98 yz400 knocking!!

ok, i ride the ever friendly xr400 while my older bro just purchased a 98 yz400. Now when he comes to my house to show me the bike i noticed it was a little hard to start the once it fired up it was good to go. But once it started i noticed a distinct knocking sound. now the knocking sound did quiet down slightly as it warmed up it seemed. Then as it got hot the header started to glow red. Now that seems sorts odd to me that it would be that hot. Now i rode the bike and it had tons of power and was slow reving at all. so no loss of power that i can tell. One thing that he told me was that the previous owner had a wr pipe on it to quiet it down. I mean that thing is toned down, At idle all you here is motor. i cant be sure where the knocking is coming from but it sounds like its from the bottom side of the engine. so could the pipe just be corking the crud outta this motor and making the pipe glow as well as the knocking? or maybe the timing has been advanced and is causing the bike to run hot? Or are these just me thinking optimistically? i dont really work on motorcycles so my knowledge of them is pretty limited. so any help you could lend would be appreciated. thanks

Check the jetting, sounds way too lean. I've seen pipes glow red before and it was the jetting. Could be that pipe too. I dunno about the knock, eek.

sounds like the crank needs a new rod bearing maybe.

Check the jetting, sounds way too lean. I've seen pipes glow red before and it was the jetting. Could be that pipe too. I dunno about the knock, eek.

See I was gonna say rich. May not have been rejeted when the wr silencer was installed and is running rich. He said its hard to start. :cry:

Glowing headpipe isn't all that uncommon. Dont let them sit idling and it wont glow :cry: Seriousely :cry: It needs air over the pipe to keep it from glowing.

The knock could be many things but rod or crank bearings are the 1st thing that comes to mind.

now if it is the rod or crank bearing should it be ridden or should we just change the bearings as soon as possible.

you may end up having to swap the crank and the rod out. usually the rod end gets too distorted, and the crank may not be able to be ground down to make it perfectly round again. That and having to press the crank halves apart and back together again is not a job for most folks.

They sell the crank and the rod as a set fairly cheap for what you get.

ok how much would the bearings cost with installatio?. or in worst case scenario how much would it cost for a new rod and crank set?

The early 400s had an issue where the counter balancer key would wear and cause a loud heavy knocking similar to the sound that a rod knock makes. It's a very common problem but it's relatively easy to replace the key and quiet things down.

Check the service manual and you'll see how the assembly goes together.

That certainly sounds a lot cheaper and easier than the whole crank.

See I was gonna say rich. May not have been rejeted when the wr silencer was installed and is running rich. He said its hard to start. :cry:

Glowing headpipe isn't all that uncommon. Dont let them sit idling and it wont glow :cry: Seriousely :cry: It needs air over the pipe to keep it from glowing.

Usually too rich of bikes will run cooler. Too lean makes them run very hot which might make the headpipe glow. True, it's not good to let them sit and idle. This is very high performance motor we're talking about!

ok while i type this my bro is at the local yamaha place. They are tellin him that he needs a new piston. And that it is fine to ride for now. hopefully hes not gettin jacked around. hes bringin the bike over again tonight so we can start tearin it down and check out some things. ill let you nkow how it goes.

Unless you have reason to suspect something other than a counterbalance key, replace it first. This is common like RR said, and is many times the cause of unneeded engine work. The key is like $3 USD, plus a gasket, and some oil and coolant. My buddies 98 had this same problem, it sounded horrible. 1 hour later it sounded fine. Key replaced problem solved.

PK

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