99 RT vibration

Hey guys. Got a puzzler for you. My 99 RT, 75k miles, has developed a pretty strong vibration at higher RPM's. So far I've rebuilt the transmission and replaced the clutch pack. While I was in there I checked the fly wheel for any movement, its solid. Throttle bodies are balanced and the valves are set. I also checked the alternator belt, that's ok too. When I'm doing 75 mph, pull in the clutch and drop the RPM's the vibration stops. Increasing the RPM's again with the clutch still in the vibration starts again. Any insights will be greatly appreciated. Thanks, Gregory

One direction you may not immediately think of is an intake vacuum leak. Check all the rubber gaskets and O-rings in your intake system; you can also spray starting fluid around all the spots where rubber seals things together, while the bike is running, to see if the idle goes up. Also if you still have your evaporative emissions system you should check the hoses going to the underside of the throttle bodies. They crack over time.
I just went through a month-long ordeal of chasing vibration problems and eventually figured out it was a vacuum leak at one of the big brass screws.

Thanks for the reply. I tossed the evap. canister a long time ago so that's not an issue. If there is a leak in the intake somewhere I dont think I would be able to sync up the throttle bodies but I'll double check them anyway. About the only thing left to check is the drive shaft and u-joints but any and all suggestions are greatly appreciated. Thanks again, Gregory

gregory said:

If there is a leak in the intake somewhere I dont think I would be able to sync up the throttle bodies but I'll double check them anyway. About the only thing left to check is the drive shaft and u-joints but any and all suggestions are greatly appreciated.

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I was able to get the vacuum "synchronized" between my throttle bodies almost perfectly even with a vacuum leak -- having a small air leak doesn't prevent you from adjusting the two of them to match vacuum on a gauge at all. What it does do is make your "synchronized" vacuum gauge reading inaccurate. When you adjust them to match visibly on the gauge with a leak on one side, what you're really doing is adjusting them to be out of sync, but the vacuum gauge can't tell you that because the airflow it senses at both vacuum ports is still equal. You are right that a large enough vacuum leak might prevent synchronization entirely, but a small one can very much still be within the range of adjustment, and very much cause a significant vibration problem.

That's why you should carefully check every rubber joint between the airbox and the intake ports, including both the large O-rings around the back of the throttle bodies and the small O-rings around the big brass idle screws.
From what I've read, the most frequent troublemakers are the rubber insulators that hold the throttle bodies onto the engine. Heat cycling and UV exposure will eventually cause cracks, and even a very small crack underneath the clamp can cause a sync issue. The only way to properly inspect them is to pull each throttle body back out of them. On my bike they were both cracked when I purchased it.

I believe you've already ruled out the possibility of the driveshaft and its U-joints, as well as tire or brake issues, because you said the vibration goes away when you pull the clutch lever at speed.

My '97 does the strangest thing and I never could find an explanation.

If the oil is at the top my sight-glass, I get a fairly high frequency vibration at highway speeds.

If the sight-glass is in the middle, no issues!

dieselyoda said:

My '97 does the strangest thing and I never could find an explanation.

If the oil is at the top my sight-glass, I get a fairly high frequency vibration at highway speeds.

If the sight-glass is in the middle, no issues!

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Ditto on this. Higher than the midpoint on the oil sight glass resulted in more vibration from my 850, too. So that may be another contributor.