Lean Carb Back-fire

[malcolm2]

That is what I am calling it....

Bus had been running great. 5 years of work, and now after running only about 3 full tanks of gas i get some problems.

Factory set up for new-ish Weber 40 IDFs in my 2 liter type 4. Specs by our former friends at AirCooled.net. I set the Best Lean Idle, and the Snail. No Load, just parked and idling it still is running great. I can rev up passed 4000 rpm and the engine is smooth. But when i start driving, I get some pops. Mainly as I shift gears. As i let off the gas and mash the clutch.... 2 to 3 and 3 to 4. Pop, pop, pop, then i get the revs back and cruise and all it well. What would cause this popping only under load?

I added a video of the PARKED and IDLING. Please, let me hear some suggestions. TheSamba folks suggest, re-timing and checking the valves.

 

[K@rlos]

I’d look for an air leak or a blocked idle jet first

 

[sparkywig]

Sounds like inlet manifold leaks.
Spray a bit of brake clean/easy start/etc around the base of the carbs and inlet manifolds. If the revs change you found your leak.

 

[ozziedog]

^^^ ^^^ ^^^

Wot they said. But I’d still do your valves first anyway unless you got hydraulic ones. Not sure if I could hear a bit of hissing or if I’d imagined it then believed it.

Ozziedog,,,,,,,,,,,bit toooo clean in there for my liking

 

[W1K1]

Clean out your idle jet, parked idling is a different circuit than actually driving on the idle jet. It could be lean on the idle jet too, just because it is set for lean best idle doesn’t mean it’s the correct idle jet to drive on. https://web.archive.org/web/20240412174151/https://www.aircooled.net/vw-carburetors-jetting-102/