Sport C gearboxes are remarkably fragile if they are treated 'Gently'. Most likely the bike's PO, (Or one of them.) was a posing softcock. I recently responded to another query about a newly 'Found' Sport C on another board so to save me repeating myself I hope you won't mind if I C&P even if it seems a bit odd?
Right. A few pointers on the Carb Sports.
First thing. Drain the gearbox oil. if there is anything but the very lightest 'fuzz' on the magnetic plug your first task is to pull the box and take it to bits. Sorry, but if you do it now it will save pain later on. The Carb Sports use a straight cut gearbox with only three dog engagement. they also don't have a cush drive in the rear wheel and their face cam shock absorber on the input shaft has a 'stacked belville washer' spring that is VERY unforgiving. If the bike has spent ANY time being ridden by a softcock there is a good chance that the pinions will be beginning to hammer the case hardening off some of the teeth. 5th goes first, which is lucky because it's the only gear you can replace individually. Then 3rd because this works twice as hard on the clustershaft as its where the power comes in as well as driving out to the output shaft in 3rd. IMHO It's worth going into the box anyway as the 3205ATN9 beaings on the front of the input shaft and rear of the output shaft are shite. Replace with 3205 AC3's which instead of 9 balls per race and a plastic, polyanamide, cage have 14 balls per race and a sepentine metal cage! Both are rated to over 10,000 RPM. If you're spinning your crank at over 10,000 RPM your oil pump will be cavitating and your big ends will last seconds, before that happens it'll probably chuck out a rod! You'll NEVER overtax the 3205AC3's While you're in there you can shim up the selector drum for a better change too. I'd recommend puting in a helically cut 5th with 5 dog engagement from a later Cali instead of the straight cut 3 dog 5th. You won't notice the difference and if you're riding hard in the twisties you'll never use 5th anyway!
Gearbox strip and shim instructions, with pics, are available at Greg Bender's wonderfull 'This Old Tractor' site in the archived Guzzitech dk section.
http://www.thisoldtr...m/guzzitech.dk/
Make ABSOLUTELY SURE that the drive shaft trunnions are aligned correctly. The crosses have to work in the same plane. If they are misalligned you will bust splines and destroy gears.
Carburetion. Your bike is stock. Stock they run a pressurised airbox. While this is completely pointless from a power point of view until you are going well in excess of 100mph it IS important to remember if you choose to fit pod filters and dump the airbox. To work correctly the carb float bowls are vented to the AIRBOX, fuel is PUSHED into the venturi by pressure acting on the surface of the fuel in the float bowl. To work correctly the pressure has to be equal to that in the airbox, not atmospheric. If you simply remove the airbox you HAVE to re-jet/needle/atomiser and slide or it'll run like a three legged hippo with a spear up its chuff. Keeping the airbox will allow you to get the jetting right without too much faffing about. You won't gain a lot with pods anyway so I wouldn't bother. Also the brown, anodized DellOrto carb bodies have a peculiar habit of suddenly, and for no discernable reason, *Swelling*! This isn't visible obviously but what happens is that the pillars that the float pin goes through seem to 'Grow' and this leads to the float being 'Grabbed' so it can't pivot. As a result the needle and seat leak and the float bowls over-fill causing the bike to slowly become too rich at idle and eventually die! The fix is stupidly easy. Remove the float pivot, float and needle and make a couple of qick swipes across the inside of the 'Pillars' with a smooth file to remove a thou or two of material. End of problem. Floats will now move freely so the needle closes the seat and it won't over-fuel at idle. Make sure that there is at least 2mm of free play in the 'Choke' cables.
Clutch cable also must have at least 2mm of play at the arm at the back of the gearbox. Otherwise you ride the thrust bearing and it'll fail.
Most important of all. Ride it like you stole it! NOTHING damages a Carb Sport more than treating it like spun glass. The NEED to be flogged hard and put away wet! That's just the nature of the beast!!!!
pete
PS. My guess is that the gearbox work was done to correct failed case hardening or bearings. Chances are it now has a helically cut box in. this can be seen by taking out the filler plug and having a squizz at the gears.
). Bike has 14K and a bunch of recent work including trans overhaul with closer gears by Hecht and a hotter cam (trying to find out which one was installed. I was told idle is a tad higher due to new cam). Carbs seem to be stock Dellorto 40's. Deep sump and external oil filter added. Frame was powder coated. Rear end was resealed and vented. New WP rear coil. New front disks.
