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MC How-To

Step by step: From general maintenance to complete restorations, we share tips and tricks for working on classic bikes.


Tune Up a BMW /5 Airhead Engine

Last time, we showed you how to adjust the valves on a BMW /5 airhead twin. Valve adjustment is the first step in a comprehensive tune up, followed by replacing the ignition points, spark plugs, air filter, and setting timing. While replacing ignition points and setting ignition timing can be a laborious process on some bikes — Kawasaki triples spring to mind — it’s extremely straight-forward on BMW’s /5 twins.

A few points are worth noting. First, never remove the front engine cover without first disconnecting the battery. On /5 airhead BMWs, the alternator is located directly behind the front cover; if the battery’s connected you can potentially short the alternator by grounding it with the aluminum cover.

BikeMasterSecondly, if you need to rotate the engine, never try to spin it using the 10mm nut on the automatic advance unit: There’s a high probability you’ll snap the threaded snout, which is an extension of the camshaft. Instead, use a 6mm Allen wrench in the end of the alternator, and even then make sure the spark plugs are out so you’re not working against compression.

One thing we didn’t cover in the photos is the felt pad that lubricates the ignition points rubbing block by lightly oiling the camshaft. Oil the felt pad by squeezing a small amount of high temp grease into it or lightly soaking it with straight 30w oil, with the emphasis on lightly. You don’t want it flinging off grease or oil but rather slowly giving it up.

Additionally, keen eyes will notice a missing spring on the automatic advance unit on our bike. These bikes often have huge miles on them, so it’s not unheard of for one of the two springs to stretch enough that it falls off, as did one of ours. We found it inside the case, and it proved too far gone to reuse. That made accurate ignition timing on our bike a bit of challenge, and we retimed the engine after getting a new set of springs from BMW. Fortunately, they’re readily available, but they’re not exactly cheap at $12 each.

It doesn’t take much to tune an old airhead: just an air filter, a set of spark plugs, new points and a new condenser; valve cover gaskets and oil filter are optional.

That brings us to the money side of this How-To, and thanks to the /5’s simplicity it doesn’t take especially deep pockets to keep the engine running happily. A set of ignition points from BMW runs $21.67, a condenser is $16.68, air filter $17.48, and spark plugs are $5.93 each. We also changed our oil, the oil filter costing $18.62 from BMW. A note on spark plugs: Bosch no longer supplies the stock W6DC plug, only offering the WR6DC, the “R” standing for resistor. BMW dealers still stock the correct plug, and while opinions differ on whether it matters, we kept to stock. If your ignition system is marginal, too much resistance can result in a poor spark and poor running.

The average weekend warrior should be able to complete this tune-up in a few hours. Nothing about it is particularly difficult; just take your time and work through methodically. As ever, have a good shop manual at hand to aid in parts identification and proper torque specs. And when you’re done, go for a nice ride; you’ll have earned it!

1. To begin, carefully pull the spark plug leads off of the spark plugs and using a socket wrench remove the plugs from the engine.

 

2. Next, disconnect the negative battery lead. Remove the single bolt holding the horn to the frame. Once the horn’s unbolted it’s easy to disconnect the electrical wires. Don’t worry which goes where, it doesn’t matter.

 

3. Remove the three Allen bolts securing the front cover to the engine, one at roughly 9 o’clock, another at 3 o’clock and a third at 6 o’clock.

 

4. Next, carefully remove the front engine cover by pulling it straight forward until it clears the alternator and down. It should come right off.

 

5. With the cover removed, remove the single screw at approximately the 1 o’clock position securing the ignition points to the points plate.

 

6. Note the two screws securing the plate, one at 12 o’clock (visible here) and the other at roughly 6 o’clock. Remove the lower screw and clip that hold the points wiring loom in place and the points should fall free.

 

7. Next, disconnect the ignition lead at the condenser and remove the single screw securing the condenser to the engine and the protective wiring loom sheath to the condenser.

 

8. With the points and condenser removed (shown here kept loosely together), disconnect the points lead from the condenser, pull the lead from the protective sheath, then feed the new points lead back up through the sheath. Screw the new condenser in place.

 

9. Install the new points ensuring the points pivot engages the plate. Don’t forget the clip that holds the points lead in the roughly 6 o’clock position, as visible here. This screw and the opposing screw shown in step No. 6 secure the points plate to the engine. You’ll loosen both of these when timing the engine.

 

10. Using a 6mm Allen wrench rotate the crankshaft so the heel of the points cam contacts the ignition points block and lifts the points arm open. Rotate it through as necessary and stop at maximum lift.

 

11. Next, set the points gap to 0.016in. Draw the feeler gauge between the contacts; you should feel a slight drag with no slop. Adjust by loosening the single points set screw and moving the points left or right as necessary.

 

12. Remember, the points return spring puts tension on the points arm so only loosen the points screw just enough to move the points assembly or the points will close up. Once the points are set and secured, confirm the gap on your new spark plugs and install them.

 

13. Next, install the new air filter. Loosen the left carburetor at the rubber intake boot and loosen both clamps on the plastic carburetor air intake. Rotate the carb and the plastic intake toward you and downward and they’ll pull off.

 

14. Remove the long screw that goes through the engine cover immediately above the air outlet. Loosen and lift the gas tank far enough to remove the side cover, with the choke cable attached. Replace the air filter and cover, and the carb and intake, rolling the carb and intake into place together.

 

15. Now it’s time to check the timing. First, remove the black rubber plug on the side of the engine block, just behind the left carburetor. With the front cover still removed, reconnect the battery and warm up the engine, getting it to idle around 700-850rpm.

 

16. Check the timing with a timing light. If it’s correct, you’ll see an “S” with a horizontal bar above it in the middle of the window. If the “S” is at the top of the window the ignition needs to be advanced, if it’s at the bottom it needs to be retarded. Stop the engine, loosen the two points plate screws and rotate the plate left to advance and right to retard. You may have to do this several times.

 

17. Once the timing is set, check full advance by running the engine up to 3,000rpm. If the automatic advance is working correctly you’ll see an “F” mark in the middle of the window. Deviations here are usually caused by a sticking advance unit, which is fairly common as the springs wear out. Cleaning the advance unit can often help.

 

18. Finally, disconnect the battery, reinstall the front cover and the horn, then reconnect the battery. We also changed our oil, so the last thing we did was top off the oil before buttoning everything up and riding away. That’s it!