1982 CB900F KEIHIN Carb tune

[Downunder Wonder]

Chasing some advice on tuning the 4 KEIHIN Carbs on my CB900.

It was in storage for 5 years, and not running when I bought it.

Carbs needed cleaning, to get rid of the old dried up fuel. After that it ran, sort of.

Below 4500 RPM it coughs, misfires and splutters and has poor acceleration.

1000 RPM idle is lumpy and most time dies after a few seconds. Stays running if I set the idle up to about 2000 RPM

Hit 4500 RPM and it comes alive and pulls like a freight train.

Would appreciate any advice and how to tune for good bottom end performance

 

[brettp]

Idle problems can be from the pilot jets (wrong size, clogged) and carb sync. Mid range sounds like a problem with your needles and/or valve pistons. Are you running a stock set up?

 

[Downunder Wonder]

G'day Brett,

Thanks for your reply.

Set up is stock. It had a 4 into 1 exhaust system fitted when I aquired it. But I also got the standard dual exhaust and refitted that. At the same time I got a carb rebuild kit, with standard jets and needles. Stripped and cleaned the carbs and rebuilt them with the standard bits.

I haven't tried balancing. So I'll look into that.

Would pilot jet needle adjust do anything for me? They are set to the standard 2 turns out from completely closed.

Needles are standard, and the pistons move freely.

Is there any sort of tuning manual where I could learn ore about these carbs? It's the first time I have seen KEIHIN's. Have worked with multiple SU carbs on autos, but this is 1st time on a bike

 

[brettp]

All of that sounds good. Your idle mixture screw is set well enough to at least idle. A poor idle that coughs and burps sounds like a carb sync to me. Do you have a stock air box or do you have pods?

 

[Downunder Wonder]

Hey Brett, I really appreciate your help.

Air box is stock. Read the other day that this engine doesn't like Pods, but that's not the problem.

Put the vacuum guage on it yesterday, and the needle fluctuates wildly. Doesn't settle down to a steady indication. Same on all 4 carbs. Maybe leaky rubbers beteen the carbs and the head. But all 4 doing the same thing???

 

[brettp]

Okay, a couple things to go through here. Your carburetor sink vacuum gauges should have an option to reduce the vacuum on the line so the needle doesn't bounce as much. If it doesn't, you'll have to do your best to adjust the carbs so that the needles bounce within the same range. Now, the needles will bounce a lot at idle but as you give the bike throttle they should settle down. But idle is the real goal.

 

[Downunder Wonder]

Got it. Thank you again!!

 

[Downunder Wonder]

Okay, a couple things to go through here. Your carburetor sink vacuum gauges should have an option to reduce the vacuum on the line so the needle doesn't bounce as much. If it doesn't, you'll have to do your best to adjust the carbs so that the needles bounce within the same range. Now, the needles will bounce a lot at idle but as you give the bike throttle they should settle down. But idle is the real goal.

G'day Brett,

Took me a few days to get hold of a multi vacuum gauge set with adjustment screws.
I used it today and am really puzzled by what I see.
I have seen plenty of vacuum gauge setups on dual carb setups on various 4 cylinder cars, but this CB900 behaves different to anything I have seen before.

If it works I'll attach photos and a video, with sound on the video.

I have never seen vaqcuum gauges reading as low as what I saw today. I'm used to seeing vacuum readings around 50 on a good engine. This one barely gets above zero. 7 at Carb 1, 12 at Carb 2 (the reference Carb, 10 at Carb 3 and 5 at Carb 4. This with the engine idling at 1000 RPM.

Increase revs to 3000 RPM and I see a slight increase of around 2 on each cylinder

So I checked compressions. 120 to 125 on all 4

I checked valve timing. Correct as described in the Clymers manual

And I checked ignition timing. Carrect as per the manual.

All 4 plugs are clean, slightly black, so maybe a little rich, But not bad.

I'm waiting on a ColorTune plug, so I can see the actual flame color in each cylinder. Poor man's way of getting flame temperatures.

One other data point. On 3 recent 1-1/2 hour rides, moderately hilly country, cruising along quietly, not pushing it, It has turned in 45 mpg

The attached has photos of the 4 Vacuum gauges. Tried to post a video, but the forum won't let me.

 

[brettp]

Mine jumped around between 0 and 20. If the needles aren't moving much, I'd say the vacuum lines on the gauges are closed off or the valve adjustment is off.
You said the valves were within spec for the manual. Did you use. 003" or. 005"?

 

[Downunder Wonder]

Hey Brett,
My reference to Valves and the manual was to say that the valve TIMING is per the book.

Not sure what you are referring to with the .003 and .005 numbers

 

[brettp]

Ah. I am talking valve lash/clearance. Putting feeler gauges between the cam and valve shims. If your valves are too tight they will affect your compression. In fact, they might be the number one variable to impact your compression.

 

[Downunder Wonder]

Got it.
Hey, I really appreciate you taking the time to educate this "Newbie".
Cam covers off again and I'lll do some measuring.
Compressions are good, so I know the valves are closing properly. But too much clearance could lead to delayed opening, which ties with what the Vacuum Gauge labels are telling me.

 

[Downunder Wonder]

Got it.
Hey, I really appreciate you taking the time to educate this "Newbie".
Cam covers off again and I'lll do some measuring.
Compressions are good, so I know the valves are closing properly. But too much clearance could lead to delayed opening, which ties with what the Vacuum Gauge labels are telling me.

 

[brettp]

If your clearances are good, then you can at least check that off the list of things to check for any other reason.
If your vacuum gauges still don't seem to register the vacuum correctly, hard to tell if it would be the gauges. But that's where my mind would go.
What does the whole gauge look like?

 

[Downunder Wonder]

If the attachment to my earlier message came through, there are 4 photos, on of each vacuum gauge. There are 4 identical gauges mounted on a backing plate. Each gauge has a rubber hose, connecting it to the associated cylinder. Each gauge has an adjusting screw, to allow closing off the vacuum connection sufficient to smooth out the needle vibrations.

I also have a single gauge, left over from my SU Carb tuning days. I used that to check each cylinder individually. I see the same wild needle vibration and when I squeeze of the connecting hose, I see the same low readings as on the 4 gauge set

 

[brettp]

They did come through. I was curious to see that adjusting screw.
Try letting the needles bounce a bit. As long as the cylinders are bouncing in the same range, you're good. Here is what I did:
1. Let needles bounce a bit, trying to make sure the bouncing was not wildly different than any other. Adjust a carb, blip throttle.
2. Ensure all bouncing in same range.
3. Rev to 2k or so and hold for a couple seconds, looking at needles.
4. Let off throttle, adjust where necessary.
Go to step 2.