"thumping" in 4th gear?

I have a constant, rhythmic, mild thump throughout 4th gear that I can feel through the foot pegs. This has been going on for the last few rides. Never had this happen before.
Can anyone shed some light on what is causing it?

 

I'd start by putting the bike on the center stand and peeling back the u-joint boot and look there for looseness.

Also check your tires for a flat spot.

First inspect visually for any debris, then lay your hand on the tire and rotate it under your hand. Should be able to feel it if it's there.

Then go about making sure all the nuts, bolts and bearings are tight.

 

Because you have isolated it to one gear only I would say you have either a damaged gear or debris stuck in the gear itself. If you haven't opened the engine up lately like added oil or anything you might be looking at a disintegrating chain guide. To check for this drain your oil through a screen or sifter and check for small bits of plastic. Good luck

 

Hm! a mild thump you said - what is the frequency? 2 times a sec.?
If you are running in 4th gear i will expect a high frequency sound if it's something in the engine or gearbox.

 

Roughly 2x a second is about right.
The frequency reminds me of driving with a nail in a car tire.
Im going to ride it home in about an hour. I will pay extra close attention to its behavior.

The thump is independent from engaged/disengaged clutch.
Higher RPM only modestly increases freq. of thump, and "impact" of thump. dropping to 3rd or up shifting to 5th and the thumps go away.

 

Ok. Riding on it more and paying closer attention has this becoming apparent at nearly exactly 4,500 rpms. Its not associated with any gear specifically, but the rpms. Any use at 4,500 has it rearing its head.
I kept it around 4,000 and it was avoided. At about 5k, nothing.
I feel it threw the footpeg (mostly the left).

 

Im kinda thinking tci box just because thats about the rpm that the advance kicks in. But man it could be a number of things plug cap coil bad connection low battery. Does it do it when there is no load as in when on the center stand in neutral at around 4500 rpm with the engine at operating temperature

 

Run your hand over the Rear Wheel and see if the rear Wheel is "Cupped"

A Cupped rear Tire will make the whole Bike vibrate when it hits the right frequency in a "Cruising Gear".

Next:

Place the Bike in 1st and have an assistant "Rock" the Rear Wheel while you very carefully LISTEN to the Boot at the rear of the Engine for noises coming from the U-Joint.

 

I'd think the cupped tire senario would only be noticable when leaning in a turn - and it would be in any gear. I'm be more inclined to think it's a u joint vibration - seeing as how it's somewhat rhythmic.

 

I'm leaning at the ujoint too. It's right next to the left peg.

A cupped tire, depending on where it's cupped and how badly can show signs at only certain speeds or speed ranges. While going straight or turning. If the tire were cupped on the sidewall you'd have to get it leaned over to feel it.

 

It's engine RPM not gear or road speed related.

Tight valve or need for a carb sync could also contribute.

 

Anyone have a picture of a cupped tire??? I'm interested in seeing what this would look like not that I have this issue, just curious

 

http://www.rattlebars.com/tirewear/wear05.html

 

Redshirt do you have access to a timing light, Multi meter , Plug socket and a ten pound sledge hammer?

 

I see says the blind man, really similar to tire hopping in NHRA by that description

thank you much!

 

You can actually quite often "feel" a cupped tire by running your hand over the tread surface, before you'll "see" it.

 

and the answer is:

Cupping!

Gave it proper psi and ran smooth like buttah.
Thanks for all the help, gang.

Now I know what cupping is like. You seasoned riders are a boon to us new timers.

 

Was it the front back or both tires?

 

A truly "cupped" tire is worn unevenly resulting in a wavy "scooped" pattern where the term cupping comes from.

If your "thump" went away once you put the proper amount of air in the tire, it's not cupped, it was just way too soft. Once a tire is truly "cupped" just airing it up won't fix it, it has to be replaced.

 

Ohhhh. The front was a little low and the rear was moderately low. I guess I caught it before it wore too much.