I've recently got my CB72 to idle fairly consistently at about 1k (see thread in restoration section). However checking the timing with a strobe to double check my feeble attempts at static timing and I can see that the timing is pretty much spot on but it jumps about a little bit (particularly on the left hand cylinder), on revving to anything above idle it advances immediately and doesn't immediately retard after the throttle is back on its stop, this means the idle speed stays at about 1500 for a couple of seconds, sometimes longer.
I've read the thread on the advance springs and the neoprene but I didn't notice the state of the springs when I had the cam out - lesson learnt there. It's pretty quiet though so I'm assuming the neoprene is in place.
Should the igintion advance immediately? I thought it only advanced higher up the rev range.
I've still not run this bike up the road since the rebuild but it's nearly there so it may be hardly noticeable when riding or when the engine is properly hot.
Timing at idle unsteady
LM probably has a more accurate answer, but I was under the impression that the ignition advances between tickover and 3,000 by which point it is fully advanced.
The weights should return to position as the revs drop unless the springs are weak or stretched/worn. That cyclic variation is probably what results in the clack-clack that LM refers to when the rubber is gone. If the timing is a problem for you, the springs should be replaced.
Hi,
I have electronic ignition, and a pick up on the crank, but I encountered the same phenomenon. Found out the the the periodic change in rpm at idling can be found in the carburettor, to be precisely, in the float valve. It has everything to do with opening and closing of a "sticky" float valve. That will change the hight of the fuel, which, in reaction will change the rpm. Jensen assembly of Japanese motorcycles requires great peace of mind (Pirsig)
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