Cold Starting problems: 

• In a cold climate, what happens is that the plug does not get enough charge from the coil to ignite the fuel.
• If fuel is kept running and the engine is still being primed & kicked, primed & kicked, all that is happening is that more fuel is being sucked into the cylinder. Now, the fuel here is pre mixed and thus has a mixture of oil and petrol, which comes in and clogs the spark plug and cylinder. The fuel evaporates but the oil residual is left behind and blanks out the plug. Additionally, especially with Jikov carbs, flooding becomes more susceptible
• Shutting off the fuel tap ensures that no more oil comes and dampens the plug which as it is, is receiving lesser spark. Furious kicking after that ensures that all
  oil and fuel is blown out. And thus ignition is achieved.
• That apart, while pumping, air is compressed in the cylinder and thus the temperature in the cylinder goes up - thus facilitating ignition.
• The pumping action causes a suction thereby drawing more fuel which helps spontaneous ignition (the choke may be used for the same)
• We do not have a decompression lever on our machines (eg. Enfield, Luna). So a single direct kick may not usually fire up the bike. Hence, 'pumping' the kicker for a while, will allow the piston to move up and down manually quite a bit - thereby giving enough momentum to for the final firm kick to fire the engine.
• In cold conditions, the pistons may be tight inside the cylinder due to contraction and "pumping" frees the movement of the piston so that kick starting would be easy

Hot Starting problems:

• Could be caused by vapor lock - this happens in bikes like the old Ducattis and in old carbureted cars. 
• What may happen, is that the heat causes the vapor to form a film in the fuel path, which is bad 
• Wait for the engine to cool down or just use the choke 

Rain Starting problems:

• In certain cases, the carburetor covers do not cover the carb properly and hence are exposed to the rain showers (also as in the case of washing with a pressure hose)
• In heavy rains (or after a thorough wash) water sometimes enters the carb, giving rise to starting problems. In this case the carb will have to be cleaned out
• Rain may get to the electricals if they are not well protected
• The side covers of the engines of our bikes are not very well packed. Therefore, the points are vulnerable to heavy rain. This can be solved by placing thick packing above the assembly so as to protect the points from rain.... almost all our vehicles have it.
• A quick trick here in case of a sudden downpour, after which the bike refuses to start, is to remove the spark plug, pour in a couple of spoonful of petrol and then refit the plug. The bike will start immediately.

Possible reasons about sudden starting problems -

• Certain additives in the new petrol types nowadays can cause the bikes to overflow.
• Point settings (where existing) may be disturbed if a lot of distance has been traveled without having set the point properly. A lot of speeding and running through rough terrain, can cause the screw, that holds the point to loosen up a bit
• The points need to be kept clean too.
• In the most common cases, the fuel flow line should be checked. If the petrol tank is dirty and if a filter is not used, the chances are that the carb could be choked.

Troubleshooting...  

1) The most essential part - the heart of the bike, the Carburetor decides how much fuel has to flow through and how much air should be allowed  in for ignition. When bikes don't start, the fault is usually  found out here.  

• If the air filter is clogged and  a lot of white smoke is emitted, once started,  it means that the carb is getting flooded.  
• Get the float, the valve, the carb needle and the carb setting checked. 
• In some cases a spacer could have been placed for the float which might lead to starting problems.
• If required, clean the carb thoroughly with diesel or kerosene.
• The carb should not be too lean tuned, ie. the oil fuel ratio should not be set to be too lean. Also the main jet may have to be replaced to a larger one, thereby giving a richer mixture.
• After reassembling the carb, check for overflow by blowing into the petrol inlet tube by holding the carb upside down. 
• In order to avoid an overflow (flooding of the carb) get into the habit of shutting the petrol off just few meters before stopping. Switch off the petrol tap, wait a couple of seconds and then turn off the ignition.
• In the mornings, just before kicking, twist the throttle a couple of times, prime the engine by pumping the kicker a couple of times, again twist the throttle a couple of times and then kick start. This could also be tried first with the fuel tap off while priming. Then turn it on, watch the fuel run and give a firm kick to fire the engine.  Alternatively in extreme cases, turn the idling screw one full round (clockwise), try tickling the carb, then pumping the kicker a couple of times with out switching on the ignition. Finally, turn on the ignition key and kick start.

2) Check the fuel tank for dirt or water. Ensure that the petrol filter in the petrol supply tube line is clean too.

3) Check the spark plug for carbon. If it must have accumulated enough carbon (with the bike running rich), then clean it. If it looks as if its going bad, get a new one.

4)  For those bikes with point settings, if the problem still persists then probably the points' timing is not right. Get the timings properly set up by an experienced hand.

5) Check the wiring for any short circuit or earthing, on the live wire to the ignition coil or the switch or anywhere else.

6) Check the condenser and ignition coil unit (under the fuel tank)

7) Check the starting/running coil inside the magneto core.

8) For those bikes with CDI systems, get the CDI Unit checked.

9) Get the piston and cylinder checked for enough compression. At times the piston rings might have worn out or even the piston itself. If so, get a rebore with the original escorts piston.

From the drawing board of the Yezdi / Jawa Company, with regards to cold starting problems -

The bikes which had been tunned for extra efficiency were the first bikes which encountered starting problems when the engine was cold. The reason, being that these bikes burn a lean mixture, ie. fuel to air ratio is less then recommended.

To start these bikes, the use of the choke was mandatory, if carbs with choke had been fitted. Here the choke reduces the air going into the block thereby allowing more petrol to flow into the block.

Most of the Yezdi bikes had the tickle type carbs, which allowed the carb to over flow petrol into the block there by increasing the fuel percentage (providing rich mixture).

All the Yezdi bikes (except the CDI versions) have the ignition switch with 4 positions:

1 - Start

2 - City lights

3 - Full head light 

4 -Starting on battery

Most of the Yezdi bikes were sold with a battery.

When the ignition switch is turned onto position 4, the CB point and condenser are disconnected from the magnet and are given an electric supply from the battery. This supply from the battery gives a very strong spark, which is capable of burning fuel very easily. As soon as the vehicles starts and idles properly, the ignition can be switched back to position 1 for regular riding.

As long as the bike is drawing electric current from the battery, the kind of spark at the spark plug end is very similar to that of spark in Maruti 800.

with inputs from Koustbh Patil, Venkat, Ajai & other Yezdi clubbers