Tune Up a BMW /5 Airhead Engine


| 4/13/2015 11:14:00 AM


Tags: November/December 2014, BMW, How-To,

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.

keith
4/23/2015 7:36:11 AM

I have this bike. I have a devil of a time setting the point gap as it is only possible with the advance unit in place, which obscures the points. It is then a very tight space and the job is done by trial and error which causes unhappiness. This article shows setting the points without the advance in place but that also removes the lifting cam. What am I missing? Thanks! Keith





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