Steve,
The cause can be one of many, as follows, starting with the easiset...
air leak into the exhaust pipe, usually a rust pin hole, often in the
silencer; poor fit of the clamp at the exhaust pipe to manifold joint,
letting in air,( you often hear it blowing a little hear, use some
sealant;) manifold gasket broken, often on the underside so you cannot
see it, at one of the exhaust manifold ports; this can also be loose
manifold bolts;sticking exhaust valve, burnt out exhaust valve beginning
to show up, ( do a compression check to find it;) weights in the advance
& retard in the distributor sticking, or their springs worn out; loose
connection in the points allowing a rogue spark.
One of those should be the culprit, no doubt others will reply as well.
Neil.
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