VM30 Mikuni's and/or what engine mods on a 1980 XS250. What gains?

[XSpeed]

Hi,
I'm tinkering with my XS250E and considering how to make it rooly, rooly fast I'm considering simply bolting on a 400 top end, but there's also something cool about keeping it as a 250. So I'm wondering what I could realistically expect in terms of performance gain from fitting an appropriately tuned set of Mikuni VM30 carbs. Maybe mild head porting??

So who's done what mods to an XS or similar and what was your experience?

Cheers for any input.

 

[der_nanno]

I built a XS400 for vintage road racing a few years ago. There's not a lot you can do.

1) different camshaft - MegaCycle does a few grinds
2) slightly bigger (inlet) valves (from a XT600)
3) some minor porting, but said ports are fairly good, except for around the valve seats
4) bigger carbs - I ran 33mm flatlslides, but everything will be better than the stock CV-stuff
5) different exhaust 40mm O.D., 37mm I.D. was a pretty nice combo (and at least in Europe is easy to get)
6) check the squish-gap - with such HUGE squish areas, quite a bit can be gained, by tightening them up - more in the field of detonation prevention than outright power
7) ignition: some sort of electronic ignition is very useful, as once you get closer to 10k rpm, the contacts tend to bounce a lot, making ignition timing rather error prone. (You don't need something overly fancy - a Dyna-S or CHYME-138 will easily do the trick.)

 

[spotty]

Greg, you are a man of many surprises, really, racing an xs400 !!! Undoubtedly the least promising basis for a race bike ever

Xspeed, listen to what this man has to say, despite being quite European his opinions and knowledge are respected by many
Check out some of his own threads to see his work, worth the time

 

[der_nanno]

Too many kind words, Mr. Spotty. And yes I tried. In the end I built a 450 (claiming to be 500) version with GS500E pistons, but it was still too heavy and not really competitive in the class it was in. It was cheap, ratty and good fun though and to be honest: rather pretty. (On my blog its build was described as the XS400 RatRacer.)