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E: Engine Comet will not start when hot

everiman

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.
 

Robert Watson

Well Known and Active Website User
VOC Member
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.
 

CollingsBob

Well Known and Active Website User
VOC Member
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..
 

Magnetoman

Well Known and Active Website User
VOC Member
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

Active Website User
VOC Member
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

Well Known and Active Website User
VOC Member
Yes capacitor is first port of call
Over in UK we have this solution
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

Well Known and 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.
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

Well Known and Active Website User
VOC Member
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.
 

Jim Bush

Active Website User
VOC Member
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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oexing

Well Known and Active Website User
VOC Member
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

Well Known and Active Website User
VOC Member
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

Well Known and Active Website User
VOC Member
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

Well Known and Active Website User
VOC Member
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

Website User
Non-VOC Member
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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