Thanks for the responses.
I understand the desire to retard the timing BECAUSE of pump has, but I'm not running pump gas so I set the timing to spec. Regardless changing timing doesn't help it get out of the hole if I fall off the pipe in top gear or if I under shift running in 4th to 6th gear. Timing has been set anywhere from 1.8 to 2.3 mm (on purpose) with no real change regarding the flat spot. I can tell you if the timing accidentally gets set to 3mm BTDC on pump gas it will pre-ignite and ruin a piston in about 30 seconds at WOT. That was a disappointing day. Seizing up at 90 MPH isn't fun, but I avoided any major drama because I heard it coming. The piston was toast but I didn't damage the cylinder (or head) on the offending side that was miss timed so the cost was contained to two new pistons.
The comments about the porting matching the chamber are spot on. If I had the $$ and the time I would have a set of custom chambers made and do a bunch of dyno time with a knowledgeable tuner, but that's not going to happen (at least not yet).
So the suggestion regarding jetting for the setup I have is the target I'm aiming for. I realize it may just be what it is and I'll have to downshift for freeway overpasses. My wife's car requires the same thing.
As a side note, there is a dramatic difference between the construction of the Factory chambers and the Millennium chambers in addition to the Factory chambers being beat to hell. The flat spot exists with either set installed. The only thing that changes with swapping chambers is the jetting. The Millennium chambers allow for a fatter main jet at WOT. With the current porting, the power is between 7 to 10K and then signs off pretty quick at 10.5K (as measured by the stock tachometer).
The Millennium chambers also seem to contribute to the noticeably better pull from about 2.5K to 4.5K. The motor runs really well when jetted right for that RPM and throttle range. Makes for nice mellow operation at commuting speeds on surface streets. But then from 4.5 to 6k it's too rich and 4-strokes at part throttle unless I'm in a taller gear. It can pull itself past the flat spot with more throttle up to 4th gear but will bog and lose speed with more throttle in top gear. If I nurse the throttle I can sometimes increase speed but generally more throttle results in less speed on an incline.
I might try the crossover tube. The manifolds aren't that expensive. I guess that first started with the Banshee motors and has been found to benefit the air-cooled RD's as well.
I've also read about Dave's mod, although I thought that was aimed at R5 motors and carbs (no reed cage models). I feel if I went so fat on the pilot it would ruin how well the motor runs below 4K at cracked throttle but maybe the different needle makes up for it. I did buy a leaner needle set, but installed I couldn't even open the throttle without an immediate stall. Maybe with 55 pilots that would solve that problem?
I do also have the stock steel reeds and have thought about reinstalling them to see what the effect would be. Yesterday I read that modified YZ 125 reeds are a better alternative to the Boyesen two stage reeds. $46 for a set of four pair on Economy Cycle, enough for two motors.
2012 Honda NC700X
2006 Gas Gas 300 Pro
2003 Kawasaki KDX220R
1976 Yamaha RD400C
caryder screwed with this post 07-07-2012 at 03:15 PM