!@#$ Buzzing at 4500+ rpm on Xj750rj

Alright, i know a number of people have had this issue ;p

Below 4k rpm the bike is very smooth, hardly any valve noise.
4500-5k a slight hint, then like clockwork 5100 rpm ish i get a buzzy vibration. It travels up through the fram to the tank mainly.

Things i have done:

Found one of the exhaust headers seemed loose, reset them all

I pulled the automatic chain tensioner, it was clean, oiled it up anyway. I did notice the stopper on the front was a bit warped, but suspected it would be normal for a 20 yr old rubber part that's under constant strain.

Chain guide did not show any real wear, small grooves from the chain edges but no severe chips etc.

I have the valve cover off currently

and check the valve clearances, all seem reasonable (within spec) one of the intakes was a little tight, but right at .004 inch

Did not notice any scoring or maring on the cam

there didnt seem to be ANY left/right play on the cam

Items of concern i am left with:

1.) Weak valve spring ?

2.) cam chain stretched out ? I didnt see a max length, but the auto tensioner is towards the end of it's reach (looked like it was bout 75% out ) cold i can lift it off the top guide (t shaped) 1/4 an inch with some pressure, but it's too slack feeling.

How long it too long

3.) Loose exhaust header still ? I noticed the when i removed/retorqued the outter header rings that on some of them the little metal rings between the alum outter ring and the header pipe were free to move, and on others seemed bound up.. what is the sealing operation on the headers ? Also i would expect to be able to pick this up at idle and it not be such an on or off symptom.

4.) dumb question on cam play.. best way to read ?

5.) slightly bent valve ?

anything i missed from the prevous threads ? open to suggestions I am fairly mechanical but dont want to go deep into the engine for giggles ;p

 

No check tightness of all engine mount bolts in the list!

 

Also check the mounting bolt for the exhaust collector box. It may be what made the exhaust headers loosen up.

 

How many miles are on the bike?

 

Unknown # of miles, the speedo died and the PO swapped it.

It's wierd, hardly any noise up to 4k, then s tiny amount of tapping, the right at 5k it gets noisy/vibes.

I'll check the exhaust again, pull it all the way off etc. I notice it has a nice bubbly sound on decceleration.. would that support an exhaust leak ?

 

Yessir ... and it wouldn't have to be very large. Exhaust leaks have a wide variety of sound. You really hear the bad ones; like the guy in the car with the muffler hanging off, ready to dislodge and kill a biker.

Some are not so loud. They can also be very haunting. There one moment; gone the next. Loose parts contribute to the haunting ones. Joints at the collector can go bad. They aren't mechanically loose, in the sence that a bolt isn't tight enough. They are loose-fitting. The inside diameter of the exhaust gasket loses its fight with time and the pipe isn't really grabbed tightly in that gasket any more.

Those are the ones that make the "Baloons popping" sounds during hard engine braking. The same can be said for the infamous Yamaha Exhaust Flange Donuts. The ones that come with the famous "Brake Light Guarantee."

Guaranteed to work fine until the first time your brake light comes on.

When the Donuts get fried it sounds like an engine knock. Seasoned riders get the big-bucks shivers thinking they just spun a big-end bearing.
You can take those donuts out and make them glazed donuts and they'll work right, again.

The glaze you'll need is "Permatex High-temp Spray-a-Gasket.

Set the gaskets on nails so when you spray them they won't stick to the newspaper. Two ... three coats on one side ... flip 'em and do the other side.

While the donuts are cooling!!!

Grab a tuna fish can and an artist-style paint brush you don't want any more. Shoot some of that fine "Permatex High-Temp Spray-a-Gasket" into the can ... so you have enough to paint a couple of coats on the cylinder head ports where the gasket seats.

Now, without getting any of the really sticky spray-a-gasket on your fingers ... place all four of your glazed donuts back in the head.

(Note: It is impossible not to get this stuff on your fingers. If you are thinking about wearing latex shop gloves to prevent the sticky-fingers deal ... have your significant other videotape you trying. She can send-in the funny video and win the ten thousand bucks.)

Take 'em out. Spray 'em good. Paint the head seats. Stick-up your fingers. Ruin good clothes. Look silly at dinner with that stuff all over your fingers.

But, quiet the exhaust.

Being a biker's fun, huh?

 

Thanks Rick !!

I am 99% sure my exhaust isnt stock, it's a 4 into 2, but doesnt have a crossover.

The "donuts" your talking about are the steel rings behind the exhaust flanges that bolt up to the head right ?

*i'm having to compare my exhaust to the Fische *

Thanks again

 

Did you check for any cam chain abrasion on the inside of the valve cover? I had a similar buzz at 3000-4000 rpm and it was due to the front guide being broken about 1 inch below the valve cover surface. When I looked at the inside of the valve cover, there was a small burnished area just in front of the front cam chain sprocket.