Noisy XJ - buzzing/grinding at 1500 rpm, nuetral or in gear.

Hey guys, I'm back.

You know, you guys are cool, but I wish I could stop having to come here all the time to sort out issues with my bike.

I have an '82 XJ750J which I've brought to this forum before to ask for help solving odd problems.

The newest problem is something that only appeared after my recent escapade with the cam tensioner.
Click here for that thread.
Brace yourself for that one. It's an adventure (or it was for me anyway).

At a certain rpm (about 1500-1800) the engine makes a buzzing/grinding noise. I just uploaded the helmet cam video I captured as I was setting off to go to a meeting. The buzzing was so bad when I stopped and started again at stop signs that I turned around and went back home after only a few blocks. I wasn't about to put this bike on the highway just to have it blow up or something.

Click here for the video.

I've noticed the sound before, but it only seemed to happen when pulling into a parking lot after a long ride. My guess is it only happens to the engine when it's warm, which it was when I captured that video.

I poked around the collector covers and some other metal bits to see if anything was loose. The right side of the collector is already missing the front two inches or so of its cover. I thought maybe another part is shaking loose, but everything feels quite solid.

If this problem turns out to be just something loose and rattling, then this thread will just be a reference (with the attached video) of what that particular piece sounds like when its loose.

Until I can be sure of that, any ideas as to what might cause this sound from inside the engine? I think it sounds pretty clear in the video so hopefully the more audio oriented of you will be able to point out what might be wrong.

 

1. You shouldn't need to touch the throttle at all when starting (even from cold). Revisit the carbs.

2. The buzz sounds to me like it's coming from the exhaust. Something's loose. Could be one of the chrome shields on the collector. Usw he heel f your hand on the cold exhaust and start slapping unitl you hear the noise and can pinpoint it. Also double check that the collector hanger bolt is tight, and that the rubber isolator is still there.

 

So something that I left out of the video was that it actually took way longer to start than the video shows. At one point I moved the petcock to prime and gas started pouring through the fuel filter for quite a while before stopping, almost as though the carbs were empty.

I've noticed this before and it only does it when being started hot. It doesn't run as though it's starved for fuel when I finish a ride, and the oil doesn't seem to have gas in it, so I have no idea where all that gas in the carbs is going. My best guess is it may be evaporating from the hot carbs, but that seems like a stretch.

I had the exhaust off for my aforementioned escapade and I tightened everything down to be pretty snug (being careful with the old bolts and studs of course). The headers feel rock solid and don't move, though I'll give them some whacks later to see if I can pinpoint the sound.

Unfortunately I'm still working on getting a job and even then I probably wont start working until the summer. Hopefully then I can buy a couple of the items I need to really tune up the carbs (or just the parts to build a good manometer and YICS tool and whatnot).

Somewhat unrelated, but what really bothers me is the starter clutch slipping. I have a freshly rebuilt starter, new solenoid and battery, 15w40 Shell Rotella oil (not the synthetic kind), and the starter still slips when the engine is hot, but only when hot: it grabs and holds on tight when cold. I really can't afford to split the case and start replacing major components of the engine. That's what worried me about this sound. If a bearing or something goes then the bike is essentially totaled for me and it'll have to become someone else's project.

The only reason I have to use throttle normally is because the starter will slip after just a moment so I need to make that first ignition count. In the video I think I had to saw at the throttle because I was essentially still priming the carbs, which goes right back to the issue of why did the carbs empty out after sitting hot for 30 minutes?

 

Man, sorry about the melodramatic essays. I'll make this one brief.

Tapped headers/collector v. lightly with rubber mallet (hand wasn't hard enough)
Loud hissing sound from collector
Chrome collector covers solid
Bolts tight
Isolator in place
Put pressure on different parts and tapped - still made sound
Can't pinpoint sound - all over collector
What do?

So, what do you think of that format? I think it's more succinct.

It almost sounds like there's something loose inside the collector. Maybe a baffle has broken free? I guess I might have to live with it for now if that's the case. At least then it's not something bad.

 

Illinois ...how close are you to Chicago area?

 

Chicago is a two hour drive away. I live near Bloomington, Normal.