Engine vibration at 4500 RPM

My xj650G (1980) starts to vibrate at around 4500 RPM and then once past 4700 or so it stops. In 5th gear that puts it around 65 MPH which could get annoying for a trip. Is this typical for this machine? My xj750 doesn't do that.

 

If by a vibration you mean a buzz that you feel more than hear I had the same thing on my 82.

Some will tell you that making sure you have all those little rubber bits in the fins will help, but I never tried it.

 

All XJs seem to have a "harmonic range" around those RPMs, however some are much more pronounced than others.

My '81 550 (25K miles, and a rather hard life before I rescued it) does it WAY MORE than my '83 550 (7K miles by a handful of owners, no real "previous life" to speak of.) It's barely perceptable on the '83.

Both bikes are in a pretty much perfect state of tune; both have the same equipment (stock) and both have the little rubber blocks in the fins. (I thought those were to prevent the fins from "singing.") Both have the same aftermarket, unweighted handlebars.

I'm thinking motor mounts.

At least in my case; PO of the '81 rode it 18 miles of back country gravel roads every day (in season) for the four years he had it. Everything I've replaced on the '83 has been replaced on the '81; motor mounts are all I can think of that's left.

 

U-Joint in poor condition might do that.
Worth a peek.

 

every material has a resonant frequency. When an external periodic frequency matches this materials natural structural (resonant) frequency, occilation occurs. Say for example an external periodic frequency of 100 hertz (equal to 6000 rpm from an engine) and natural structural frequency of a metal (your frame or casing or a component etc.) being 100 hertz, you get occilation/vibration. This is why the tacoma narrows bridge collapsed, you can tune your guitar through harmonics from another string, and why a tuning fork sounds when its note is sounded.

 

Look and see if you have the Anti-resonant Rubber Wedges in the Cooling Fins.
They're simple. But effective.

 

I noticed a vibration ,or rattle on my 82 550. I started touching everything that I could safely reach while riding. Turned out to be a loose headlight mount I think (it was a couple years ago). Point being, Loose bolts, mounts, or something rubbing could be the cause. Get touchy feely with her and see what you find.

 

Thanks everyone. It's definitely a feel more than a noise. At first I thought I was reaching the end of the power curve and I was saying to myself, man this bike has close ratio gears, but then I punched through it and it was fine. I'll check the fin rubbers and then try to fell around while driving.

 

Funny, right after I replied to this post, I started getting that noise again, turned out to be my gas tank mount bolt. I left it finger tight and it vibrated loose. The washer was making "the noise" in every gear around 3500 rpm's.

 

I had a persistent "ringing" at certain rpms last summer; drove me crazy for a couple weeks.

Then my 9-year old shop foreman noticed the RELAY hanging from its harness and barely touching the valve cover.

The rubber mounting sleeve had broken, and the safety relay was just touching the valve cover, dancing around making all sorts of sounds at certain rpm. (Quick fix with a black zip tie.)

I still think the "felt" vibration as the age and wear and tear on the bike increases could be the motor mounts.

 

I had the 61-67 mph buzz in my first 1981 Maxim 650 that I purchased new in late 1980. Have the exact same thing in my Midnight 650.

The speed limit was 55 when I got the first Maxim and is 70 now, so I really didn't have a problem staying out of the 61-67mph range either go 'round.

 

most of my vibration went away as I slowly got her tuned right.
Last week I greased the steering bearings and that made a diff as well. Next will be the swingarm bearing and a look at the u-joints.
As for buzzzing, I removed the rubber fin absorbers once and that was enough! talk about noise. I even found some heavy duty rubber bushings and cut some more to repalce any that where missing.