horrible vibration between 4k and 4.5k rpm

[nezzer]

My bike recently developed a severe rattle / rasping between 4000 and 4500 rpm. This occurs in every gear. As far I as can tell it happens right away when the bike is cold and continues once it is warmed up.

My bike had been making a clicking noise for a while. My cam chain was out of adjustment so I figured I needed a new chain. I put off replacing the cam chain until the day this vibration started occurring. I pretty much parked the bike that day until I replaced the cam chain. The problem seemed to start overnight.

Now I have the new chain installed. I also checked the valve gaps and they are close, but not perfect. The clicking still occurs but some other threads indicate it might be a bad exhaust header gasket or blocked crankcase breather hose. I will certainly check both those items.

I am curious if anyone has any ideas on what might be causing this extreme vibration?

It is undoubtedly coming from the engine and not wheels or suspension. If I run it up to past 4500 rpm, then pull in the clutch, it will still vibrate badly as it drops down from 4500 to 4000 rpm. Below 4000 everything is smooth as silk. Above 4500 it is a little rough but not bad at all.

I appreciate the help.

[bikerphil]

vibration

Loose or cracked exhaust system may cause vibrations. Also, rubber mounted muffler strap might do this.

[rockjok]

I've got an 80 LG that had a vibration similar to what you describe. After lotsa worry that it was a serious problem I finally traced to a simple crack in the heat shield on my left hand exhaust. Under hard accelleration it felt like the engine was about to blow up.

Rob

[TADracer]

Dont just concentrate on the motor. In fact, the sound that 'comes from the motor' may be something else being echoed off of fairing, frame, tank, etc. I had a similar problem that I swore was the motor. Turned out to be the edge of the tank vibrating against the clutch cable. Moved the cable a bit and problem solved. Unless something goes terribly wrong inside the case, the motor should not make any appreciable changes in mechanical sound. So study everything before swearing its the motor.

[0011mixam]

What TADracer said!

My mirrors buzz at about 2000 rpm and my RH horn buzzes at around 3500 rpm (not the left one though, which is kind of interesting). Everything has a natural frequency, and when the engine rpm corresponds to that frequency, you will get a resonance. Natural frequency is affected by the mass and the stiffness of the particular item. Therefore, if you can find out what's buzzing at your particular rpm, you may be able to change the natural frequency by altering the torque on the mounting bolts, changing the mounting location, etc.

[nezzer]

Now that you mention it rockjok I did recently try to tighten up my left hand exhaust heat shield. It was rattling at idle. It turned out all three bolts were loose. Two of the bolt heads broke off when I applied .001 ft lbs of force to them. The last one was fine and was tightened. I wouldnt be surpised at all if this was the problem since I am down to only 1 screw holding it on.

I guess its time to bite the bullet and extract the remains of those 2 rusty bolts from the exhaust so I can properly secure the sheild. Well, maybe I will shove a wad of aluminum foil between the exhaust and shield first just to make sure the noise does indeed go away. I will let you know what I come up with. thanks for the help.

[nezzer]

Well I just removed the heat shield with the missing bolts. No change. I went ahead and replaced all 4 exhaust gaskets. That helped tremendously. The vibration between 4000 and 4500 rpm has been dramatically reduced. I can still feel it but I might not have noticed it was I not specifically looking for it. The clicking noise is also much quieter, although still clearly audible. I am not going to worry about the clicking until next season when I do the valve shims again. Hopefully that will fix it.

On a side note, when removing the old exhaust I used loads of liquid wrench. I would not be surprised if this was the first time the exhaust system was apart. When putting everything back together I coated the threads of the exhaust studs with liquid wrench to make it easier to correctly hit the torque specification. Once I fired up the bike it started smoking like crazy. It got worse once it fully warmed up. It was a white smoke coming from the exhaust headers and near the exhaust gasket area all though there was so much of it the smoke seemed to be coming from the entire top end. There was no smoke coming out the exhaust pipes. I guess there was still plenty of liquid wrench residue on the pipes and engine case. After about 15 the smoke subsided. That was a nervous 15 minutes for me trying to figure out if I had really screwed something up this time.

So far the bike seems much smoother, although I have to say it feels slower. Maybe it just SEEMS slower since everything is running so much smoother. Either way I am just happy the bike seems to be running well.

Thanks for the help.

[Ivan]

horrible vibration between 4k and 4.5k rpm

I have a similar situation. I reckon its the "viagra effect" of all the power setting up a resonance with..... well you get the idea.

On a serious note, get some antisieze compound for the exhaust bolts. It is a real headache saver. Also use it on spark plugs, it helps them ground better. I have had engine misses simply cured with a dab of antisieze on the threads. Yeah, it stumped me for about 3 days. Also great for single wire sensors for the same reason. I have a tube about the size of a travel size toothpaste, and it has lasted me for several years.

As for finding a buzz, Get a mechanics stethoscope. I don't think they are terribly expensive, and will help you find the noise much quicker. I have an electronic one, called an EnginEar, much more expensive, but sensitive enough to find leaks on tires that won't show in a bubble tank.

Something like this should do fine

[subike]

Try using a long handled screwdriver as a stethescope.
Put handle to you ear, hold point on motor, move it around till you hear what you are looking for. Crankshafts and gearboxes in neutral can sound alarming at first lol. Works same as the fancy tools, and well...cost?

[nezzer]

Well I was incorrect when I said the vibration went away. I replaced the exhaust gaskets and took it for a quick test ride. Everything seemed fine so I figured the problem was gone. Later I took it for a longer ride when it got fully warmed up and the problem was back.

I had already removed the left hand exhaust heat shield so I went ahead and removed the right hand one also. That is when I noticed a crack in the exhaust pipe just underneath the heat shield. The crack looked like it went about 60% of the way around the pipe. I am surprised given the severity of the crack that the the bike was not louder.



After finding this crack I gave the rest of the exhaust system a close examination. I found a failed weld on the other exhaust pipe on one of the 3 heat shield supports. One of the 2 welds that holds the support to the pipe had failed.


I took the entire exhaust off the bike so I could take it to a shop. While taking it off the exhaust cracked all the way through and fell in 2 pieced. Uh oh.

The only positive note was that it was cracked in such a way that it only fit back together 1 way; like a jigsaw puzzle. I took the exhaust to a shop and had them weld everything back together. They did a very nice job and the exhaust seems very secure.

I went for a 200 mile ride today and I am happy to report that the vibration is completely gone. There is still a slight clicking but besides that the bike is running great.

More photos here.

[CatatonicBug]

It's stuff like this that makes me apreciate the "mechanical" nature of these machines, as opposed to the more modern vehicles with all of their computerized gadgetry. At least when something breaks on these things, all you have to do is start opening things up untill you actually SEE the broken item. It's so much easier to fix stuff when you can touch, see, or feel the problem and not have to worry about computer chips.