Hi new to this site.

[MEASURE TWICE]

1944 Whizzer Model H on a Lincoln-badged Schwinn - Motorcycle Classics

Perhaps no other American-made machine gave as many young folks their first taste of powered two-wheeling as did the Whizzer.

www.motorcycleclassics.com

"138cc 4-stroke side valve engine "

I did wonder that too til I did some searching.

Wiki Cross Section View, See Diagram that has valves still on top, but intake exhaust only slightly lower down from the head:

Flathead engine - Wikipedia

en.wikipedia.org

https://en.wikipedia.org/wiki/Flathead_engine

Showing later model to sell one with side valve a 1949 job:

Whizzer - 138cc four-stroke single cylinder from the USA - 1949

Whizzer 138cc four-stroke single cylinder. Built in 1949. The Whizzer comes with a purchase agreement with engine and frame numbers for the registration inspection. The Whizzer runs well and is very fun to drive. Side-valve single cylinder four-stroke motorcycle with belt drive. This vehicle...

www.catawiki.com

MT

 

[Greg58]

The engine is a 4-cycle in block valve style like a Briggs.

 

[curtisfox]

Sounds like a fuel issue, start by cleaning the tank then, change the lines to the carb, including the filter, and the shutoff valve flowing right.
I just went through this on a lawn mower, I blew the motor ( Tecumseh ) and put a Briggs in with a new carb. Cleaned the tank and would only run a bit and stop, so change the lines and filter and it started right up.................Curt