Noticed a rattle on my RT1.
It started as soon as I kicked it up and was there ever so quiet on the idle. 
Warmed up ,still there neither getting louder or changing frequency. 
Give the throttle a blip ,got louder and as the revs built . Took it for a quick scutch round the block and as the load built  on the motor,it went quiet. 
Meanwhile back at Ted's towers the rattle returned .
By chance I took the side cover off to access the rotor. 
WTF ! Found the rattle.
it has a mechanical advance unit on . There's fine rust deposits in the access holes on the rotor and spring . I can wiggle the arm backwards and forwards so I'm wondering if it's on it's way out.
 Never seen that arrangement before on two or four strokes. 
Any comments please. 

That design was first on certain RT's and then later on the DT175.  Retards timing for starting but should be full advance at a fairly low RPM...  just above idle if not at idle. 

(Electric start AT's have a different style advance mechanism but it does the same thing.)

So Mark ,it will be rattling on idle after the firing phase then. Maybe wants a cote of looking at. Probably as dry as a nunns chuff.

I hate to say this but you're reminding me of Yamaha's MAJOR design fault....  no stereo system to turn up loud.  Which is what I normally do when I hear a funny noise or rattle. 

Couldn't hurt to check out just to be safe...  or use the provisions to wedge the mechanism open when setting timing and see if rattle stops.  Other solutions might be to turn up the tick-over speed a little... or my favorite...  remove the exhaust baffle. 

I finally found the RT advance diagram...  might not be easy to safely temp wedge full advance and run.  175's have a a couple of holes you align and insert a small stick to hold full advance.

Reason I mentioned doing that is rattles can be hard to positively identify...  if mechanism locked open and rattle is still there, it might be something else.

Forgot the pic:
 



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By coincidence, I am just about to set the timing on my RT1 B, and wondered about locking the advance / retard lever open. I will try the "bent wire" technique first. What I did wonder though is the distance to TDC when timing is stated in my Clymers as 2.9mm with full advance. Is there a measurement for timing with the lever at rest, so to speak?

Spec is only at full advance.  Like I said, once engine is running it should be at full advance so that's what's critical.

I'll be interested to hear if the play was audible in that advance mechanism as i doubt it would be. It's a good system but just more to wear out. The tail of the arm wears where it engages the cam. My RT1 had it & was super easy to start combined with the de-compressor. Later cdi off a dt400 or "79 dt250 is good as they electronically retard for starting. Sorry, getting sidetracked. Our farm early model AG175's had that system too. With wear of the parts timing is a bit of a guess & if you strobed it running you'd probably find it different to statically, which might be part of the reason they did away with it. But notorious kick back rt2 & rt3 could sure benefit from that first system. Also the points are set further out on the plate due to the fat cam if ya ordering some & might be a different number to non advancing model points.

It's definitley the advance unit. Removed the cover and had it running .
When it's stopped you can rattle the unit around.
You can replicate the noise .