Hey all. Long winded post from a newbie. You have been warned!
Just installed a 27 tooth front sprocket on an "almost stock" '05 Tomos Arrow. Prior to the sprocket, I had already installed the Tomos "performance" exhaust (which is pretty mediocre), upjetted from a 56 to a 58 main jet, replaced the stock airbox with a conical filter, replaced the cheap oil the dealer put in with Amsoil synthetic (2 stroke and gearbox), and replaced the stock spark plug with an NGK BR9ES (one range hotter than stock). I saw a gain of about 10KM/H (6MPH) over stock after doing these mods. After upgrading the front sprocket from a 26 tooth to a 27 tooth, I gained an additional 5KM/H (3MPH) with no noticeable loss in acceleration or mid-range. So as things sit right now, I have an average top speed (measured by GPS, on flat land, no wind, 180lb rider, and based on an average of 5 samples) of about 70KM/H (43MPH). Now if only I could get my hands on a 22 tooth sprocket for the rear. Theoretically, that would give me an extra 6KM/H (4MPH) top speed, and I doubt that torque or acceleration would suffer much since this thing has TONS of both to spare. Oh yeah, and a REAL pipe would be nice too! Anyway, if anyone else out there is contemplating a front sprocket install on an Arrow, here's a little "how to" guide:
1) Loosen off the chain tension adjusters to make the chain as slack as possible, remove the left crank arm, put a piece of broomstick between one of the rear spokes and the TOP of the swing arm to immobilize the rear wheel, and disconnect the spark plug wire (just to be safe).
2) Remove the covers (plastic thing that hides the wiring, plastic cover on oil pump, and of course the actual engine cover). I didn't disconnect the oil pump since I was using a hand tool to do the cutting and grinding of the engine cover; I just propped the cover up out of the way and covered the oil pump with a rag to keep debris out of it.
3) Use a small metal scraper ($5 in the paint section at Home Depot) and a hammer to bend the locking washer flat.
4) You will need a 30mm wrench to get the nut off ($12 at TSC). Don't waste your time with vise grips, adjustable wrenches, etc. Use the proper wrench! Once the nut and washer are off, the old sprocket should just slide right off.
5) The actual process of getting the chain on the new sprocket and then mounting the sprocket requires equal parts finesse and muscle. The chain will be TIGHT. Remove the broomstick from the rear wheel so you can work the chain and have the locking washer and nut handy so you can start threading it on there once you have got the sprocket in position (you will need to hold it in place while you start tightening the nut). Once the nut is finger tight, put the broomstick between one of the rear spokes and the BOTTOM of the swing arm to again immobilize the rear wheel. Wrench the nut on good and tight, bend the locking washer back into position, and remove the broomstick from the rear wheel. Give the rear wheel a few spins with your hand to be sure everything is turning smoothly and tighten up the chain tension adjusters while you're at it.
6) Forget grinding the inside of the cover. You will be cutting out a good sized chunk from where the channel for the oil lines starts, down to the little triangular gussets, and then across and up just past the "rib" that is located toward the bottom of the cover. If you are using a Dremmel tool for this, use the re-enforced cut off wheels and go slowly unless you like having the cut off wheels break apart and go flying off in all directions. Grind the sharp edges when you are done cutting, and clean up all the pretty metal dust that this procedure leaves behind.
7) Give the rear wheel a few more spins with your hand and put a little grease on the new sprocket. Put the engine cover back on making sure you have the oil pump clutch assembly lined up properly (there's a mark inside the cover for horizontal alignment) and give the rear wheel a few more spins to be sure nothing is making contact inside the cover. Connect the spark plug wire, and start the engine. Ensure that the oil pump is working and that everything is running normally. Put the remaining covers back on, strap on a helmet, and go for a ride.
Hope someone out there finds this useful. Will try to keep my next post shorter.