Dick,
Actually the effect that mike describes, (a blowback through the
carbs), is
an indication that an exhaust valve "isn't" opening! If we assume that a
completed cycle is when the piston returns to TDC, (exhaust out, and another
gulp of fuel and air just coming in), then the result of a closed exhaust
valve can be imagined. If the exhaust valve doesn't open then the cylinder
will be under compression when the intake valve opens. This doesn't just
affect the cylinder in question. All other cylinders are disrupted by the
fact that the intake manifold is a cross over affair, so proper fuel
distribution cannot occur for all cylinders. Years ago I machined my own set
of roller rockers at a ratio of 1.7:1. This increased the valve opening by
.101. I machined them from aluminum, and made the prototype set a little on
the light side. Within 1000 miles one of the exhaust rockers broke. The
engine was so disrupted by the back pressure on the intake that it would
barely run. I redesigned the rockers and never had another problem.
Mike,
As far as what could have caused your problem...Only an examination by a
knowledgeable mechanic will tell, but Dick is absolutely right about the
fact that in order for a compression tester to properly read, the valves
must be operating.
Jim
74 TR-6
Tampa Bay Area
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