Rattling noise when starting

Anvill

Anvill

Joined
Feb 26, 2022
Messages
92
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Hi all,

I'm happy to say my MKIII rebuild is complete and generally the bike is so far running really well. One thing I'm experiencing however, which didn't happen before the rebuild, is an occasional nasty rattly noise when I engage the starter. Its tending to make this noise the first start attempt and then it fires right up on the second attempt. Strange it didn't do this before, but perhaps the starter rebuild (presto 2 brush) plus a lithium battery is putting new strain on the system. Is this what a worn sprag sounds like? Would appreciate any suggestions.
 
Best take the primary cover off and try some starting to see if you can identify the culprit. As well as the sprag the chain tensioner may be late in tensioning. Of course with no oil the tensioner will not tension but if the noise is the same and does not go away 2nd start plus the chain is slack then you know the tensioner needs attention.
 
L.A.B. said:
It's more likely to be the anti-backfire device slipping (which it is supposed to do).
Click to expand...
If it was the AB device, is there an obvious reason why it would be doing so much slipping now versus pre rebuild? I’ve heard not to mess with these and I have made no adjustments to mine.
 
The device does wear out, evident from my recent rebuild. The balls wear a ramp from the hole and release sooner + you have more torque then before. Barely visible in pic but on the right side of holes

Rattling noise when starting
 
Would someone be willing to describe the sound of a slipping AB device? I’ll also try to capture a video of the sound from my bike for you guys to have a listen to.
 
Is it possible your tensioners are bleeding down while the bike is not being used and causing the primary chain to rattle until it fills up with oil again?
 
I have had the backfire gear loose its torque setting which sounds similar to the described sound.
 
Deets55 said:
Is it possible your tensioners are bleeding down while the bike is not being used and causing the primary chain to rattle until it fills up with oil again?
Click to expand...
That's certainly possible in addition to whatever else is happening, but the sound is quite close to what LAB described. It is directly tied to the starter motor engaging, and doesn't happen once the bike is running.
 
possm said:
I have had the backfire gear loose its torque setting which sounds similar to the described sound.
Click to expand...
Interesting, well if it gets worse I'll consider reading through the Old Britts adjustment guide.

For now though, I've found that kicking it over once before I hit the starter mostly prevents the effect. Perhaps the AB had already lost its setting as you describe and I'm only realizing it now because my compression is a lot higher after my engine rebuild. One pass on the kickstart loosens things up just enough for the starter to engage correctly. With this technique it now starts right up off one button press, which is a heck of a lot better than pre rebuild!
 

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