Yamaha YZ250 2 stroke super low end power modification

Modified Yamaha YZ250 2 stroke including:
1.Squish clearance 1,2mm, head machined on lathe to compensate inner piston height volume after cylinder lowering by 2mm.
2. Modified head dome for lower compression, 3 mm material removed
3. Shawed off cylinder by 2mm
4. Modified motocross governor, 3 shims only: starting from ball mechanism- 1mm shim from YZ250, then 0,8 shim from YZ250X, another 0,8 shim and YZ250X governor spring. 4th shim removed.
5. Governor rod shortened by 2mm, welded together in the middle
6. Governor system rod cover 2mm removed material from bottom
7. Ignition retarded almost to maximum
8. GYTR big flywheel magnet on ignition.
Jetting is close to standart 2007 year YZ250. Main jet 180. 3rd clip. 48 Pilot jet.

[EarthSurferUSA]

I never rode the stuff, but it is popular today. I would not be surprised if these engine mods fit well for the new, "Hard Enduro" type riding. (trials like riding on a dirt bike IMO).

[EarthSurferUSA]

Yes, 2mm is a lot, but you can space it back up if you desired. Your transfer port time/area is lower also, and you may be able to find more top end with the stock pipe by raising them, so they can fill the cylinder better at higher rpm's, with out losing much if any low rpm power. But that is hard to go back if you cut those roofs. IMO, you went pretty extreme for low end power, (but that is what you wanted), with exhaust port duration maybe approaching trials bikes. I love good low end power too, but I don't want to sacrifice much of the good top end power the stock bikes, (the only good power IMO). One big factor you might have wanted to try is adding a bit of length to the header section of your pipe, (I would have to look at the formula and your RPM target to know how much.) to match the lower porting target, and maybe you would not have had to lower the cylinder as much to get that lower RPM boost and retain some of the top rpm, (I know the stock pipe is not matched to the porting now, and the low end would be even a lot more powerful.). I know in the trails I hit, (sand whoops and sand berms with not many long straights.), that set up would be a blast, and easy to ride. Watch your compression ratio. If you did not open the chamber with a 2mm cylinder drop and your squish clearance, you would have a lot more compression than pump gas, (any maybe high octane race gas.), could tolerate IMO.
I port and stroke little 2 stroke engines for a living in Michigan USA so I am familiar some with the effects of changes. Thanks for showing. Would love to see it run.
Doug in Michigan USA.