The mechanical ignition advance mechanism changes when the spark plug fires as engine speed increases or decreases. As rpm increases, centrifugal force swings the advance weights outward, rotating the cam a few degrees (typically between 15 to 30 depending on design) to advance ignition timing to fire the spark plug well before the piston reaches top dead center (TDC). As rpm drops back to idle, the weights relax and timing retards.
For points-ignition motorcycles, this mechanism must operate smoothly. If it sticks, ignition timing becomes inconsistent and the motorcycle can develop frustrating symptoms that look like carburetor problems but are not.
Important note on springs: Springs can wear, but we do not recommend tightening or bending springs to “fix” the issue. Changing spring tension changes when the weights swing out and alters the spark advance curve. If the springs are truly worn, the best fix is a used mechanism that is in good shape.
Common symptoms of a dirty or stuck advance mechanism
| Symptom | What the advance mechanism may be doing | What that changes |
|---|---|---|
| High or “hanging” idle that won’t return to normal | Stuck near full advance (weights stuck out) | Timing stays advanced when it should return to idle timing |
| Motorcycle won’t rev cleanly / feels lazy coming onto the throttle | Stuck retarded (weights not advancing) | Timing stays too late as rpm increases |
| In-between weirdness: inconsistent idle and inconsistent revving | Sticking partway through its travel | Timing “floats” instead of following a smooth advance curve |
Before you disassemble: find the index marks
Many vintage Honda advance mechanisms allow the points cam to be removed and reinstalled 180° out of phase in reference to the base plate of the mechanism. If that happens, you will never get the timing set correctly or the engine to start.
Before taking anything apart, locate:
An index mark on the points cam (often a dot or a short line on the cam edge), and
A reference feature on the base (often a half-round notch, or a small drilled hole)
312 Ignition Advance
369 Ignition Advance
323 Ignition Advance
These marks are not always “dot exactly lines up with notch.” On some mechanisms, the reference features are offset by a few degrees. The key is to note the relationship before disassembly and duplicate it on reassembly.
Rule of thumb: If you imagine a horizontal line through the mechanism, the cam mark and the base reference tend to be on the same side of that line when assembled correctly.
Tools and supplies
Solvent/degreaser and shop towels
Small brush
Small flat screwdriver or pick
Bearing grease or a quality multi-purpose grease suitable for metal-to-metal sliding contact
A clean work surface (small shims are easy to lose)
Step-by-step: clean and lubricate the advance mechanism
1) Disassemble carefully and keep parts in order
Remove the advance mechanism from the motorcycle per the factory procedure for your model.
Before removing the points cam, confirm and document the cam index mark and the base reference feature orientation.
Disassemble the unit carefully and keep:
Weights in their original positions
Springs in their original positions
Any shims/washers with the correct posts/pivots
2) Clean out the old grease
Wipe and degrease all accessible surfaces until old grease is removed.
If your mechanism has a central cavity, look for a machined groove inside the chamber. This groove acts as a grease reservoir.
Scrape and clean the groove until it is free of dried grease. Dried grease in this groove is a common reason the mechanism stops moving smoothly.
3) Inspect wear points
Inspect the following before re-greasing:
Weight pivot posts: look for galling, corrosion, or heavy wear
Weights: ensure they pivot freely without binding
Springs: verify they are intact and return the weights smoothly
Shims/washers: verify they are present and not badly deformed
Cam movement: confirm the cam plate moves through its travel smoothly
If anything is severely worn or binding even when clean, replacement is often the most reliable option.
4) Lubricate correctly and reassemble
Apply a thin coat of grease to the weight pivot posts.
If shims are used on the posts, reinstall the shims and use a small dab of grease to help hold the shim in place during assembly.
Pack fresh grease into the central cavity, including the machined grease reservoir groove. You want enough grease to lubricate and provide a reserve, but not so much that it becomes a mess. The internal reservoir groove is there to hold extra grease.
Apply grease to the sliding surfaces where the cam plate rotates relative to the base.
Reassemble the mechanism in the correct orientation:
Match the cam’s dot/line to the base’s notch/hole relationship you recorded when it was disassembled.
5) Function check before reinstalling on the motorcycle
Move the weights outward by hand. The cam should rotate smoothly into the advanced position.
Release the weights. They should return smoothly to the relaxed position without sticking.
Repeat several times. The action should feel consistent and even.
If the mechanism does not snap back cleanly or feels sticky, re-clean and re-check for dried grease, mechanical wear, or worn out springs.
FAQ
What is the mechanical advance mechanism supposed to do?
It automatically advances ignition timing as rpm rises by swinging weights outward via centrifugal force, advancing the points cam a few degrees.
What rpm should full advance happen at?
Many of these mechanisms are designed to reach full advance around ~3,500 rpm, but exact values are model-specific. Use this as a general sanity check, not a universal spec.
Can I tighten the advance springs to fix a high idle or poor running?
We do not recommend tightening springs. Spring tension affects when the weights swing out and changes the spark advance curve. If springs are worn, replace the mechanism with a good unit.
Why does the points cam orientation matter?
Because the cam can often be installed 180° out of phase. If the cam is out of phase, ignition timing will never sync up correctly no matter how much you adjust.
What should I grease?
Grease the weight pivot points and the internal cam plate movement surfaces, and clean/pack the grease inside the cam reservoir groove if your unit has one.