I have a Briggs and Stratton Vanguard vertical shaft 18HP on a John Deere SST18. Last year a valve sleeve came loose. As a result I had it in a shop to replace the entire head assembly. I had removed the deck from the mower to repaint it while the mower was in the shop. I got the mower back, but it was a while until I put the deck back on to use it. The engine seemed to run fine when I got it back, but once I put the deck on the engine spits, sputters, smokes (as if the choke were closed), and looses a lot of power when there is a heavier cutting load. Also when it gets like this the engine looses manifold vacuum. I can tell because the steering control uses manifold vacuum to control the drive valves. Taking it back is not an option as the repairman retired shortly after repairing my engine. Does this engine have a governor and perhaps it is giving it too rich of mixer under load?
The main jet is too big , not seated properly or missing all together.
Or the choke is not retracting completely when you throttle back after starting.
check the position of the control cable where you clamp it to the engine.
We will need the model numbers off the engine for any more suggestions.
FWIW the governor closes down the throttle butterfly.
