E: Engine Comet will not start when hot

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Jim Bush

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Since the experts point away from the magneto - one thing that may help is checking the needle jet for wear, if it is oval the mixture gets very rich. Check with a #36 drill for a 106 jet. Some say replace the needle at same time - personally have not seen worn needles to the point of affecting running.

I had a '50 650 Twin Triumph with 276 single carb - it would start and run fine when cold, but hot, the idle went was all over the place, high rpms, stalling, etc., ran well on 1/4 up throttle openings. Replaced the needle jet and it was like a new bike - steady/even idle, crisp take off.
 

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Chris Launders

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I had a Comet with exactly the same symptoms, had the mag rebuilt, it made virtually no difference, never did get it to start reliably when hot before I got a twin and sold the Comet.
 

oexing

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The capacitor may be shot but the coil could be as well. You should check maximum spark length from ht lead to earth in cold condition and do same check with hot engine, half an hour run required.
The contact breaker will show heavy spark fires with a bad capacitor and a lot of wear from this but I had to replace several coils in the past, not a problem from capacitors but just old age of coils.

Vic
 

Magnetoman

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It's the mag. Absolute classic problem. Runs perfectly until stopped but will not restart. Classic condenser problem.
I hate to disagree, but...

These symptoms are not the classic condenser problem. The classic condenser problem is due to the chemical degradation of the material between the plates resulting in the resistance of the condenser rapidly decreasing with rising temperature. Because of the finite resistance the condenser no longer serves as a good high frequency current shunt when the points open. As a result, arcing occurs across the points, which both messes with the timing as well as dissipates energy that is needed at the spark plug. With this type of condenser problem the bike runs very badly when warm. When the condenser cools down, all is (temporarily) well again. If a bike runs well when it is warm, but doesn't start when warm, it is not the classic condenser problem.

If you look at the points in subdued lighting, with a good condenser you only will see tiny pinpricks of light across the points whether hot or cold. With a bad condenser there will be bright flashes across the points. If the condenser is at the point in its inevitable death spiral where the resistance is high at room temperature, but relatively small at elevated temperature (nb. 2 megohm at 400 VDC may sound high, but a condenser with that value will result in noticeable degradation of the running, and by the time it has dropped to 300 kohms you will be walking home).

Again, the symptoms described are consistent with a carburetor problem, and are not consistent with a magneto problem.
 

brian gains

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During my 'getting to know you' period with the Comet it had the classic bad condenser issue and would die when hot then OK when cool, but even that state of affairs got gradually worse.
Had the mag rebuilt and all seemed OK initially but then would not start when hot again!, it turned out I was running way rich on the piolet jet, nice and rich to cold start but too much once warm.
ps have used the Easy Cap condenser replacement on a mag dyno with great results.
 

oexing

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As written above, check the length of sparks with hot engine as compared to what you get with a cold magneto. If that check shows up a big difference you should have a look at the contacts for wear due to excessive sparking, visible with a running engine. If that looks good you can bet it is a duff coil, typical to be dependend on temperature. So then no need to deal with the carburettor , it is the mag. I don´t bore you with stories about bad coils, just check as described.

Vic
 

gregg-k

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This is my very first post, so I hope it is of help.
Like Kerry, my personal experience says that the problem is most likely being caused by the mag, but I agree with Magnetoman's suggestion that one failure mechanism of a duff capacitor (an open circuit) usually causes tell-tale arcing at the contact breaker points. However, if a capacitor fails to a shorted condition, it will not cause arcing, but will behave the same as if you had pushed the kill button.
There are many things that can cause a mag to stop working, and besides a duff capacitor, another common failure mechanism is electrical leakage within the mag's HT winding. This leakage gets worse with increasing temperature, and shunts away the energy needed to create a good spark. Again, this leakage will kill the spark, but will not create arcing at the contact breaker points. Also, given this leakage, replacing the capacitor will not resolve the root cause, which is addressed by an armature rewind.
Everiman's original post says the carb has been thoroughly cleaned, and that the engine runs well until such time as it heats up and is allowed to stop. The fact that one can see a spark at that point is a bit of a red herring because there may not be enough spark energy to ignite the mixture under compression.

So rather than debating all the things that could be wrong, let me suggest a few tests to try:
- is your tank vent clear? When hte problem happens, do you still have a good flow of fuel if you remove the hose from the carb? (I know you'll be careful when doing this ...)
- try a *new*, non-resistor spark plug. I've had more bad plugs the past few years than i care to think.
- if you have a "suppressor" spark plug cap and/or carbon HT lead fitted, bin them and replace with a proper stranded copper lead and straight through cap.
- reduce the plug gap, see if the engine "death" takes a little longer to happen
- when the engine refuses to re-start, cool the mag as quickly as you can. Suggest you take a Thermos of ice-cold water and towel with you, and when the problem surfaces, soak the towel and wrap it over the mag to cool it off. With the towel in place, pour any remaining water on the towel so as to let the mag cool down as quickly as you can. When doing this, be careful not to let the water get into the mag.
If your engine now re-starts, you have some pretty compelling reasons to suspect the mag. If not, you may have a different problem.

Give these suggestions a try and let us know what you find.
Hoping this is of help,
.. Gregg
 
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