Why is my top speed 70 mph?

[ame="https://www.youtube.com/watch?v=RNgjS5mHI0E"]How to find your differential gear ratio without stamps, tags, or tear down - YouTube[/ame]

Easy way to determine gear ratio.
I also doubt that's the problem but its nice to know regardless.

He did say "the carb is open". That should be clarified, meaning, make sure the throttle is opening ALL the way. Likely need to disconnect the linkage to do this.

Not to make light of the OPs problem but its interesting watching people play this game.

To the OP - you will get great help here if and only if you read carefully and do what has been suggested, then report the findings.

Of course there are sometimes a lot of suggestions. Do the very simple easy things first. It only takes a few minutes to verify the carb is opening, check the air filter, check exhaust pressure.