Starting problems

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Hello,
I have a Cafe Racer based on a -61 Dominator with a -74 Commando engine. The engine is fitted with a single Mikuni carb and Boyer Mk III ignition.

The problem is that it is impossible to get it started without an external battery (car battery). The battery installed is very small, about half size of the "normal" batteries used so I should think about 6Ah.

So what have I done:
I have checked the wiring four times, made new wirings and soldered all the connections, bought a new battery, bought new coils (6V).
I have done a seach in your forum for similar problems and got some tips that I have checked.

So what I suspect now is that the battery is not powerful enough to get it started. I have kept a battery charger connected over-night so the "internal" battery is fully charged.

It starts quite easily with an external battery connected but this is not very convenient if you are going for a drive :D

I start it on the external battery, let it warm up, shut down the engine, connect the internal battery and it fires quite happily.
Then I take it for a drive, stops to get a coffee and after some half an hour, it won't start.

There is absolutely no room for a larger battery, and as far as I have found out, the generator is charging.

Spring has arrived so the matter is urgent :D so what I am thinking now is to install the old points ignition again.

Anybody who has had this problem before?

regards
Gunnar
 

L.A.B.

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Hello Gunnar,

Have you checked what the voltage is at the Boyer feed wire? Boyer ignitions are voltage sensitive and don't work well (or at all) below 10V. Have you (as a test) tried connecting both sides of the Boyer unit directly to the battery? As the ignition and kill switches can cause some voltage drop, installing a relay could help there.

Have you got a 2MC starting capacitor fitted, as theoretically the bike should then start even without a battery connected at all provided the alternator is in good shape?
 
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Hello,
No I haven't checked but I think you may be on to something here.
With my warmed-up engine and fully carged battery it fires on the first kick.
It is only when I have to fiddle around, using maybe 10 minutes I get problems.
I do not have a starting capacitor fitted, but I will have a go at your advice and let you know.

Thank you.
Gunnar
 
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A small battery shouldn't be a problem. It might just be that you heve been unlucky and have had two defective examples.

Do you have a volt meter ? It's really necessary if you want to do a quick check.

A 12v battery should read slightly above 12v when fully charged and if you then start the engine and bring the revs up a bit then you should certainly see a reading above 13.5v

What are the symptoms when it won't start ? Does it kick back and pop and bang a lot ? Those are the usual symptoms if a Boyer is seeing low volts.
 

L.A.B.

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Has the ballast resistor been left connected to the ignition (coils) circuit by any chance?

Does the battery stay fairly well charged? Or does it go flat quickly?
 
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I have checked the voltage on the battery, and when running at around 12-1400 rpm it shows a bit above 13 volts. But the battery doesn't hold charge very well. And it kicks back, pops etc. when it won't start.

Seem like I have to check the whole wiring system to see if the are any shorts or something else wrong.

The ballast resistor has been removed.

Maybe the best thing is to make a complete new wiring harness and get rid of the old rectifier and install an electronic one.
 

L.A.B.

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Have you checked the rectifier and Zener diode? As a fault with either part could cause the battery to discharge.
With everything switched off, you could disconnect the main Neg. wire from the battery (or remove the fuse) and connect an ammeter between the two connections, the meter should show zero amps (or connect up a tail light (5W) bulb instead between the battery and the main wire) if the bulb lights or glows then there would seem to be some current draining somewhere?
If so, start unplugging the various connections until the light goes out, that should give a clue to where the problem is (if there is one?).
 
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Hello Gunnar,

This may sound like the bleedin'obvious but have you checked your Ignition Timing also.

Mike
 
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