Ironhead Air Cleaner, Carb, Spark Plugs, and Valve Adjustment

Kevin has wrapped up servicing the rear end of his ’77 Sportster. He is now ready to move forward with inspection and service of the Ironhead air cleaner box and carburetor.

It is not common to see a Harley-Davidson from the ’70s that still has a fully functioning and unadulterated air box and carburetor. This Ironhead Sportster may look a little rough, but it is very much intact and fully operational. Kevin removes the Ironhead air cleaner cover and the old foam air filter. The filter will be replaced upon reassembly. The backing plate is removed next.

The Ironhead air cleaner plate is secured with three Phillips head screws and a support bracket on the back right. The Phillips screws lock onto the backing plate with star washers that will also be replaced upon reassembly. With the Ironhead air cleaner assembly out of the way, Kevin inspects the stock Keihin carburetor as well as the intake manifold.

He inspects the accelerator pump, the petcock, and the throttle cable for proper function. The plastic fuel inlet and fuel hose are inspected for cracks or leaks. The intake seals and bands are inspected as well as float bowl seal and overflow hose. Kevin takes a moment to explain how to set the carburetor mixture screw as well as proper jetting for the stock carburetor.

Next, the old spark plugs are removed and inspected prior to being discarded. Kevin explains how to read your spark plugs so that you can determine how your motorcycle is truly running. Black or “sooty” spark plugs tell you that a motorcycle is running rich. White spark plugs are an indication of a lean running condition. Ideally, you hope to achieve a chocolate-brown coloring.

Lastly, Kevin gives us a demonstration on valve adjustment. Valve adjustment must be done with the engine cold. The pushrod tube retaining clips are removed, allowing for the pushrod tubes to be pushed up. This exposes the bottom end of the pushrods and the top adjusting section of the solid tappets.

The tappet that is being adjusted must be all the way down on the cams base circle prior to adjustment or inspection. When the tappet is adjusted correctly, you should be able to turn the pushrod with your fingers but with a light degree of resistance. Once all four valves are adjusted, the pushrod tubes are reassembled, the motorcycle is returned to neutral, and a fresh set of spark plugs are correctly gapped and installed.

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