Hi guys,
Hope you've all had a great Christmas and I wish you a happy new year from over the pond.
It's taken a while for my mate to find time to look at my XS in terms of mapping the cam, comp test, leakdown test, valve timing. Anyway, we did it all ove r the Christmas break and the results are:
Compression test: LH cyl: 145psi RH cyl: 145 psi (actual figures read from guage)
Leakdown Test: LH cyl: 6% RH cyl: 6% (actual figures read from guage)
Valve timing was checked and found to be correct, as in the hole in the cam on the RH side was vertical when piston at TDC.
Valve Lift
LH Cyl: Inlet lift: .407" (Lash: 0.006") Ex. Lift: .402" (Lash: 0.012") - see attached file of map.
According to Bob Bertaut's document, detailing 'actual cam lift' for a 1980 stock XS650 motor, the 256 motor has a cam lift of 0.300" and the 447 a lift of 0.314". My motor, being a 1980 XS650 SG, is, I presume a 447 motor, so the stock cam would have had a 0.314" lift OEM. So, allowing for rocker arm ratio (1.27) and my 0.006" inlet valve lash, a stock cam with my lash should produce 0.393" of valve lift i.e (0.314"x1.27)-0.006". As my cam's actual lift is 0.407" this suggests the base circle on my cam has been ground by approx. 0.014", which I guess is a mild grind, as Daryl Hutcheon stated it had. So, now I have a better idea of my cam. Overlap and duration can be determined from my cam map, but I haven't got my head around the implications of these issues and how they might affect my motor's performance.
Since my earlier thread comments I went back to the original carb set up (VM34's), when I acquired the bike, being:
PJ: 32.5
MJ: 190
NJ: 159 P6
JN: 6F9 (@ p3)
Slide: 2.5
On the above settings, when I first got the bike it felt gutless and wouldn't make 75mph, albeit on an 18/38 final drive setup which is a bit undergeared, but had a nice tickover with airscrews at around 1 1/4 turns out. After trying alternative MJ's and PJ's and getting nowhere, I went back to the above settings but now it won't seem to settle down to a nice tickover: annoying. I've currently got an 18/33T final drive set up which I believe approximates to the 650 stock ratio and I still can't get it to roll on past 4500rpm in top gear on a ride, so still runs out of steam at around 75mph.
Given the above, unless I'm misreading everything, and tbh this is all a strange world to me yet, then my poor performance issues may well be down to issues regarding ignition timing, final drive gearing or the aftermarket exhaust. Ignition timing when checked with a strobe a couple of months ago seemed ballpark right (about 3-5 degrees advanced).
So, armed with all this information I'm still not sure where to look to improve things. Everyone, even MikesXS, suggest fitting the big bore kit offers significant performance improvement, but I'm not seeing it, although I didn't start with a stock motor, so have no baseline for comparison. Your thoughts would be much appreciated guys.