FCR Tuning – How to Adjust the AS Air Screw and PS Pilot Screw

FCR Tuning – How to Adjust the AS Air Screw and PS Pilot Screw

I will write about my recent method of adjusting the FCR screw.
I apologize if I have written anything wrong or irrelevant.
There may be better ways, but this will be useful for CBX1000 owners who are starting to tune their FCR. Adjusting the 6-cylinder screw and synchronizing with a vacuum gauge may seem like a high hurdle, but once you get used to it, it's not so much difficult as it is a hassle because there are so many of them.
As for the cycle,
① Carburetor synchronization
② PS adjustment
③ AS adjustment

Repeat ① to ③ until you are satisfied.
As a premise, I will write about a motorcycle equipped with an air-fuel ratio meter and a digital tachometer.

① First, synchronize the carburetors properly.

This is something that the godly mechanic teacher who taught me about FCR taught me, but it starts with getting the synchronization right.
Some mechanics say it won't go out of sync even if you don't worry about it too much,
but there's no harm in making precise adjustments.
If you're using a multi-vacuum gauge, be sure to calibrate the needles of each gauge carefully.
If you're using a two-pronged throttle wire, adjust the height when you first open the throttle and the height of the throttle valve when fully open
by using the idle stop screw and the tension of the throttle wire.
I feel that synchronization helps the rotation speed return better if you lower the rpm slightly when idling.

②Pilot screw adjustment

Regarding the screw adjustment, I know that it's okay to adjust each cylinder separately, but I adjust the pilot screw to the same position. You can use a digital tachometer to find the position where the idling increases for each cylinder because you can't judge by sound, but I don't feel that it's very meaningful.
By the way, if you adjust each cylinder separately, it's difficult to see the change unless you take a note of when the RPM increases, return the screw to the position before adjustment, and then look at the next cylinder. And when adjusting the pilot screw, it's difficult to see the change in RPM in the first place.
As for the specific adjustment, I adjust the air-fuel ratio at idle so that it's between 12.5 and 13.0 using the air-fuel ratio meter. It is also affected by other factors such as the air screw and oil temperature, so I check the pilot screw adjustment after touching other things and redo it several times.
Previously, I would adjust while checking the starting performance when cold during test runs and over two days, but
I can generally find the correct screw return position quickly.

③Air screw adjustment

Is it better to adjust each cylinder separately? Or is it better to adjust all cylinders to the same return? It may be a matter of preference. In the case of the CBX1000, I am not able to judge by sound.
I use a digital tachometer to find the point where the RPM increases for each cylinder.
For example, if the starting point is one turn back and the point where the RPM increases for cylinder 1 is 1.30 minutes back, I note that position. Next, I turn cylinder 1 back.
Then I search for the return position for cylinder 2. I repeat this process six times,
making adjustments for each cylinder. However, in the case of my CBX1000, the error was within about 10 minutes,
so I adjust the whole thing by turning it back 5 minutes at a time at the same time. In other words, I return all cylinders to the same amount. Then, when I find the point where the RPM is high, I adjust it in the 10 minutes rich direction,
and the air screw adjustment is complete.
Then I synchronize again.

Around the joint between the idle port and the slow port

It is probably around the junction of the idle port and the slow port.
There are cases where the air-fuel ratio changes suddenly when you open the throttle just a little.
I couldn't adjust it properly with either the pilot screw or the air screw.
By changing the straight diameter I was able to make a satisfactory adjustment.
If something seems strange when starting off, check the air-fuel ratio gauge and you will find that the area is lean.
It is hard to notice the rich direction.

Then return to carburetor synchronization in step ①

Repeat this at least twice and you will achieve excellent tuning with the throttle open at 1/4 or less.
If the jet selection is not perfect, you will need to repeat this process several times, changing the jet as you go.
In the past, I would make adjustments while repeating test runs, but recently I no longer do this. It runs smoothly, and even when racing the RPMs drop steadily, which feels great.

Caution when selecting slow jet

If the idle is set high with the slow jet rich, the pilot screw will not work. The pilot screw will not work even if the idle is not set high.
If the engine idles perfectly with the pilot screw fully closed,
the slow jet is too large. Conversely, if the pilot screw is turned back three turns and the air-fuel ratio gauge indicates 15 or higher,
the slow jet size is too small.
Otherwise, the idle may be unstable because the fuel supply cannot keep up with a paper-type fuel filter. Or the angle of the fuel hose may make it difficult to idle for long periods of time.
If the engine is not running well immediately after replacing the slow jet,
the slow jet may be clogged.

For FCR beginners – If you are replacing the SJ, you may also want to try changing the ST diameter of the JN.

If you just look at the headline, it looks like some kind of code or a presentation by a fancy person.
Translated, it is:

[If you replace the slow jet, you can also try changing the straight diameter of the jet needle. ]

The CBX1000 has six cylinders. It is what is commonly known as a 6-cylinder engine.

When replacing the SJ, the air-fuel ratio at low throttle opening changes too much.

If you think of it as fine tuning, changing only the straight diameter of the jet needle will not change much.

It is assumed that beginners do not have many jet numbers, but replacing the slow jet may increase the risk of unnecessary work.

For beginners who have many jet numbers, it may be better to try changing the jet needle first.

This is for when the situation is bad when starting.

If adjusting the screw or changing the slow jet does not improve the situation, changing the straight diameter or turning off the acceleration pump may improve the situation.

If tuning the FCR does not work, it may be because of preconceived notions.

If you are in trouble, please look into various things other than my blog and try various things.

It's also a good idea to email the person who has made the information public and ask them.

FCR Tuning – How to Adjust the AS Air Screw and PS Pilot Screw【summary】

It may not be perfect, but in the case of my CBX1000, I have been able to ride it without any particular trouble.
I have spent less time adjusting the screws during test rides.
Also, I give high priority to visible numbers, so there are few elements that depend on years of experience or intuition. Especially with carburetors, beginners will probably think that if the engine is too lean, it will break or stop working.
If the engine is too lean, you will not be able to start.
If the engine is running and you suddenly lean it, the engine will break.
It's been so cold recently that I haven't been able to ride. I think everyone is in maintenance mode by now.
Don't let the cold get you down and enjoy your motorcycle life.
See you again!

CBX1000 Photo Gallery

We will post a selection of photos from past posts, mainly of the tank being removed.
Sometimes we need to attach and detach the carburetor while we're out and about.