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Broken Cases in 450F

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Posted by: KDXFreestyle---------------------

Hi guys I work at a dealership in MA and today a guy brought back his 05 YZ450F because he said he rode it for 5 mins, then it shot a flame out of the exhaust (backfired?) and then on the opposite side of the motor right in line with the kickstarter something happened, and now there is a 2 inch hole or so.

anyways... my point is...

This isn't a normal thing for thumpers is it? or is this a manufacturing problem?
Im going to be buying this same bike within a few months when I get the money, even though most people are telling me to go with a 2 stroke because a 4 stroke WILL blow up. (my theory behind thumpers blowing up is that people either ride them like a 2 stroke holding them wide open off the rev limiter, or they think they are maintence free, when they really need more maintence than a 2 stroke.

Thanks for the replies.. Im hoping this isn't a scare for me in buying a new bike.




Posted by: Treejumper---------------------

Well i've had mine for 2 years and a ton of hours on it with no holes in my case. Sounds like something wasnt right from the get go on that motor.


Posted by: ironworker---------------------

Quote:
Originally Posted by Treejumper
Well i've had mine for 2 years and a ton of hours on it with no holes in my case. Sounds like something wasnt right from the get go on that motor.


Well said Treejumper,
I will be purchasing my 4th new thumper in the past 2 years and not one of them have had a problem, not 1 problem.

I dont think you could get a better bike.

Good Luck.


Posted by: KDXFreestyle---------------------

Anyways.. The mechanic had the engine taken apart, and it needs all new cases and bearings. Turns out the kickstart gear tooth broke off, got wedged in between 2 gears, and then something shot out through the cases after the motor fired.

The mechanic working on it keeps telling me the thumpers are JUNK! and they WILL blow up no matter what.. it is just a matter of time. and also that they are too much money to maintain. 4 times the amount. I think he is just against them. Everyone is trying to convince me to get a 250. i really like the 4 strokes, but why so many problems????


Posted by: john3_16---------------------

First off I have to agree that something must have been wrong with that bike from the factory...

Quote:
but why so many problems????



O.k., back to your question...There's many more things that are a potential cause of failure...These 4 strokes rev higher to match the peak power of a 2 stroke 250 so there is greater potential for piston wear..Not only that you have to consider the sheer number of parts inside of the 4 stroke motor that rely on each other to keep things running...

Of course you're always going to have guys that have no problems........but.......The probability for a major failure happening due to the vass number of interdependant parts is greater on the 4 stroke and this is why you see more problems with a high performance 4 stroke than a 2 stroke....


Posted by: KDXFreestyle---------------------

is it worth it to buy one then in my position?


Posted by: john3_16---------------------

Well, if you want one I'd buy one....Just make an effort to do periodic maintenance to avoid possible issues...From all the reports I've heard, Yamaha makes the most reliable MX 4 stroke motor...The Honda is close but it is advised to go with aftermarket kibblewhite valves and springs...There's a thread on here about that if you would like doing a search on kibblewhite...

I really think that the bike you mentioned had something wrong with it right from the factory so I wouldn't let that particular event worry you...I'd look at what happens under normal circumstances, such as the extra maintenance required to keep it in top running shape....Just remember, with all those parts, if one part fails it has the potential to destroy your motor.


Posted by: Solid State---------------------

A buddy of mine has a YZ-450F and was pitted next to me at Budds Creek last season. After the first moto, he came back to the pits and said his bike was making a strange noise. We looked at his motor and their was a 2 inch hole in the case by the kick starter shaft! He rode his son's 250F for the next moto. Yamaha replaced both cases for free - was only a few weeks old. By the may, I have a YZ-250 2-stroke and can't come close to his speed. He has passed me on the straights and even pinned I can't catch him. I think we are similarly capable riders. That 4-stroke hooks up like mad. Then again, maybe I stink?



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