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Engine doesn't start puzzle (HARD)

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opcb5
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Engine doesn't start puzzle (HARD)

Post by opcb5 on Fri 04 Mar 2016, 1:22 pm

I have a 2001 CB 500 with 150K km on it which always starts without any problem. Ok, *almost* always. Very rarely, say 3-4 times a year, the following intriguing sequence of events takes place:
1) Engine doesn't start at first attempt.
2) I keep trying with the starter, but the engine doesn't start.
3) To avoid killing the battery I change tactics.
4) With the keys in the "ON" position, I put in 6TH gear and a I slowly push the bike for 5-6 meters, so that the engine makes just a few turns.
(I'm assuming there is no slope to take advantage of).
5) Now I try again with the starter and the engine starts *immediately*!!!

What happens during step 4???
I'd be really curious to understand that!
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eternally_troubled
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Re: Engine doesn't start puzzle (HARD)

Post by eternally_troubled on Fri 04 Mar 2016, 1:33 pm

Hello.

I'm not really sure what happens either.

If you starter motor doesn't move at all when you press the button then moving the bike forward in gear can move the starter motor onto a different commutator segment which can sometimes solve the problem and allow the bike to start - except that this doesn't sound like your problem.


Is it possible that your fuel-tap vacuum tube is failing? A weak vacuum will stop the correct amount of fuel flowing out of the tap.

Your 'step 4' pushing the bike forward and turning the engine over may just be enough extra rotations of the engine to allow some more fuel/air mixture in which encourages the engine to fire normally on the next attempt.
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opcb5
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Re: Engine doesn't start puzzle (HARD)

Post by opcb5 on Fri 04 Mar 2016, 2:22 pm

Hi,
the starter motor works perfectly from the beginning.

The fuel-tap vacuum tube is an interesting hypothesis! Thanks.
Unfortunately (or I should rather say "fortunately") this event is so rare that is more or less impossible to study it in a sistematic way :-)

What puzzles me is the following: whatever effect I achieve by "making the engine turn by pushing the bike", why don't I get the same effect by "making the engine turn using the starter"?
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wornsprokets
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Re: Engine doesn't start puzzle (HARD)

Post by wornsprokets on Fri 04 Mar 2016, 6:31 pm

Is it possible there is corrosion in starter relay connection, my cb had it, my fireblade had it in ignition and starter button, clean contacts especially at starter relay solenoid my cb had loads of it there  ,

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