help with chain thunk problem

[LSN]

Hello,
I need advice on what is probably a simple fix:
After assemblingmy new T50, I was unsure of right length for the chain.
So, I had a local bike shop repairman, install it. At the shop, everything seemed right and tight when crank was turned.

But, when I got the bike home, I discover that, with each turn of the crank, the chain goes "thunk" against the first tooth of the front sprocket. Why? Is the chain too loose? The chain seems to be properly aligned over the sprockets, and to shift properly into each gear. And just turning the crank doesn't make a "thunk." But when pedaling, on almost every rotation..."thunk."

Also, the LBS repariman says that the back brake pads barely touch the back disk. What adjustent do I need?

I appreciate your help with what may seem like obvious problems. My nearest bent bike shop declined my request to tune my T50 because it was purchased online instead thru their shop. They referred me to this forum instead.

 

[Don1]

check for a sticky chain link

 

[dtseng]

check for a sticky chain link

This can happen at the "quick link" if it is not properly installed, or when cutting the chain, one link may be slightly bent. To find this sticky link, ask someone to lift up the wheel, observe the chain link leaving the chain ring tooth or lower derailleur guide wheel. When you find it, bend that link very slightly back and forth to free it.

 

[super slim]

Check that the chain joining link is fully engaged!
When you hear the "Clunk" ,stop pedalling, and stop the bike, and check the chain at the crank AND the cassette, for stiff or not complete chain joint!

Breaking and joining using a chain link.

Using a pin to join a chain

 

[Jim Zimmerman]

LSN, it appears that nobody has addressed your problem. You specified that the "thunk" happens on every stroke of the crank, not randomly as the chain rotates around everything. That tells me you have a problem with the chainring, not a problem with the chain (as everybody else here seems to think).

In fact, it sounds like you actually know which tooth is causing the problem. Have you removed the chain and rotated the chainring, inspecting the alignment of the teeth? I suspect you might have one bent tooth on that chainring and that would show up when you rotate the sprocket. The easiest and best way to correct the problem would be to completely replace the chainring, but I have used vice grips and repaired a bent chainring in my past. It wasn't perfect but I didn't have any more problems with it.

Sorry about the late response. I hope you have already fixed the problem by now, and would be interested to hear back from you what your solution was.