Paul,
That is annoying, isn't it? When I picked up my '69, brand new, that's how
the engine behaved. Slow to return to idle rpm. Caused by the emissions
control equipment, primary the gulp valve and the throttle butterfly poppet
valve - both dump fresh air into the intake manifold on overrun. The reason
they do that is to prevent backfiring in the exhaust system. And, of
course, the reason that the backfire occurs is the fuel-rich mixture being
exhausted upon overrun meeting the fresh charge of oxygen from the air pump.
Bang. Pop. Pow. Also annoying. The fresh air (diverted from the air pump
by the gulp valve and from the poppet valves dilutes that fuel-air mix
enough to prevent the backfiring.
That characteristic was the primary reason I removed all the emission
control equipment from my car - to included replacing the throttle discs
with solid ones. I know of no other solution, and defer to wiser folks for
that answer.
Bill
> 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. It fires
> right up, idles great, and runs smooth and docile.
> But it does something that I find strange:
>
> If I'm at idle, and I blip the throttle, the engine
> responds quickly and revs freely, but when I lift off
> the pedal, the revs kinda "hang" up there and come
> back down to idle somewhat slowly. Not REAL slowly,
> but just a little slowly... Is this a function of the
> air pump or something? My old B was a 1963, and it
> didn't do this at all. One thought is that it could
> be the simple fact that it's a 5-main engine and I'm
> used to the more responsive 3-main. Another thought
> is that it's the air pump and related emissions
> controls stuff, and "desmogging" it might help
> (although I am loathe to do that, since it's running
> so nice and since I would feel bad about yanking
> everything off a good original engine), and a third
> thought I had is that the dashpot oil might be too
> thick or something (in which case a recommendation on
> what to use would be wonderful...)
>
> Any thoughts? Do all later B's do this?
>
> Thanks again,
> Paul
> 1971 MGB Tourer
> (Still searching for a nickname, but leaning towards
> "The Pumpkin", since it's blaze...)
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