If I go out for a run and get the car up to temp then stop and turn the ignition off I get a rtaher embarrasing bang out the exhaust anything up to a few seconds after switching off.

If I purposly stall the car I can avoid the backfire.

I have a B18 with twin SU's.

Thoughts are -
Worn exhaust valve
Air leak in spindles of carb,or from brake servo
Carb fault
Ignition fault.

any ideas what to check first?

Thanks

What you have here, is unburnt fuel entering a hot exhaust. The exhaust itself can be hot enough to ignite the mixture, so there's no need to wonder where the spark is coming from.

The old SUs did not have an idle cut-off solenoid. Shutting off the ignition does not shut off fuel flow. What stops the fuel from flowing, is the stopping of the engine. When you cut the ignition, the fuel keeps mixing with the air in the carbs as normal for however long it takes that engine to spin to a stop.

So, I've got to go with the idle speed theory. An engine that's idling too fast will take longer to stop after you cut the ignition, and so it will pump more air-fuel mixture into the exhaust.

Other tuning factors can aggravate this by providing, say, a hotter-than-normal exhaust or an excessive amount of unburnt fuel to begin with. So, as always, be sure to check your basic tuning stuff.

Most of the time we just sort of shrug and accept it when the engine won't idle all the way down to spec... alas, in this case I think you might be forced to actually do something about it. Finding root causes, fixing air leaks and then going back and fixing the newly-upset mixture, all that stuff.

Good luck with it!

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

Now that you stole the thread, have you ever taken the radiator to a shop for cleaning? You could have a restriction in it that causes the high temps.

My cable slipped and the cabin heat turned on while in the humid 90Fs. It took 20 minutes to get the cabin back to tolerable.

Klaus
--
Always willing to listen, just not able to take direction.

Sorry for the thread Hi-jacking, my backfire issue seemed similar when I started the post-reply and I got carried away.

As for the radiator, no I have not taken it to a shop for cleaning yet, I'll have to add that to my list once I get the backfiring/dieseling/timing issue resolved. Thanks.

Driving home last night, though, the engine started to output a low level of varying power while trying to maintain a constant road speed (varying to the point of jerking the car). My initial thoughts would be that the timing issues have fouled a spark plug or two, but alas it'll have to wait until I have some time on Saturday to really examine the plugs as I attempt to perform a compression test and valve check. Any other thoughts on what to take a look at would be greatly appreciated.
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Andy - '66 122s, '89 Bronco, '03 Trailblazer

I have a Weber carb. Mine used to buck at constant speed with a light throttle, cause by very dirty carbs. Right where the gas goes down to the center ventury. A lot of carb cleaner solved that issue.

Klaus
--
Always willing to listen, just not able to take direction.

Last time I saw this problem, it was caused by a highly restrictive exhaust, possibly a collapsed muffler.

Have you done the usual things like check the tappets & timing?

--
Three 164's, Two 144's, One 142 & a partridge in a pear tree.

Is the idle set higher than spec?

Not that I'm aware of - but I'll check thanks

Have you retro fitted an electric fan?

It has a 'period' kenlow fitted - yes. Why? Thanks

Some of the fans are known to cause this. If they're running when the engine switches off it tempotarily acts as a generator, back feeds into the system and causes an ignition pop.

It's also a party trick on the ignitor type ignitions.

Do check the basic timing settings though.

I can't imagine why a fan would make a difference, but I'll stand by for an answer. I'll go with a high idle speed, or your carbs are letting a little fuel past them that goes into the combustion chamber and explodes on a hot spot. As B20 Paul has indicated, are your valves and timing set correctly?