Clunk at Idle

Hi Folks,

I haven't been overly active here, but I try to keep up. I get my Norton out every year but unfortunately haven't put a lot of miles on in the last couple years. Last year I put some money into a belt primary drive, 3 phase charging system and new electronic gauges. Nice upgrades, running great.
I noticed a fairly loud clunk noise at idle. It goes away as soon as the revs increase. Clutch in or out makes no difference. Center stand, side stand or on its tires, no difference. I think this noise has been there for years, why I chose to suddenly pay attention to it is beyond me, or maybe it's getting louder.
Last fall I put my mechanics stethoscope on and it's loudest at the back end of the primary case. Verified that the primary is not touching the z plate at all, plenty of room around it. Trans mounts were verified to be tight. Iso's checked and slightly adjusted, no change. Dave Taylor head steady is only a few years old and in good shape. While looking for answers I found some information regarding timing. I re-checked the timing with a strobe and it was right on (Boyer). Got a degree wheel and verified the scale in the primary case is accurate. Didn't suspect a timing issue, but wanted to eliminate the possibility.
So, any ideas what else it could be? It sounds like the primary case is hitting the z plate or something else, but I sure can't find anything. It's pretty lumpy at idle so I'm not surprised it goes away as soon as the revs increase. Where else should I look? Anything with the swing arm?

Thanks,
Ben

Could the clutch wheel be contacting the inner primary?

Clutch centre nut coming loose, or the bearing on its way out?

On a previous thread, it was mentioned cases with very little clearance to the swingarm. Definitely worth a look, especially if the back of the primary shakes when idling.

Another thought, is your belt tight enough? Maybe its slapping at idle, but as soon as revved that would disappear.
There was a similar thread quite recently which turned out to be that.

EDIT

Found it..

I ran it with the primary cover off quite a bit when I added the belt drive. Its the CNW kit, new basket and bearing, hardened clutch centre, etc. I checked for the belt slapping and didn't find anything. Took a look at the swing arm to inner case clearance with a mirror tonight, looks like plenty of clearance, but I'll confirm when its running. Most of this was done last year. I'll go over everything again, but keep the ideas coming. Also, its a clunk, not a clank or rattle.

It was the belt slapping....
Checked everything, took the primary cover off and started it up, put pressure on the belt with a ratchet extension, effectively tightening the belt temporarily, noise went away. Foolish me. I'll make the adjustment and carry on. Thanks!!

Brly said:

It was the belt slapping....
Checked everything, took the primary cover off and started it up, put pressure on the belt with a ratchet extension, effectively tightening the belt temporarily, noise went away. Foolish me. I'll make the adjustment and carry on. Thanks!!

Click to expand...

Good to hear that it’s a nice simple fix.
Dave

Brly said:

It was the belt slapping....
Checked everything, took the primary cover off and started it up, put pressure on the belt with a ratchet extension, effectively tightening the belt temporarily, noise went away. Foolish me. I'll make the adjustment and carry on. Thanks!!

Click to expand...

I have the same setup, same problem, same solution. Glad you got it sorted!

Thanks guys. A little embarrassing, I thought I had verified last year that it wasn't belt slap. Oh well. I set it for 90° twist when cold, but have since read that may not be accurate. I'll snug it up a bit and then check it when hot.

Yes I had the same issue with the CNW starter and belt drive.

But also set the idle for 1000 rpm minimum.

Lower than that isn't good for the camshafts and the belt slaps more too.

Thanks for that. I have the idle set just under 1000. I'll bump it up a little.
Made the adjustment last night but didn't have time to start it up. Should get it wrapped up tonight.

By the way, no need to feel embarrassed. Better to ask than call for a recovery truck. - That IS embarrassing !!

Just be sure to check how tight the belt is when the engine is at full heat. The belt gets tighter when hot. A lot tighter.

Onder said:

Just be sure to check how tight the belt is when the engine is at full heat. The belt gets tighter when hot. A lot tighter.

Click to expand...

better too loose than too tight.
IMHO.

Understood. I think I went overboard with how loose I set it when I installed it last year. Snugged it up a bit last night and will verify when hot.