From what I've read the yellow spring sounds like it's probably a dalton and an after market spring. but that won't change your engagement. Secondary spring only changes shift-out and back-shift of the transmission based on load on the driveline, if there is alot of load the secondary stays shut longer keeping you in a lower gear etc. The yellow spring probably isn't stock but isn't the issue you are chasing.
Where did you order the wet clutch shoes from?
What doorfx said, you can buy a cheap tach for about $45 from most shops, I got mine through a dealers generic parts supplier, it looks exactly like this $7 one from wish.com now I wouldn't recommend getting anything from wish.com if you want it this year, but this is essentially what you are looking for, single wire, no batteries, fully sealed unit. Mine has been on my machine for 10 years now and not failed me yet, been under water, abused hit with stuff, pressure washed etc.
Here is a moose one from Revco
Providing quality and performance at a price you can afford is what we strive for, that's where the real value is.
revco.ca
Running a carbureted machine I couldn't imagine not having a Tach and an easy adjust PMS screw anymore, -10c I turn my pms out a bit for more fuel, 0c I start closing it up a bit, +10c close it a bit more. I can make trail side adjustments based off tach RPM +/- 10 rpm. makes a big difference in getting the most out of my 400. the hardest part about getting dealing with it is all the first degree burns on my hand from the cylinder. I can even adjust for elevation.