Ken Looney asks about his:
> 68 MGB-GT.
> Specifically,
> when I begin to accelerate, about a block away from
> where I started the rpm's will fall to nothing
> (needle bouces off 0) so hard that you fall forward
> in the seat, then it backfires and the entire process
> begins again. Now, let's say I don't accelerate
> at quite the same rate, let's say I just ease up
> to speed and maintain that speed with just a little
> throttle, things seem fine. Also no matter what,
> the way it idles and runs (when it's not backfiring)
> is what I would think is perfect for this car.
Fuel pump. That'd be the first place to look, anyway;
your fuel pump can't keep up under load, and when you
suck the float bowls dry the car sputters. As long as
you keep out of the gas, the bowls won't be emptied
faster than the pump can fill them, so if you ease up
to speed or idle it, it's perfect.
Other possible problems: Did you leave the springs out
of the dashpots after cleaning/rebuilding the carbs?
Are they so completely devoid of oil that they're
bouncing straight up when you drop the hammer, and
the resulting drop in engine vacuum kills your flow
rate (not to mention the signal to the gas in the
venturis)?
There's a very small likelihood that your vacuum
advance unit in the distributor is sticking, since
the problem you describe seems tied to load on
the engine, but I'd check the carbs first (you
can look in them, and you probably already have oil
in the garage, right?). The fuel pump is the
most likely culprit; I was having a similar problem
on my '71, where an older fuel pump couldn't deliver
at high enough pressure for high-speed running.
The car acted as though it was out of gas; a few
moments of staying off the gas, letting the bowls
fill up by coasting, would cure this.
You can probably find fuel pump flow rates in one
of your manuals; it's usually tested by letting
the pump run unencumbered for a specified amount
of time, then measuring the amount that it pumps.
Obviously you'll need to know the flow rate so
that you have a big enough bucket...
> What have I missed? I did have the points set
> so tight the car could not run past idle so
> please leave no stone unturned.
>
> In the what I've done lately category, the
> answer is nothing, before this happened and
> these after:
>
> -new coil
> -new dist. cap
> -new button
> -new plugs (split fires)
> -new wires
> -checked compression
> -checked valves
>
> If you have a clue I would really appreciate it.
>
> ken (a tinker-er who has been really lucky) looney
> ________________________________________________________
>
> Ken Looney kjlooney@ssdkjl.b17c.ingr.com
> Systems Engineer 205-730-7702 Ext
>
>
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