I’m looking for help with tuning a custom …747 TBI system. This arrangement runs very nicely except during deceleration when I “blip” the throttle to double clutch downshift it has a “flat spot” and the revs don’t want to come up.
Some background first; this is originally a 1960 Rochester fuel injection plenum converted to TBI by custom machining two TBI injector housings and inserting them before the throttle in the Rochester inlet. The object was to disguise it to fool the judges at car shows and so far that has worked. The original 3” throttle plate was way too large in that “tip in” made it too sensitive to drive without laying rubber every time trying to take off smoothly so I reduced the throttle bore to 2.120” which is much more controllable. In normal cruise or hard acceleration the throttle response is instant when jumping on it.
The only difference, that I can see, is this flat spot seems to happen only following very high manifold vacuum where using the engine for braking and dropping down gears. I thought this vacuum was cutting the mixture too much so I doubled the pulse time at 0 and 20 Kpa in the accel enrichment table but to no effect. Note; in early experimentation, using a stock twin bore throttle body, it did the same thing. Looking for ideas. Thanks.
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Originally Posted by dave w
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