Mauser wrote: It's a bit hard to quantify, I hold the brake and push down on the forks they go down as you would expect but they seem lazy to return when I stop pressing, they return a couple of inches then stop but if I lift the barsd lightly or use the side stand they will pop up another couple....?

What you describe is how all of mine are.
 

Thanks Bart, I have the workshop manual but couldn't see any measurements for the spring length....I'll pop the cap and see how long they are

Hi Mauser, you mentioned it's quite difficult to gauge the amount of unwanted sag on your front forks (as Mark T said a little is healthy). You could try putting a couple of cable ties on the stanchions just above the dust seals, then when you depress the suspension you'll get an idea of the 'rebound' sprung distance. Also when you then assist the return by lifting the handlebars up you can see/measure the remaining travel in your forks. These observations along with measuring the spring lengths will help decide how long you need to make your fabricated spacers. Hope this helps and good luck with it all.

Ok popped the spring out and it measures 435mm so not million miles of stock 438mm.
Then did the cable tie as suggested and here are the results, front brake hard on and pushing bike forward....there is about 50mm or 2" of relax after compression is released, then lifting bike on stand this increases to 100mm or 4"
Maybe this is normal ?

Mauser, from the info you've supplied you are saying the springs return 50mm of the travel and you can then lift the bars and get a further 50mm of travel ? If so then that doesn't sound right to me. I would expect it to be more like 85mm rebound from the springs and approx 15mm 'extra' when you lift the bars. You don't mention whether initially you had overhauled i.e. stripped the internals of the forks prior to this problem occurring? Have you got the right viscosity oil and correct volumes in each fork? If all this is/was correct then consider rechecking the fork replacement section of your manual, as my manual was very specific about the order in which you retighten the various nuts involved presumably to ensure 'binding' doesn't occur. Good luck with it all

Just a small point here. If you've changed the fork seals and not used genuine or high quallity ones you can get a surprising amount of sticktion .
Couple that with old springs and you lose some boing from the springs action. I would agree too with around 15 to 20 mm for front sag. Don't fret too much if it's not bang on as you're not giving it a handful of brake before hitting a bend at 85 mph as the rear shocks are more akin to a space hopper than real suspension.

Genuine seals etc, I guess that's as good as it ever was 50 years ago, it's not a problem I only ride on tarmac and a bit of grass every now and again lol...just thought I would ask the question.

Hi again Mauser, I've done some testing on my DT250B forks and have the following results: 84mm was the amount of rebound on the forks when I  held the brake on and pushed my 12st through the bars. When I then lifted the bike the 'extra' extention on the forks read 92mm i.e. a further 8mm of travel.  Hope this helps. Andy.

Yours seems to rebound almost fully

Yep, they do spring back to within 8mm of full extension i.e. having sprung back 84mm and settled if I then put a jack under the engine bash plate (rear of the bike is tethered to M/c ramp) and raise the front wheel well off the ground the forks extend a further 8mm. Put another way the weight of my bike compresses the front forks by 8mm.