mx 250

[rohan]

Can any one tell me the original horse power rating for a mx250a 1974 model, and while we are chatting, any standard ways of gaining more power

[cyclegod]

Raise exhaust port by 1mm to 34mm from top of cylinder, raise boost port (above intake port) to 52mm from top of cylinder X 30mm wide, and lower intake port by 1mm to be 104mm from top of cylinder, use 78-92 CR125 carbon-tech reed petals on stock reed block (low tensile for bottom end or high tensile for top end) Circle-f pipe. I also have a webco head on mine plus PVL ignition, it makes good all round power although i was thinking my next upgrade would be a 34 or 36mm carb. I can't find HP or torque figures anywhere but my logic is that if it "feels" strong, goes hard, and is fast enough to test your personal fear factor then its making the right amount of HP.

[DJRacing]

The specs I have for the MX250A  had 31BHP @ 7500rpm and 3.03kgm torque @ 7000rpm. A vm30sc carb, timing is  2.3mm BTDC, gearing 51/14 and a dry weight of 101kg.

[vandy010]

carby wise, my manual says a 34mm is standard.
has anyone here run a 36 or 38?

[mx250]

Can any one tell me the original horse power rating for a mx250a 1974 model, and while we are chatting, any standard ways of gaining more power

You should specify how much you want to spend and what characteristics you want/need.

The easy/cheap/reverseable stuff

1/ OEM was 30mm carb (painted black). The best cheapest easiest thing you will ever do is put a 34mm mikuni on. -$50-100 on evilbay.

2/ Webco head give better squish and higher compression (I think), but regardless why, is worthwhile. -l$50-100 on evilbay - i would guess but would only rarely come up.

3/ cut off back off the piston skirt for better breathing. Use a DT (two rings and cheaper/easier to find) piston regullarly.

Dearer,

1/ a modern designed chamber and
2/porting as above.

I doubt the value of the PVL at $600+ unless you have a  known prob. Definitely last on my list.

Cheers, Graeme.

[cyclegod]

2 Webco heads on Aussie ebay only 2 weeks ago and a guy in Florida has a N.O.S one (his ad #3118473 is on the vintagemx.us website swapmeet)

As for the PVL ignition, I would only recommend one if you are constantly having problems with the old O.E.M stuff (remember it is 32-34 years old  ) I got lucky and found a new unused PVL on ebay and only paid $260US.

If anyone is already running a 34mm carb what are your jetting specs???, fuel/oil mix ratio

[Freakshow]

thats weird all my MXA have VM34m carbies stock and all the info i pick up says thats stock for all MXa.  the 73 and the first batches of A's did have the poxy 30 but the japs pushed them out for #028218 on the 250 and # 024833 on the 360 - if its got ANY THING ELSE ON IT now, THROW IT IN THE BIN OR SELL IT TO NATHAN.

THEY recognised the Vm34 made the thing run better so they changed it as a spare part in feb 1974 in japan and all new spare parts where shipped out as the 34mm and as a kit so unless you bike was a slow mover or yamaha australia change the tank and platics it should have the code vm34 stamped on the carby body high up on the side, or marked with numbers 36563 or 36462

NO other size carby has made the things run better,a few high end teams  tested the begeezza out of them on the salt flats for years with the YZA/b and they couldnt get anything to run as sweet as the 34 could- " thats in print "

so save you time and money on anything other than a 34mm, you can still get em new for $150 out of the USA so dont wast you time with used ebay shitters unless your paying peanuts for them.

Vm34 on a 250cc  start your Jets around #50 pilot #270 main on the 360cc start with a #300.  keep working your way back until its leans up for your track. 30:1 is what i use in all my bikes and have never had a problem 74 - 01 models, expensive and cheap oil seems the same to me currently using that shell stuff piston is cleaner than my dinner plates :OP
 
I bought a new PVL for MY mx off ebay for $175 used once ( one could ask why) i bought it after a coil scare and the fix price was $200 just for the charge coil alone - cheap insurance - couldnt tell you if they rock yet as it still sitting under my lounge in a box, just waiting for the next time that 30 yr old coil gets smart with me again