Try setting the dipswitches manually to 7.5x100

Coppermines have locked multipliers and whatever is set via the multi dipswitch is ignored - you can only affect the fsb.
The SL4MF cpu is of the older stepping cC0 but I am not sure if that matters.
http://www.cpu-world.com/sspec/SL/SL4MF.html
I have a later cD0 version here, the SL52R, and that one runs fine on a BX board. That later stepping runs on 1.75volt and you could try to raise the voltage on your cpu accordingly but I would not have too much hope because I have not been very successful with fiddleing with early Coppermines. You could also try a PCI videocard, maybe the AGP overclock causes a problem.

i already tried 100FSB and 133FSB, but nope 😒
Voltage can only be set in bios, also tested with 1.7, 1.75 and even 1.9.

will test with a later AGP card and PCI, good idea

You could also try to find out what the board does with a native fsb 100 cpu @100 & overclocked. If it behaves correctly then (if I were you) I would try to find a cpu with cD0 stepping, they are really different.

with 100FSB i can get into the bios, but than the system hangs. tested various AGP and PCI cards. I wonder how other people manage to run a Tualatin on it, which also has 133 FSB

Question is: did you try 100FSB with a >100FSB< cpu ? If that does not work there is likely(?) something wrong with the board.

nope, because i don't have one 😀 only a celeron which is 66

Memory timings.

don't suppose that, running on auto

Which is set by SPD for 100mhz mode.

Cover the pins of the CPU itself, (not through bios selection) to force the CPU to run at 100mhz. I have a PIII 1ghz that will not post on a 400bx whatever I do. By connecting B21 to ground it forces 100mhz and will boot. If I force 100mhz through Bios it will not post.

I have this board since 2003 and a tualatin cpu on it ( also get world record of overclock... it done post and do memtest at 1950mhz 150mfb on a 1.3 tualatin cpu with lid on, just polished)
I kept as my retropc with win98 on it...

For everyday use, 1610MHz was nice for him at 1.6v

see: https://www.hardware.fr/myocdb.com/fiche11395.html

it's strange that it not work, did you try just reset, put automatic, put minimum on the board (1 memory slot at a time, cpu, good heatsink, + 1 agp or pci video, no sound card, no lan, no sata card...)
IT IS sensitive to AGP and pci card, I got a lot of them not work at overclocked AGP (matrox G400...), and finish with a generic ATI 8500 32mb card... that can do overclock easy 😉

Yes, copermine cpu is supposed work direct without pin mods or tualatin mod socket (check if some pins was not modded,cut on socket...)
You said have last modded bios... it should recognize it

To say, those board are same base as the p3bf slot and they are reliable over time, my friend have a retro asus p3fb and still works fine too
they not suffer from bad capacitor as ABIT, MSI... mine looks brand new 😉
I notices that the lock gen chip was burning hot, I put a glued radiator on since buying (it's the component that get the most hot)
On high overclock, transistor and coil get REAL hot, it need good air flow around cpu

I like it a lot, it have an isa slot to match old soundcard
When we did lan play, I was able play a day without crash (all my friend had crash every 3-4 hours of play...)

My system are:
Asus CUBX with ata66 (disabled)
celeron tualatin 1.3 good stepping SL6JT (I was tried all of them, it's the best stepping of all tualatin cpu for overclock)
3 row of very good 7 nanoseconds micro tech pc 133 cas 2/2/2 memory (2x128 and 1x256 =512mb)
promose sata HDD card, 2x160wd 7200 HDD
Dlink rj45 card
awe64 gold

The Serpent Rider wrote:

Memory timings.

check if it's at least pc 133... and good reliable brand

Also, I want an info, can you tell me the numbers on the clock generator chip? ICS-XXXXX (I always wanted to know the numbers of this chip, mine have a radiator glued on it since very long time...) 😉

Another reason to have extra procs on hand of different ratings so there would at least be something to get by with instead of this trouble.