1974 MX100 Help

Give it a small shot of starting fluid (ether) through the back of the carb with the throttle open. Close the throttle, and give it a kick. If it starts, fuel delivery is still not right in the carb.

Are the points clean and adjusted correctly ? Exhaust pipe clear ?

Are you getting spark? Those condensers tend to go bad and if the points are even the slightest bit dirty, they won't start. Which plug are you using? You may want try try a colder plug, like a B7ES. Is the bottom end clean? You could have a bad crank seal and if you're not getting spark, then you've been loading up the bottom end with raw fuel when it gets past the rings.

A B7ES is the stock plug and I would not go any colder with a B8ES or B9 because the heat range is not the cause of the problem. Do a compression test first to see if you even have enough compression to start and go from there. Any time they sit like that you will need to completely dissemble the carb and clean it and replace any orings and make sure the needle isnt stuck closed same with the petcock. Also check to see if the air filter has broken down and is falling apart, the foam disintegrates after a while. You need 3 basic things to run fuel,air and fire so if you have spark then you need to make sure you are getting the other two.

For what it's worth, you can have spark, and still not get it to run. Went through this recently with my old Indian Boy Racer. Nice blue spark, new points, condenser, only a few pops and farts. Closed the plug gap down quite a bit, and it would fire for a second or two, but not really run.

By passed the complete magneto system, using a known good coil and a 12V motorcycle battery. Basically, just the points on the plate was the only part used in this test system.

It fired on the first kick. Did not try to mess with the old ignition much. Bought a CDI system for it, and it runs great, starts easy. The old system showed spark, but must have been on the weak side.

I helped a friend with his old 74 MX100 years ago. The best thing to happen to it was installing a later model RT100 CDI system on it. Direct fit.
74 MX100 engine short tracker I restored for a friend years ago. The RT100 system was used on this bike.