At 12:44 PM 5/7/03 -0700, Paul M. wrote:
>.... I have an issue with the way the engine responds off throttle that I
>find perplexing.
>
>This car is a 1971 tourer, and the engine compartment is remarkably
>unmolsted. All of the factory emissions equipment is in place and working
>well. ....
>
>.... when I lift off the pedal, the revs kinda "hang" up there and come
>back down to idle somewhat slowly. ....
This is normal for that car. But you can fix it !!!
By 1971 the carburetors had an overrun valve. This is a small spring
loaded poppet valve which is part of the throttle plate. When you lift off
the throttle at hiogh engine speed the intake manifold vacuum goes quite
high, and this overrun valve opens to allow some air flow through the
throttle plate. The result is similar to holding the throttle slightly
open. The intention is reduce the condition of high vacuum on overrun
which causes overly rich fuel mixture, resulting in excess hydrocarbon
emissions. The same condition also contributes to backfiring in the
exhaust system.
Sometimes with increasing age the spring weakens, and the overrun valve
stays open more than necessary. Some people would also like to defeat the
overrun valve to return to the condition of more engine braking which they
have been accustomed to with earlier cars. The solution is to solder the
little poppet valve to the throttle plate so it can't open, or to replace
the throttle plate with the earlier part without the valve. After that you
may anitcipate the return of the crackling backfire sound in the exhaust on
overrun at high engine speed.
Barney Gaylord
1958 MGA with an attitude (and nice crackle in the exhaust)
http://MGAguru.com
/// or try http://www.team.net/cgi-bin/majorcool
/// Archives at http://www.team.net/archive
|