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There’s little more frustrating than an old motorcycle that won’t start. If you’ve rebuilt a project motorcycle and it still won’t start, that just adds insult to injury.
Your old motorcycle will likely either have an electric starter only or a kick starter and an electric starter.
If you’ve got a kickstarter, you can sometimes put off fixing electric starter problems for a while – as a faulty electric starter system will kill your battery, while a kickstarter usually won’t.
For your bike to start up and run properly, you’ll need to:
- Check that your electrical components and charging system is in order.
- Set the point gap.
- Set the ignition timing.
- Set the valve clearance.
Check out this page to learn how to sync the carbs on these motorcycles.
How to adjust the point gap, valve clearance, and ignition timing on a Honda CB750
CB750 Point Gap Adjustment:
- Check that the points surfaces are not pitted or burnt – file them or replace them if they are.
- Turn the crankshaft clockwise until the points open to the biggest gap.
- Insert a feeler gauge to check that the gap is between .3 and .4 mm.
- Adjust the points by loosening the contact breaker plate lock screw and moving the
- plate until the gap is in the correct position.
CB750 Ignition timing:
- Start with the 1-4 points.
- Connect your timing light power cord to the battery and ground the ground cable on the frame.
- Attach the spark plug lead from the timing light to the spark plug wire for the 1 or 4 cylinder.
- Remove the points cover.
- Start the engine and bring it to idle.
- Make sure the points plate is loose enough to adjust but tight enough to stay in one place.
- Point the timing light at the view hole in the points plate and confirm that the F mark is aligned with the timing mark on the engine case.
- Bring the engine up to about 2,500 rpm to check that the parallel advance timing marks line up with the timing mark on the engine case.
- Check the same for cylinders 2-3.
- To adjust the timing for 1-4, carefully and slowly rotate the points plate. Rotating clockwise with retard the timing, rotating the plate counterclockwise will advance it.
- To adjust the timing for 2-3, loosen the 2 base screws for the 2-3 points plate and adjust up or down.
- Tighten the screws and confirm the timing.
CB750 Valve Clearance (Engine Cold):
- Remove the engine head cover or the tappet covers.
- Rotate the crankshaft clockwise until cylinder 1 intake tappet retracts completely and beings to rise. Check that the timing index mark on the engine case lines up with the “T” mark. Piston 1 should be a top dead center here.
- In this position, you can take the tappet clearance measurements for the intake tappets on cylinders 1 and 3 and the exhaust tappets on cylinders 1 and 2.
- With a feeler gauge, check that the clearance between the adjustment screw and the valve stem are:
- Intake: .05mm
- Exhaust: .08mm
- Adjust the clearance by loosening the tapping screw lock nut and turn the adjusting screw.
- Turn the crankshaft 360 degrees until piston 4 reaches TDC.
- Check the clearance for intake tappets on 2 and 4 and exhaust tappets on 3 and 4.
- Reinstall the gaskets and covers.
How to adjust the point gap, valve clearance, and ignition timing on a Yamaha XS650
XS650 Points gap adjustment:
- Remove the points cover.
- Check that the point surfaces are in good condition. File or replace if necessary.
- Turn the crankshaft to check that the gap at the widest position is between .3 and .4 mm.
- Adjust the gap by loosening the points screws and moving the contact point holding pate.
XS650 Ignition timing:
- Start with the rightmost cylinder.
- Connect your timing light to the battery, ground the ground cable to the frame, and connect the spark plug lead to the corresponding wire.
- Start and warm up the engine.
- Aim your timing light through the timing viewport.
- Check that the F mark lines up with the timing index mark.
- Bring the engine up to about 3,500 rpm. Check that the advance marks are in line with the index mark.
- To adjust, loosen the breaker plate screws and the the plate until the timing marks align. Retighten the screws and confirm timing.
- Repeat on the next cylinder.
XS650 Valve clearance (Engine Cold):
- Remove the tappet covers and the generator cover.
- Turn the crankshaft to line up the rotor with the T mark on the stator to place the pistons at TDC.
- With a feeler gauge check that the clearances are within the following specs:
- Intake tappet: .15mm
- Exhaust tappet: .10mm
- To adjust clearance, loosen the adjuster screw lock nut. Turn the screw to change the clearance.
- Recheck clearance once the screws and lock nut have been tightened.
How to adjust the point gap, valve clearance, and ignition timing on a Kawasaki KZ650
KZ650 Points Gap Adjustment:
- Remove the points cover.
- Check that the point surfaces are clean and in good condition. Replace or file if necessary.
- Use a 17mm wrench to turn the crankshaft clockwise until the points open to their widest gap.
- With a feeler gauge, check that the gap is between .3 and .4mm.
- To adjust the point gap, loosen the breaker base screws so you can move the base.
- Adjust the gap and tighten the screws, recheck the gap thickness.
- Repeat for the next set of points.
KZ650 Ignition Timing:
- Connect your timing light to the battery, ground the ground cable to the frame, and connect the spark plug lead to spark plug wire 1 or 4.
- Start and warm up the engine.
- Point the timing light at the timing viewport.
- Check that the F mark on the timing advancer lines up with the index mark on the engine.
- Bring the rpms up to check that the parallel advance marks line up with the index mark.
- To adjust, loosen the adjusting screws to move the plate until the indexes line up.
- Repeat for cylinders 2-3.
KZ650 Valve Clearance (Engine Cold):
- Remove the cylinder head cover.
- Use a 17mm wrench to turn the crankshaft clockwise until the Ex mark on the exhaust side cam lines points to the front and is aligned with the cylinder head surface.
- Insert a feeler thickness gauge between the cam and the valve lifter between the two valves that have clearance. It should be about .08 to .18mm.
- Turn the crankshaft ½ rotation to check 2-4 exhaust valves.
- Turn the crankshaft so that the T mark on the intake cam sprocket is facing the rear and is aligned with the cylinder head surface.
- Measure the valves with clearance.
- Turn the crankshaft ½ rotation and repeat for the other two valves.
- To adjust, you’ll need to add new shims under the lifter buckets.
- Remove the camshaft.
- Remove the valve lifter buckets and measure the existing shim thickness for the valves with improper clearance.
- Replace the shim with a size that would bring clearing into the specified limits.
- Reinstall the buckets and camshaft and repeat the measurement procedure.
Troubleshooting the Motorcycle Ignition System
If you’re bike won’t start at all, you’ll want to check that the electrical components and starter motor (electric start) for your motorcycle ignition system are working properly.
Check you battery, coils, and wires for continuity and any voltage drops.
Check that your starter solenoid is working.
If your motorcycle has been sitting for a long time and has some miles on it, it may not be starting due to a gunked up starter motor. Luckily, starter motors are very easy to clean and sometimes a quick cleaning will be all your motorcycle needs to start up again. Overtime and from the heat that the engine puts off, the o-rings and seals inside your starter motor can break down and its internals can get gunked up.
Electronic Ignitions for Old Japanese Motorcycles
If you’re tired of messing with and adjusting your points, there are plenty of aftermarket electronic ignition systems for these old Japanese bikes. Installing them is usually fairly straightforward. Typically, you’ll remove the points plate, replace the shaft on the timing advancer and install the magnetic/electronic pickups.
If your project motorcycle came complete and all the parts seem to be in working order, it’s a good idea to try to get it running properly with its original points system.
If your motorcycle came to you in pieces or if ignition parts are broken, it may be a good time to upgrade to an electronic ignition. Along with the electronic ignition, you’ll usually need new coils with a different resistance than your stock coils as well as new plug wires and caps.