This week one of my stick coils has failed (which of course has made the bike run rough, and rattle more) so whilst I wait for the replacement coil to turn up I thought I would take a look at the reason why the throttle bodies were so noisy, so I remove the left hand one and stripped it down.
It turns out that the cause is a worn out bearing bush. Each end of the shaft runs on a split oilite bush 8mm Id and 10mm OD. One is 12mm long the other 8.6mm, the shorter one being the TPS sensor end. I pulled out both of them by screwing in a M9 taper tap and drifting it out with a thin rod and small hammer.
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I measured the throttle shaft and found between 0.1 and 0.15mm wear in it - not a lot. The bushes however were much more worn. Actually the short one wasn't bad at all but the long one at the quadrant end was worse and a bit oval too. Removing the bushes essentially trashed them so I turned a couple of new ones from Acetal. This engineering plastic is very chemically inert and doesn't absorb moisture like for instance nylon. I turned the OD and reamed the ID, but I had to ream it again after I pressed in the bushes because the thin walls were too flexible.
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After that I re-assembled the throttle body and there's no discernible radial play plus the action is very smooth.
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After this I had my dinner and got to thinking that the bushes might be available as a standard part - and they are (at least the 12mm long one is). Cheap too so I have ordered some. I was intending to do the right hand throttle body with the same bushes but now I have found these off-the shelf parts I'm going to wait and use those instead.
https://www.bearingboys.co.uk/Split-Bea ... hes-1024-c
I'll report back when I have fitted the new bushes.
Pete.
