Another way to make an injection port is to just install a Schrader valve (fancy name for a car tire stem). If you have a needle for your syringe you permanently remove the stem from the tire valve. The syringe needle will fit through the stem-less valve (of course the "valve" is no longer a valve once the stem is removed). The plastic screw on cap for the stem is more than strong enough to contain the combustion pressures.
You can get a metal tire valve from a car tire shop for a buck or so.
Here is an example
http://www.inpharmix.com/jps/Sluka_Spud-zooka.html
Another example, left end, upper part of the chamber, that's a chrome truck tire stem with metal cap that I inject fuel through using a syringe:
Or you can go totally ghetto. Drill a hole in the chamber just a hair bigger than the needle. Inject the fuel and cover the little hole with a piece of tape. Surprisingly, the tap will be just fine. (Just avoid putting a body part too close to the tape covered hole just in case.
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I suspect you have some problem in your math and that is messing up your fueling. The volume of a cylinder is pi*radius^2*length. Note that it is the radius, not the diameter, in the formula. Or just post the dimensions, internal diameter and length.
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