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[Clement]

Hi All

2003 cast iron Bullet 500

After a good start and riding 50 kms or so the engine starts to miss initially occasionally and then only firing intermittently with some big bangs before dying . If I put a new spark plug in it will be okay for a while and then the process starts all over again. Spark plug sooty despite moving circlip up on needle carburetor. Condenser okay, ignition timing okay, points gap okay, battery okay (12.88 volts, alternator charging), HT coil tested and seems to be okay. Tried NGKB8ES, NGKB7ES (slightly hotter),
Champion N7YC but they all give up although Champion seems to last a bit longer. I wondered if it was a loose connection somewhere but engine readily responding after fitting new plug (despite being hot)throws doubt on this theory. It is this intermittent failure after a 45 minutes ride or so which makes diagnosis tricky. Any ideas welcome. Clement.

[Bullet Whisperer]

Perhaps your air filter [if fitted] is dirty, needle jet is too big, or worn and / or main jet too big, float height too high?

[stinkwheel]

What you describe sounds like what can happen when the battery is going flat. Since you've checked that's ok, have a look at the frame earths. They can get pretty rusty and make poor/intermittant contact. The main one is usually a ring terminal behind the battery box.

A condenser breaking down can also cause similar issues.

In any case, I'd go looking for an electrical problem first off. The conking out with bangs sounds like an engine 8-stroking (only fires every fourth revolution and igniting the first fuel charge in the exhaust).

[papasmurf]

Faulty coil?

[jawa-enfield]

Coil indeed can be a cause, internal short circuit when it gets warm.
What kind of ignition is installed and are you sure fuel flows freely?(no vacuum in tank, or clogged fuel line?))

When the engine runs, and there are a lot of yellow sparks between the points, the condensator is bad.
and with a bad condensator, the strangest things cvan happen.

[p]

I've had this with weak coil - although not on my RE; check if it feels too hot when fault happens, mine was fine when cool but as it warmed up it failed, time taken to change plugs allow it to cool down and work again.
As it happens, mine was on twin boat engines, both did it almost simultaneously despite independent electrics, they ate coils until I put series resistor in each, never failed again. Strangely Lucas tested my selection of coils and deemed them good.

[Andy C]

If you have a sooty plug I would suspect a fuelling issue - you dont say what carb, I would be inclined to take the carb off , stripstrip it and give it a good internal clean / check float level - does the float have a leak?

Is the choke operating correctly?

Does not sound like the coil to me, but could be wrong of course.

[Clement]

Thanks for your comments so far. Carb is a VM28; replaced the carb with an amal just to try; same problem; no problems with (new) carb, choke or fuel I think; the fact that it starts to conk out and comes back to life after inserting a new plug indicated to me that something is not right with either the regulator or ignition coil? It must be the electrics somewhere. I still have to try re-using the plug on which it conked out, if those plugs are not working anymore it would mean that I have too high a loading on the plug? Can that happen? Cheers. Clement

[windmill john]

My 350 suddenly stopped after a few miles. After a while it started again. This happened a few times.
After a few suggestions on here, we narrowed it down to a bit of carb gasket in the bowl, plug or plug cap.

I removed gasket bit and fitted new plug and cap.
You’ve tried carb, so try a new plug cap.


John

[stinkwheel]

A duff condenser can kill plugs pretty quick-sharp.