| [GMCnet] Vacuum / Knocking / Timing? [message #104744] |
Mon, 01 November 2010 03:58  |
980
Messages: 192
Registered: July 2010
Location: United States
Karma:
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Senior Member |
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The coach has been running pretty well but I had an interesting problem
tonight. I just replaced the intake manifold gasket with the blocked
exhaust crossover version. While in there I straightened out a bunch of
vacuum lines. (It used to whistle when I'd coast at speed!) and the
distributor was loose so we tightened that down and shot the timing.
Most of my trips have been somewhat short since then and it ran great
even over a big hill between here and Pacifica. But, when I made it
over 20 miles or so and started climbing a hill, the timing went way out
and the engine started knocking whenever I tried to get power out of
it. Especially on hills.
We pulled it over and checked the vacuum lines. We moved some related
diaphragms and checked out the rotor cap, which looked fine.
We though we'd freed a jammed advance linkage and it drove great for a
while. Until about 15 miles later, then it started knocking again but
not as much as before. We made our destination, but about 20 miles on
the return trip it came back. I now think that we didn't unjam anything
but perhaps the engine had just cooled down enough. So far my two
theories are that the "Christmas tree" looking vacuum valve on the front
of the intake manifold isn't right when it gets hot or that the timing
needs to be shot differently to cope with the vacuum advance on a GMC.
Going after it again in the morning. Your insights appreciated.
...The pumpkin made it fine and was a big hit!
Desmond
--
1977 GMC ex-Palm Beach, 26-3
Treasure Island, CA
KC6VHG, KAG0675 "980"
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