Topic: Vibrations

[Scout]

Hopefully my questions are getting less stupid as I learn my way around the bike, and I have to say, it's been a lot of fun. Last year most I've ever done mechanically was changing the sparkplugs... Now I can tak apart/put back together a carb

Anyway, yesterday was a big day for my K8. I took the carb off, replaced the float gaskets and needles, adjusted the float level, cleaned them a bit again. Put them back on, then checked the valves (no adjustment needed), synced the carbs (with Carbtune II) and again, didn't need any adjustment. Also cleaned the points, and replaced the plugs. Anyway, the bike is almost perfect...

When going over 60mph ( I think that's around 4 - 4.5k RPMs) the vibrations are a bit pronouced, especially through the footpegs. Since I've adjusted/checked pretty much everything I can think of, is there anything else I can do?

Scout

[scott_cb650]

Choose one of the following (IMHO):

- Speed up
- Slow down
- get thicker gloves and socks
- find a female passenger that like it
- get softer grips and pegs.

I've found that there are different rpm ranges at which my bike runs smoother than others.

[Bob Wessner]

Not sure it would change much else, but have you adjusted the cam chain tensioner? Vibrations are something of a subjective thing and some folks are more sensitive to them than others. A certain level of "buzzbies," as I call them, are to be expected.

[Scout]

Ok, I guess I should make myself clearer

I know that pretty much every bike has certain amount of vibrations. However, I'm taking about vibrations that shake the mirrors to the point of not being able to see much out of them and certain RPMs, not some little annoying tingling... It's ridable, but a bit obtrusive...

As far as the cam chain tension, I adjusted it following the procedure in the Honda service manual - start the engine, warm it up, let it idle, unscrew the adjustment nut, and screw it back in. I don't really know if it did anything though... Can I remove the whole cam chain tensioner assembly without any fuss to clean it? Maybe I'll do that...

Scout

[wheelie]

I'm having similar vibrations @ 4800-5100 rpms.
 I'm pretty new to this engine so I don't know if this is 'characteristic' of the big four but I would like to hear more on the matter.

It's interesting to me that your and my K8's are the same year and model?!

[Lumbee]

...the sweet spot for my 78F is right around 4200 RPMs, which puts me at around 68mph (smaller sproketin the rear).  You really get a feel for finding that sweet spot on longer trips.  You could go with different sprokets to lower the RPM's at higher speeds, also maybe try retorqueing the engine bolts.  Other than that I think its just the nature of the inline 4...

[wheelie]

I'm wondering if the vibration comes in all gears or just final drive? It's tune-up day for me anyway so I will check the cam chain, head bolts, and look for anything obvious. Also, I will try to 'float' in each gear at the rpm trouble spot and see what happens. Will report to the thread later...

[78 k550]

Have you checked your chain it might have one frozen link. Something to check. Swing arm nuts torqued front and rear.

Paul

[wheelie]

Well, im addition to the normal tune-up, I did a chassis once-over checking the chain, swing arm and engine mounts (I was hopeful to find a loose mount.)  Nothing unusual.

My question to the Honda sages here on SOHC4 is:  What the h**l is happening? Since (so far) nobody seems all too concerned about the issue, I can only guess that it hasn't really been traced as the cause of greater problems. "oh yeah, mine does that too" seems to be the theme.

However, from an engineering and materials standpoint (I'm an expert in neither), acceptance of excessive vibration doesn't sit well with me. Something is happening here. My intuition tells me that if it is simply some quality of resonance caused by reciporicating masses ( the cranks, pistons, cam, rockers, etc) that it's less of a concern (unless, of course, you're the Honda engineer designing the beast.) My real worry is that it's an indicator of head, crank or trans bearing wear combined with excessive endplay and that a little harmonic resonance is setting up (think spinning pie plate) which feels like an excellent way to damage the engine.

In final drive, my 78k cruises between 67 and 72 within the offending RPM range. I'm sure you can see that I'd really prefer to not be working around this speed range for the life of the cycle, especially if I can't be sure that I'm not harming the engine.

I'm afraid only a techincal explanation will satisfy. Please help!

Thanks much..

[volthause]

Everything has a resonant frequency, including your frame. The engine is putting out a freqency, and it could be that the harmonic content of the engine at 68-72 mph begins to get the frame involved in a sympathetic resonance.

[Bob Wessner]

Could even vary depending on whether you are riding single, double, how much you weigh and where you sit relative to the center of gravity.. who knows.

[TwoTired]

You are aware that piston speed is not constant.  It is fastest at midstroke with the crank at 90 degrees of rotation.  The pistons are at their slowest at 0 and 180 of crankshaft rotation.  The change in piston speed sets up harmonic vibrations at certain RPMs when materials masses and pulses reach synchronicity.

Engines build up carbon deposits on their piston tops that changes their mass and can magnify the vibration produced by the changing piston speeds.

Lastly, if your bike is uncharacteristic in its vibration, any rotating mass could be out of balance;  IE, Alternator rotor, clutch,  etc.  You can also get drive chain related vibration, particularly if it is dry or not well matched with the wear of the sprockets.

Is the mirror vibration what you're are trying to solve?  Bar end weights and/or convex mirrors can help that.

Cheers,

[byidesign]

my   CB650SC   never liked 4400 to 4900 rpm anything over, or
      under most pronounced in high gear.
      went up to a 17T sprocket, to change where it cruised at
        never bothered the engine performance,
         just annoying.
               hope it helps
                        Bruce

[BobbyR]

Some Tires can cause vibration. From the post it almost seems speed dependant. Quick check could be bring her up to the vibration speed. Pull in the clutch and get off the gas. If it vibrates while coasting, it is not the engine.