In my Jeep Owner's Bible and other books and writing, I break down engine troubleshooting to basics. First, the engine must meet these four criteria before you consider tuning:
1) Normal compression at each cylinder.
2) Correct valve lift at each valve.
3) Correct valve timing.
4) Normal oil pressure and no engine knocks.
If the engine meets these basics, then any problem related to starting would be outside the "long block" (the block assembly with all internal parts and the cylinder head with all valve mechanism pieces).
There must be no intake manifold leaks or vacuum leaks at the carburetor base gasket, throttle shaft and vacuum circuits or EGR valve. Vacuum leaks can be tested with the engine idling and use of a lower-volatility penetrant like WD40. (Warning: Avoid spraying this on high heat surfaces or near sparks!) A light mist sprayed at the fittings, intake manifold gasket, the carburetor base gasket and near the throttle shaft will cause engine speed to change if there is a vacuum leak.
Now we can address "tune issues". The tune must meet these basic requirements:
1) Adequate fuel supply, both fuel volume and pressure.
2) Adequate spark and ignition at the correct time.
3) Normal air/fuel mixtures from the carburetor.
Breaking these tune needs down to trouble areas, the fuel supply must provide enough gasoline to the carburetor. Poor fuel supply to the carburetor can be caused by a clogged tank sock, a dirty/clogged fuel filter, a defective fuel pump or a camshaft fuel pump lobe that is flat.
Overall fuel flow to the carburetor can be tested with a "T" fitting placed at the carburetor side of the fuel filter. The pump can be tested by cranking the engine with the coil wire removed (completely) from the coil and distributor cap. With a fuel pressure test gauge attached, you can check the pressure at the "T". Add a piece of fuel hose to the "T", and you can test fuel volume by running fuel into a safe container (away from heat or sparks). If pressure and volume test low, move to the fuel pump side of the filter and repeat your tests. Normal pressure and volume at the pump side of the filter means a clogged fuel filter.
Note: If you need to know the fuel pump pressure and volume required, I can furnish these details.
Air/fuel mixtures are usually either correct or not with a carburetor. If the BBD Carter has a Sole-Vac metering system, however, a defective Sole-Vac system can throw off the A/F ratios. (Non-U.S. versions of the BBD may not have this setup.) A sticking carburetor needle or low float height can also create fuel supply or mixture issues—and fuel starvation. I would look at the fuel filter and pump volume first.
See my BBD Carter rebuild details at the magazine: http://www.4wdmechan...Carburetor.html. There are many details and additional carburetor tuning comments at this illustrated article.
Spark, as you share, should not be an issue here. If you do not find a problem with the fuel side troubleshooting, we can go into ignition troubleshooting. The magazine has a number of 4.2L carburetor, tuning and EFI conversion articles to study. Type these words into the Search box: Carter BBD.
Lastly, don't overlook checking the spark plugs for either a rich or lean condition, gasoline fouling and other signs. If the engine won't start, turn off the ignition and remove a spark plug. (Protect your hands from heat!) Look for a wet plug, sooty plug or completely dry plug, each is a clue...