Technical Q and A for classic motorcycle maintenance and repair.
Editor’s note: If you’re having trouble with that old Suzuki, BSA or BMW, Keith Fellenstein is your guy. From motorcycle tuning tips to detailed motorcycle engine repair, he can draw from a wealth of experience to help guide you to success. Send questions to: Keith’s Garage, 1503 SW 42nd St., Topeka, KS 66609, or send an e-mail with “Keith’s Garage” as your subject.
Triumph 350 Engine Build
Q: I am presently building a Triumph 650 engine that has a pre-unit bottom half and 9-stud unit barrels and head. The problem I have is the marks on the timing gears that came with the engine (in bits) do not match the images in my manual, so I have to time the valves with a degree disc and dial gauge. The gears are on the cams, and I have a manual that gives the valve timing figures for the T120 that this engine spec is now close to, i.e., T120 cams, etc. Can you please talk me through the process, remembering that I only have the idler pinion to adjust with as the gears are on the cams and I don’t have a tool to pull them off? Can I simply set the cams and crank at the spec BTDC (both top-dead-center) and slip in the idler pinion? I realize that each cam/crank BTDC must be done individually, and I can do this using paint marks as the gears rotate, or also using a dial gauge on the cam follower. Do I set the cam as soon as it moves the finger on the dial? - Albert/via email
A: The information I have shows the pre-unit and unit 650 twins with the same valve-timing specs. The intake valve opens 34 degrees before top center and closes 55 degrees after bottom center. The exhaust valve opens 48 degrees before bottom center and closes 27 degrees after top center. Using your dial gauge as you plan, check both measurements and see how they line up with the degree wheel. You should plan on checking the valves through a couple of complete rotations to make sure you have them set exactly. Being one tooth off one way or the other shouldn’t prevent the machine from running, but it will change the torque/horsepower curve. To answer your question about the use of the dial gauge, the manufacturer usually states a valve clearance to be used when timing the cams. It is usually larger than the running clearance you set when tuning up the bike. In the case of your Triumph 650 engine they specified 0.5mm for valve timing. When using the timing clearance, you would check the degree wheel when the dial indicator moves to see if it matches the manufacturer’s numbers. MC