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Backfire question

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SilentUnicorn

SilentUnicorn

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My 122 Volvo will backfire about 10 seconds after i shut it off. I know how it happens, fuel in the hot exhaust. The car is SU equiped. It has no vac advance. I am just trying to figure a couse of action to make it stop. My thinking is at this point either the mechanica advance is sticking a bit, or the oil in the carb dampers is to light. otherwise the car seems to be running ok. i have put about 220+ miles on it in the last week or so, and i might even upgrade it daily driver status /ubbthreads/images/graemlins/smile.gif
so any thought or ideas will be considered and welcome

thanx
mark
 
Is the engine still "running on" 10 seconds after you turn it off or has it stopped when the backfire occurs?

I doubt that timing is the issue in either case - the plugs would have stopped firing. The ignition is happening because something else is hot enough to light the fuel/air mix.

You could try a higher octane gas but that doesn't solve the problem.

It's usually carbon deposits that remain hot, usually in the cylinders (engine still running) or in the exhaust system (engine has stopped). This also can happen due to sharp edges in the combustion chambers which sometimes happen when DPO has replaced long-reach plugs with shorter ones.
 
I've seen this in cars with leaky exhaust systems.
I'm not positive on how it works, but I imagine air gets into the hot exhaust (through the leak) and fires off any nearby, unspent fuel.
 
It could be timing if it was slow enough to cause the engine to run hot. It could also be very advanced timing. In any case, that must be known and set as a first step. Be certain the float level is correct. Be sure the plugs are correct in terms of heat range. Ensure the engine temperature is normal.
Running on is a result always of high temperature/pressure in the combustion chamber. The mixture self ignites which is why it's called dieseling. So long as the four little vacuum pumps are moving up and down, fuel will continue to be drawn into the cylinders, and the process continues until the temperature/pressure decreases below the threshold.
 
Mark -

Shot in the dark here: In addition to the above, if your SU's are equipped with those little valves in the butterflies, make sure they're sealing completely. If not, fuel mixture could get sucked in when you shut it off.

Mickey
 
[ QUOTE ]
I've seen this in cars with leaky exhaust systems.
I'm not positive on how it works, but I imagine air gets into the hot exhaust (through the leak) and fires off any nearby, unspent fuel.

[/ QUOTE ]

Absolutely. If the mixture is a little rich and there's an exhaust leak, you have all need for a nice backfire in the exhaust manifold or even further back.
 
But TEN seconds is a LOOONG time from key off to detonation, IMO. It'd seem it's drawing fuel after shutoff, vaporising it and SOMETHING well down the exhaust is setting it off. Float levels too high is about the "best guess" I can make for what Mark describes.

The car is too old to have the poppett valves in the throttle plates ~unless~ it's been refitted with newer carbs.
 
Thanx for the brain food fellas, the engine is completely stopped when the backfire occurs. Exhaust is new from the manifold back. i will check on the plug length. I will look and see if it has poppet valves(dont think it has) i checked the float levels and they looked good, will double check. Tonight i was checking things out a little bit and discoverd that the manifold connection is a little weak, this will be the first thing i fix, asap.

keep the ideas flowing.


thanx
mark
 
Got IT VROOOMMM VROOM
started at the basics. Dwell,timing (it was set waay late) then the carbs , which i had jacked up to far to compensate for the late timing. it seem sto be alot better now. will keep ya posted tho...

I did almost have a *moment* while setting the dwell. I had removed the rotor and set about tweeking the gap, popped the cap back on and hit the key. No start. Oops where the cap? spent 20 minutes looking for that blasted thing. Even tried a lucas rotor. that didnt work....... finnally found that littl rascal on the other side of the engine block, hanging on the oil pressure line...WHew /ubbthreads/images/graemlins/eek.gif



mark
 
[ QUOTE ]
I did almost have a *moment* while setting the dwell. I had removed the rotor and set about tweeking the gap, popped the cap back on and hit the key. No start. :
mark

[/ QUOTE ]

Gee, Mark, like none of us have ever done that before! /ubbthreads/images/graemlins/grin.gif /ubbthreads/images/graemlins/blush.gif
That's almost as bad as forgetting to remove the rotor, and spinning the engine with the screwdriver on the points. Makes a nice "snap" when the rotor hits the screwdriver and shreds itself. I've only heard rumors of this happening, of course. /ubbthreads/images/graemlins/cryin.gif
Jeff
 
Mine has always been the disappearing rotor trick.... usually I find it's crawled itself up onto the workbench somehow... after a period of serious head scratching and numerous words Basil won't allow here.


...rotors got LEGS!

Glad ya got it sorted, Mark. It's ALWAYS th' "dumb stuff". /ubbthreads/images/graemlins/jester.gif
 
Hi Mark be sure and check the butterfly valves in the carbs. If they have the little spring loaded "Anti Run On Valves" in them solder them up and you will be good to go. Additionally, the B18/20 engines like the Bosch Super Type W7BC Spark Plugs.---Fwiw---Keoke
 
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