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Flip-Up Honda Chrome Fuel Cap Gasket Replacement Procedures

Following are procedures that will greatly decrease the effort required to replace the fuel cap sealing gasket used on the ubiquitous Honda chrome flip-up fuel tank filler lids used on many Honda CB and CL 350, 360, 400, 450, 500, 550 and 750cc models throughout the 1970s.

After 50 years of service or neglect, the sealing washers become brittle and cracked, no longer functioning to effectively seal the fuel tank from leaking gasoline out of the tank, or letting air/water into the tank.

Often the underside of the chrome flip cap can become rusted, stained and dirty and cleaning it in-situ is not an easy or completely effective operation.

Trying to replace the sealing gasket without knowing the following “secret procedures” (not published by Honda anywhere that I could find); can result in hours of unnecessary effort and strain.

Replacing the sealing gasket as detailed below is usually simple and quick.

Click here for the full procedure document

Jerrys 750 Plugs Fouled

Every now and then when you tinker on bikes you run into a very clear example of how carbon fouled spark plugs can drastically affect how well an engine will run.

Honda 50 Cub Horn

Recently repaired a small horn off a Honda 50 Cub. There isn't much to go wrong in a small horn, in this case the contact points were cleaned and the points tension screw was adjusted.

Oil Pressure Switch Honda CB750

Had always been curious about the components of a motorcycle oil pressure switch and having to replace a faulty switch provided an opportunity to take one apart to see how it supposed to work.

Oil Sump Screen Honda S90

For our next club engine rebuild meeting we spot welded a stainless mesh to the oil sump screen. Originally, the screen was made with a rubber gasket edge on each side but the part is no longer available, this will work well with the addition of a rubber gasket edge.

Battery Chargeing Check

We recently did some electrical work on a vintage Honda and as well as a couple broken connectors, we found a few that that were completely uninsulated due to the plastic tubes being simply gone. One of the connectors showed what we believe could be evidence of a small flame having happened. We aren't yet 100% certain but we discovered the voltage output of the charging system is a bit high at the mid / high engine speeds, during our test we were getting almost 15.2 volts which may have pushed the current higher than what the wire harness is rated for. We would like to have the bike back in the shop again to see if the voltage regulator can be adjusted to lower the charging voltage to about 14.5 volts maximum.

Ignition Points Honda CB350

Until fully electronic ignition became the norm for motorcycles, mechanical contact breakers, most often called "ignition points" were the means by which a spark was generated across the gap of a sparkplug.
The basic theory is that when a set of points is closed, it is completing the ground side of an electric circuit which then provides a voltage across (and current through) the primary winding of an ignition coil creating a magnetic field in the core of the ignition coil. When the points open and the current flow suddenly stops through the primary winding, the magnetic field collapses in the core briefly inducing a very large voltage across the secondary winding, strong enough to jump across the sparkplug gap.
The period of time that the points are closed and allowing current flow was often referred to as the DWELL angle.

The condenser in an ignition points circuit is there to act as "surge reservoir" and absorb the electrical energy that will attempt to back-flow and arc across the points when they break contact, thereby preventing damage to the points.
On any engine that uses ignition breaker points, the "points" affects the dwell period though it is seldom thought of that way, the workshop manuals generally recommend setting the points gap to 0.012 in - 0.014 in (0.3 mm - 0.5 mm) and that's all you need to worry about. For the fanatical mechanic armed with a dwell meter, rather than simply adjust the points using a feeler gauge, the dwell meter can be connected across the points and if the dwell specs are available setting the points gap with a dwell meter is much more accurate.
Once the correct points gap or dwell is set, the timing of when the points break contact is then adjusted referring to timing indicator marks on the crankshaft and engine case.
Setting the timing on a single cylinder engine is easy, but things get a little trickier with two or more cylinders. On a single cylinder engine you will have one set of points mounted on a movable backing plate, the plate can be rotated clockwise or counter-clockwise to adjust the timing of when the spark occurs at the plug.
On a multi-cylinder engine, the ideal arrangement would be to have a separate timing plate for each set of ignition points in order to achieve accurate timing of spark independently for each cylinder. We have seen this on some twin cylinder bikes, but things can get complicated and crowded when dealing with more than 2 cylinders. Many of the older twin motorcycle engines used only a single timing plate with 2 sets of points, adjusting the mounting plate for one cylinder affected the timing of the other cylinder.
If the design and manufacture was accurate, setting up the timing for one cylinder would have the spark timing happen for the other cylinder dead-on or very close, depending on how accurately the points gap was set. With this arrangement of 2 points on a single timing plate, the gap of the second set of points can be adjusted slightly to achieve dead-on timing. This is the type of arrangement seen in the photo of a '71 Honda CB-350.
While tuning up a CB-350 we shot video of the points operating, slowing it down helps to see the points cam rotating and pushing open each set.

Roller Clutch Animation
Roller clutch animation
How one type of motorcycle starter clutch works. A very effective and reliable starter system, so simple it can be difficult to understand at first.