CU backfiring

[Johnmck]

Hi
had van moted recently and they had to mess about with carbs to get it through emmissions test. Been running fine but not out much, took for run on moterway fine but after a half hour stop struggled to have power for about 2 miles the ran fine. Now few days later is running ok but has started backfiring.
Check plugs looked ok light grey, any ideas what might be cause?
Thanks

[HarryMann]

1) They've weakened it off too much on the idle/crossover screw (to get emissions).. if so silly,people as older vans do not need to meet modern criteria, maybe the on-line DVLA shtupid criteris these days..

2) Air leak in inlet tract....

3) Exhaust leak

4) Ignition retarded too far

That';s usually what's going, on, burns too slow or ignited too late, so still burning when exhaust valve opens, if its not that, and actually firing through carb or inlet, that's another matter

Also can be plugs (too hot, that is they're too 'soft') or exhaust valves burning

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[BOXY]

One other thing to check is the rubber tee & rubber elbow at the ends of the metal pipe that runs across the back of the bulkhead. They connect the servo & the inlet manifolds together. If they're split they'll draw air in and the engine will pop under load.