JohnZissler@aol.com asked:
"can someone explain why you need to mess about with
the carb needles when you change an exhaust. "
John
I have an SAH Triumptune (part of Moss now days)header
and dual exhaust set up on my car. When it was first
put on the car it ran horribly until the needles and
springs were changed in the carbs (HS2Es). My car is a
UK model ('73 1296cc) so your may need to make similar
changes. For the specifics I higly recommend calling
the Richmond location of Moss and talking to Terry
Hurrell (son of founder Sid Hurrell). He has always
been most helpful to me in the past, he authoured the
Triumohtune performance book, the bible of Triumph
performance tuning IMHO.
The reason for retuning is that the new exhaust has
different reversion characteristics and also flows
more. More flow out = more flow in = leaner mixture.
Reversion is often confused with back pressure. When
the exhaust gasses meet the baffles in the muffler a
reverse pressure wave is sent back through the
exhaust. For any given setup there must be some rpm
range where this will cause some negative effects in
the combustion chamber. The rpm that this occurs at
varies according to the exhaust design and the
camshaft specs - the more overlap the harder it will
be to tune the carbs. Back pressure is never desirable
and always reduces the amount of power from the motor.
HTH
Neil
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