<VV> Tune ups (turbo hesitation)

[craig nicol]

From: "Dave Thompson" <dave.thompson at verizon.net>
"Dave asked:
<snip> I just have one issue that bugs me. Every once in a while, when I
stomp on it, the engine coughs once and continues just fine. It almost
sounds like I'm momentarily turning off the ignition. Kind of like: (How do
I describe it in text?) "Wa a aaaaa".  When it happens, it seems to happen
whenever there is a quick change in RPM. It never happens during sustained
RPM." <unsnip> Dave

Craig replies:
Is your choke working properly?  Do you have a fresh gasket on the carb top
cover?
Acceleration enrichment in the YH happens through the usual accelerator pump
squirt, but that is the secondary system.  The primary system is the choke
plate, which for lack of a better term, "Blinks" for a moment when the
engine vacuum drops at the sudden opening of the throttle. After that, the
pump shot occurs, which continues the enrichment. Both of these systems
require a good vacuum signal, which AFIK, is routed through the top cover of
the carb.  If the signal is bad (leaky cover) or the choke is adjusted too
far "lean" the blink won't happen and you might get a cough.