Are you sure the firing order is right?? You know that 289, 302 and 351W parts interchange. You can install the cams in one another. This will change the firing order.
Next where is the intial timing?? If you're not sure. Pull #1 plug, turn the engine over till #1 is on compression stroke. Line up the timing marks and install dist. to fire on #1. Many, many are installed wrong. Very common problem.
How did you adjust valves?? If they are too tight you will get backfire. Easy enough to check.
Pull valve covers, turn engine over by hand and do each valve adjustment seperatly.
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1968 Cougar
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