In a message dated 97-04-17 12:13:19 EDT, you write:
<< John McEwen writes:
> On a motorcycle, when any changes are made to increase the flow of exhaust
> gases, the carbs must be richened to accommodate the increased air flow
> through the engine. You may have created an excessively lean engine with
> the new system. While you can adjust it at idle, the carbs cannot
> accommodate higher revs.
I must disagree. Within a certain usable range, the CD nature of SU
carbs automatically compensates for changes in flow.
Right?
>>
I'll have to disagree with you and side with John,
ANY thing you do to increase the gas flow into, through or out of the
engine will effect the mixture setting, even on a Constant Depression carb
(Ie. SUs).
What you said about a "certain usable range" is true as far as it goes,
but, that usable range is pretty much limited to the given state of the
engine. Changes in intake, exhaust, cam, ports, cam, etc. can and will effect
the mixture to the point of poor driviability.
Case in point. I replace the exhaust from the "Y" pipe rearward on my 72
GT with relatively straight 1.75" pipe and a straight through muffler
(debatable nomenclature) and a 2" tail pipe. It threw the mixture off such
that 75 MPH or so was all she would do even with a stiff tail wind. A plug
reading showed all four cylinders lean at the top end. A minor adjustment in
needles was all it took to rectify the problem.
Point is, the SU can and does a great job compensating for variation is
driving conditions, atmospheric and weather conditions and altitude (up to a
point), once components are changed, they are outside their baliwick. Until
suitable corrections are made.
just my tuppence worth
Rick
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