Don’t be too eager in congratulating yourself on a success!
After my SRX400 stopped starting last year, I gave the carb a service and replaced the jet, float, needle and seals. No spark, so replaced the plug and the coil for good measure. It started this evening! …wait for it.
Dieseling!
First time I’ve seen that happen in years.
I never knew this could happen. Trust you to have a fault I’ve never seen before. 
Umm, how is that possible? What’s powering the spark?
Electrical short somewhere.
An SRX400 will not produce enough compression to cause dieseling, its not engine run on caused by incorrect carburation either. Engine run on and dieseling have a very lumpy tickover and the engine will stall as soon as its put under load, as me_groovy has correctly diagnosed its an electrical fault. Focusing on the symptoms - Ignition off engine continues to run, kill switch off engine continues to run, the common factor and prime suspect here is the engine stop relay and/or its associated wiring.
Quick patent it, me_groovy has broken the laws of physics and created the perpetual motion machine!
Update: found a bunch of corrosion on the kill switch. Cleaned it up as best I can. It’s riveted so disassembly and repair is impossible, as is finding a replacement.
Also realised I can start it without the key in too.
Oppps.
Plenty of generic ones to choose from.
https://www.dirtbikexpress.co.uk/bike_parts/motocross_engine_parts/switches__and__kill_buttons
have you checked if it is the killswitch, it can look bad but work.
test it with a multimeter, its just a switch. same for the bike, either short out or leave open the connections in the loom without the switch connected and see what happens
Did exactly that. Get continuity switched both ways.
Took the front sprocket cover off tonight to extract the suspect feed. It’s perfectly fine. Under the cover was filthy though so spent a while scraping all the gunk off.
