Regarding the spring for the mono shock on my 75 MX 400: I have seen unmarked, green, white, red and yellow  springs for sale. Please explain each  color and if there are any other colors available. And what is my best choice for a 190lb. rider. I currently have an NOS white spring installed.   

Yamaha had a service bulletin in 1975.  For the MX400B:

Yellow - 3.8 kg/mm or 213 lb/in spring rate - preload 60 lbs - for light rider 100 to 130lbs
White - 3.8 kg/mm or 213 lb/in spring rate - preload 101 lbs - for middleweight rider 135 to 160 lbs
Plain no paint - 4.0 kg/mm or 224 lb/in spring rate - preload 101 lbs - for heavyweight rider over 160 lbs

Spring that came on the bike was the plain and standard nitrogen pressure of 285 psi

They also show a "general" chart with very light yellow/white spring (125), a yellow/blue spring that has the same rate as the plain spring but only 62 lbs preload. 

Finally, they show a blue spring with a rate of 4.2 kg/mm or 235 lb/in and a preload of 110 lbs.

Sounds like plain or blue is what you need?

They also talk about adjusting the nitrogen pressure between 180 psi and 425 psi to fine tune the rear rate.

If you look at the 1976 YZ250C and YZ400C parts lists on Partzilla you will find other colors listed along with the basic metric spring rate.  Not sure if those springs would fit your bike but if they do, you have a guideline with the above info. If I can find some more detailed specs on the 1976 springs I will update.

 

Holy Crap! That's a lot of info. Thank you Mark.   

So further reading and research tells me the bike could have been a serious MX contender OR a outstanding trail ride based completely on the correct spring and preload combo. Preload could only be
set by lowering or raising the nitrogen pressure in the mono shock. Which at that time required special equipment only available at a very small number of Dealers. This certainly hampered Privateer racers and trail riders efforts to get the full potential from this bike. Given the speed of development and innovation by the Big 3 at the time somebody had to be left behind , temporarily. I found one Tech at a shop 65 miles from me who said he could do it but his current shop did not have the equipment. Later redesigns of the mono shock took away the " membrane housing" on the swing arm end thus ending this adjustability.

"Preload could only be set by lowering or raising the nitrogen pressure in the mono shock"

Not completely correct.  Yamaha sold springs with the same spring rate, but different preload.  For example the yellow spring had 60 lbs preload and the white spring had 101 lbs preload, same spring rate of 213 lbs/in.   The plain and blue springs also had the same spring rate but different preloads. 

Getting a nitrogen bottle and regulator is not as difficult (or expensive) as it used to be.  And rebuildable nitrogen shocks are becoming common on Jeeps and trucks...  Fox, King, etc.  So there might be a shop near you that has the nitrogen and equipment.  Even a Ford dealer might?  The Raptor has had Fox shocks since 2009.  (Ford dealer probably just replaces shocks though)

Even mountain bikes have gas-charged shocks.  So a good bicycle shop should have the equipment. 

UTV's also use these type shocks.

You can even get a small high pressure manual pump...  the problem is it uses air and not nitrogen...  nitrogen pressure is much more stable at different temps than air.  Which reminds me that tire shops often fill tires with nitrogen so there's yet another potential option. 

Yamaha probably used a needle-type valve for filling...  I don't remember on the early monoshocks.  Search ebay for something like "fox shock nitrogen needle" for tons of options for needles, and fill valves.  You can get a needle for a few dollars so if a shop you find only has a schrader-type connector on their nitrogen, you can bring the needle adapter...  but anyone that can service nitrogen shocks should have the needle. 

Some paintball guns use nitrogen so there's another source for filling or maybe even a  refillable bottle and regulator to DIY.

Lots of options these days.  Hopefully you can find one close by!



 

Early monoshock did use the needle to fill... 

Amazon has a nitrogen regulator for $29.95.  Needle is just a few dollars more.  Industrial gas supply would have nitrogen bottles for sale or rent or you might find another source like paintball, mountain bike shop, or off road shop that sells nitrogen bottles. 

With the Yamaha needle system you are pushing a sharp needle through a solid piece of rubber about 5mm diameter & 10mm long [guessing size & length from 45 years ago].
They used the same system on reservoir caps & also when we had to drill & tap a tapered thread to fit a genuine valve in something that never had a valve where the shock gas seal 'fat' Oring was leaking & the shock then went like a Pogo stick a gas got up in the oil.
So what i'm saying is put a drop of oil on the neede "&" down the hole to the rubber. reason being it struggles to push through the rubber & i had one case [one too many] where the rubber pushed out [pushed inside to the gas cavity] of the cap & totaly ruined my day haha.

For what it's worth, almost all mountain bike shocks still use air, so that the user can adjust the sag by adjusting volume pressure with a high pressure shock pump. Depending on your location, a lot of snowmobile shocks use nitrogen, and for whatever reason, most places that service snowmobile shocks typically do so at a fraction of mountain bike and motorcycle servicing prices. Even if they're all fox shocks being serviced, go figure.

When I asked the bike shops about filling my old bike shock ,they answered like they had no interest. Maybe I should have said it's for my 2022 Raptor. Did not know all of those Nitro shock applications were refillable. I thought they were sealed units to prevent owner tampering. Thank you all for the corrections.