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Losing Compression at high rpm

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Hello! I need some help with diagnosing my motorcycle.

I have a 2000 XR650L.

The previous owner told me that at one point the bike was not given oil changes and the piston was damaged. The piston was bored out making it a "660". From what I understand no problems from there on.

However, recently I was cruising at 65 on the highway when my chain suddenly broke. The whip from the chain broke open the air box, ripped off the slider and chain guards, and tore up the back sprocket.

So, I installed a new airbox, air filter, new front sprocket, rear sprocket and a new chain. I thought I was back in business and was testing the bike out. Everything was running fine for about ten miles going below 55. When i tried to bring the bike up to higher speeds it totally lost compression at 65 or it just completely bogged down. I pulled over to inspect the bike and everything seemed fine, the bike never shut off. Afterward when I took off it would begin to bog down again at 35mph, especially when i would throttle it quickly. I have tried the bike one other time since then and would notice it skipping or bogging down even at 25, again especially when i try to throttle it quickly,

This is my first motorcycle so I am on a steep learning curve and do not have much knowledge to come up with a diagnosis and solutions.

If anyone has any ideas and suggestions it would be very much appreciated!! I can wait to get this baby dialed in again! Thank you

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It has to be something other than "losing compression" at high engine speeds.  It sure sounds like it's low on oil.  I've experienced that one personally when I ran my own bike out of oil, and it behaved much as you've described.  The engine had to be completely torn down and rebuilt with a new piston and connecting rod to fix that issue.

And how on earth did you break a chain?  Did the master link fail?

Go check the oil right now and let us know how much is in there.

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Sorry. I forgot to mention I just did an oil change when the bike was inactive from the chain break. So the oil should be fine. It was, however, pretty low when I changed it. So, it wasnt in great shape while I was running it before the chain break. 

I have no clue why the chain broke. I was running it on my normal 9 mile cummute going 65 like I usually do. The bike seems its happiest at 65... I was thinking maybe the master link failed because I could not find it afterward.  

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If there isn't still a master link on the chain, then that's what failed.  Was it a rivet type master link or a clip master link?  I don't trust clip master links, although that's what my bike had when I got it and I waited until the chain was good and worn before switching over.

Any chance the oil filter got put in backward?

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It is fuel mix related.... trust me I know look at the old air box for drilled holes/parts that were taken off, anything that is different if the bike was bored over it was re jetted and had airbox opened to bring in more air. The bike runs ok when you roll onto the throttle slow but still sputters when you just full throttle it its almost like your hitting the kill switch? Am I right on? If so your not getting enough air and the bike is flooding because there is too much fuel comming in and not enough air. 

Edited by yz400fmaniac

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Yes! I definitely had holes drilled into my old air box! Exactly it runs okay while gently rolling the throttle and sputters on full throttle. Feels exactly like there is too much fuel.

So i should just drill some holes into my air box?

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And also was the air box broke from the chain or cut open from the previous owner..... you said it made a hole a chain would most likley crack it if it was a hole im guessing it was cut by previous owner to let in more air.

Drill holes or just tape old one and return the one you bought 

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Scott, I am not sure what type of chain the old one was. My new one is a master link chain, I am also a bit skeptical of it since I just recently had a chain break!

I will check the oil filter, but I believe i put it in correctly 

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The air box was completely broken open and destroyed by the chain when it broke... But, I do remember looking at it and there were a lot of holes drilled into the old air box. There was some tape on the holes however, Im not sure if they were all covered up though

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Also make sure you put the master link clip on so that the way the chain rotates, if the chain hits something it is pushing the clip on not pushing it off if that makes sense

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It will take some trial and error to get the air box dialed in, but dont make my mistake lol and drill too many you can always drill more but you cant undrill!!! And if you ran it low on oil and you starven it of oil it would smoke like a 2 stroke and be knocking, I have had an xl600 before and At first i mistook valve tap for rod knock, keep that bike topped up on oil and it should literally go forever those motors are pretty much bullet proof.

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OK Great! Im excited to try it out! Hope it works!! When my bike was broken down, and I noticed it was very low on oil, I started it and it would throw a little black smoke when I gave it a good rev in neutral, the valves also sounded a little un-lubricated... But since I changed the oil it doesn't throw any black smoke like that and it sounds solid. So I'm pretty sure oil is not the problem. Thankfully! This baby is all lubed up now

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Would you recommend that I drill a few holes, then take her out for a short ride and see if she's still getting flooded. If so then drill more? keep doing that until no more flooding?

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Before you go chopping up your airbox , just try riding it with the left sidecover off , it'll never get more air than that.

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2 hours ago, yz400fmaniac said:

It will take some trial and error to get the air box dialed in, but dont make my mistake lol and drill too many you can always drill more but you cant undrill!!!

And if you ran it low on oil and you starven it of oil it would smoke like a 2 stroke and be knocking, I have had an xl600 before and At first i mistook valve tap for rod knock, keep that bike topped up on oil and it should literally go forever those motors are pretty much bullet proof.

Duct tape is as good as undrilling the holes. ;)

When I ran my XR650L out of oil and seized it, there was no smoking and no knocking.  At least there wasn't any knocking until it was dead and I tried bump-starting the thing.  Then there was knocking as the piston started to come apart.

Edited by ScottRNelson

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I second joerc51 on my dr650 when i tejetted all i did was take the sidecover off, should have led with that try that first, and if it was puffing black smoke that is also a sign of running rich, black smoke usually means unburnt fuel so there is a good chance that when you get it dialed in it will have more power, after you do this if there is hanging idle, that means too much air not enough fuel. Remember this rich is better than lean, so if it still puffs a little black it is ok and even a little blue is ok too it wont hurt anything, if it smokes a little blue just think of it as extra lubrication but if it is like a 2 stroke thats a little much. but all bikes burn oil just depends on how much and always always always check oil before riding. 30 seconds is much better than 1200 in repairs. Check air filter often also and a little trick buy mineral spitits clean air filter in it than save it, its reusable wash filter with soap and water and coat in belray or oil of choice and thats it!!! Thats pretty much all engine maintinance other than valves man i love air cooled bikes and if your not sure about something youtube it. Its the best recource.

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Hey guys! SO i took the cover off the airbag to test the theory and it turns out not enough air is not the problem! The bike did the same thing with bogging down when throttling hard... I think what happened is that I've gotten a little bit of dirt in my carb from when I was changing the air box... So i guess my main jet is just getting clogged!!

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16 minutes ago, ulikemike said:

Hey guys! SO i took the cover off the airbag to test the theory and it turns out not enough air is not the problem! The bike did the same thing with bogging down when throttling hard... I think what happened is that I've gotten a little bit of dirt in my carb from when I was changing the air box... So i guess my main jet is just getting clogged!!

Your bike has an airbag?

If dirt got into your carburetor jets, you should add a fuel filter between the tank and the carburetor.

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