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E: Engine Comet will not start when hot


Active Website User
VOC Member
Right, so I have been dealing with this pretty much since I have owned this bike, a 1954 comet. The bike starts easily when it is cold, choke slides closed 3 pokes on the tickler.
Once is it is warmed up it will not start until it has cooled down.
The bike runs great if I can manage to not kill the engine, power is excellent, no problems exceeding 60 mph.
There is no misfiring once off idle, but it will not idle for more than a few seconds.
The spark plug looks OK, a bit rich, but no different than my other bikes, Norton Commando, Yamaha XT500, both of which start easily hot or cold.
There is always spark when checked, hot or cold, magneto timing is where it should be.
It seems to be carburetor, everything else including cam timing has been checked and what it should be.
It is very easy to mess up the carburetor so that it will not start, hot or cold. The set up that enables cold starting is slide lifted about 1/16 or a bit less, air screw turned out 1 1/4 turns, anything deviating from this setting will not work hot or cold.
The carburetor is the original 229 amal, it has been thoroughly cleaned ultrasonically, all the fuel and air passages in the jet block are clean and open.
It is just about impossible to resolve the problem given that once the bike has started and warmed up, I try an adjustment, it dies, and then won't start again until cold, so back to square one.
My next move will be to replace the carburetor with something else, probably a Mark II, but before I do that I want to check with the forum to see if there is something I am missing.

Bill Thomas

Well Known and Active Website User
VOC Member
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.


Active Website User
VOC Member
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.


Website User
Non-VOC Member
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


Well Known and Active Website User
VOC Member
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.


Well Known and Active Website User
VOC Member
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



Well Known and Active Website User
VOC Member
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.

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