FCR Tuning – Full Throttle! How lean can the main jet go? Will the engine hold up?

FCR Tuning – Full Throttle! How lean can the main jet go? Will the engine hold up?

his is a story about how lean I've managed to go with the main jet in the past.

First, let me clarify that I have not pushed the engine to the point of failure. Therefore, this testing is not comprehensive. The reason it's not comprehensive is that I haven't tested it to the point where the piston melts, which is what I mean by "engine failure." There might also be cases where the spark plug melts, but pushing it to the breaking point is something that’s difficult unless you're a circuit user.

I'll provide a guide to lean tuning based on past dates and combinations of jet needles and main jets. This report will confirm that the CBX1000 I tuned did not break down, indicating that it’s possible to go this lean without problems. Through this tuning, you should be able to identify the limit for how lean the main jet can be safely adjusted. The leanest main jet I have on hand is size 102.

I’ll be writing this in a column-style format. So, let's get started!

The past lean main jet sizes.

Even MJ-102 is Fine

2021-10-24
MJ-102
JN-90FTS#2
Full-throttle air-fuel ratio: around 14.5

2021-08-27
MJ-102
JN-90FTQ#1

2021-09-11
MJ-102
JN-90FTS#2

These are records from a few years ago. At the time, I wasn’t particularly concerned with the main jet sizes, but looking back now, I realize that I was using quite a lean jet size. Currently, I’m using MJ-120. Throughout the year, I’ve been tuning the main jet within a range of 115 to 138, constantly tweaking it.

Although the jet size doesn’t change proportionally, I typically see a change in the air-fuel ratio of about 0.8 to 1.0 when moving from MJ-102 to MJ-112.

Symptoms of Lean Tuning

With the FCR, tuning is fundamentally based on throttle position.

Regardless of throttle position, a lean air-fuel ratio will produce similar symptoms. The engine will rev up quickly but lose power. This is especially noticeable during takeoff or at low throttle openings, where it becomes difficult to launch properly. When you're already at a certain speed and open the throttle, there’s no sense of acceleration.

On the other hand, if the mixture is too rich, the engine feels bogged down and doesn’t run smoothly. If the throttle is fully open and the mixture is lean, the engine will rev energetically as it heads toward top speed. I believe that the engine, revving excessively with such momentum, could cause the piston to melt.

If acceleration or top speed isn’t improving, the air-fuel ratio might be too lean.

The Moment When the Piston Melts

The moment when the piston melts is generally when the throttle is fully open.

It happens in an instant, with no warning signs. Scary…

A piston can melt during a full-throttle run lasting about ten seconds or more, or during a sudden engine brake from high speeds. Extreme ignition timing can also cause it. My CBX1000 has been running on an MJ10 for over 10 years. Although, with my current understanding, it’s a lean setting, the engine hasn’t failed.

The standard heat range for the spark plug in the CBX1000 is 8.

For NGK spark plugs, the default for the CBX1000 is DR8EA. According to the service manual, a heat range of 7 is recommended for colder regions. Generally, it is advised to use an 8.

For racing applications, a heat range of 9 is sometimes used. If you increase the engine's compression or continuously operate at high RPMs, where the plug tends to retain more heat, you may need to adjust the heat range.

A lower number indicates a higher heat range, meaning the plug gets hotter and has a stronger spark but is more prone to retaining heat. Increasing the heat range above the recommended value might sometimes make the bike feel like it's running better. However, there is a risk of the plug melting, so please be cautious.

For FCR Beginners – Simple Selection of Main Jet Size

Typically, the method involves either seeking advice from others based on past data or tuning while checking the air-fuel ratio with a gauge during test rides. Alternatively, using other motorcycles of similar make and model as references is also a standard practice.

The main jet primarily affects the throttle’s full open position. It might seem challenging to select the jet size during test rides. From my experience, changing the main jet can significantly alter acceleration, even just between 1/2 to 3/4 throttle openings. If you select a main jet that works well in the 1/2 to 3/4 throttle range, you are less likely to choose an incorrect size for full throttle.

Testing full throttle on large displacement motorcycles can be difficult. Therefore, tuning the main jet based on the 1/2 to 3/4 throttle range is a good approach. However, this does not guarantee perfect tuning. Even if it's not perfect, FCR will still provide excellent acceleration.

Keep in mind that the air-fuel ratio changes with temperature and humidity. The best setting today might not be the best tomorrow. As long as the main jet is not drastically off, it should not significantly affect the engine's performance.

FCR Tuning – Full Throttle! How lean can the main jet go? Will the engine hold up?【summary】

For the CBX1000, using a main jet size of 102 will not cause the piston to melt.

It is unlikely to cause engine failure with a jet size of 102. Even when running lean in summer, it's advisable to stop around a jet size of 115. The main jets for the CB-F series also seem to be of similar sizes.

When running lean, it’s better to start with a jet size around 110 and monitor the performance. Installing an air-fuel ratio gauge allows you to quickly check the ratio at full throttle.

If you don’t have an air-fuel ratio gauge but want to know how lean you can safely go with the MJ, I hope this information is helpful.

See you again!

FCR Main Jet Replacement – Photo Gallery

To replace the main jet on the FCR, remove the drain bolt located at the bottom of the chamber. Be sure to turn off the fuel cock to prevent gasoline from continuously spilling. Additionally, protect the frame with rags or similar materials to prevent gasoline from spilling onto it during the process.