Wisconson engine questions.

Hello, I'm a novice trying to get an old walk behind Ditch Witch C-99 model running and have some questions. I've been informed it's the Wisconson AENL D-series engine.

Can anyone tell me what production year the engine might be? It's a one-cylinder engine. Embossed on the block is AA-91-A, stamped and offset before the embossed characters is BB- and after the embossed characters is a 2. The combined stamped and embossed characters (if meant to be taken together) would read BB-AA-91-A 2.

Also, I'm getting an occasional violent jerk back on the pull start, and the best I'm getting so far is a single fire in the exhaust or through the air filter. I've confirmed I don't have a damaged flywheel key, recoil spring is okay, bench tested a newly rebuilt magneto, alligned 'X' mark on block's internal gear tooth with sight hole when installing the magneto, cleaned the carb, and replaced the plug with piston at TDC so evidence is pointing toward valve adjustment or stuck valve.

I'm guessing access to the valve springs is behind the carburator like the area circled in red in the image below (which is not my exact engine).

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AA-91-A is the basic block for standard AENL engine. Access to adjust the valve's is the rectangular plate above the magneto, below exhaust and behind carburetor. You can find information by looking here " www.m-and-d.com/pdfs/Wisconsin/AEN_AENL_AENS_instr_parts.pdf ".

You need to make sure your piston is at tdc on the compression stroke. Then see where your rotor
is pointing. Hal

Thank you, Jo Bird, that manual has a wealth of information.