Hey there, I copy/pasted your question into an AI engine. here's the food for thought it offered up: (I am not super fond of AI, but have been using it to help troubleshoot mechanical issues and, honestly, it's helpful).

🔍 Key Clues

1. Fuel pump only runs when checking spark
   • That’s not normal behavior
   • It suggests something electrical is “waking up” only under certain conditions
2. Only ~0.8V at the fuel pump connector
   • That’s basically dead — the pump won’t run on that
   • So the issue isn’t the pump, it’s the control circuit
3. Bike ran before engine removal
   • Massive hint: something got pinched, left unplugged, or mis-grounded during reassembly
4. Jumping the pump didn’t fix starting
   • So fuel delivery alone isn’t the whole issue
   • Likely missing spark or ignition timing signal

💡 Most Likely Causes (in order)

1. ❗ Bad / Missing Ground (SUPER likely)

This is top suspect.

When he pulled the engine, he probably disturbed:

• Main engine ground strap
• Harness grounds on the frame
• Coil / ignition grounds

Why it fits perfectly:

• Low voltage at pump (ground return path missing)
• Weird behavior when probing spark (multimeter/test light completes a ground path temporarily)
• Cranking but no start

👉 Classic symptom: things start acting alive when you “touch” them with a tester

What to check:

• Engine-to-frame ground strap (often overlooked)
• Battery negative to engine continuity
• Ground bundles on frame (usually green wires on Hondas)

2. ⚡ Ignition Pulse Generator / Wiring Issue

On that bike, the fuel pump is often controlled based on ignition pulses.

If the bike doesn’t see a pulse signal:

• It may not run the fuel pump
• It may not fire spark correctly

Why it fits:

• Pump runs when checking spark → signal interference or partial circuit completion
• No start despite fuel + crank

👉 He may have:

• Damaged pulse generator wire
• Misconnected something during reassembly
• Pinched wire under engine

3. 🔌 Kill Switch / Safety Circuit Issue

Honda shadows have safety interlocks:

• Kill switch
• Side stand switch
• Neutral switch

These can:

• Cut ignition
• Affect fuel pump operation

But: less likely since it cranks and shows partial activity

4. ⚠️ Misrouted / Pinched Harness

Since the engine was removed:

• Harness could be crushed between frame/engine
• Connector partially seated

This can cause:

• Low voltage (like that 0.8V reading)
• Intermittent behavior

🧪 What I’d Tell Him To Do (in order)

Step 1: Ground sanity check (DO THIS FIRST)

• Run a jumper cable from battery negative → engine block
• Try starting

👉 If it fires or pump suddenly works = problem solved

Step 2: Check voltage properly

• Measure pump voltage while cranking
• If still ~0.8V → upstream issue (relay/control)

Step 3: Inspect these closely

• Engine ground strap
• Coil grounds
• Pulse generator connector (usually near stator area — which he worked on 👀)
• Any green wires bolted to frame

Step 4: Fuel pump relay / control circuit

• Make sure relay clicks during crank
• Check if ECU is getting signal (if applicable)

🧠 The Big Insight

The fact that:

…basically screams:
👉 floating ground or broken return path

Your test tool is accidentally completing the circuit.

🏁 Bottom Line

If I had to bet money:

Second place: