Dealing with the CB500/550 Flat Spot

If you’ve ridden lots of other bikes, you’ve probably noticed a “flat spot” between about 3500-4800 RPM on the “mid-four”. This is caused by several factors, all relating to certain things Honda wanted this bike to do:

  1. It was supposed to idle well and start very easily. This requiired an idle A/F mixture of about 14.25:1, a little rich.
  2. It was supposed to be quiet. Long intake runners (distance between the carb slide and intake valve) helped this.
  3. It was supposed to be easy to maintain and not leak fuel, so that carbs were to sit horizontally.
  4. It was supposed to be even and smooth through the whole throttle range.
  5. It was supposed to cruise easily at highway speeds 55-75MPH and get good MPG.

It did all these things, and very well. But, the tradeoffs of design caused by the long intake runners (#2) and the angled direction change into the heads (#3) made #4 and #5 harder to obtain. The richer idle had to lean out at running speeds to get good MPG. To smooth all of these things out a little, the spark advance curve was made quick, quicker than the other fours of the day. The result: between 3500-4800 RPM (or so), the mixture was slipping from richer-than-normal to normal while the spark advancer had already reached full timing. It made the torque curve flatten out in that range, where most bikes are just getting stronger.

Here’s some simple things you can try to smooth over this “smoothie” feature.

  1. Raise the jet needle in the carb slides one notch. Install a 10-size smaller main jet at the same time (5 size smaller on last-year CB550). Switch from the standard D7E (NGK) or X22E (ND) sparkplugs to the D8E (X24E) at this same time. Check the color of your plugs to make sure it does not get too lean, which can happen if you’ve installed indivdual air filters and/or headers that actually work (most did not), or longer, megaphone-type mufflers.
  2. Advance the timing 2 degrees static, but cut off one turn from the springs on the advancer and reshape the end of the next coil so the springs will fit back onto the advancer mounts. This slows the advance curve about 5%-8%, depending on the year of your bike.
  3. Add 4 teeth to the rear sprocket. This raises the RPM a little, which shifts the lower-than-normal torque curve downward a little to a point where the torque-vs-acceleration is not so noticeable.
  4. Test out your octane ratings. Start with a tank of Regular, then try Mid-Grade, then premium. You’ll see how they affect this “flat spot”.

Here’s some harder things you can do to smooth this anomaly while increasing the power a little:

  1. Smooth the insides of the intake runners and match the ports.
  2. Polish the intake valves.
  3. Shorten the intake valve guide bosses about 2-4mm. Narrow them in the flow direction, but don’t get thinner than 50% of the original thickness.
  4. Change from the stock air filter (paper) to a K&N filter. Open up the airbox intake holes about 25%.
  5. Install a cam with 7 degrees more duration and advance it 3-4 degrees. Action Fours used to make one of these in the early 1970s.