i have a 1980 dump truck that makes a popping noise like its back firing and then in 3rd gear it loses power and throttle?
you have a valve problem I would sugest checking your valve lash. this is it back fires or pope on giving it gas. you might even having a timing issue but on this old of a motor it is a good thing to pull the valve covers and give the lash a good once over
Change spark plugs sounds like they are miss firing and check air filter.
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