Auto decomp 400 starting procedure?

I am a relative noob to the YZ400F. I have a '99 that i just swapped to a 450 exhaust cam, properly timed and shimmed, and a 2005 450 carburetor swap.  The carb swap made it a whole new bike,

 I am almost sorry that I performed the cam swap. Before the swap, a cold start consisted of 4 kicks choke on, decomp lever pulled, a couple twists of the throttle, then one kick just after TDC. I have the easily adjustable fuel screw, and I have spent some time dialing it in for good idle and rideability. 

After the swap, I am kicking quite a few times with the choke knob out, walking away frustrated, and kicking some more, twisting the throttle a few times, getting some pops out of the muffler, then it finally starts and idles for 20-30 seconds before I shove the choke in. 

My last big Yammy was a YZ490. I really dig this water pumping 4 stroke, I just wish it would start as easily as before the cam swap. 1999 YZ400F.jpg


What starting procedure works best for you other decomp swapped 400 people?



"Normal" procedure is to crank the throttle from 2-4 times, then kick with the choke on, no throttle or maybe very slightly above idle.  If it's really cold, you may need to repeat this a couple of times.  


One thing many folks miss if they aren't familiar with auto decompression is that there is still a kind of "drill" to follow.  It's much simpler, though.  Push the starter down until you hit compression ("the hard spot") and stop.  Reposition the starter at the top and kick through from there instead of some random position, and don't outsmart yourself by trying to push past TDC to start.  Put it up against compression and go right from there.


Once you get it warmed up, get your pilot jet/screw adjusted correctly.  Makes a big difference in starting. 

I haven't had the 400 yet but on 450's do not open or pump the throttle when starting. Also when you swapped the carb did you reject it or leave the old jets in it? Pilot jet and fuel screw like grayracer513 said. Finally are you positive the cam is timed correctly. I remember hearing that the 450 cam isn't time the same as the 400 cam.

Yes, it's timed correctly per the TT instructions. The jets are the ones that came in the 450 carburetor. I have the pilot screw out pretty far, so it needs a fatter jet.  I'll try the procedure per  GR13 . Thank for the replies. -Ed

... on 450's do not open or pump the throttle when starting. 


Fail to twist the throttle on mine while cold starting, and you'll be busy for quite some time before you ride it.  The cold engine needs a jolt. 

Mine have all been opposite, if you touch the throttle you'll have to kick a few extra times. I know not all bikes are the same so you gotta find what works. My yz250 was a joke to start, I had to wick the throttle when I kicked it

Well, it's been almost a year. I have this one dialed in so that I can start it with one kick cold. Four twists of throttle, one good kick with the choke pulled out. Thanks for the help guys!

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