Bad Vibrations

[raistian77]

Ok gurus, have a problem that is driving me bananas.
Have a 1979 GS850G that begins to
noticable vibrate at 3,500RPM then
annoyingly vibrate at 4,000-4,5000RPM then
painfuly vibrate at 4,500-6,000RPM.

I have
Set valve clearances
Rebuilt carbs (full strip and dip)
Replaced all rubber cusions (gas tank,footpegs,ect)
Placed foam vibration dampening rubber (we use it at work for generator pads) between downtubes and fairing bracket and between fairing bracket and fairing.
Set timing (have a Dyna-S comming Monday I hate setting points, never get it just right and with me putting 80-100miles a day on the bike have to adjust them to much)
Checked motor mount bolts
Synced carbs
Set mixture high idle using point pickup tachometer and colortune
new wires,plugs and coils
Torque checked all hardware

Even though this is a engine caused vibration (it does it in netural) I also balanced the tires and tore the swingarm down to inspect the driveshaft for looseness or damage.

Is there a procedure for ensuring the motor mounts are set properly? I know some bikes you loosen the mount bolts and run the motor as a set rpm to seat the motor.

 

[JEEPRUSTY]

Placed foam vibration dampening rubber (we use it at work for generator pads) between downtubes and fairing bracket and between fairing bracket and fairing


maybe the motor was designed for solid mounts
I would try it without any deviation from oem style mounting.

 

[raistian77]

Placed foam vibration dampening rubber (we use it at work for generator pads) between downtubes and fairing bracket and between fairing bracket and fairing


maybe the motor was designed for solid mounts
I would try it without any deviation from oem style mounting.

Aftermarket fairing, foam rubber is recommended by Pacifico to ensure the fairing does not "tuning fork " the normal engine vibration.

 

[tom203]

Does it do this with clutch pulled in? How about in neutral if you rev it?

 

[raistian77]

Also the PO placed what looks like liquid 'bar snake' in the handlebar. That should dampen any vibrations in the bar.

 

[raistian77]

Does it do this with clutch pulled in? How about in neutral if you rev it?

Will do it in neutral with clutch in or out.

 

[JEEPRUSTY]

Aftermarket fairing, foam rubber is recommended by Pacifico to ensure the fairing does not "tuning fork " the normal engine vibration.

ahh I see well then I saw a thread a while ago about some sides having longer bolts then others maybe this is your problem and things arent tight

I mean the bolts are tight but mounting plates not fully seated.

I think the thread dealt with this breaking cases though so it likely does not apply.

 

[raistian77]

Also Steve told me the 850s were not prone to twisting the crank as they do not have the raw power to do it.

 

[bobthebiker88]

are you POSITIVE its firing on all 4 cylinders? thats kind of my impression is that you've got a motor that isnt firing on all 4.

 

[raistian77]

thats what i keep comming back to. But yeah, all 4 do fire plug chops show good mixture.

 

[tkent02]

Also Steve told me the 850s were not prone to twisting the crank as they do not have the raw power to do it.

This is true, not likely but it may be possible, if a PO was abusive enough. Have you checked that #1 and #4 pistons come up simultaneously?

 

[raistian77]

This is true, not likely but it may be possible, if a PO was abusive enough. Have you checked that #1 and #4 pistons come up simultaneously?

What I did is remove 1 and 4 plugs bring 1 to TDC measure with depth micometer then compare to 4. diffrence is almost nill.

 

[Nessism]

One other thing to try is to loosen all the engine mount bolts after supporting the engine with a jack of some sort. You want the engine to be centered within the mounting hardware clearance, not binding on any particular mount. After you have the engine centered then carefully snug up all the bolts a little at a time so the position doesn't get disturbed. Read this trick from Joe Minton, magazine wrench guru from back when our GS bikes were current. I don't know how much it will help but its worth a try.

Good luck.

 

[raistian77]

One other thing to try is to loosen all the engine mount bolts after supporting the engine with a jack of some sort. You want the engine to be centered within the mounting hardware clearance, not binding on any particular mount. After you have the engine centered then carefully snug up all the bolts a little at a time so the position doesn't get disturbed. Read this trick from Joe Minton, magazine wrench guru from back when our GS bikes were current. I don't know how much it will help but its worth a try.

Good luck.

Now that is what I am looking for. I will try it, I was not sure off the sequence for centering the motor.

 

[SqDancerLynn1]

What did you use to sync the carbs ? I did a mechanical sync on my 850, always ran good. Did a sync with motion pro set up. Made the engine noticeably smother..

 

[gssuzukixxx]

My '79 GS850 had rattle/vibration until I discovered one of the horns under the tank was slightly touching the tank. When I moved the horn away from the tank the rattling stopped.

Brian

 

[raistian77]

What did you use to sync the carbs ? I did a mechanical sync on my 850, always ran good. Did a sync with motion pro set up. Made the engine noticeably smother..

same tool, Steve double checked with merc sticks

 

[bwringer]

Hmm... this is definitely odd. This should be one of the smoother 4-cylinder engines.

Have you hooked up a timing light to verify that 1/4 and 2/3 are firing at the correct times?

If you have, for example, 1/4 timed right but 2/3 is off, you can get some strange vibrations. I'd also hook up the timing light's pickup to each plug wire and verify that it's firing as expected and the wires are hooked up correctly.

 

[tkent02]

What I did is remove 1 and 4 plugs bring 1 to TDC measure with depth micometer then compare to 4. diffrence is almost nill.

Measure with the pistons half way down, a few degrees of twist will give a measurable difference in piston height then. When they are all the way up, even a large twist won't displace the pistons much. The difference should be absolutely zero.

 

[BadBillyB]

Yeah, I was thinking the same thing. The piston dwells at TDC for a couple degrees. Does, I hate to say, sound like a twisted crank. If you knew someone with a really nice degree wheel and a piston stop, you might see it on the degree wheel...Comparing cylinder 1 to cylinder 4...absolute TDC...Short of that, the method used by crank builders is to run a precision rod through all four connecting rods at the same time....Billy