How to Tune the Honda CB 750by Michelle Schaefer
The Honda CB750 was released in the US in January 1969 and almost instantly changed the motorcycling world forever. Featuring the first mass produced in-line four cylinder engine, hydraulic disc brakes, and weighing a mere 480 pounds, the CB 750 quickly became a class by itself. Production rose from an initial 25 units a day to over three thousand units a month at the height of production. To keep your Honda CB750 operating at peak performance, a comprehensive tune-up every 4,000 miles is recommended.
Locate the air box under the fuel tank and next to the carburetors. Remove the two hold down screws and release the bottom of the air box. Remove the air cleaner element and throw it away. Install a new air cleaner elements into the air box. Fasten the bottom of the air box with the hold down screws.
Start the engine and bring it up to normal operating temperature. Shut down the engine. Loosen all four spark plug wires and remove the spark plugs. Examine the spark plugs for burned electrodes, raw gasoline, excessive carbon or other problems which may indicate an internal engine problem. If any plug indicates a problem with that cylinder, then that problem must be corrected first before any further tune-up can be done.
Connect the compression gauge to the number one cylinder. The number one cylinder is the one farthest to the left as you look at the motorcylcle while sitting on it. Press the starter button and turn the engine over for 4 to 7 seconds. Record the reading on the compression gauge.
Remove the compression gauge from the number one cylinder. Repeat Step 3 for each of the remaining cylinders. The actual readings are not as important as the difference between the cylinders. If the readings deviate more than 10 psi then there is a problem with the piston rings, valves or carbon build up.
Gap four new spark plugs to 0.24 to 0.28 using the spark plug gapping tool. Place a small drop of oil onto the spark plug threads and install the plugs into the cylinder head hand tight. Tighten the spark plugs and additional 1/4 to 1/2 turn with the spark plug wrench. Replace the spark plug wires. Over-tightening the spark plugs will destroy the plug gasket and may damage the machine threads in the spark plug hole.
Remove the ignition cover and connect a tachometer to the ignition lead. Connect the inductive timing light pick-up to the number one cylinder. Connect the timing light power leads to the battery. Start the engine and set the idle speed to 1,000 rpm.
Loosen the three screws at the outer edge of the timing base plate. Shine the timing light strobe into the window of the timing base. Rotate the points plate until the "1.4 F-1" mark lines up with the fixed pointer. Tighten the timing base plate screws securely when the timing has been adjusted correctly. Disconnect the timing light and tachometer. Replace the ignition cover.
Remove the seat of the motorcycle and turn off the fuel supply valve. Remove the side covers and the gas tank hold down bolt. Lift the tank up and to the rear to remove.
Remove the four vacuum plugs from the carburetors. Each carburetor has one vacuum plug. Connect the multi-port manometer by attaching the vacuum leads of the tool to the ports on the carburetors. Ensure that the lines are connected in order from one to four on the manometer.
Start the engine. Record the readings of each cylinder. The multi-port manometer will measure vacuum in "inches". If any cylinder deviates more than 2.4 inches from the others, then adjustment is required. The number 2 carburetor is not adjustable and will be used as a fixed reference for all other adjustments.
Locate the adjustment screw and lock nut between the number 1 and 2 carburetor. Loosen the lock nut and turn the adjustment screw until number 1 reads less than 2.4 inches difference from the number 2 carburetor. Tighten the adjuster screw lock nut without changing the adjustment.
Place the fuel tank back on the motorcycle temporarily. Turn the fuel valve on and allow the gas to flow for at least 1 minute to refill the carburetor bowls. Remove the gas tank and set it aside.
Repeat Steps 10 to 12 for making adjustments to carburetors 3 and 4 if required.
Turn off the engine and disconnect the multi-port manometer. Replace the carburetor vacuum port plugs, gas tank, seat and side covers.
- During a tune up is a great time to change the oils in the motorcycle. For a complete service change the engine oil, oil filter and front fork oil at this time.
- Engine exhaust fumes are toxic. Work only in a well ventilated are.
- Engine exhaust pipes are very hot during and after engine operation. Serious burns may result from touching a hot pipe with exposed skin.
Items you will need
- 18-mm spark plug wrench
- Metric socket set
- Metric combination wrench set
- Cross-tip screwdriver
- Air cleaner element
- 4 spark plugs
- Spark plug gapping tool
- Compression gauge
- Inductive timing light
- Multi-port manometer
- "Honda CB750 SOHC Fours 1969-1978"; Clymer; 1978
- modern sports motorcycle image by terex from Fotolia.com