Yz450f stall problem

Hoping someone with a lot more experience than myself might be able to help me out. 

Make: YZ450F 2012 model EFI


  • idles for a minute or two and then stalls
  • when hot and I back off the throttle it stalls
  • after stalling when hot, takes lots of kicks to start again, when i remove the plug and dry and clean it the bike will start more easily

What I had done to it before it started this:

  • bike seized due to  chain tensioner wore out and travelled up into the cam chain and head.
  • replaced piston and cylinder (aftermarket Athena installed)
  • assembled head with new valves and springs
  • new cam chain
  • set valve clearance
  • replaced clutch (basket as well)

After less than 10 hours, the stalling was happening too frequently to ride without losing my calm so I took it to the shop again. They spent seven hours trying to diagnose the fault with no results.

  • reset cam timing and checked flywheel key
  • found small amount of water in oil
  • cleaned injector and air mixture screw
  • checked crankshaft position sensor
  • checked coil and cap
  • replaced spark plugi
  • inspected TPS, APS, atmospheric pressure sensor and kill switch

It was getting too expensive to leave at the shop any longer so I am trying to have a crack at it myself. Does anyone have any ideas? I'm going to try to do a compression test or leak down test this week. Also, has anyone made up the test harnesses for this engine?... I'm not keen to buy a gytr tuner to test the sensors again if I can avoid it.

Any help would be greatly appreciated.

do you have an aftermarket tune on this mate?

maybe try resetting the tune to standard and see if it stops. you will then eliminate the posssible causes

How did they clean the injector? It's been my experience that most  dealers are used to parts swapping for diagnosing, and don't know how to properly clean an injector.... 

The fuel pump filter should have been changed many hours ago. Seach on ebay for a replacement filter and / or filter and pump.  

Your symptoms lean toward a damaged/failing fuel pump, and if that pump has been trying to work through the same 7 year old filter, I would say yes that is for sure your problem.

Edited by THE KRAN

Thanks for your both of your replies. Sorry that I've taken so long to get back to it, my work means I travel an awful lot.

The shop I last took it to reset the map on it and said that it was previously on a stock mapping anyway. 

I rechecked the cleanliness of the injector, it looked fine. Also I disassembled the fuel pump and cleaned the filter in it. It wasn't particularly dirty, but I did manage to get it a lot cleaner.

I just did a pressure test on the fuel pump and it was generating over 20psi, my little gauge didn't read any more than that. It also had a strong and constant flow. I'll check the specs in another thread and see what people usually get from the pump. I'll still start looking for a new pump and filter.

I also tried to do a compression test the normal way, but assume that the auto decomp has hindered me getting an accurate reading. I'm just making up a leak down test kit now to try that instead and should be able to perform it tomorrow. I'll pop the results up as soon as I have.




Looks like the manual specs for fuel pressure are much higher than I had anticipated. It should be up around 328kPa. I'll get a gauge that can work at that range and try again.

10 hours ago, Hewie said:

Looks like the manual specs for fuel pressure are much higher than I had anticipated. It should be up around 328kPa. I'll get a gauge that can work at that range and try again.

Yeah, over here you can rent a FI pressure gauge at most autopart stores. You test with water for 5+ minutes to see if it will hold 50psi for at least that long. 

There are videos on you tube showing the pump pressure start to fail as the pump warms up, dropping psi over 50% over time, and creating a sputtering nozzle squirt.

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