exhaust sound around 3,800 - 4,000 rpms in all gears

I'm very new to motorcycles but I've been slowly getting my 81 xj650 in shape from a very bad PO. So I've swapped shims, vacuum synced, colortuned and throughout out all of this I've noticed this noise around the 3800-4000 rpm range that really sounds bad, as if the engine is hitting a 'max' of some sort prematurely but it quickly drops off after that. It follows the growl sound of the rpms up perfectly and then just gets very high pitched in that small range. When you linger in that zone it is really annoying. I'm sure I should be riding in a higher rpm anyway, but still, I often hit that spot and it worries me. Anybody have experience with something similar or am I just making something out of nothing?

While on this note, is the bike supposed to sound so shrill when riding? I'm making an effort to ride around 6k but it often feels like I'm on the verge of the bike exploding and it causes me to cruise around 4 or 5k. Even passersby look over to see if I'll explode. Must mention I am running 4 into 1 exhaust.

 

Hi Squidjob, check your exhaust pipes and mufflers for any holes, use a mirror to get a look all the way around. I wouldn't worry about blowing your engine up, unless there is a problem with your oil pump. If you have a non "YICS" engine, it will accelerate moderately until 4000 rpm and then it will "come up on the cam", it's sorta like turning on some Nitrous oxide. There may also be something loose which is vibrating. That's all I can come up with.

 

My exhaust as far as I know has no holes, thats why I swapped the original 4 into 2 for the 4 into 1. I'll check tomorrow and make sure though. I did have a leak at the header after I installed the new/old 4 into 1 that I thought was something in the engine. Once I tightened up the culprit it went away. I'm just weirded out by the fact it happens between 200 rpms then abruptly goes back to the good feeling/sounding acceleration. If I speed through gears its really not an issue but its still there, only when I'm p**** footing and cant accelerate and have to hang in certain rpms does it bug me. What I would relate it to is a sound of 2 stages. As if the acceleration hits a max at 4,000k then transitions to something else after that. Pretty bizarre, but if enough of you can assure me that the bike should sound like this I'll proudly scare the pedestrians as I zip around.

 

Well... That's disconcerting.

Have you ever taken the bike up to 9.5k? People not used to high revving engines often get spooked but running a motor so fast for so long; what helps people feel ok about 6k is learning what 9.5k feels like.

If something is genuinely wrong, though, that could be something bad. Maybe the camshaft moved or is loose or something? No idea.

There is a commonly described resonant frequency at 5k, +/- 200rpm's. Is there a chance that's what you are describing, and your tachometer is slightly off? While the vibration is usually minor, I know that when I had an improperly secured exhaust baffle for a while, that vibration would feel so terrible that I started doing all kinds of goofy shift techniques to avoid hitting that spot in the engine range--going to 4.5, then shifting down to rev it up, speeding up, then shifting back up again... Or shifting up when at 5.5 and I wanted to slow down. Felt terrible.

I'd check that the exhaust is all very well secured--go over all your bolts and such. Also, just guessing, but I imagine other things being shaken by that vibration would do the trick--check the engine mount points?

Lastly, as for noise, the few times I've driven without a helmet kinda freaked me out because the bike sounds so loud and menacing on the highway at high revs. It really does sound intimidating if you can fully hear it when driving.

 

Is it the famous "middle gear whine?" Is the sound more of a "chirp" at 3000 rpm under load? All XJs have a "middle gear whine" (in quotes because it's actually coming from the crank/clutch basket gear). Although it's usually noted from 3000-4000 rpm under load. If you haven't heard that before it can be a bit disconcerting, buy the "middle gear whine" is a normal engine sound.

 

My bike makes a sound similar to what you describe in the same RPM range. It's done it since it was just over a year old. I am certain it's a rattle with the exhaust. Hasn't broke in 29 years.

 

Most 4 to 1 exhaust systems have a removable baffle in the muffler. That baffle has fiberglass packing around it, over time it blows out making that rattling sound. Simple solution it to remove it and repack it. I have to repack my XS11 about every 3 years because it starts that rattling noise.