... with idle timing too far retarded, you'll experience hard starting and carbs popping out of the boots...
Have another cup of coffee this deary sunday morning Grizzly bear, advanced idle timing will pop the carbs off not so much retarded....
No, Gary, I'm wide awake and fully caffeinated, and know whereof I speak. Every time I've dealt with XS650 carbs popping out of the boots it's been a case of worn bob weights with advance timing correctly set and idle timing way too far retarded...
Ooooooh, this is a good'n. Let's get into it.
Numerous posts about this carb backfire, *chuff*, *pop* issue, and I've experienced it myself.
Been thinking on this for years. An interesting conundrum.
Consider that the XS650 points ignition has about 90° dwell, which translates to roughly 180° at the crank. If the idle timing is about 15° BTDC, which is when the points open, then the points are closing 180° before that, about 15° BBDC. if the timing is too retarded, say about 5° BTDC, then the points closure would be about 5° BBDC.
Note that the intake valve is still open when the points close.
Now, consider at what point in the start attempt when this backfire, *chuff*, *pop* occurs.
I've only experienced this a few times on my XS1b, but it has ALWAYS occurred at the END of a kickstart attempt. NEVER during the kickstart cycle, only AFTER the engine has spun, then come to rest.
I'm thinking that, as the engine spins down, with its last bit of angular momentum (rotational energy):
- Pistons approach BDC, one intake valve is closing,
- Points close, coil energizes,
- Pistons continue traveling past BDC,
- An intake valve closes, that cylinder enters compression stroke,
- Piston travels upward, beginning compression,
- *grunt*, not enuff energy to compete compression cycle,
- There's some compression pressure in there, fighting the piston's last gasp,
- Engine rotation stops,
- Residual compression pressure forces piston back down,
- Engine now rotating backwards,
- Intake valve reopens,
- Piston passes BDC in reverse,
- The currently closed points now open,
- *spark*,
- *blammo*
If the ignition timing is correct, the crank/piston will have to travel backwards at least 15° past BDC for the points to open. There may not be enuff rotational energy for the engine to spring back this far. If the timing is retarded, that would be an advanced condition for a backwards turning engine. If the retarded timing was at 5° BTDC, the backwards points opening would be closer to BDC, only 5° away, and that reversing engine may actually reach that position. Points open. Backfire thru opened intake valve.
This supports Grizld1's position,
And my experiences, since I prefer an overly retarded idle timing.
Is this viable? Does it make *any* sense at all?
I suppose the only way to know for sure is to closely monitor the points cam, see if any rearward motion, and if its enuff to re-open a point...