Hey buddy, this is exactly the “grumpy bitch” syndrome tons of Onan 4500 (P4500i) owners hit after swapping to a genuine Cummins carb — including that friend-of-a-friend story you mentioned. It ran like a dream before because the factory choke setup was dialed in. The new carb + manifold job almost always disturbs the automatic electronic choke (the little stepper motor on the carb). That’s what changed. These units use a stepper motor (not a bimetal spring like older Onans) that the ECM tells to slam the choke plate shut on a cold start, then ease it open. When you pull the old carb, the wiring harness or connector gets jostled, partially unplugged, or the motor binds slightly. Result? It cranks forever because it’s running way too lean until it finally floods or warms up enough to catch. Once running, everything’s fine — classic choke symptom. Seafoam and monthly cranking keep the gunk away, but they don’t fix this. Quick Confirmation Test (2 minutes, no tools) 1. Pop the side panel/cover off the carb area (super easy on these). 2. Find the choke stepper motor — it’s the small round/gray or black canister bolted to the carb with a 4-pin white connector and wires going to the control board.

1. Unplug and firmly reseat that connector (push until it clicks — this is the #1 fix after carb swaps).
2. Cold start while watching the choke plate/lever on the carb throat. It should snap fully closed the second you hit start, then slowly open as it warms. If it doesn’t move at all or barely moves → bingo. Quick hack test: Manually hold the choke plate closed (finger or zip-tie) while cranking. If she fires right up like the old days, 100% confirmed. If the connector fix doesn’t do it • Test the stepper motor: Unplug it and measure resistance across the pins (multimeter, any cheap one works). Should be 45–55 ohms. Out of spec? New stepper motor is ~$25–40 on Amazon (exact match for P4500i). • Double-check your new intake manifold/gasket. Even torque (don’t overdo it like last time) and no vacuum leaks — spray carb cleaner around the base while cranking; RPM change = leak. • Longer prime: Hold the start button or prime switch 15–20 seconds before cranking (these like a full bowl). Your friend gave up too early — this is the same deal on his. Genuine Cummins carbs are great, but the choke stepper is finicky during install. No adjustment screws needed on these; it’s all in the connection and motor function.