I have been using a TW brush type timer for a little over 2 years now. I have developed a high speed miss that I suspect is coming from the timer. I have cleaned out the timer but it still misses.
How much of the brush should remain before replacing with a new one?
Mike, that's hard to say, I would suspect if it's at least half the original length and it's still making contact at the inside end of the spring, it should be good. Are there any high spots where the brush makes contact inside the timer cap? If so, maybe that could cause a miss? Or, are you absolutely sure your cam seal isn't seeping any oil? Only weak point about a TW is they don't like any oil on 'em. I bought my first one the week Tony came out with 'em, ran it two years before replacing the brush, put over 3K miles on it, the brush was only an eighth wore maybe. But I had a seal leak, so just replaced the brush while I had the whole thing opened up.
Well, I opened it up again and took out the brush. I compared it to a new one and it was less than half its original length, so I replaced it.
This solved my high speed miss. The new brush was a real pain to install; hopefully I won't have to open it up again in another 2 years!
Mike, my TW timer has been great and used for many 1000's of miles. However, I have just looked at my timer yesterday prior to a big tour coming up. The brush was worn down to where the spring wire is embedded in the brush. This has caused damage to the insulating ring and the segments. This brush did not last as long as the first one. I agree, it is difficult to change the brush over. Would not like to have to do it on the side of the road. Like any other timer, it pays to remove it and heck it out and service it.