I have a 1977 Javelin 85hp. Once a hated stepchild but now a beloved member of the family. If you are not familiar with this particular motor, it can be a bear to start when cold. Once warm, will roar at the touch of the key. That problem can seem like no spark but it likely is a more basic issue. If you search around here you'll find lots of info on this no-start issue.
I had the same problem as you - the boat's PO had no issue starting the thing but it was impossible for me. She'd just turn and turn and turn with no kick at all. Then I learned "her temper" as someone else here put it so accurately.
Here's what I've learned after a full season last year of frustrations at the launch:
1. This motor is very easy to flood. I was holding the choke for far too long and soaking the thing to death. My current start procedure is lift start lever to START position (NOT all the way up, see #2), a quick 2 second try with choke (key pressed in), wait, another 2 sec try with choke out, wait, then another with choke in. Usually will sputter and die with that...I'm patient and try again. Sometimes bumping the choke when it goes helps, after 10-20 seconds of running roll the throttle back and allow it to warm up. Do NOT overchoke. Also make sure the choke plates are well aligned and lay good and tight when closed. You need to remove the intake cover plate to see this.
2. START lever position: Start lever all the way up will almost guarantee a flood for me. That always showed up as gas smell and wet plugs. Here's what I did: open the motor and look for the moving plate that connects the two carbs, it has a cam that rests against it. You will see three lines that the cam lines up with through the throttle movement. The cam needs to line up with the middle one that says "START" next to it. I raised the start lever until the cam lined up with this START line, then marked that position on the throttle console (in my case the arm is just about 2/3s up). This has greatly helped with successful starts.
3. Clean plugs are a must.
4. RPM: This engine needs to turn at least 300RPM for the power pack to allow spark. Good solid spin from a good starter/cables/battery combination are essential.
5. Make sure the fuel primer is pumped up well (good n hard). Also I stick with non-ethanol fuel. Up here that is Shell 91.
I originally thought no spark too, so I made a spark tester with an old distributor cap. I made 4 "electrodes" out of 18ga copper wire and attached them inside to each of the 4 terminals, pointing at the center. Then adjusted each to 7/16" from the center, attached a cable and alligator clip on the center terminal to ground to the block, attached the 4 coil wires and let er rip. Worked great just sitting up on top of the motor so I could see from the front. You can also make a similar tester with a wood block and nails, guys here have done it that way too.
Sorry for the long post, but it's been a long journey and I want to give back.
