A lot of low-end mowers fitted with 'classic' engines were not equipped with a throttle control. Not much point when there's only 3.5 HP available!
If it runs smoothly, but slowly, then all that has happened is the bracket that secures the front end of govenor spring has been bent - probably by mowing under a bush or similar.
You need to physically bend it back out - thereby increasing the spring tension. Don't overdo it - should top out at about 3600RPM. You really need a tachometer unless you know what 3600RPM sounds like.
Phil Saunders, September 2008
