06 YZF450 kickstarter implodes cracks cases

Sunday one of my friends was getting ready to go out for practice at our last race of the year. it was about 40 degrees out. He twisted the throttle a few times then kicked it over. It fired, back fired then died and he looked down and saw the cases cracked all around the kickstarter. He had this happen on his 04 as well.

any idea what happened or why. I have been considering an 06 or 07 YZF450 but having 2nd thoughts.

thanks, joe

my only thought is that he is giving it gas while kicking causing the bike to back fire.i have heard this will break the kick start stop inside the engine.

not sure if he twisted the throttle when kicking or just before but I used to do the 2 quick blips of the throttle on my 04 crf450 when cold then kick it and it almost always fired in 1-2 kicks. I had read it on transworld or mx action or something. just gets some gas in the carb I guess.

I'm no mechanic, just the basic maintenance and bolt on stuff but why would that hurt the bike to this degree?

not many responses on this one. I posted it on another site and same thing.

thanks, joe

That may be because it's really pretty rare. Mine kicked back once so hard it nearly broke my foot right through my boot with no damage done to the bike.

Hard to imagine twisting the throttle during kicking would cause any problems (besides flooding and a sore leg) as this is a normal routine for some people. He has had this problem on a different bike also? Have you ever seen anyone else with this problem? I would say,dont let him kick your bike! I weigh 250 and sometimes twist to start with no problems. The kickstart hits the peg well before any kind of rotational limit in the mechanism.

I've been on this board for several years and have never heard of this happening. I would say your friend just has really crappy luck. I definately wouldn't let it stop you from getting a Yamaha if that's what you want.

I called Fat Max racing. they do engines and suspensions here in the midwest. Eric said he had never seen this and would have to have the bike to figure it out as to what happened and why. he said yamaha's are in his experience at the top when it comes to 450 reliability.

I rode this bike briefly the day before it failed just on an oval track but could tell right away it is much lighter and turns better than past YZF's I've rode. i'd like to get one and since this is apparently very rare, I expect next spring I'll be on one.

Still curious why this happened to him twice though. I have not noticed anything unusual when he starts his bike and we pit together all the time.

thanks for the responses, Joe

My brother had this very same thing happen to his 03 YZ450. It cost him $1200 to get it fixed. Ouch!! I know for a fact that he did not twist the throttle, because I was watching him. All I know is that his starter gear was all messed up and the case was completely cracked around the starter gear.


2003 WR450

Yes this WAS happening to some on the WR and YZ 450!

It would backfire so hard it would crack the case when the kicker returned to the stop with force!


I've had 5 of these bikes and I've never had anything like that happen to me and I've ridden them in everything from snow to 110 degree heat. That is either a anomaly or your buddy was born under the sign of the Turd. That's some bad luck...

It was definitely an issue on pre-06 YZ450's. My friend at work, a former 125 Englishtown pro, had his frag like that after about 4 hours of riding. It cracked the kickstart gear in half and then it would push out and damage whatever was around it. In his case things went from bad to really bad when the shop that rebuilt it failed to find a stray part and it got into the drive mechanism for the oil pump, disabling it and then the whole engine smoked. Yamaha warrantied the whole thing but, believe me, it was one hell of a fight.

He posted his complaints on here. You may be able to find it with a search on "kickstart gear" or some derivation thereof.


if I did not mention it this happened on his 06 this time. the other was an 04.

good news my friend Dave e-mailed me today that Yamaha was taking care of the parts. Not sure exactly what the labor part will cost but he's working that out with his dealer who actually went to bat for him with Yamaha. He buys all his bikes there and has a pretty good relationship with the owner. to me that says a lot about Yamaha. Even though it was a 2006 he did race it this year and it had no binding warranty when it failed.

I just posted something along these lines with my 04. I pulled the casing apart to find the ratchet wheel which is behind the gear in two peices and it put a couple of small scrapes in the casing, (lucky nothing blew a hole through the casing) There is small tab that comes off this piece and is bolted into the casing with two small bolts. One of those bolts is actually bent. I am going to replace the kick idlegear, ratchet wheel and the snap rings and spacers but I am not sure if the shaft is damaged. I know you can roll the shaft on glass and that will show if it not true, has anyone had this problem and had to change the shaft as well? I was trying to get the parts in the next two days for this weekends race but that does not look like it will happen.

I think I just kicked it to hard, although when it stalled there was a small backfire after the first kick and it broke on the 3rd one. I am going to check with OEM on this site and see if I can get the parts..

Grayracer, I tried to get the impeller off first but no luck and ended up pulling the casing off with it.

04 YZ 450

Factory Connection

W/B Alum Pro

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