YZ450F Hard to Start

45 replies to this topic
  • wags450

Posted March 21, 2011 - 12:09 PM


I did a search but couldnt find anything like this on there.

I have an 08 YZ450F pretty much all stock 40 hours on it. I bought it used a couple months ago and it started 1 kick maybe 2 every time.

The other day I did some maintenance on it:
New Twin Air Filter
New Spark Plug
Change Oil/Filter

and thats it.

Now bike takes forever to kick and you have to do a couple things to get it to go. No choke, throttle cracked and sometimes hot start pulled in. After it starts you have to pull the choke to keep it going. Once its hot and been running a while it starts back up 1 maybe 2 kicks.

What could it be?

Any thoughts? Jetting is stock.

  • brentn

Posted March 21, 2011 - 01:15 PM


There were a couple times that it was about -10 out or so and I was using maxima FFT for the air filter. Stuff is really good, but was just too viscus in the cold and the bike would not start. I use UNI now for cold riding and save the maxima for the summer.

Just one possible reason why you might be having troubles starting the bike, temp and air filter oil.

  • wags450

Posted March 21, 2011 - 01:17 PM


Hahahaha thanks for the reply I dont think thats the issue. Im in Southern California and I think the coldest it had gotten was about 40 something and i wasnt trying to start it then. Mostly starting it in the 60's and 70's.

-10 cant say that I am envious.

  • brentn

Posted March 21, 2011 - 01:20 PM


Didn't even see where you were at lol.

  • grayracer513

Posted March 21, 2011 - 01:24 PM


Possibly over-oiled your new filter, and or installed it without letting the oil "dry" out and drew some into an air jet.

  • wags450

Posted March 21, 2011 - 01:24 PM


Possibly over-oiled your new filter, and or installed it without letting the oil "dry" out and drew some into an air jet.

Brand new pre-oiled filter from Twin Air

  • Dylan8kawasaki

Posted March 21, 2011 - 01:30 PM


They come in a sealed package, if you read the MXA write-up on the yz450f they also mention the bike doesn't like wet or over oiled airfilters. I would make sure the filter is 'dry'.

  • wanawakeboard

Posted March 21, 2011 - 01:41 PM


check your plug gap?

  • Dylan8kawasaki

Posted March 21, 2011 - 01:55 PM


check your plug gap?

If it was plug gap I don't think the bike would ever start or it would run rough... Just my opinion...

  • wags450

Posted March 21, 2011 - 01:57 PM


Gaps good filter is dry. Could it be a bad batch of gas?

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  • chris856

Posted March 21, 2011 - 02:58 PM


how long has the gas been in there? i would just pull the bowl off the carb to make sure its not gummed up from sitting.

  • wanawakeboard

Posted March 21, 2011 - 03:20 PM


doubtful on the gas being bad but possible, any chance the fuel screw has backed off or vibrated out? sounds silly but worth a check, and make sure your airbox is closed all the way...

  • wags450

Posted March 21, 2011 - 03:26 PM


Gas is fresh i ride every week. Filled the tank 3/16 maintenance 3/18 Now the issues. Not sure about the fuel screw. How would suggest checking?

Turn it all the way in and back it out again?

  • joeprunc

Posted March 21, 2011 - 03:46 PM


With the fuel screw, count the revolutions while you tighten it. Most air/fuel screws should be around 1 1/2 to 2 turns out. The previous poster that mentioned this thinks that maybe the screw rattled loose and it might possibly be 4 or 5 turns out or maybe even missing. there is a spring, washer, and o ring that keeps pressure and a seal between the screw & carb.

I am really interested in what your find, please keep me posted, as my 03 YZ250F is seeing the EXACT same symptoms and problems. Except I did a whole top end, cleaned the carb, new TwinAir filter, spark plug (heat number 9). Maybe I over oiled the filter and gunked a jet or two. I ordered a whole new replacement set of jets and am going to replace them one at a time till I find the cure. I may even try to swap out and use my old UNI filter. And maybe even put a new plug in (heat #8 which was what was in previously). My timing and valves are dead on so I know that isn't the issue. So between the work you did and the work I did I think I can safely single it to the air filter, carb, or spark plug. I'm 90% sure its the Pilot Jet, but I will have to wait and see.

I was itching to ride the other week and went out, it took 10 minutes to start the bike, but never an issue restarting while it was hot. I rode very close to the truck and made sure I was on a steep downhill before killing it. First ride since the rebuilt and it had a hell of a lot more kick and power! After a 5-10 minute settle/cool it was a bitch to start again, but fine after riding for a bit.

I will keep you posted if I hear or fix anything. Here is what I've done so far with no success, to maybe help you a bit.
  • Played with the air fuel screw, at 1/2 incremental turns till 3 turns out.
  • Changed the spark plug with the old one. But the old one was shot.
  • Re cleaned the carb with carb cleaner.
  • Replaced the carb slide valve plate gasket (mine was toast)
  • Checked the timing
  • Checked the valve clearance
  • Checked the spark (have not checked the spark timing, but if it ran great it should be dead on)
  • Installed new foot pegs & ASV levers :thumbsup: (of course this wasn't going to help, but it made me feel a little better about the bike...now I just need to figure out this starting issue.

  • wanawakeboard

Posted March 21, 2011 - 05:19 PM


joe is right, once you turn the screw in and confirm
A) the screw is there AND
:thumbsup: how many turns out it is
Long shot here but maybe the new filter is letting more air through throwing off your jetting a little, maybe you need another half turn out on that fuel screw(more fuel)
but before you do that make sure you know exactly where it is now so if it does not help you can restore the screw to the exact same position it is now.

  • ttr250dude

Posted March 21, 2011 - 05:48 PM


As for the air filter, take it out and get as much oil as u can out of it by using paper towels. Inside and out. I usually spend a good 5 mins 'drying' out the filter with paper towels. Be thorough.

  • wags450

Posted March 22, 2011 - 06:19 AM


I plan on checking this out later today since I have the day off. Ill let you know what I find. Thanks for the tips

  • joeprunc

Posted March 22, 2011 - 07:07 AM


The UPS man should be delivering the majority of my replacement jets and carb gaskets today. I think I am going to try to start it without an air filter and see if that helps any, but yes it may be an over oiled filter (though I oiled it myself and am usually fairly modest with the oil).

  • skull1971

Posted March 22, 2011 - 10:24 AM


Do you have an after market mixture screw?? I put one in my 450 and it's kinda hard to start now too. It use to start first kick, or half a kick, now it's 3, 5, 8 or more till it pisses me off. I found that if I crack throttle it's better. I'm going to put the stock mixture screw back in and see if it acts any better.

  • usaf_yz426

Posted March 22, 2011 - 10:44 AM


could it be your valves are out of spec???

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