Front end seems bouncy

Gentlemen I find this strange
XS 650 SE US Custom 1980
I have two bikes one fork Fine one Stiff and Bouncy.
A few years ago i rebuilt it the bouncy one .. I dont think there hardly ever was the right amount of Oil in the left leg.
The tube was scratched .. But i never suffered that much .. Gliding around cruising small roads No problems.
No wobbling or more than expected handling problems.
First years after hard braking it could stick a bit down. But came loose when giving gas.
So OK It was a cheap crashed bike everything might not be perfectly straight any more
One spring I rebuilt the fork since i knew the other one was much better.
New tubes cleaning , seals , polishing fairly good job. Better than average

I read the manual Clymer 5.7 oz ( if i read it right ) 10 w -30
Exactly the right amount oil .. in
AS the saying goes in Computing read the F ..n manual
Still worthless damping .. And I also bought a cheap tire that added to the Bouncing at a certain low speed.
" Real things have imperfections "
I drove it .. a while 2 years perhaps and came up on this thread.
This week.
I then tried to comprehend what oil to use --Even Bought the SAE 30 Oil
After googling differences in Oil I could not find any substantial difference ..
In the temperatures the bike is used here in Sweden.

So I decided to top up from the 5.7 oz to the 7.5 recommended here.
I was skeptical to say the least ..
We are talking Japanese manufacturing in its prime -- Quality attention was legendary.
We are talking a large well known company with more or less the best people in the world.
I Have been working at VOLVO here in Sweden and there were some very skilled engineers.
Rarely they got it wrong at all and virtually never sold anything bad.
from 5.7 to 7.5 is a rather large step
My impression is that i was much better with 7.5 oz oil instead of 5.7.
After a short ride
HOW can it be so ?? Are there other specs for different years.
HOW did anyone figure this out ?
is 7.5 oz the best amount or 8 ... 9 ???
is other thinner oils better.
I have been interested in Motorcycles for many years but never heard of this before
Usually the book / manual are in the right region but apparently not here ?
1.3 x more oil works ... Jawdropping
 
Yes, the later 35mm forks do work better with a bit more than the stock amount of oil. I use the recommended 10w fork oil but a little less than 7.5 ounces. I run 6.5 to 7 ounces. I also do the Minton Mods to the damper rods. My '78 has those mods, MikesXS progressive springs, and about 7 ounces of oil. On my '83, I have the same Minton Mods but stock springs with added preload spacers. I had 7 ounces of oil in them but they rode a bit harsh so I lowered the oil level a little and shortened the spacers a bit. I drew out just over an ounce of oil total for both fork legs so I'm probably running 6.5 or 6 5/8 ounces now. Actually, I set the oil levels 6" from the tops of the tubes, springs out, forks compressed.
 
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