E: Engine Comet will not start when hot

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Bill Thomas

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That was funny, Maybe not !, With the one we did, It had points I had not seen before,
It was 5 years ago so I am not sure, You could reverse the moving point ?.
But some were proper points and some were dummy, Something like that.
So we reversed it and it started super, Ran for a very short time and would not start again,
It was not a proper point and just burnt over, But it proved we were on the right track, I had another points base in my tank bag, But it was handed wrong , But still worked OK.
I learnt a lot that day, Made me feel good.
 

Chris Launders

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Gregg, yes it is, but if you are converting a K2F to run on a Comet you would have the right points platform already.
 

SteveW

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Rebuild the mag then look at the carb. This is a classic mag/bad cap. problem. I had an issue w/ mine it ran perfectly until it reached 50 MPH then it would fail under the increased compression.
 

gregg-k

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Just to add a couple details to Bill Thomas's posting:
- the keying of the mag's rotary contact breaker platform is different on clockwise mags vs anti-clock
- with the points sets made for the later "low inertia" breakers, you can indeed flip over the rocker to use it in whichever direction you want, but the fixed part will have only one tungsten contact, so as Bill says, if used for the incorrect rotation the "incorrect side" will burn away.
- with the points made for the earlier brass breakers, the original Lucas rockers are not reversible, though they can be modded to work if you have a 1/4" counterbore with a 4mm pilot. If you do this, you will need to replace the thrust pad for the hold down finger, since the original Lucas pad will be too small.
- The current repro points sets, despite being of somewhat poor quality, are made to run in either direction. Before using them, be prepared to spend time aligning the contact faces and correcting the rubbing block heel.

.. Gregg
 

greg brillus

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The original brass points assemblies are the best by far.......on the later reversible ones, I've seen the earth contact come loose causing a misfire that is very difficult to find.
 

Roslyn

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taking a slight detour from the main thread:
in Magnetoman's third photo of a section through the carb, do the washers in red have a specified thickness, is final fuel level a suck it and see approach , I guess the fibre washers are prone to some compression ?.
I have the , what seems usual state of affairs, where once assembled fuel weeps out of the top of the slot through the threads of the lower carb body seen in photo 2, once I'm motoring the issue is no concern. Is it a case of 'they all do that' or indication of a poorly assembled / badly adjusted carb'?.
Fibre washer is a fibre washing it just has to be in good order the right diameter and not delaminated (falling apart) oh one other thing it has to be done up really tight also checked every so often if the bike is left standing for a while as the fibre as washers they do shrink dry out and the float bowl can come loose some thing you should always keep a eye after replacing the washers.
 

Chris.R

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I know nothing about quantum mechanics my problem is with the 1954 Comet not starting from cold she has been rebuilt fitted with an Amal remote float 276 carb refurbished no amount of fiddling with it makes any difference, I am sure of the valve timing being correct do I have a mag problem I am wondering, when I bought the bike she had been rescued from a museum when sold at auction I bought her from the dealer who had had her for three years he said he had the magneto re furbished by the MAGMAN I gather he was working from a base in the midlands UK. the bike has been in my garage being rebuilt for 2 years so inactive for a five year period since the magneto was rebuilt. This morning it took 20 minutes to get the bike started not much different since I first ran her in late August 10 minutes would not be unusual firing on starting seems intermittent but once started she runs and sounds good. Any ideas the magneto was the only part I had assumed good

Chris.
 

Magnetoman

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the dealer who had had her for three years he said he had the magneto re furbished by the MAGMAN I gather he was working from a base in the midlands UK.
Simply removing the armature from the body results in a permanent loss in magnetism that only can be restored by an appropriate electromagnet. My electromagnet weighs ~250 lbs. and I know that not every magneto rebuilder has one.

A magneto will still work with the reduced magnetism in its chunk of Alnico, it just will require spinning it faster before it generates the necessary voltage to bridge the spark plug gap. Once running, the engine typically spins the magento fast enough that there are no further issues (the faster the magneto spins, the higher the voltage it generates).

I know nothing about your MAGMAN, but it's possible that even if he rebuilt the magneto and used the right condenser, he didn't have the necessary electromagnet.
 

RobGorLin

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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
Have to agree with Gregg - had a similar issue with my Comet - after about 10 miles it would start missing - eventually stop and no restart for at least 1 hour.
Magneto rewind fixed the issue
Gordon Lindsay
 

kerry

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Ironically I took a Mag in to our local chap yesterday in Letchworth, it will be ready in just 3-4 days and cost around £200, It was taken off a Venom that had a thorespark fitted when the mag started the usual dying when hot, the thorespark unit has not been very successful in this instance so back to the good old mag.
Kerry.
 
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