The Service Dept

THE SERVICE DEPT

JODY NICHOLAS

POWER AND SPEED?

I am stationed in Rota, Spain, with the U.S. Navy. I’ve owned my 1971 Triumph Trophy 650 since May of 1971 and have been contented with my purchase in every way but one. I bought it with the thought in mind that I would use it mostly as a trail bike on “fire trail” type ground; this was my mistake though, because I haven’t found any such territory around worth spending the time to travel over. Instead, I have used it strictly as a road bike. Being that the Trophy has a different size gearbox sprocket than the Triumph Tiger and Bonneville, its engine and power are being wasted on the road. I would like to convert my Trophy to a road bike like the Triumph Tiger 650, thus enabling me to reduce engine wear and put the power this engine can produce to use.

The Trophy has the same size rear sprocket (47 teeth) as the Tiger, and I think that if I could change the rear sprocket, it would solve my problem. I don’t know, though, if my rear hub will take a different size sprocket, and I would like to avoid, if possible, tearing into the gearbox.

Which sprocket should I change for the most beneficial effect, giving me both take off power and higher top end? How many teeth should this new sprocket have to give me the effect mentioned above? How much, if at all, should the chain be shortened? If logical common sense wouldn’t tell me, would you give a breakdown of the procedure?

Timothy W. Stone Rota, Spain

Triumph has three Trophy models that are sold in the United States, the TR-6, TR-6R and the TR-6C. From your description I assume that you have a TR-6C with upswept exhaust pipes and probably Dunlop trials universal pattern tires.

The TR-6 and TR-6R models are fitted with a gearbox (countershaft) sprocket having 19 teeth and a final overall gear ratio of 4.96:1 (with a 47-tooth rear sprocket) in top gear, whearas the TR-6C has an 18-tooth gearbox sprocket and a final overall gear ratio of 5.31:1. Using a hypothetical tire size (which should closely approximate the size of your rear tire) and computing the top speeds of both gear ratios at 6500 rpm where maximum power is produced, we find that the bike geared 4.96 will run 98 mph and the one geared 5.31 will trip the lights at about 92 mph. Not much difference, is there?

The 47-tooth rear wheel sprocket fitted to your machine is just about as small as you can go, using the standard brake drum, and the other alternative is to change the gearbox sprocket from an 18-tooth to a 19-tooth. In order to change the gearbox sprocket you will have to remove the primary chaincase, the alternator assembly and the clutch to get at the “gearbox sprocket detachable cover” which will have to be removed to expose the gearbox sprocket. In order to do the job properly you should have Triumph special tools, part number D1861 (clutch hub extractor) and part number Z36 (gearbox nut box spanner). These tools are found in Triumph service shops here in the U.S. but they may not be available to you in Spain. Also, you may find it difficult to locate a gearbox sprocket there and would have to order one from a Triumph dealer in England, most likely.

Changing from a small countershaft sprocket to a larger one will give slightly more top speed and more relaxed cruising, and a smaller countershaft sprocket will give more acceleration at the expense of reduced top speed. The gearbox sprocket that is most suitable for the purpose your Triumph was designed for is the standard 18-tooth item. If you think it’s worth the trouble to change the gearbox sprocket for a nominal 6mph increase in top on how to port this two-stroke? for normal touring in Spain, I’d leave the gearing the way it is.

BIG AT-1

I have heard that someone manufactures a 200cc kit for the Yamaha CT-1. I now have a AT-1 that I would like to enlarge. I feel that the drivetrain will handle the 175cc kit that is available, but I wonder about the larger kit. The transmission seems strong (no problems in 7000 miles).

Do you feel that the AT-1 drivetrain could handle the 200cc kit? Do you know who markets this kit? Also, do you have any publications on how to port this two stroke?

R.S. Kistler Rolla, Mo.

I don’t know of anyone making a kit to convert an AT-1 Yamaha to 200cc, but the 175cc Yamaha cylinder and head will bolt right on to the AT-1 crankcase with no modifications. Because the CT-1 piston assembly is larger and heavier than the AT-1 assembly, the engine’s balance factor will be changed somewhat, but not enough to warrant having the crankshaft rebalanced unless you plan on high rpm racing as a steady diet for the machine. The point of maximum vibration in the rev range will be changed somewhat, but the overall vibration shouldn’t be much greater than it is now.

You will find that the electric starter will be reluctant to spin the engine over with the CT-1 top end installed, but installing a compression release and then using it when starting the engine will obviate this problem.

Porting a two-cycle engine is not a particularly complicated process, but it does require more than a hammer, chisel and a rattail file! Several special tools are required to do a first-class job, not to mention the knowledge of where to remove metal from the cylinder wall and/or piston. It’s best to leave this operation to a specialist. [O]

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