OK guys, I bought a custom built XS650 Street scrambler a couple of years or so ago and from the outset was disappointed, to say the least, by its lacklustre performance. However, after two years of heartaches, frustrations and expense, which led to almost selling the bike at a loss just to rid myself of a problem, perseverance has won through and I now have a bike which performs way beyond my expectations. I thought the story might be of interest.
So, the bike I bought, rather, imported from Australia, is a 1980 XS650G customised as a street scrambler. It has a 270 rephased motor, built by the late Daryl Hutcheon, who specialised in building true 270 rephased XS motors and having his own cams made for them. It had a 2-1 rev mega exhaust and a Probe electronic ignition and VM34 carbs.
When rode, the bike was gutless, just flat and uninteresting, topping out at around 70-75mph!! Wasn't sure what the problem was, so talked to Howard Smedley, who founded Smedspeed. Howard suggested I measure the cam profile and get back to him, as he wanted to rule this out. I duly measured and plotted the cam lift curves for inlet/exhaust and discovered that overlap was 93 degrees. Howard confirmed that the cam was mis-ground and from this Smedspeed offered to get one of their 'hot street cams' ground for me, a one off in effect, due to it being a 270 rephase. This was during covid, so took a while to arrive.
Once fitted, I called upon the help of a friend, with decades of experience tuning standard through to very highly tuned Triumph twins to help me get the bike set up properly. After setting the cam and ignition timing we stuck it on his rolling road and fired it up to see how it performed. Before we got chance to set up the carbs the Probe ignition had a catastrophic failure. Dead as a doornail. As the guy who made these stopped production some time ago, and cannot be contacted, I had to find a suitable alternative. Not many out there for 270 motors. So I approached Pipes N Stuff who kindly agreed to make me a one off TCI to 270 degrees. I installed it and got back to testing, which didn't go well. The bike was now misfiring heavily from half throttle and above. So bad it wasn't safe to ride while misfiring. It just seemed, at that point in time, like the bike was fighting me every step of the way.
So, not sure of what the problem was, and having checked the cam and ignition timing several times, I began the process of trying combinations of PJs, MJs, NJs, JNs and needle slots, all to no avail. The severe misfiring above half throttle, around 4000rpm upwards, remained.
As my stock of Mikuni brassware was limited, my friend suggested we stick a pair of his Amal MK2 Concentrics on my bike, borrowed off a 750 T140 with hot street cam, as he had lots of brass ware for these, so hopefully getting us a tuning result. But no such luck. Try as we might, so jetting combinations had any effect whatsoever on the misfiring. From this we deduced we needed to look elsewhere for the cause of the problem. Our attentions turned to the aftermarket 2-1 exhaust.
We swapped this out for a stock XS650F exhaust system, but I cut off the balance pipes and welded up the resulting openings. Not expecting much I headed out on the road again. Result: misfiring totally gone, pulled quite well, just a slight stutter pulling away from idle and a bit flat midrange. At this point the needle was in slot 3 (from top). I decided to raise the needle one slot to see if a bit more fuel would help. It didn't, it made it worse. So i decided to lean it up two slots and that made all the difference. Now it pulled smoothly throughout. I changed pilot from 20 to 25 and this got rid of the idle stutter. I was very happy, as it looked like we'd turned a corner, finally, after a long rocky road of testing different carbs and brassware. My only problem now was that the stock pipes looked crap on the bike and killed the street scrambler look. So it was looking like I would have to get a set of 2-2 pipes made, with twin rev megas on one side, calculating headers and megas based on motor and cam parameters (my mate would do that part of the job).
However, in the meantime, we took apart the mega I had on the bike originally and it appeared that the internals were preventing the gases doing what they needed to do (according to my knowledgeable mate - great having one of those, haha!). Effectively it was a straight through system with the rev meg bit doing little. So he proceeded to do a simple mod (cut away the through pipe and blanked off the cut end up by the rev cone end, and then re-assembled it. I stuck the whole 2-1 system back on the bike, with modified mega, and today gave it a test run, fully expecting the heavy misfiring to return. BUT....it didn't. Instead, the bike was utterly transformed. Unbelievably so. From idle it pulled like a train, VERY hard right through the throttle range. So hard I struggled to keep up changing gear, seriously! In what appeared no time at all, literally, I was hitting 85-90 in fourth! Just like that. In fact the bike was accelerating that fast it quite shocked me, hard to say from one who recently owned a 1290 Superduke and always rode on sport mode. That bike always gave me an ear to ear grin when I nailed it, and that grin has returned once again on my XS. Great news indeed, after such a long and rocky road. So glad I didn't sell it now.
So all I wanted to do was to share my experience, frustrations and struggles and say to those out there having similar performance issues: stick at it, don't give up because you WILL resolve it if you want it hard enough.
Now all I have to do is decide how important the clutch slip is (only when I nail it). The clutch basket has been machined to allow 7 standard frictions, which in my case are EBC race frictions, as used on early Yam R1's; they fit straight in.
Anyway, apologies for such a long post, but it's been a long road travelled, but got a bike that is truly of awesome performance now.
This is my new 'beast'
So, the bike I bought, rather, imported from Australia, is a 1980 XS650G customised as a street scrambler. It has a 270 rephased motor, built by the late Daryl Hutcheon, who specialised in building true 270 rephased XS motors and having his own cams made for them. It had a 2-1 rev mega exhaust and a Probe electronic ignition and VM34 carbs.
When rode, the bike was gutless, just flat and uninteresting, topping out at around 70-75mph!! Wasn't sure what the problem was, so talked to Howard Smedley, who founded Smedspeed. Howard suggested I measure the cam profile and get back to him, as he wanted to rule this out. I duly measured and plotted the cam lift curves for inlet/exhaust and discovered that overlap was 93 degrees. Howard confirmed that the cam was mis-ground and from this Smedspeed offered to get one of their 'hot street cams' ground for me, a one off in effect, due to it being a 270 rephase. This was during covid, so took a while to arrive.
Once fitted, I called upon the help of a friend, with decades of experience tuning standard through to very highly tuned Triumph twins to help me get the bike set up properly. After setting the cam and ignition timing we stuck it on his rolling road and fired it up to see how it performed. Before we got chance to set up the carbs the Probe ignition had a catastrophic failure. Dead as a doornail. As the guy who made these stopped production some time ago, and cannot be contacted, I had to find a suitable alternative. Not many out there for 270 motors. So I approached Pipes N Stuff who kindly agreed to make me a one off TCI to 270 degrees. I installed it and got back to testing, which didn't go well. The bike was now misfiring heavily from half throttle and above. So bad it wasn't safe to ride while misfiring. It just seemed, at that point in time, like the bike was fighting me every step of the way.
So, not sure of what the problem was, and having checked the cam and ignition timing several times, I began the process of trying combinations of PJs, MJs, NJs, JNs and needle slots, all to no avail. The severe misfiring above half throttle, around 4000rpm upwards, remained.
As my stock of Mikuni brassware was limited, my friend suggested we stick a pair of his Amal MK2 Concentrics on my bike, borrowed off a 750 T140 with hot street cam, as he had lots of brass ware for these, so hopefully getting us a tuning result. But no such luck. Try as we might, so jetting combinations had any effect whatsoever on the misfiring. From this we deduced we needed to look elsewhere for the cause of the problem. Our attentions turned to the aftermarket 2-1 exhaust.
We swapped this out for a stock XS650F exhaust system, but I cut off the balance pipes and welded up the resulting openings. Not expecting much I headed out on the road again. Result: misfiring totally gone, pulled quite well, just a slight stutter pulling away from idle and a bit flat midrange. At this point the needle was in slot 3 (from top). I decided to raise the needle one slot to see if a bit more fuel would help. It didn't, it made it worse. So i decided to lean it up two slots and that made all the difference. Now it pulled smoothly throughout. I changed pilot from 20 to 25 and this got rid of the idle stutter. I was very happy, as it looked like we'd turned a corner, finally, after a long rocky road of testing different carbs and brassware. My only problem now was that the stock pipes looked crap on the bike and killed the street scrambler look. So it was looking like I would have to get a set of 2-2 pipes made, with twin rev megas on one side, calculating headers and megas based on motor and cam parameters (my mate would do that part of the job).
However, in the meantime, we took apart the mega I had on the bike originally and it appeared that the internals were preventing the gases doing what they needed to do (according to my knowledgeable mate - great having one of those, haha!). Effectively it was a straight through system with the rev meg bit doing little. So he proceeded to do a simple mod (cut away the through pipe and blanked off the cut end up by the rev cone end, and then re-assembled it. I stuck the whole 2-1 system back on the bike, with modified mega, and today gave it a test run, fully expecting the heavy misfiring to return. BUT....it didn't. Instead, the bike was utterly transformed. Unbelievably so. From idle it pulled like a train, VERY hard right through the throttle range. So hard I struggled to keep up changing gear, seriously! In what appeared no time at all, literally, I was hitting 85-90 in fourth! Just like that. In fact the bike was accelerating that fast it quite shocked me, hard to say from one who recently owned a 1290 Superduke and always rode on sport mode. That bike always gave me an ear to ear grin when I nailed it, and that grin has returned once again on my XS. Great news indeed, after such a long and rocky road. So glad I didn't sell it now.
So all I wanted to do was to share my experience, frustrations and struggles and say to those out there having similar performance issues: stick at it, don't give up because you WILL resolve it if you want it hard enough.
Now all I have to do is decide how important the clutch slip is (only when I nail it). The clutch basket has been machined to allow 7 standard frictions, which in my case are EBC race frictions, as used on early Yam R1's; they fit straight in.
Anyway, apologies for such a long post, but it's been a long road travelled, but got a bike that is truly of awesome performance now.
This is my new 'beast'
