That type of cap on the gearbox is usually for adjusting the preload on the worm of a worm gear drive.

However, most worm drives will not allow the sector gear (tines shaft) to rotate the worm (input shaft), so I'm not sure what you have.

As for backfiring, need to know model and spec number of engine. If it is a Briggs, and older than 1983, you should check condenser (moot issue if electronic ignition) and on any Briggs I'd check valve operation and clearances.

Thank you for your response. The back firing has been an issue with both engines. That's why I am suspious of the worm gears. I can't tell how much drag there should be on the tines when there is no power applied. I am thinking that maybe there is a bearing that is worn or broke that puts too much stress on the engine thus making it run hotter than normal and back firing. When the tiller is under load, it settles down, however, I don't see much action on the governor in either case? The carb linkage on the new engine is a straight pull to the carb with no spring attached. Maybe I've answered my own question?

Ariens M 901012 Need High Gear for Transmission #001208 No longer available from Ariens Can anyone help me Jim