Hi

I have a very original 175mx including original rear shock which feels very soft now and having seen a pic of me on it (my profile pic), I really need to stiffen it. I was wondering if anyone knew of a stiffer upgrade from another model that will work

Found an old post -  yamaha-enduros.com/index.php/forum/1977-...-suspension-upgrades 
Markt posted  a late 70s YZ100 will fit and also DT250/400 spring, hi mark, will these both be stiffer and reduce sag?
Any other options for stiffer springs?

P.S havent been on the forum for a long time but will probably change soon as I am about to pick up a DT250 in boxes, might have a spring to try 

The type of shock those would have, wear out inside & the gas escapes past the gas/oil separating piston & mixes with the oil.
This turns it into an undamped pogo stick. I'm guess that that is what's going on & could be quite wrong. But had experience with DT175's around that time.
The spring is unlikely to loose its tension through old age--well maybe a little but should be adjustable to tension it up a little.
How does it feel?--all springy & bouncy--or slow acting like it 'has' damping.
The spring adjuster should be a thingy with room to turn it with a big C spanner or big grips etc. It'll adjust in big clicks & maybe only 3 or 4 clicks. 
All a guess by me though.

Using the 250/400 spring was an modification recommendation I read about in an old magazine. The manuals in the Tech Library have the spring rates so you can compare. It does make a difference.

You can use the spring or the whole shock from the 77-78 DT250/400. (Possibly more years) The reason I mentioned the YZ100 was the better piston and easier to rebuild due to having a nitrogen valve... plus they were available and cheap at the time.

Thanks, its got good damping but just feels soft.  Probably more to do with being heavier these days Hopefully I'll get the DT250 and experiment with the shock from that

I ended up putting a preload spacer on my DT125 shock, mainly because I could not find a 250 spring for under £150. (that would be from a probable knackered shock).

It was turned from £15 of aluminium extrusion.

I have no idea if it will damage the spring long term, I have assurances that it shouldn't.

Has reduced the sag by a good foot. It is much firmer and does not wallow through corners and bumps anywhere near as much as it used to.