What went wrong?

This sucks! :cry:

I have an '03 YZ450 that suddenly quit running right, half way through my last ride (it was making a snapping sound on hard acceleration). I posted a description of my problems here at TT, but no one was able to come up with any causes that I hadn't already looked into, so I took it into a dealer to have them take a look. We talked over all of the things that I had tried to troubleshoot as possibilities (bad gas, needing a new sparkplug, cleaning the carb, etc.), and I decided to have them do a leak-down test on it. They said I had 15% flow through the exhaust valve (uh-oh!). So they tore into it, and I just found out that it is going to need $1100 worth of repairs!

They are telling me that the exhaust valves had excess carbon built up on them, keeping them from seating correctly. This caused more damage to the engine, and they are recommending that I replace the valves and get a complete new cylinder (with piston and rings, of course).

OK, here is the pisser – nobody can tell me what went wrong! I could understand if I did something dumb, but they can't give me a reason for the damage! I bought the bike brand new, have ridden it less than 15 times (estimating 30 hours on the engine). I had the valves checked two rides ago, and they were well within tolerance (it was still running fine then). I only run super unleaded gas (sometimes mixing 50-50 with 110 octane race gas). I never go more than three rides without an oil change, and I use Yamalube. I clean the air filter every time I change the oil (more if it needs it).

I am NOT flaming the 450 here, I still love the bike and was planning on getting an '06 YZ450 (well, I'll probably have to wait until '07 after this bill … ouch!), although I do have to say that my confidence has really been shaken on the reliability of the bike.

Anybody have any insight on what might have gone wrong? Other than somehow I just got a lemon? :cry:

I am not expert rider but I do know a lot about engines. So you did all of your normal maintenance and somehow this unfortunate event happened. There were excess carbon build-up on the valves which prevented it from fully closing. OK, here is my assessment. Assuming the bike was perfect in the very beginning. When the air filter was cleaned and installed, the lithium soap base grease didn't fully seal the lip of the air filter thus causing unfiltered air into the carburetor. This contaminated air started scoring your cylinder walls. The piston rings started to lose its seal between the cylinder walls. Oil started seeping into the combustion chamber thereby creating carbon. I would call it a slow death to your engine since you didn't know that the air filter was not fully contained. When I cleaned my filter the first time in my 04 YZ450, I was worried about the seal between the foam filter and the plastic body. I used a lot of grease around it but this is all I can do to prevent unfiltered air. This is maybe why you need a complete rebuild of the engine.

I am not expert rider but I do know a lot about engines. So you did all of your normal maintenance and somehow this unfortunate event happened. There were excess carbon build-up on the valves which prevented it from fully closing. OK, here is my assessment. Assuming the bike was perfect in the very beginning. When the air filter was cleaned and installed, the lithium soap base grease didn't fully seal the lip of the air filter thus causing unfiltered air into the carburetor. This contaminated air started scoring your cylinder walls. The piston rings started to lose its seal between the cylinder walls. Oil started seeping into the combustion chamber thereby creating carbon. I would call it a slow death to your engine since you didn't know that the air filter was not fully contained. When I cleaned my filter the first time in my 04 YZ450, I was worried about the seal between the foam filter and the plastic body. I used a lot of grease around it but this is all I can do to prevent unfiltered air. This is maybe why you need a complete rebuild of the engine.

sounds like a logical explination

... OK, here is my assessment. Assuming the bike was perfect in the very beginning. When the air filter was cleaned and installed, the lithium soap base grease didn't fully seal the lip of the air filter thus causing unfiltered air into the carburetor. This contaminated air started scoring your cylinder walls. The piston rings started to lose its seal between the cylinder walls. Oil started seeping into the combustion chamber thereby creating carbon. I would call it a slow death to your engine since you didn't know that the air filter was not fully contained. ...

Thanks for the suggestion. :cry:

Yeah, that makes sense, but I hope that isn't what caused the problem. I was always very careful to grease the leading edge of my air filter when I replaced it, too. I guess it is possible that some grit made it in there anyway.

It still seems unlikely to me, though. When I examined at the rings, they didn't seem to have any abnormal wear, and there wasn't any evidence of blowby. Wouldn't there be a lot of blowby in this case? The cylinder was slightly scored (which is why the shop suggested that I replace it), but I have since had three people (each with a LOT of experience working on these bikes) examine the cylinder, and all three said they wouldn't replace it.

Besides, you would think a lot more people would have trouble if it was that easy to screw them up ... :cry:

If this IS the cause, how would I avoid having the same problem in the future? Is there some sign that I might have missed?

Any other ideas out there?

It only has to happen once and as I said, it is a slow death. I couldn't think of a way to check the seal. It might have happened to my bike already and it is just waiting to show the symptoms. A clamp on filter like Uni-clamp may do the trick. These filters clamp on to the flange of the Keihin carb. But for now, I use a lot of grease. It is a crappy design, cheap for the manufacturer, expensive for us.

If thats what happend then there would be a shiza loud of blue smoke comeing out. I think your dealer is not telling the full truth.

I find it hard to believe that the engine is destroyed like that. With a little scoring, do you REALLY need to replace the cylinder? What about re-sleeving it? There are tons of options....the dealer knows only one option, replace everything that doesn't look new with new OEM parts. I think you have more options here. Why would you need to replace all the valves, unless they all showed signs of wear on them? Are they, or is it just the one exhaust valve with carbon build up? Can it be cleaned? I'm just thinking a little cheaper solution here....I'm no racer, and can deal with a bike that's down on power if it is financially driven....

Get a 2nd opinion from someone that is reputable at 4stroke maint/rebuild in your area....Ask other riders in your area, a lot of them have tuners they go to. The dealer has given you the worst case news, now go find the right opinon....and you will have something to compare to..... :cry:

Lets us know what they say ....mine is just like yours :cry:

take your bike to an independent shop i had my 03 top end rebuilt for 700 bucks, including having the cyclinder sent off and redone

the exhaust valves get carbon build up from unburnt\partially burnt fuel. It's not from oil, it's from crappy gas.

Sounds like you gettin a hose'n to me.

Thanks for all of the input -- I appreciate it! :cry:

Butta: I thought about re-sleeving it, but my dealer told me that the cost was within a few dollars to just get a new cylinder, so that sounded like the best way to go. However, I took Ga426Owner and rljjredhed's advice and asked around to several different reputable tuners, and I think I will just re-use the cylinder as is. It has very slight scoring, but three for three said they would run it without any worries. So that helps the pocketbook a lot! Also, it's only the exhaust valves that have the buildup on them. I took them with me to show, too, and they are definitely caked with carbon, so they do have to be replaced. I guess they can't be cleaned without removing a protective coating that they need to work correctly.

YZ250F Rider: I am very interested to hear more about bad gas possibly being the cause. I only run super unleaded from Chevron in the bike (although I have occasionally run 110 race gas mixed 50-50 with the super, too). Around here, Chevron has a good rep for their gasoline, but I have wondered if I should use something else? What gas do you use? Also, could the jetting be at fault? I tried several jetting configuations because the bike always seemed to be a little off, but at the time of the trouble I had settled on running it pretty close to stock (I had upped the main jet to a 170 and I had dropped the needle one position). There was never any indication that things were way off (no blue smoke), and that just doesn't seem like it would be enough of a change to do that much damage ...

If anyone else out there has an idea, please chime in -- I'd really like to figure out what caused this! :cry:

If you have a BP station around try their amoco 93 octane, or maybe shell's new v-power gas. Both use a lot of detergents. Your bike isnt the first yzf to get goobered up valves.

The problem is the valves themselves. The titanium passes the heat so rapidly that the valve tends to cause the exhaust to condense on them as they are the coolest part in the exhaust path.

Sounds like your engine ingested something that it wasn't supposed to. I doubt if "greasing the lip" caused problems (I never grease my filter and have never have any top end problems), but if the filter lip wasn't seated properly, your engine would be toast in a short period of time. Just a thought. :cry:

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!


Register a new account

Sign in

Already have an account? Sign in here.


Sign In Now