Other things to check are the starter motor and solenoid......if they are faulty then it is possible that the voltage to the spark plug isn't high enough to generate a big enough spark to start the engine efficiently. Make sure that the spark plugs are correctly set with the correct gap. Also make sure that the contact points to the battery are clean. If not clean them with a bit of sand paper. You'd be amazed at how often dirty contacts are the problem. And make sure they are tightly connected to the battery. Check the distributor is working properly and that the ignition timing is not out. Any of the above can cause difficulty starting.
Otherwise look for the standard engine problems that can make starting difficult.......such as poor fuel air flow (look at the condition of the plugs to gauge that first)/ check fuel pipe/ filters and carbs for any blockages. Poor compression also. If the engine runs efficiently when running...doesn't stall etc then an electrical problem as above is more likely. Modern engines don't need to warm up to get going as in the old days so starting the engine should never be a problem.
The best advice I can give short of looking at your engine directly is that you buy a Haynes manual for your van. It will not only give you detailed possible causes of faults but it will also give you the specs by which to measure for performance and fault finding. There will be a whole section on solving the causes of poor starting, both electrical and engine based, and you can work through that to find the fault. The battery and spark plugs are only part of the ignition system and it may well be that your old plugs were fine. Hope that helps.
Angela
February 2008
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