I have a Lifan 70CC engine. The intake hole into the engine is 20MM. Is it possible to install an intake manifold with the same flange size but a larger diameter? The inner diameter of the intake manifold is 24mm. Trying to reroute the carb.

 

In theory you want the intake manifold to be fairly close to the port in the head and carb bore sizes but in practice it won't make much difference to airflow. Intake manifold length also makes some difference, and racing engines have intake lengths "tuned" to suit the rev range where the power is required.

 

Thanks, I was able to find a 22MM so its a bit closer in size.