Jeff Johnson wrote:
> Gernot;
> If all else seems correct, check for a minor exhaust leak near the manifold.
> That can cause a backfire even if everything else is dead-on.
>
> Jeff Johnson
>
> Gernot Vonhoegen wrote:
>
> > Quick question, the TR has a slight backfire on overrun. I checked the
> > points gap and static timing should be ok. There is a couple of
> > disconnected vacuum lines (except the one for the advance) but they are
> > properly blocked off. 74 emmision control california car twin stromberg,
> > no emission stuff left.
Jeff is quite right on the exhaust leak. There is only one condition which
causes a backfire in the exhaust system--unburned fuel in the presence of air.
The carburetors always leak a bit of fuel with the throttle plates closed
suddenly, and a small leak in the exhaust introduces air which can cause the
fuel to burn in the pipe.
However, I think your problem is related to those randomly blocked-off vacuum
tubes. Like most US emissions standards cars, the Triumph was equipped with an
assortment of devices meant to make the car meet emissions and still be
driveable. If your car is one with air injection, and the diverter valve has
been bypassed, then the cause of your trouble is clear. That valve is meant to
stop injection of fresh air into the exhaust manifold on deceleration by pumping
output of the air pump to atmosphere during decel. Otherwise, it continues to
push fresh air into the admittedly small stream of unburned fuel pulled into the
cylinders by manifold vacuum. And, bang, a backfire occurs.
Try to find an emissions plumbing map of your year car and put the vacuum
plumbing back right, and see what happens. At least that will tell you what may
be going wrong, in terms of the stock configuration.
Cheers.
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