RPM Rough Spots & Vibrations

[buickyouth]

1979 XS1100SF

I've done a lot of work to my bike recently and am starting to look at fine tuning the things that seem like they could be tweaked a bit. One of the main things I've noticed is the sort of hiccups that I'm getting (spikes and drops in the RPMs) and changes in engine vibrations.

As far as the hiccups (which sound a lot like popping corn) go, I bought the bike with individual air filters on the carbs. I hear the pops and spikes in the tach a lot more when the bike is low on fuel (1.5g or less) and mostly during deceleration. But, it still happens when I'm maintaining a set speed but doesn't happen as frequently.

Good, Okay, or Bad Vibrations:
I know that the power band for the bike starts at around 3-3.5k RPMs. I usually ride at the lower end of that. Still on the highway it's nice to go a little faster than 60-65 sometimes. There is a noticable change in the engine vibration right around 4.5k and then it seems to smooth out a little if I push it past it. I'm not sure if this is something that is an indication of a larger problem or if it's par for the course.

What RPM does everyone else ride around.
Highway?
Around town?

[Dragonrider]

There is a noticable change in the engine vibration right around 4.5k and then it seems to smooth out a little if I push it past it.

That's pretty normal for an in-line 4...

[randy]

buickyouth,

Yes, it smooths out just as you pass through mach 1. Oh, wait, that's the X-1.

The XS like to run betwen 4k and 6k, actually works more efficient in that range. I just did my own 1000 mi test.

One of the guys posted about MPG vs. RPM a while back and I thought I'd test his theory. During the july 4 rally, I avoided 5th gear entirely and my fuel mileage went UP by 5MPG (almost 10%!). Go figure.

[TopCatGr58]

Hey Buick,

Like Randy said, the engine likes to run in the mid to upper rpms and it's more efficient there! Get it out of the basement, and turn the throttle more, and stay in the lower gears more, don't be afraid to use the throttle, and use the leverage that the lower gears provide for better response.

I was riding 2 up in that NY rally, and only shifted into 5th when we were on the main 65mph highway. Thru the twisties and hills, used mostly 2nd and 3rd, in the 35-50mph range, only used 4th above 50mph. These are powerful engines, and will vibrate in varying amounts depending on load and rpm range. Forget the trying to get it running "glass on hood" smooth, and run the thing!!!
T.C.

[Brasco18]

I have a noticeable vibration at 4.5 -4.8k as well. It is very apparent to me while riding at highway speeds. It seems to happen right around 70 mph in fifth gear. I don't notice any change in performance, nor anything in the handling. It is just a noise and I can feel it a bit in vibration. It didn't used to be so apparent. I recently put on several hundred miles at 70-80mph. If this vibration is 'common' in inline fours, why would it get more apparent, not just remain consistent?

[boyat68]

Brasco18:

One thing that happens is the tightening or loosening of various things after doing maintenance on the bike. This action changes the vibration dynamics of the components and cause sympathetic vibrations as each item has a natural frequency. For instance, I repaired my fairing, (cracks in the mounting) and used new bolts and rubber washers, I had to loosen the mounting brackets and shift them to make the fairing line up correctly. I have a loud buzz in the fairing in third gear and low rpms that I have to fix by loosening and tightening the mounting bolts. Perhaps changing the mounting rubber.

[Brasco18]

I know it doesn't matter what gear I am in. When I approach 5k, the metal-sounding buzz starts. Because it goes away over 5k, it has to be a resonating problem. Something on my bike must have a frequency that matches the engine at around 4800 rpms and begins resonating. Maybe I need to go through and tighten the bolts/screws and such. I do know that one of the head cover bolts is loose as there is a minute leak there.

[DELL82]

I have a buzz also in my XJ1100J. Found it was the motor guard hitting one of the header pipes. Using a rubber mallet I hit the header pipe toward the engine mount allowing a dent to develop in it. Buzz stopped.
Might try looking very close at the pipes and engine parts that are contacting things on the frame.

[petejw]

the 4-4500 rpm range on these bikes
do have a vibration, i noticed when i was
mucking around with the exhaust baffles,
the less restriction i had, the more the vibration
became worse, cam chain adj can also help
minimise vibration.

[ClarkGriswald]

Ive noticed a few of my covers, the timing one in particular rings like a bell. Certainly something like that could sympathize with the right frequencies. Also under that timing cover, that whole advance mech that spins, has a pretty good sound to it on my bike. I think I hear more noise from that area but I wonder if its not just my primary chain flopping and the sound is more intense there in that spot. Yamaha makes musical instruments ya know.. mabye its a timbali!

[bikerphil]

timing cover

Hey Clark, that's an interesting point you have about that "ringing" noise. I was looking thru my extra parts and I found a timing cover with a rubber molded sound damper inside. I was wondering if yours had this or not?

[ClarkGriswald]

Nope its bare inside, and I was already playing the detective show triangle music on it when I discovered it.. (I pretty much do that to anything that rings including the rear brake rotor of an xs1100 rear wheel that was at a cycle shop I was at recently, the guy was changing the rear tire.. and dropped a wrench on it.. so I had to show him ahhaha).

[bikerphil]

cover

Check this one out. I'm going to try it tomorrow and see if it quiets things down a bit. My engine does the same thing.

[ClarkGriswald]

Wow how about that.. interesting. Not sure what to think about that.. mabye yamaha figured it out after a while too hehe.. Its just like when you find that "right note" in the bathroom that makes itself 10 times louder than any other. The resonant frequency of that room/space. My chain is pretty loose, I can tell now that Ive been thru most of the drivetrain and know that each of the componenets like the final drive and middledrive box have very little play and almost zero in the shaft (splines). You can move my rear tire back and forth quite a bit in 1st gear on the centerstand.. its all between the middledrive box and the crank.. mostly chain though.

[bikerphil]

cover

Yeah, the cover in the pic is the original one from my 79F, but it's scratched to $h!t. If this thing quiets it down, I may be tempted to remove the rubber and transplant it into my good bare cover. Mabye it would be possible to pour some of that Plastisol coating in there, into the indentation and make a new one. Looks like I'm getting my hopes too high on this, lol.