E: Engine Comet will not start when hot

[everiman]

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.

 

[Robert Watson]

Not likely anything to do with the carb.
This is classic capacitor failure of an old magneto. Two solutions.
1) take the mag off and have it rebuilt
2) replace the mag with something modern

If you are not running a magneto what ignition do you have

Robert

PM me if you want and we can make contact. I am in Vancouver. I won't put contact details on the public forum.

 

[Bazlerker]

I second Watson’s opinion..it’s the magneto. I can highly recommend Spyder Integrated technologies in Ottawa for a thorough rebuild. PM me for Gregg Kricorissian’s contact info..

 

[vibrac]

Yes capacitor is first port of call
Over in UK we have this solution

Brightspark Magnetos - EasyCap condenser

Thats what I would do if mine went off perhaps you have a clever guy who can slip a replaceable capacitor in...

 

[Magnetoman]

I disagree with the condenser diagnosis. If the condenser were the cause of it not re-starting when hot, it also would cause missing when running, but the owner says it runs great. The symptoms are not consistent with a bad condenser. Although 90% of carburetor problems are electrical, this seems very likely to be one of the 10% that actually is due to the carburetor.

It is somewhat suspicious that the behavior is so sensitive to the air screw so it is well worth checking the fuel level. If there is an issue with the float needle sealing it will cause the level to be too high in a static (engine off) condition. However, the same issue can still leave the level OK when running because the vibration helps it "wiggle" into place and seal. A too-high fuel level will cause the mixture to be rich, which is what you want it to be for ease of starting when the engine is cold. However, when hot, the rich mixture makes it difficult or impossible to start.

Whether or not this is the explanation for the problem you are having remains to be seen, but it does have the advantage that it explains all of the symptoms you described. You'll have to make a fixture to bring the fuel to a piece of clear plastic tubing to check the fuel level. If it is too high, a very gentle lap of the float needle in the seat using the finest polish (e.g. Simichrome) could cure this nagging issue.

 

[Gerry Clarke]

If this is a clip mounted carb., it is worth carefully checking that the float chamber is in fact truly vertical. AMAL carbs. of this type are not super sensitive to settings and one's first instinct screams to have the magneto (if that's what you have) overhauled by a true expert like Dave Lindsley. We also have a 1954 Comet with original carb., and it should tick over reliably and consistently at very low revs. like a gas engine. In fact, this is the only way to enact a noiseless gear selection with the Burman gearbox.

Gerry

 

[Roslyn]

Yes capacitor is first port of call
Over in UK we have this solution

Brightspark Magnetos - EasyCap condenser

Thats what I would do if mine went off perhaps you have a clever guy who can slip a replaceable capacitor in...

They are only get you home repair as to use it full time you have remove the internal capacitor and was told this my local expert Martin Dryer of Hanham Bristol and hot magnetos do lose there magnetism once cooled down they work ok had experience of this on a pre war Douglas many years ago.

 

[Roslyn]

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.

I had mag rebuildt three years ago and the internal capacitor was loose caused lots of problems to point i very nearly gave and sold the bike at the time but got it going once it was sorted as it was not cheap at the time but worth it and was a good learning curve for me.

 

[Magnetoman]

Magnetos cause plenty of problems for plenty of people. But, I feel it's worth repeating that the symptoms everiman describes are not consistent with a magneto problem, but are consistent with a carburetor problem.

 

[kerry]

Friend of mine has a MAC would start and run perfectly just like you describe, but if stopped no way it would restart until cold, it was the Magneto