Service
Service
JETTING 101 COOKED XR BEST USED BII(E INSEAM ISSUES FEEDBACK LOOP
WAKING UP A SCRAMBLER
Q: I want to install a 2-into-1 Arrow exhaust on my 2007 Triumph Scrambler. I don't want an attermarket airbox, though, because I am not trying to make this a loud performance monster. I just want to enhance the looks and sound and save a few pounds here and there. Renthal handlebars, aluminum rear sprocket, and chain guard are the modifications to date. Jetting will need to be changed. What is the jetting change you recommend? The only change I plan on making to the airbox is to pull the rubber snorkel out and keep the stock air filter.
KEVIN TAYLOR FORT COLLINS, CO
A:While we don't have direct experience rejetting a Triumph Scrambler for the Arrow exhaust, the changes you describe warrant modifying every circuit in the carburetors to optimize fueling.
The easiest way is to purchase a jet kit such as the Dynojet 5121 ($85.65), which the company says suits a bike with a stock airbox/filter and aftermarket exhaust. If you have reasonable mechanical aptitude, the installation is straightforward and the bike should run well with minor seat-of-the-pants tuning. If you want to get the most from the tunability offered by the kit, make sure the ignition system is in perfect condition and take your bike to a dyno with an air/fuel ratio meter to get a baseline with the machine bone stock.
After that, install the exhaust and run it again noting the change in air/fuel ratio at various rpm and loads. Once you've done this, you will have a pretty good idea of where fuel needs to be added or taken away, and can use the jet kit to modify the fuel curve to suit your new setup. If you're intending to ride the bike in and around Fort Collins and its 5,000-foot altitude, you will need a leaner-thannormal setup. For riding at your elevation and up, Dynojet recommends a 112 main jet, the needle in groove #3 and two and a half turns out on the mixture screw.
A good rule of thumb on jet sizes is that for every 3,000 feet of elevation gain, go down one jet size.
WELL-DONE XR
Q: I purchased a used 1995 Honda XR250R with low hours. On a trip to the mountains, I jetted the bike too lean, got it real hot, and it started smoking. I had a local bike shop bore the cylinder and order a Wiseco piston. They also did a valve job and installed new valve seals. After reassembling the bike, I went through normal break-in procedures. It started and ran well but smoked (blue) like a two-stroke after about five minutes of running. The bike shop said I just needed to put more miles on it. After I did, it still smoked, so I tore it back down. The shop checked everything again, rehoned the jug, and said I needed to put at least 500 miles on it. Honda mechanics say I needed to toss the Wiseco and put a Honda piston in, which I did.
The following is a list of things I have tried, all to no avail: 1) had machine shop check head for cracks and warpage.
Had new seals again installed; 2) honed cylinder, installed new rings, and verified end gap and ring-end stagger; 3) temporarily removed muffler to see if oil-soaked; 4) temporarily removed air filter; 5) verified crankcase breather clear of blockage; and 6) honed cylinder and installed Honda piston and rings.
I currently have about 400 miles on the last rebuild. When I first start the bike, it doesn't smoke at all. But as soon as it gets warm, it starts smoking. The bike pulls real strong, and I just check the oil often and add what it's burned.
I'm at a loss.
JERRY LEWIS LUBBOCK, TX
A:Unfortunately, what you have experienced is something we colloquially call a "scorched cylinder." The cylinder got so hot during your ill-fated jetting attempt that it changed the properties of the metal. The properties of the metals in both the cylinder and rings have to be quite exact to balance friction, sealing, and oil scraping. This impromptu heat treatment went exactly the wrong way, causing a cylinder wall that will not seal properly with the rings no matter how much it is bored. Very often in a case like this, the cylinder head itself warps as well, which can cause head-gasket sealing problems. The only corrective action is to replace the cylinder with a new unit and verify that the cylinder head has a perfectly flat surface.
GOT A MECHANICAL OR TECHNICAL PROBLEM with your beloved ride? Perhaps we can help. Contact us at cwservice@cycleworld.com with your questions. We cannot guarantee a reply to every inquiry.
INSEAM ISSUES
Q: After riding, 20 I have some decided years away to get from back into it. I recently obtained my license here in Southern California, and based on Blake Conner's November 2012 review of the Honda NC700X, I've decided to have a very serious look at this bike.
Just yesterday, I had an opportunity to sit on one and "kick the tires," if you will, at a local dealer. I'm 5-foot-8 with a 31-inch inseam, and I could barely touch the ground with the balls of my feet. I looked at various options like shaving the seat, lowering kits, and even a seat replacement. I don't want to give up comfort, as I plan to ride long distances, so the lowering kits were my direction. Soupy's makes an adjustable one that is of interest, but will I need to lower the front suspension as well? Will only 1 or 2 inches lower make a significant difference? I would think that in a very short time my "sea legs" will come back and I'll be able to return to standard settings. I would very much like to make this bike fit me and would greatly appreciate any advice, suggestions or comments.
ALEX FRANCO IRVINE, CA
A: Consider the seat modification your main path to confidence. Once you gain said confidence, there are many aftermarket seat modifiers that can build it back up by installing different density foams or even a special gel to make it even more comfortable for long journeys. While there is nothing wrong with attempting to tailor this machine to your physiology, there are potential ramifications.
Lowering a machine more than 1 inch at either end is an extreme that's best avoided. When the chassis is lowered in front by sliding it down on the fork tubes, it affects steering geometry and ground clearance and sometimes limits suspension travel. A lowering link can have similar effects, while also altering the swingarm angle, shock linkage progression, and sometimes suspension travel. Either of these modifications can result in an unsafe motorcycle. Further, if the bike is lowered too much, fenders can make contact with the frame at full compression. All of which can destabilize your bike. Changing the swingarm angle can adversely affect traction and might cause issues with chain wear and/or adjustment. Also, be aware that lowering both ends of the bike in unison will make it much easier for hard parts such as footpegs or exhaust to touch when at full lean. The NC700X isn't a bad platform for these modifications because it has a fair amount of suspension travel. So try the lowered seat first and attempt to get your "sea legs" back in that way. It's a relatively inexpensive fix, and it's easily changed back to stock with no adverse consequence on handling.
COMPRESSION LESSON
Q: I'm currently doing a top end job on my 1992 Buell RSS1200. The engine is a stock Harley Sportster 1200 as installed by Buell. Wiseco offers two pistons for this engine. One is stock 9.o:1 compression; the other is lo.5:i. Would raising the compression on this otherwise stock engine cause problems?
HANS BERTELSEN FROM CYCLEWORLD.COM
A: There is a high likelihood that the engine will detonate like a carpet bomb if the only change you make is installing lo.5:1 pistons. While it might seem like a reasonable compression number compared to the 13.5:1 (and above) ratios of many modern, liquid-cooled sportbikes, raising an aircooled engine, such as the one in your Sportster, to lo.5:1 is a large step. As part of a more involved hop-up program that includes new cams, a good valve job, and a completely new ignition system, higher compression is a very effective part of the recipe. Although you might be able to get away with just the 10.5:1 pistons and a properly tuned ignition, you'll likely gain more usable power (and not have to worry about detonation) by upgrading to higher-performance cams, a valve job, and 9.o:1 pistons.
FEEDBACK LOOP
Q: I recently read that CW reader Mark Wolfe took his BMW F800ST in to get tires changed, and when he left the dealership, a fork tube was leaking (Service, October 2013). Itmayverywell have been the dealer's fault, as the fork tubes may have been pulled by the tightening of the axle. When tightening front axles, it is not uncommon, if done incorrectly, for the fork slider to get pulled at an angle in the fork tube thus causing the seal to leak fluid. I worked at a dealership for manyyears and this happened quite frequently. Hope this helps.
JOE O'AREZZO MIDOLETOWN, VA
A: That's a great point because it's easy to take something as simple as a wheel change for granted. The order in which the front axle bolts are tightened is critical. The axle should be drawn through the fork and wheel, then the main axle nut (or bolt) on the end should be just snugged (but not to full torque) to ensure that the axle is seated against the wheel! spacer assembly. Next, torque to spec the pinch bolt(s) on the same fork leg that you just snugged the axle nut/bolt against. After this, release the machine from the stand and push down on the front suspension a few times to ensure the male and female parts of the fork legs are normalized, and then torque the remaining pinch bolts and axle nut to specification. This drastically reduces the chance that the fork legs get pulled out of alignment during wheel installation. After torquing all of these fasteners, pump the brakes to seat the pads and push down on the suspension to ensure smooth action.
