[Jay In Milpitas]

Ralph, does the engine reach red-line in fourth and fifth gear? Obviously it doesn't in sixth or you would be near 80 mph.

But always start with basic maintenance steps.
1. Valve clearance.
2. Test compression.
3. Ignition system:
a. Spark plug condition.
b. Ignition timing (should NOT really be a problem).
c. Spark quality (fat blue spark).
4. Air filter clean, no obstructions (mice like to nest in them).
5. Fuel system.
a. Filter clean, good gas flow.
b. Carburetor clean, adjusted, throttle opens fully.

Those steps, in order. Applies to any four stroke engine.
Good luck and happy hunting.