Troubleshooting carbs

[AfricaHunterCBX]

Hi all,

I'm Nick — a Dutch guy living in Japan, new to this forum and a new CBX owner.

I've recently started working on reviving my father-in-law’s CBX, which had been sitting in storage for about 20 years. Fortunately, he kept it indoors and covered with linen, so the overall condition was better than expected.

So far, I’ve removed the carbs and replaced all the rubber components, float needles, and jets in the float bowls. I thoroughly cleaned all the passages using carb cleaner and an ultrasonic bath. I set the pilot screws to 1 1/4 turns out and bench-synced the carbs to match carb #4. I also checked the valve clearances, and surprisingly, they were still within spec — likely because the bike wasn’t ridden much after its last maintenance.

After reinstalling the carbs and fitting the exhaust with new gaskets, I managed to get the bike started — a great first step toward getting it roadworthy again. However, before I reattach the airbox and finish buttoning everything up, I’d like to resolve a fueling issue that I suspect is carb-related.

Current issue:
The engine seems to be running lean — it stalls when revving to around 4000 RPM or when blipping the throttle. If I spray parts cleaner into the carbs while opening the throttle, it pulls through, which reinforces my suspicion that it's running too lean. I've checked for vacuum leaks around the manifold boots and cylinder head (which I had removed for the valve check), but couldn’t detect any leakage.

The slides move freely and have the expected resistance when tested. All passages seem clear, and the carbs were relatively clean to begin with since they were stored dry(ish).

At this point, my main suspicion is that the absence of the airbox is affecting the air/fuel ratio. From what I understand, the CBX is more sensitive to intake conditions compared to other bikes. My Africa Twin, for example, runs fine without the airbox, but that's a rugged workhorse — whereas the CBX is more of a finely tuned racehorse.

So my question is:
Has anyone here run into similar issues when testing the CBX without the airbox? Is it even possible to properly troubleshoot these carbs without the airbox and filter installed?

Any insights or advice would be greatly appreciated!

どうぞ宜しくお願い致します。

Kind regards,
Nick

Video of current situation:
https://youtube.com/shorts/T-k9cDDN6nU?feature=share

[CBX-tras]

I typically enlarge the pilot jet to #38 and set the mixture screws to 1-3/4 turns out (factory spec), providing a tiny more fuel to help the overall performance.
The airbox and filter box along with the PAPER filter provide the proper vacuum signal for the lift slides. Remove or modify them in any way will result with poor performance.
You didn't mention the year but if it's a '9 through '81 the pilot jets are pressed in and require extraction. If it's a '82 those jets are screwed in.
Simple but special tool and "no drilling, no tapping" method that I developed to extract the early jets here:
https://www.kmcarburetor.com/products/h ... moval-tool

[daves79x]

You mentioned replacing the float valves and jets. I hope you used genuine Keihin items. The aftermarket valves and jets that come in most kits are junk. As are the o-rings. I'm talking Keyster/K&L/similar. If you did not seriously re-adjust your floats after installing aftermarket float valves, they will be very likely far too low. Plus they will weep. The dimensions of the aftermarket ones are different. You should clean and re-use all original brass that's possible, including the metering needles and needle jets (not brass).

As JR said, running the bike without the airbox on does not give you a true picture. It will be far too lean.

You should also have wet bench tested the carbs for a few hours. Checking for leaks/weeps, plus verifying accelerator pump operation.

Dave

[CBX-tras]

BTW, if you apply the choke and it gets better, you're too lean. You're making it richer when you spray the carb cleaner.

If the circuit highlighted in the photo that is fed through the pilot jet, is compromised in any way, overall performance will be down.

[AfricaHunterCBX]

Thank you all for your feedback.
The model is a 82. I will go back in and check the float height.
After ensuring those are all correct I will once install it with airbox and new airfilter (paper) and test if it pulls through the whole rev range.

[Larry Zimmer]

A bit of a side note of my experience: I ave an 82. It has always had a slight flat spot between 3000 and 3400. Just enough to be noticeable. When pulling 'hard', it is not noticed. Only when doing a slow throttle advance through that range do I notice it. Just thought I would mention this to you. My rig is total stock.

[CBX-tras]

A bit of a side note of my experience: I ave an 82. It has always had a slight flat spot between 3000 and 3400. Just enough to be noticeable. When pulling 'hard', it is not noticed. Only when doing a slow throttle advance through that range do I notice it. Just thought I would mention this to you. My rig is total stock.

Larry,
A single, 0.5mm shim washer under the slide needle will cure this condition.
DM me your address and I'll mail you some.
My totally stock 1100F was the same way.

[Jeff Bennetts]

A bit of a side note of my experience: I ave an 82. It has always had a slight flat spot between 3000 and 3400. Just enough to be noticeable. When pulling 'hard', it is not noticed. Only when doing a slow throttle advance through that range do I notice it. Just thought I would mention this to you. My rig is total stock.

Larry,
A single, 0.5mm shim washer under the slide needle will cure this condition.
DM me your address and I'll mail you some.
My totally stock 1100F was the same way.

I believe the carb mixture screw washers are close to 0.5 mm, used to shim the needles with them but got away from doing that after enlarging the idle jets to the next size seemed to eliminate the flat spot.

[CBX-tras]

I was assuming that everything else was correct.

[Jeff Bennetts]

I was assuming that everything else was correct.

Larry said his setup was stock, so I’m assuming he is still running the stock #35 slow jet.

[CBX-tras]

If the anti-rotation flags are still on the mixture screws and can't be adjusted richer AND the pilot jets are #35, a single shim is the easiest remedy without removing the carbs to make those changes.

[Jeff Bennetts]

If the anti-rotation flags are still on the mixture screws and can't be adjusted richer AND the pilot jets are #35, a single shim is the easiest remedy without removing the carbs to make those changes.

Yes, it’s a workable short cut if that’s how you want to go about it.

[Larry Zimmer]

Yep, carbs/jets are stock. Removed the tabs long ago. Screws set at 1 3/4 turns. As I mentioned, the 'flat' is only noticeable when doing a slow accel through that range of about 400 rpm. Shims will certainly be easier than pulling the carbs. (PIA, that is)

[AfricaHunterCBX]

Update on my issues:
After cleaning and testing carbs multiple times, I checked the needles of the carbs again, and the new ones were rubbish. They had a more aggressive pitch to them, messing up the fueling. put stock once back in after cleaning them well and just to be sure also put stock slow jets back in. Main jet always stayed stock. Next to this I had to clean the accel pump again, as old junk got back into it. The motorcycle runs much better now and just needs to be balanced.

I have my pilot screws set to the initial amount of turns (1 1/4) and as I don't have colortune or more precise tachometer I will leave them like this unless I notice some popping on decel.

Thanks all for the advice.

[daves79x]

If you carefully bench synced the carbs, they should be very closely synced on the bike. If you didn't, the idle will never be right until you get a set of gauges on them. Glad to hear it's running better!

Dave