overrun popping

[alan230752]

I have a small annoying problem,(no not the wife)
on the overrun,my car runs weaker than it should and pops
I have a wide band aem air/fuel ratio fitted
idle is 12.8 /1 which eqates to 4.5 %co
ight cruise is 15/1
wot is 11.5 to 12/1
overrun is 18/1 which is too weak
so is all ok and runs sweet
my car has strombergs,pertronix system
Background>
petronix electronic ignition
timing set dynamic at 12 degrees before
plug gaps at 35 thou, Spark Plugs NGK BP5E
fairly new distributor,vacuum unit and advance all work ok
carbs all overhauled.jets at 2.5mm below bridge ,biaq needles at 1 turn back from rich, pistons all moving down nice and freely, 20w 50 in dash pots
cam shaft clearences all correct
no air leaks at manifold or anywhere tested that all out with propane gas,besides it idles very smoothly
exhaust joints also leak free

any ideas

alan

[Giles]

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just thinking aloud really -

I have alwaysthought that the popping is unburnt fuel igniting in the exhaust manifold and is usually a manifold leak but as you have already checked would the float bowl level being a bit high cause this? but thenyou are getting a weak mixture reading so probably not!?

Another thought - Do Stags have poppet valves in the throttle butterflys? (you would think I could remember but it has been a hard day working on Karen's Mini in the sun) if they do - if the springs are weak in the poppet valves this would make the mixture go weak on overrun, try soldering them up??

just a few thoughts to start off with

[V Mad]

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Funnily enough Giles, thepurpose of the poppet valves is to prevent overrun popping. But when the springs go weak it causes problems with idling; I had to fix this problem (ie solder them closed)on a pair of SUs on a stag with RV8.

I am not sure if any of the stag strombergs had them fitted though.

[alan230752]

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i have ordinary throttle discs,no poppet valves
defo is weak as gauge reads 18/1
alan

[Mark S]

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Hi Alan,

Is the ignition timing somewhere near where it should beat higher revs? I suffered from popping on overrun on the Stag a couple of years ago, turned out to be weak/retarded spark due to a faulty condenser.

Mark.

[alan230752]

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set to factory timing at higher revs
advance curve is all ok

no condensor fitted as pertronix ignition
alan

[Sheepdip]

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I'd check carefully for air leaks. I had same problem, suspected a leaking servo (not), but found it was the breather pipe connection to the cam cover, looked ok but was a loose fit so disconnected, plugged, tried andmuch better,put a jubillee clip on it now.

Incidentally while I had the pipe disconencted and engine idling, for some reason I put my finger over rocker breather and was amazed to see oil squirt out of two of thecam cover screws, suspect they were bottoming on the threads with old gasket, seals etc. What comes out must let air in......

Hope this helps

Paul

[alan230752]

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yes that was my thoughts, air leak
i will blank off all connections and see if it is better
i assume when engine is on overun that manifold vacuum is very high,so any small air leak will be magnified
I do have a clip on the cam cover breather,tested with propane gas on idle and no increase in revs,and as I said it idles very well
will check out servo
alan