1979 GS750E - Resonant Vibration under load

[gs12tim]

Hi All

First off, as soon as I figure out how to post a pic I'll do a proper introduction of me and my GS. A little background. I just picked up a 1979 GS750E with less than 6000 miles on her. She runs great and I've double checked the timing, carb sync, and she's got good plugs and fresh fluids.

I've owned bikes more or less continuously since my first bike - coincidentally a 1979 GS750E that I bought in 1979 - so I'm pretty familiar with bikes & maintenance etc. Also rebuilt a 1978 GS550..

Anyway, here's the problem symptoms:

1) Resonant vibration (noise) from the engine or front end while accelerating or maintaining 4500 rpm under load (ie driving)

2) Same or similar noise (but louder) at any RPM and over approx 60MPH

3) Unable to replicate noise at 4500 whether in neutral or in first gear on centerstand.

Beats the heck out of me. The 4500 RPM noise is bad enough, but when I'm in any gear over third and at 4500 or greater (which is 60mph) then the noise is scary loud.

Many thanks in advance for any suggestions on a tricky problem.


Tim

[tatu]

Hi gs12tim, Welcome.
Might you have a tight spot in your chain?

[gearheadE30]

Maybe a shot in the dark, but when you hear the noise, push down on the spring loaded door covering the gas cap. Mine vibrates and makes all kinds of noise at about that rev range. Some weather stripping took care of that.

[gs12tim]

Hi

Thanks for the replies so far.

I checked the chain and it doesn't appear to have any tight spots.. in fact it's a bit loose so I'll have to adjust that.

I thought the same about the gas cap cover. As I was driving in 2nd gear at 4500 I was touching everything from the gas cover to the instrument cluster to see if there was something loose.

I'm beginning to wonder if there are two issues. Something resonating at 4500, and something different that happens over 60 mph.

If I stay in first or second gear it will accelerate smoothly and without noise (except at 4500) all the way to redline.

But once I'm at 60 mph it starts making a hell of a racket at any RPM

[UncleMike]

Check your headlight. I get bad vibrations from there. Sometimes the ring and sometimes connections vibrating against the shell. I've got some insulation in there to prevent metal on metal.

[koolaid_kid]

Anything on the bike will have a resonant frequency, you just happen to have one that finds its sweet spot at 4500 rpm.
Since it only happens under a load, I would look for something connected to the engine because the engine torques ever so slightly under a load. I would start with all the engine mounting bolts. Then I would closely examine the exhaust system. On that, I would look for either loose bolt(s) or one of the shields hitting something.
Best of luck.

[gs12tim]

UncleMike

Not sure if it's the cause, but the plastic headlight bucket is totally cracked where the headlight mounts to the bucket. I've removed it for now, and if it wasn't raining I'd do a road test. In the meantime I'm superglueing it back together. Good guess.

Koolaid-Kid

will check engine and exhaust bolts - thanks

[BassCliff]

Greetings and Salutations!!

Hi Mr. gs12tim,

I had a strange vibration noise on my bike once. I must've looked silly riding down the road touching everything I could on my bike, trying to find the source. It turned out to be the vanity cover on the front of the forks, under the headlight, covering the brake line splitter. At first I squirted some silicone caulking stuff around underneath. That helped.

I finally got around to replacing the rubber parts when rebuilding the bike after my wreck. There's no more noise now. New rubber on the headlight ears should help.



These are the bent ones I replaced after my wreck a couple of years ago.

Anyway, let me dump a TON of information on you and share some GS lovin'.

I just stopped by to welcome you to the forum in my own, special way.

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Here is your very own magical, mystical, mythical, mind-expanding "mega-welcome". Please take notice of the "Top 10 Common Issues", "Top 15 Tips For GS Happiness", the Carb Cleanup Series, and the Stator Papers. All of these tasks must be addressed in order to have a safe, reliable machine. This is what NOT to do: Top 10 Newbie Mistakes. Now let me roll out the welcome mat for you...



Please click here for your mega-welcome, chock full of tips, suggestions, links to vendors, and other information. Then feel free to visit my little BikeCliff website where I've been collecting the wisdom of this generous community. Don't forget, we like pictures! Not you, your bike!

Thanks for joining us. Keep us informed. I will put you on my prayer list.

Thank you for your indulgence,

BassCliff

[gs12tim]

Thanks for the welcome BassCliff - enjoyed reading all your tech tips.

ok. update

1) I think there are two problems. The resonance at 4500 AND under 40mph is most likely the gas cap cover. Need to look at some weatherstripping 2pts for GearheadE30 !

2) At 60 MPH (which is 4500 rpm in 5th gear) there's loud resonance. At 65 MPH (approx 5000 rpm) it turns into a loud squeal - so loud that I back off the throttle.

Could it be cables? The speedo drive? Cam tensioner?

[DanTheMan]

Check your coil bolts. I had my aftermarket coils on the 1100 loosen up (only held in by one bolt, they're longer than OEMs), and I had a horrible resonance coming from the right side of my gas tank. It sounded like it was ready to fall apart. I re-bolted them with better bolts and it's been fine since.

[koolaid_kid]

It wouldn't hurt to lube the cables either, they need it periodically anyway.

[gs12tim]

tightened up the coil bolts and took it for a test drive - still getting the resonance and the squeal. thanks for the suggestion !

[gs12tim]

lubed tach and speedo cable - still getting resonance and squeal...


Tim

[DanTheMan]

OK, I struck out on the resonance, did you check the tank mounting isolators?
I have had, in the recent past, a squeal from the speedometer drive gear on the front wheel. It almost sounded like a brake squeal. I took it off, it was rusty and slightly warped, causing the driven part to rub on the inner rubber seal. I cleaned and lubed it, straightened out the driver, and now the squeal is gone. Worth a try.

[gs12tim]

Hey DantheMan

Yup - I checked the isolators and they looked fine.

I'll go check the speedo hub. Both the speedo and tach needles move very smoothly, but it's still worth the check.

Tim