1. Like every moving component of an internal combustion engine, camshafts are deliberately timed for a specific and desired effect. Timing cams is an involved and tedious process; part of the difficulty lies in knowing what numbers to use for best results and the all-important valve-to-piston clearances that must be maintained to prevent catastrophic engine damage. To help you decide whether this is a job you would like to undertake, we'll demonstrate a step-by-step cam timing on a GSX-R750.
2. Cam timing is easiest with the engine out of the bike, but it isn't much more difficult to to do with the engine in the frame. Secure your bike and prepare it for a valve adjustment (Hands-On, June 1993). There is a need to work with the cam-chain tensioner, so remove the carburetors as well. Drain the coolant and remove the thermostat housing assembly by disconnecting the hoses at the radiator and the rear engine water pipe, and disengage the attached wires. Remove the valve cover, spark plugs and the right engine cover as well.
3. Measure your valve clearances and note them on a chart. Rotate the crankshaft to the TDC mark on cylinders 1 and 4 so that the No. 1 arrow on the exhaust cam lines up with the valve cover mating surface of the cylinder head. Loosen and remove the cam-chain tensioner cap and spring, and then the complete tensioner assembly. Next remove both camshafts (follow the suggested method in the service manual) and make any necessary shim adjustments. If you are going to slot the original cam sprockets, mark the cam and the sprocket to make reassembly easier. If aftermarket cam sprockets are to be used, slide the new sprocket over the cam (before you remove the original sprocket), match it to the cam and original sprocket and mark it. Remove the original cam sprocket bolts by holding the flat side of the cam sprocket firmly in a vice. Remount the slotted cam sprockets to the cams, centering the cam bolts in the slots.
4. Rotate the crankshaft to TDC for cylinders 1 and 4 and replace the ignition advancer with a degree wheel, using a longer bolt. Whenever the crankshaft is rotated with the cams removed, account for the cam chain so that it does not become lodged between the crankshaft and the crankcase. Fabricate a pointer out of some thick wire such as a coat hanger or welding rod. Mount the pointer with the upper right clutch case cover screw or in the 10mm top engine mount bolt hole (pictured), pointing to TDC on the wheel. Rotate the crank about 90 degrees and thread the positive stop device in either the No. 1 or No. 4 spark plug hole. Gently rotate the crankshaft forward (clockwise) until it stops and make a note of the number. Rotate the crankshaft in the opposite direction (backward, or counterclockwise) until it stops again and note that number. Add both numbers and divide by two. The derived number is where the pointer should be pointing when the crankshaft stops rotating in either direction. If it is not, use pliers to bend the pointer accordingly, and this will make sure the pointer and wheel are accurate. Remove the piston stopper and rotate the crankshaft to TDC.
5. Install the camshafts. This is a critical stage of the operation and you may want to consult the service manual. Take some time to understand and familiarize yourself with the process; apply liberal coats of assembly lube to the cam bearing surfaces and tappets. I like the Red Line Assembly lube, but theoretically anything with moly (molybdenum disulfide) in it will do fine.
With the crank at TDC for cylinders 1 and 4, rotate the exhaust cam so that the No. 1 arrow points forward (to the front valve cover mating surface) and slip the cam into place, engage the cam chain and make sure the front span of the cam chain is tight. Install the cam holders following the directions in the service manual after priming them and their bolts with assembly lube. Do not tighten the cam holders all the way down, leave a gap of about 1 or 2mm. Install the intake cam with the No. 3 arrow pointing up. Count 13 pins from the No. 2 arrow on the exhaust cam and the No. 3 arrow on the intake cam. Include the first and last pins. Observe that the cam chain is taut between the crank and the exhaust cam and the exhaust and the intake cam. Set the intake cam in its place and install the cam holders in their proper order (lube first). Do not torque the cam holders all the way down, leave about a 1 to 2mm gap like the exhaust side. Compress the cam chain tensioner plunger by releasing the ratcheting mechanism and reinstall the cam chain tensioner with a new gasket if necessary. Slide the spring in place and thread it in the cap. You should feel and hear the cam chain tensioner plunger expand to the cam chain guide. Rotate the crank in the clockwise direction very gently. If you feel any solid resistance, stop immediately and double-check your work. If all is well and all arrows point as they should, torque down the exhaust cam caps in a cross pattern to the proper specs. Rotate the engine again and repeat the process with the intake cam. After you've confirmed that all is well, tighten and torque the tensioner cap.
6. The numbers. Confer with the camshaft inspection section of the service manual for the stock timing specs. The numbers in the service manual represent the total duration of a given cam and are measured with zero clearance (a technicality that gives an impression of a bigger cam). The majority of aftermarket cam manufacturers measure at a 1mm opening. Another way to simplify the use of numbers is the term "lobe centers." In an ideal environment it should be relatively easy to measure true opening and closing with a correctly placed dial indicator gauge, but in the everyday compromises that we face it's not that easy. That is why the lobe center method is preferable to use. To find the lobe center, simply add the sum of opening and closing numbers to 180. Subtract from this sum double the smaller number and then divide it in half. Example: 28+71+180=279-(28 x 2) or 56=223 divide by 2=111.5 L.C. which is the standard intake lobe center (L.C.) of a 1995 49-state GSX-R750; 66+36+180=282-(36 x 2) or 72=210 divide by 2=105 L.C., which is the standard exhaust L.C. of a 1995 49-state GSX-R750.
For specific cam timing specs that will work for you consult someone with intimate knowledge of the subject who is willing to provide that information to you. These numbers represent the cutting edge of tuning technology and not everyone will part with them willingly.
Manufacturers of aftermarket cams supply recommended cam-timing specs with the purchase of their cams. We will offer the following suggestion: For better midrange and low-end power (roadracing and street), adjust the cams to produce a lower intake L.C. number. For more power on top (dragracing and top speed), increase the intake L.C. number. Exhaust cam timing is not as sensitive as the intake side, but the same principles apply. For fail-safe valve-to-piston clearance, look for 1mm on intake valves and 2mm on the exhaust valves.
7. Install a dial indicator gauge fitted with a long pointer on the No. 8 exhaust valve tappet (bending and custom-fitting will be necessary to achieve a pointer that won't interfere with the normal function of the cam lobe/tappet). Be sure that the valve is closed (lobe pointing away from tappet) and zero the gauge. Rotate the crank forward until the cam starts to open the valve and the dial gauge starts to move. Continue rotating until the dial gauge needle points to 1mm or 0.040 inch. Note the exact number on the degree wheel and write it on a blank work sheet. Rotate the crank backwards (making sure the dial gauge remains zeroed in) until it starts to open the valve in the other direction. This time continue to about 1.5mm or 0.060 inch, then rotate the crank forward slightly until the dial gauge points to 1mm or 0.040 inch. Make a note of this number. The first number is for the valve opening and the second is for when it closes. Now use the method we described in step No. 6 to figure out the current cam timing of the exhaust cam. If adjustment is necessary, loosen the cam sprocket bolts and rotate the crank the desired number of degrees. Once you are satisfied with the cam timing, remove the cam bolts one at a time and apply locking agent before you re-torque. The cam timing of the intake cam is done the same way as the exhaust cam and only the numbers are different.
8. Checking the valve-to-piston clearance should be done immediately after you have set a cam at a desired specification. For exhaust cams, check for minimum valve-to-piston clearance between 5 and 15 degrees before TDC. For intake cams, 5 to 15 degrees after TDC. These numbers may grow where big aftermarket cams are being used. To check the clearance, rotate the engine to the desired number and use a prying device (in the absence of an actual prying device, use a wide-bladed flat screwdriver with smoothed edges) to push down on the tappet (while the dial gauge is still engaged), observing the clearance on the dial gauge. If the clearance is sufficient (1mm intake valves, 2mm exhaust), you may continue; if it is not you may have to compromise on your cam-timing specs, or take the engine apart to make the valve pockets deeper. Without the minimum valve-to-piston clearance, there is no sense in running this motor. It will blow up.
How And Why
We contacted Yoshimura R&D; of America (909/628-4722) for a cam-timing kit (also available through Vance & Hines). It includes a degree wheel, an adapter, a positive stopper and a set of slotted cam sprockets. The tools required to undertake this job are a collection of specialized pieces and everyday hand tools. For the specialized variety, along with the degree wheel kit, a dial indicator set with a Vise-Grip-type base is needed. A pointer made out of welding rod or coat hanger wire should be an easy tool to fabricate. As for the everyday variety of tools, make sure that you have on hand 5 and 6mm Allen regular and ball-end sockets, 19mm and a long 10mm boxed-end wrenches, as well as all the everyday tools necessary to work on an engine; sockets, screwdrivers, T-handles, 1⁄4-inch drive set with a swivel 10mm socket, open ends, etc. Another invaluable tool is a Genuine Service Manual.
This story originally appeared in the April, 1996 issue of Sport Rider.