FCR Tuning – Where should I tune the main jet at full throttle?

FCR Tuning – Where should I tune the main jet at full throttle?

For small motorcycles, you may not need to worry so much, but where should you tune the main jet of a large motorcycle? There is only one answer.
[Chassis dynamo]
I can't think of anywhere other than on a chassis dynamo.
It seems that you can sign a contract for about 200,000 yen per month.
I think it's difficult for amateur DIY FCR tuning people to introduce it because it's a high hurdle and there are no places that handle it.
I think it would be better to try it on a safe straight line without traffic lights or side roads, such as an express way.
①Fix the tuning of the MJ on a straight line without traffic lights and side roads, such as an express way or bypass
②Measure the top speed and horsepower with a chassis dynamo

I think this is the flow of tuning the main jet.
I don't have a good image of chassis dynamos, so I haven't done ②, but
I think you have to try it without hesitation.
When I was riding a Z1000mk2 a long time ago, the push rod burned out every time I measured it, which was a sad experience.
This time, I will write about the selection of the main jet in my own way. I could be wrong, but I don't think it's too far off.

Taboos and things to prepare when tuning on the expressway

Do not remove or attach the FCR or throttle cable.
Removing or attaching the carburetor carries the risk of making the bike unable to be driven. It is safer to keep tuning items to a minimum on express ways and bypasses. You will undoubtedly be very disappointed if you are unable to drive the bike.
When you change the main jet, gasoline that has accumulated in the float chamber will leak.
You will need to have a measuring cup or a cloth to catch the gasoline, or something to wipe it off.
It is also best to have an O-ring for the drain bolt.

Predictable problems

Here are some serious problems you can expect when tuning the main jet.
Threading the main jet. It is possible to change the main jet with the carburetor still installed.
However, unlike the relaxed work you can do at home or in the pit, you will probably be installing and removing the main jet blind. When doing so, it is possible to imagine a problem where the main jet gets screwed off by tightening it too much. It is a good idea to test in advance how much the main jet can be threaded off.

Proper air-fuel ratio when fully open

It's just my feeling, but I try to aim for 12.5 while accelerating. It becomes lean while accelerating, and I think it also differs depending on whether it's in 4th or 5th gear or the speed range. I haven't tuned the main jet until I'm absolutely sure that this is the best way, but I try to focus on how much acceleration I get.
I don't think you should get too caught up in the numbers, so I think of them as just a guideline.
The moment it becomes lean is when you apply the engine brake or open the throttle.
An air-fuel ratio that's too rich is not good, but I think being too lean is also a problem.

Guide to selecting main jet

This is an example of when you want to make the air-fuel ratio about 0.5 rich.
Changed from MJ120 to 125.
I change the number as a guideline so that it shifts to about the same as the number of jets.
I don't think this is necessary when you want to make a big change, but changing 10 numbers at full throttle is difficult to notice, and it also has a significant impact on the acceleration feeling at about 1/2 throttle opening.
If you set the MJ to a level that gives you a good acceleration feeling at about 1/2 to 3/4 throttle opening on general roads,
you don't need to change the MJ that much, and you can just check the condition of the motorcycle when driving at full throttle. In the first place, the throttle will go through 1/2 to 3/4 before you can open it fully,
so if the throttle is finely tuned at a throttle opening below full throttle,
there is no point in driving at full throttle on the express way. When driving at full throttle, you are always accelerating. In the partial state, the motorcycle is hitting its maximum speed with the current gear.

TPO in service areas

It's time, place, and occasion when changing the main jet.

Choose a parking lot with very few people. Go quietly to a corner and change the main jet quietly. Do the work as inconspicuously as possible.

It's okay if motorcycle enthusiasts talk to you, but it's very troublesome if you get involved with a road patrolman.

Express ways are closed places unlike ordinary roads, so it's very troublesome if you get into trouble. You may also be exposed on social media with your license plate number.

It's better to avoid getting close to others as much as possible, or to take measures such as tuning with multiple people.

Troublesome others can suddenly appear from unexpected places. Someone who was talking to you in a friendly manner just a moment ago can suddenly turn into a troublesome person. At least, tuning on an express way is not illegal. There is public attention. It is important to work discreetly in a corner.

FCR for Beginners – Main Jet Tuning

It is best to start by finding a main jet that works well with a throttle opening of about 1/2 to 3/4. Even if you think that MJ110 is best, there are cases where MJ130 is far better. If you search online, you should be able to find the main jet number that is roughly suitable for your motorcycle. It's okay to make a rough selection, so find a main jet number that feels right to you, and then look for a main jet for full throttle. The answer to the main jet is whether it can achieve the top speed listed in the catalog.

FCR Tuning – Where should I tune the main jet at full throttle?【summary】

My conclusion is that it is better not to try too hard to tune the main jet when it is fully open. If you are satisfied with a throttle opening of about 1/2 to 3/4, or if you have selected a suitable main jet, then I think it will be a fine-tuning level change. Rather, I think that full-open riding is a confirmation process that the main jet is the appropriate number. There are also changes in gears and speed ranges. It also depends on how you twist the throttle. The effectiveness of the acceleration pump may also be involved.

If you find a main jet that feels good for the way you ride your motorcycle,
it doesn't have to be the best number measured completely with a chassis dynamometer.
However, tuning the FCR is a repeated AB test.

You will need to change the MJ several times before you arrive at the tuning that is best for you. It's been cold lately, so I haven't been riding my motorcycle. I hope everyone rides their motorcycles without getting sick. See you again!

CBX1000 Photo Gallery

I will post photos from the past, mainly of the area around the front master cylinder.
They are not particularly related to this blog post.