04 YZ450F, few puffs of white smoke, blew bung out, oil leak?

[2wheels-drz]

had my first race on my new(to me) 04 450. bike was running great, apart from a blutal cultch, and a very poor suspension setup of which i spent most of the day getting dialed in.

Anyway after my second last race, a mate came over and told me he had seen some puffs of white smoke coming from my exhaust on the last lap (he was directly behind). having just pulled into the paddock i took a quick look at the engine, and it seems i have blowen some kinda frost plug, or bung out of the top right on the cyclinder head(see pic). which aslo resulted in a oil leak. am not sure what this part is/does?? or what would have caused this to happen? was it me? or has something failed internaly? i am racing supermoto, and the engine was getting a hard time, it hit the limiter a few times. i did not start the bike again, and seening how i had planned on giving the top end an over haul after the first 3 races anyway, i guess i would best to get it done now, aswell as sort this problem.

Thank god i brought the old donkey aswell (yz400) she ran a dream, and i actually got my fasted lap on it, but i guess that must have been down to me already having the suspension sorted, cause it felted very under powered down the straights compared to the 450.

[King_Air]

That would be the rubber plug that plugs where the decompression linkage went on the early models without autodecompression. The head design was not changed until later years, so Yahama put in a rubber plug to close the bore. You can get a nice alumiumn one here on thumper talk for $20 USD or so.

[grayracer513]

An 18mm brass or steel cup plug (auto parts) will also work.

[2wheels-drz]

That would be the rubber plug that plugs where the decompression linkage went on the early models without autodecompression. The head design was not changed until later years, so Yahama put in a rubber plug to close the bore. You can get a nice alumiumn one here on thumper talk for $20 USD or so.

ah i see now. those alumiumn ones look nice! however, the local dealer has replacement one in stock, so am gonna go with one of them, and them make a bracket to ensure it does not blow out again...all this is after i get a new piston, timing chain and valves in need be....

but what would have caused this to happen? and is there a chance i have done major damage? cause it did run flat out for a lap just after the plug blew out.

thanks

[King_Air]

This has been a somewhat commmon occurance on these bikes. I would doubt that you have done any damage at all. Try a search and see how many others have had this happen to them. I would not worry too much about it. Get the new plug, secure with your bracket and go ride!

[grayracer513]

but what would have caused this to happen?

Usually, a blocked breather hose. Check the two metal clips that hold it to the frame and see if one of them is flattened, pinching the hose.

[2wheels-drz]

Usually, a blocked breather hose. Check the two metal clips that hold it to the frame and see if one of them is flattened, pinching the hose.

i'm running a catch tank, block hose can happen easy. will have to mod my tank with an over flow valve to stop this happening again. i'm defo gonna get the top end rebuilt, (and my forks serviced) i really don't wanna risk it, supermoto is hard enough on these engines.

many thanks.

[grayracer513]

The only risk of damage there is in this is from dirt intruding through the open bore. As likely as that may appear, it doesn't actually happen very often, and is usually minor when it does. You can open the cam box up and have a look. You should be able to see what, if anything, got in.

[2wheels-drz]

The only risk of damage there is in this is from dirt intruding through the open bore. As likely as that may appear, it doesn't actually happen very often, and is usually minor when it does. You can open the cam box up and have a look. You should be able to see what, if anything, got in.

hmm ok sounds like a good idea...but what about the fact that the puffs of white smoke defo came out of my exhaust and not from oil getting spat on to the header......i have a feelin this could be work out rather expsensive, top end rebuild and fork service, all straight after the first race, and i'm broke already.....

[grayracer513]

Fix the plug and ride it some more. See what happens.