2000 WR400F starts fine, very loud and makes no power

[tylers24]

My WR400 has had an issue that neither myself nor the shop can figure out. Bike was running beautifully until I parked it bike outside for 4 days uncovered, and afterward it starts fine but won't make any power when driving. Exhaust note is significantly louder than normal, and it will hardly reach 20 mph. This was just 3 weeks after a new spark plug and carb rebuild + re-jet.

Took it in to the shop and they checked valves and they're in spec and timing is spot on. Took apart the carb and all is as it should be. Checked intake and filter, with no blockages and carb boot is fine. Tried running the bike off of a separate fuel source with no improvement. 

Does anybody have any suggestions as to what this might be? Any and all help is appreciated.

- Tyler

 

[FORMA MOTO]

One of your exahust valves is leaking. 

Unless you don't use this bike, the head is 18 years old and worn out.... and will make that 'broken header' sound when one of the valves does not seal.

[tylers24]

Kran,

This is the suspicion that I had when I dropped the bike off, however the mech said he checked for that and all the valves and seats appeared fine. He also stated that the bike would not run if compression were so low. Also, no explanation for why the bike went from running like new to this after a few days of sitting...

That being said, I still agree with what you're saying as its the only explanation I could reason for such a drastic change in exhaust note and loss of power. If this is truly the case, would I need new head and/or valves, or is there a chance I could have the head refurbished? I would be reluctant to put a used head on the bike for obvious reasons, and the only alternative is to purchase a whole new head assembly at $800+. The only people I have found any mention of offering a refurbish are https://dynobike.com/wiseco.htm, but they are located in AUS.. Anybody been down this road and have suggestions to offer here? I saw a past thread about services offered for the 400/426 head but all links to helpful pages have since expired.

[partsguy57]

Kran,
This is the suspicion that I had when I dropped the bike off, however the mech said he checked for that and all the valves and seats appeared fine. He also stated that the bike would not run if compression were so low. Also, no explanation for why the bike went from running like new to this after a few days of sitting...
That being said, I still agree with what you're saying as its the only explanation I could reason for such a drastic change in exhaust note and loss of power. If this is truly the case, would I need new head and/or valves, or is there a chance I could have the head refurbished? I would be reluctant to put a used head on the bike for obvious reasons, and the only alternative is to purchase a whole new head assembly at $800+. The only people I have found any mention of offering a refurbish are https://dynobike.com/wiseco.htm, but they are located in AUS.. Anybody been down this road and have suggestions to offer here? I saw a past thread about services offered for the 400/426 head but all links to helpful pages have since expired.

Compression test.... Or put air to cyl .... Air will run through a leaking valve.... Always curious why some refuse to do the most basic of tests....

[FORMA MOTO]

If your mechanic has not done a leak down test....you need another mechanic.

Valve guides/seals van fail just from sitting...

[flyandride]

Do a leak down test.  And check to see if the exhaust got blocked somehow.  Pull out the baffle and see if it runs better and if there's something in their.  Critters crawl in and die.  I've found a mud daubers nest in a muffler, it wouldn't run with that.  

[tylers24]

Update: After they had run through everything with no success, they found it to be a failure of the CDI. This wasn't allowing the timing to advance. Whether this was due to moisture or what I am not sure, but they claim that a simple replacement of the CDI unit fixed the problem. I will find out when i return to the country in a few weeks.