Motorcycle Classics

Finding Top Dead Center, Seca Forks, And Prospector Problems

Check out these letters from readers about finding top dead center, Seca forks, and prospector problems.

Finding top dead center

Q: I’m having difficulty setting the timing on my Boyer-equipped 1966 Triumph Tiger T100SR. I’m using a Motion Pro degree wheel that measures 6.75 inches. I set the timing at 38 degrees BTDC as per the shop manual. No joy! I have also tried 19 degrees BTDC thinking of exhaust cam rotation. It’s been decades since I’ve done this, what am I missing?

Randy Randall/via email

A: It depends on where you have mounted the degree wheel. If you have it mounted on the crankshaft, the 38-degree measurement is correct. If you have it mounted on the camshaft end where the points cam sits, you cut that in half to 19 degrees. The best way to find exact TDC is by use of a piston stop or dial indicator. The piston stop is a rod that protrudes deep enough in from a spark plug sized adapter to stop the piston before it reaches TDC. You fit the degree wheel and stop tool and rotate the engine forward until the piston lightly reaches the piston stop. Make note of the position of the degree wheel and then rotate the engine backwards until the piston once again reaches the stop. Note the position of the wheel once again. TDC will be exactly between the two measurements. A dial indicator works also. The only problem is fixing it to the head in some way. Motion Pro sells a kit they market for 2-strokes, but I see no reason why it wouldn’t work on 4-strokes as well. It mounts in the spark plug hole. With a dial indicator you rotate the engine until the pointer reaches its highest point and then rotate the bezel to mark that as zero. Repeat a few times to home in on exact TDC, then use the degree wheel.

Seca forks

Q: I’m the original owner of a 1982 Yamaha FJ650R Seca. I resurrected it about 3 years ago after storing it for about 25 years. I love riding it again. The problem that I’m writing to you about is the uneven appearance of the front forks. It looks like some of the coating has worn off. This area can be polished, but the rest of the fork remains dull. I don’t care if the fork is bright or dull, but I would like a consistent finish. Do you have any suggestions?

Rich Borden/via email

A: Those forks were originally clear coated and the coating is worn off on most of the leg. The only way to get a consistent color across the entire leg would be to remove the rest of the clear coat, either with stripper chemicals or with steel wool. After it’s all removed you could then polish the whole leg to get a consistent shine. I’ve never found a way to reapply a good clear coat from a rattle can. After all that work, you just have to polish and wax it often.

Commando tag

Q:I really enjoy your column. It’s the first thing I read in every issue. I’m doing a frame up resto on a 1973 Norton Commando, and was wondering if there is anything wrong with removing the info tag on the steering head and reattaching over new paint, or should I mask it? Thanks!

Patrick Butson/via email

A: Thanks for the kind words. There’s nothing wrong with removing the number plate. The hardest part will be removing the screw rivets holding it in place. If you damage the plate or rivets, Old Britts in Washington state will sell you new ones for not a lot of money. You’d have to restamp the VIN number yourself. Update: Old Britts has announced their retirement, so get your orders in soon. Sales are limited to stock on hand. Help them eliminate their inventory if you can.

Prospector problems

Q: I recently discovered Motorcycle Classics Tech Corner. It said you were the go to guy for old Suzuki questions, so I thought I would ask one. I picked up a Suzuki TC125 Prospector recently. This is my first project bike and it is in OK shape. It ran at least. After getting it home, I noticed that the rear sprocket wiggles, and on closer inspection, it looks like the whole hub is loose. It also has a slow oil drip near the front sprocket. Being new to this kind of work (I only know what I have been able to read, so assume I know nothing), do you have any pointers? I am mostly hoping to avoid costly mistakes, get an idea of the order I should look at things, and could use your help on where/how to find parts for such and old bike. Thanks!

Garrett/via email

A: Your slow oil drip near the front sprocket is probably caused by a worn out oil seal. They’re not too hard to replace, and you should also look at the sprockets themselves to see if they are worn and need replacement. The rear hub is a different story. It’s probably worn shock absorber rubbers, causing the sprocket to float loose. All these parts are still available and not too expensive. Bikebandit and Partsfish have the parts you need, but neither has all the parts you need. Replacing the oil seal usually requires carefully drilling a small hole in the old seal, and using a slide hammer to pull the old seal out. The new seal can usually then be pushed into place to start and carefully tapped in using a wood block and hammer. A seal driver is ideal, but the drive shaft usually prevents you from using one here. A large enough socket or properly sized piece of PVC pipe can also be used. The key is going slow and keeping the seal square to the case.

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  • Published on Apr 1, 2021
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