Help diagnose clacking and rough idle?

Bluemeanie is correct, the noise is coming from your primary chain inside the engine. When you try to idle it to low the cylinders start to fire unevenly due to the carbs not being adjusted perfectly so the primary chain starts to rattle.
I have tried to fine tune my carbs to get it to idle lower but after spending a lot of time on it I came to realize you can't, there are to many variables. So just make sure your carbs are synched to specs and keep the idle at 1200 RPM.

baldy110 wrote:

Bluemeanie is correct, the noise is coming from your primary chain inside the engine.

That was going to be my guess as well, since my 750 twin will also let out a CLACK every once in a while at hot idle. You can feel it through the handlebars and it is very annoying.

Yeah - the guys have it right on the money, you have yor idle screw too far out. Gingerly screw it in and set the idle at 1200 - 1300 rpm's and forget about it. The bike is 30 years old.

Everyone and their mother has wondered about this at some point.

Virii wrote:

Digesting nails is a good way to describe it!

I did put some gauges on and synced the carbs (cheap set of dials off eBay). The bench sync surprisingly got it extremely close and I only needed some minor adjustments to get balanced. While diagnosing, I also put in new plugs but have never changed the wires and caps which go straight into the housing of the coils.


I've just gone out to check. I can confirm that the advance appears to return nicely but I gave it a little oil just for good measure. The sound doesn't appear to change with the clutch or gearing. Visually the holders looked okay but I did a quick spray test just to be sure and didn't notice any behavioural changes.

Raising the idle very carefully, I can say that it's just barely over around 1200 or so (eyeballing by the dash gauge which I have no way to verify) that the engine finally evens out and the sound pretty much goes away entirely which seems to be expected so I think I'm going to take that as my cue to relax.

I kind of get the same issue with the whole eating nails sound when the bike comes to an idle after a high rev:

www.youtube.com/user/KawGrazing#p/a/u/0/oRYbpdgyXZ4

Raising the idle to around 1200 does seem to help my bike too, but that will probably take a toll on fuel consumption, which is not good since I plan to do long distance riding on this bike. I'm also a bit puzzled since the carbtune gauges don't seem to read as high as others I've seen on youtube. I've been told to adjust the mixture screws until I get the highest reading on the carbtune for each cylinder. So I plan to do that next.

sakiguy wrote:

Raising the idle to around 1200 does seem to help my bike too, but that will probably take a toll on fuel consumption, which is not good since I plan to do long distance riding on this bike. I'm also a bit puzzled since the carbtune gauges don't seem to read as high as others I've seen on youtube. I've been told to adjust the mixture screws until I get the highest reading on the carbtune for each cylinder. So I plan to do that next.

From what I can see on your carbtune, you have basic compression loss (rings/valves/diluted oil). It could be late timing, or a constant air leak as well, but it sounds good revving... I won't know without seeing a timing gun check.

Adjusting your fuel/air screws will not have a dramatic effect on the vacuum numbers unless your already way off, which you would probally already know from how it rides.
If you blip the throttle hard and the tach falls below previous idle, you need more air in the pilot circuit. If it hovers above previous idle for 2+ seconds, you need m ore fuel in the pilot circuit. Its really easy to see

Setting the idle higher won't increase fuel consumption unless your stopped alot, or if you increase fuel input at idle via the air/fuel screws.

Also... Unless your tach has been rebuilt professionally in the last few years, its inacurate! You have to use an ignition source tach/timing gun for proper rpms.

Cheers! Hope this helps....