Re: p pop POP p p

[lhalliday@creo.com]

Carb engines with relatively large valve overlap were always notorious 
for this - assuming your problem is banging out the exhaust, and not 
genuine backfires out the intake manifold.

The problem is the combination of high engine RPM and closed throttle, 
which causes intake manifold vacuum to go sky high, pulling more fuel 
than usual through the carbs. The mixture goes super rich (hence black 
smoke), unburnt fuel goes out the exhaust, and if it ignites, BANG!

The throttle dashpot thingie (I don't recall what VW officially called 
it) was to try to avoid this situation, slowing the engine gradually. 
FI engines had similar gadgets, for similar reasons - D-Jetronic 
measured manifold pressure and could be fooled.

The moral of the story - check your carbs. Check the dashpot. Check 
your FI pressure sensors: for some reason you're going rich on the 
overrun. Have fun.

Laura Halliday VE7LDH         "C'est une femme mutine, assez
lhalliday@creo.com             elegante, grave et legere, ayant le
ve7ldh@amsat.org               sens du confort et du plaisir
Locator: CN89mg                en tout." - C. Deneuve