runs hot (rightmost edge of the third green mark) in both winter and summer (Ok, its in the rightmost white area on the really hot days of summer, 115+ Degrees) if I leave the heater closed. When I open the heater the temperature improves a little bit but not much, and same with turning the heater fan on to full blast with vents open. When I turn the car off, in addition to the symptoms listed above, the temp gauge needle travels off the gauge and a puddle of coolant and water is left under the car. With this being my first vintage car I have been learning maintenance and repair in a sort of "Baptism by Fire" method and being the 4th owner of this car with an unknown history, there has been a lot of baptizing (not that I'm complaining, I've enjoyed learning).
Through my trial and error repairs to combat having the heater on in the summers here (I can live without the air conditioning but having the heater on is just insult to injury), I have replaced the thermostat with one from IPD, I have flushed the cooling system, replaced the water pump (GMB brand), installed an electric fan, soldered the hole in the radiator caused by installing the fan, replaced plugs, wires, coil, points, condenser, rotor, I've changed the oil a few times, I've installed the K&N filters (which might need to be cleaned since we were hit by a haboob), and I have been tinkering with the timing since I did the ignition tune up in March. I've bounced the timing between too far advanced (around 19 or 20 BTDC using a timing light) as well as probably too retarded (around 10 or 12 degrees BTDC) and also had it in the middle (none of which seemed satisfactory for cooling and are the cause of the issue I lead into this novel with) and I've also been adjusting the idle screws to run the mixture a little richer (which in turn caused me to tamper with the timing as well). I would like to point out that as far as I can tell, there have been no performance mods to the engine itself (elec. Fan and K&N Filters don't really count), which is probably the original engine with unknown mileage and a B20 thermostat housing (I don't think the housing makes a difference but I thought I would point it out as I realized this while trying to jam a new radiator hose onto it).
At this point, I'm sure I tweaked something a little too much and that is the source of some of my issues (the overheating has been around before I started tinkering so I can't blame myself for that one, at least I don't think I can).
Anyways, I'm looking for some advice from those here at brickboard, who have read this far down my post, to try and at least nip the timing issue in the butt, and hopefully the overheating (although it being summertime, with a low of 86F I don't have high hopes). This weekend, I plan on compression testing and pulling the cover off to take a look at the valves, but is this even the right direction and what should I look for when inspecting the valves? Also, if I adjust the valves, I assume, will affect the timing which as you can tell, I know how to tamper with it, I'm just not sure I know how to set it correctly.
Thanks in advance for the help.
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Andy - '66 122s, '89 Bronco, '03 Trailblazer
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