Valves not right or bad gas


Old 01-11-06, 01:45 PM
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Join Date: Oct 2005
Posts: 31
Question Valves not right or bad gas

Have a 77 GMC truck with 250 inline 6(standard head). The ignition timing has been set according to specs, as well as RPM's and valves were adjusted by an experienced mechanic. When I accelerate to 50 mph, the valves begin to rattle. The previous owner bored engine over .60 and apparently installed an aftermarket cam. My question is do valves come out of adjustment after they have been set due to various factors? How do I correct this problem? Thank you for your time and advice/suggestions.
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Old 01-11-06, 06:42 PM
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Join Date: Mar 2005
Location: USA
Posts: 44,351
I would try adding some carb cleaner [prefferably the good kind $4+] it will probably quiet them down. Better grade of gas may also help. Sometimes the factory spec for timing isn't the ideal setting for a worn or reworked engine. If gas treatment or high test doesn't fix it you can adjust the timing slightly which should fix the problem.
Old 01-12-06, 06:55 AM
Join Date: Oct 2004
Location: Ontario
Posts: 539
if a mild cam has been instaled than valve adjustment would differ from spec also you need to find out how much vacuum you hav at idel and also what rpm the engine is at vac should be 19-21 "Hg if not valves may need readjusting just losen thenm till thay beguine to clack than tighten 1 1/4 turns should be suficent than re set timing you may need to find an old school tech that knows vacuum timing procedure

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