Popping from exhaust on overrun

Just had a trouble free run of around 200 miles in the mk7 , but she pops a bit on overrun , going down hills with the throttle closed . What causes this ?

Probably running a little rich

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And still returning almost 20 mpg , Yahoo! :grinning:.

OTOH, I’m under the impression that indicates ‘too lean’.

This is why you never want to get opinions from two economists.

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Frankly , what causes it is the very crude manner that carburetors monitor mixture, over any given rpm range. On very few cars with carburetors, did they not do a little popping out the exhaust on overrun.

Popping can come from too lean, not sure about popping from too rich, lift the carb pistons 1mm at idle and if it doesn’t die or accelerate much it’s right (if it dies the mixture is lean).

The popping is unburned mixture igniting in the exhaust manifolds, correct?

I have this pretty significantly on one of my carbureted non-jag engines, and my reading indicates that the mixture is not getting completely burned in the cylinder because it is too lean, and subsequently gets ignited in the exhaust manifold.

It seems odd to me that a mixture that was too lean to ignite in the cylinder, with a spark, could then ignite in the manifold, without a spark, but that’s what it said.

Prior to this, I always assumed the popping was due to too rich, that there was not a complete burn happening in the cylinder, and the excess was popping off in the exhaust manifold. This makes sense in my little brain, it happens on closed throttle/high vacuum situations and it seemed logical that excess fuel could be getting pulled in.

I’d love to hear the definitive explanation so that I can sleep at night. :grimacing:

In the fuel injected tuner world, intentional popping on the overrun is done by introducing excess fuel. To me that would indicate that too rich in certainly one path to popping on the overrun. There may be other conditions by which a lean mixture could also induce popping.

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I had never had very much success with the lifting pin method - a matter of controlling the amount of lift and steadiness, I expect. For tuning purposes now I use an 1/8 inch screwdriver in the carb throat, put it just under the piston and turn to lift and test. A tip I picked up from the University Motors guy on YouTube. Naturally, it requires more dismantling but better results for me. FWIW.

This is at least consistent with the bypass valves used on the Stromberg carbs. They allow extra air to bypass the throttle plates in high manifold vacuum situations (overrun) to allow the excess fuel to burn in the cylinder to reduce emissions and stop exhaust popping. If you shut down or disable these valves as many do (because they will leak if the diaphragm is cracked), you get a popping exhaust on overrun.

I had this when my 140 had a plug that tended to foul from apparently a loose guide. The overrun pulled oil down the guide and fouled the plug. Then the mixture that didn’t fire well was there to pop on deceleration. Do you have one suspect plug? If you put in a clean one will the problem stop for awhile?

All plugs seem to be firing ok , good acceleration , but I will check the plugs after another run . This will also give an indication of mixture strength .

Just 2 days ago i settled the popping on overrun at my XK 120 from 1953 .
I measured the CO level at idle . Incredibly 2,5 ppm !! I enriched the mixture by half a turn on both carbs .
Yesterday i hade a 165 mls trip over two mountain passes , which lots of downhill passages .
No popping anymore . Before the action with the carbs i checked whether the exhaust system is sealed
properly . I have not yet measured the actuell CO level .
Greetings Martin