Vibration between 3500-4000RPM

[Johnny K]

I have an 82' GS850. The rear tire was put on new and balanced in June. When placed on the centerstand the tire has no play back and forth. The bike is smooth from the start to about 3500-4000, where the center of my seat begins to vibrate and then after 4000 it is smooth as anything and no problem. Just about 3500-4000. What to look for that a wouldn't be to hard for a newb to do??? Thanks guys.

[Casino Joe]

Does it vibrate @ 3500-4000 rpm in any gear or only in one gear?


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[chef1366]

Have you synced the carbs?




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[Johnny K]

Yeah, it vibrates in all the gears and yup, my mechanic synced them a month ago. It's a little vibration that starts around 3500, and then gets stronger towards 4000. It's not a really big vibration, but noticeable if you pay attention to it. Then right after 4000 whether going smooth through the gears or hammerring on it, I can still notice it. It just feels like the rear wheel is a little whobbly at that RPM.

[Johnny K]

The vibration stops right after 4000.

[Casino Joe]

Sorry it took so long to get back to you Johnny K.

If it does it in all gears, then it is definitely not in your powertrain or rear wheel.
These parts would be moving a lot faster in fourth gear as opposed to say... second.

The vibration has to be coming from the engine as it always appears in a certain RPM range regardless of speed or gear selection.
My best guess without more info would be that either your mechanic didn't do that great a job syncing the carbs, or that you may have a small leak in the intake boot o-rings causing an imbalance at that range.

[Smokinapankake]

Try torqueing all the engine mounting bolts. I did this on my Katana and it made a world of difference.

[Johnny K]

Thats ok. Where are the intake boot o-rings exactly located. And how would I be able to know if there is a leak as I am a newb. Smokinapankake, I heard that you can overtorque the bolts and that could be a bad thing. I do not have a torque wrench, just wrenches and sockets. I will try seeing if all the bolts on the engine and everything else at a good snug. Thanks again.

[Steve]

The intake boot o-rings are located in the surface that joins the intake boots to the cylinder head.

If you have not changed them, chances are that they are still original. If they are original, they probably need to be changed. It was never mentioned in the original maintenance schedule, but they should be changed every 20-25 years.

You can get them from Robert Barr, a member of this board, very inexpensively at Cycle O-Rings. In fact, do yourself a favor and get his complete o-ring kit for your carbs, too, and change them out. You have to remove the carbs to get to the intake tubes and remove them to change those o-rings, so may as well strip the carbs down and change all of their o-rings, too.


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[Johnny K]

Thanks Steve. I will have to look at and inspect them when I get a chance. Another quest. Where do I look to get some seafoam???? I have been using Lucas upper cylinder lubricant every 3 or 4 tankfulls. In a 155ml bottle, I would use a 1/4 each 3-4 tankfull. How does this Lucas stuff compare to the seafoam that you guys highly speak about??

[Casino Joe]

Being as you're in Boston, I don't know if you have AutoZone in your area, thats where I buy SeaFoam.
It is great for removing carbon buildup in your engine.
You can run some in your gasoline to clean carburetor passages, valves and piston tops.
You can direct inject it into a hot engine through your vacuum ports to really get those valves and cylinders good.
You can also run it in oil for a short period of time before an oil change to help clean the sludge from your case.
(Actually I have never done that on my bike, not sure if it would work well with the wet clutch. But it works great in my car.)
Call around to some of the local parts stores in the yellow pages, I am sure someone will have it.

[feelergaugephil]

Try also, General motors (GM) spray Top engine, that works 100%, you'll have to go to your nearest chevy dealer to pick it up, in spray form it's about $10.

[BassCliff]

850's rule!

Mr. Johnny K,

These 850's run much better with tight intake systems. To add to the list of clean carbs, O-rings, carb boots and synchronization, have you sealed up your airbox? You probably need to replace the weatherstripping around both side covers and the top of the air filter cage. There should be no air leaks anywhere in the intake system. Make sure your air filter is clean and very lightly oiled. The K&N aerosol oil is recommended.

Have you checked your valve clearances lately? After performing this adjustment my bike it ran a lot smoother. There a forum thread here or a PDF download here.

Do you have a manual? If not, PM me.

Thank you for your indulgence,

BassCliff

[Johnny K]

I just took the intake boots off and inspected them. The back where it connects to the engine was a little rusty/paint chipping. So I am going to purchase new intake boots. I happened to buy a set of stainless steel allen bolts for my carb and engine. I have a good weather stripping on the top of my K&N filter and lightly sprayed it. I will check again as I just took the whole airbox off to get to the carbs. I do have a Clymer manual. Checking the valve clearance seems a bit overwhelming, but as I have the carbs and everything off at this point, I guess it wouldn't hurt to check.