1982 CB900F KEIHIN Tuning

[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 got 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

[Easy-X]

For starters the baseline you want to be using is what's the engine like when warmed up. And while you're heading to that point monitor the temperature of each cylinder i.e. are all of them firing? In my experience if it's really running rough you might miss you're down on one cylinder.

Low RPM rough points to the idle circuit - blocked pilot jets and/or wrong mixture screw settings. Did you do a complete carb strip down, new gaskets, etc? If so did you note the turns on the mixture screws (maybe you never touched them and they're factory set and blanked off.)

After that make sure you have proper carb cleaner, not just a can of WD40, and clean again paying special attention to the idle circuit.
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Husqvarna Vitpilen 401, Yamaha XSR700, Honda Rebel, Yamaha DT175, Suzuki SV650 (loan) Fazer 600, Keeway Superlight 125, 50cc turd scooter

[A100man]

[WD Forte]

I've worked on plenty of Keihins but usually in pairs not quads but the principle is similar.

Various stuff from memory:

Remove, strip, clean obsessively.
Change any iffy gaskets or seals.
Change em all anyway if money is no object.

Remove mixture screws and check they are not missing tips and put back fully in then unwind say 1-1/4 turns
each for intial setup.
Check vacuum fuel tap operation.
Check accelerator pump diaphragm and needle
Stick a finger in the rear and lift/drop each pisto
to see they all drop in a timely manner with no sticking.
check all butterflies have the same gap.
the ones I'm looking at have over run valves
they're supposed to enrich the mixture a wee bit to stop oppoing back on th eover run
the diaphragms in them split for a pastime in my experience on similar aged bikes and I blocked the wee feckers off and
never had issue but YMMV.

when it's up and running you'll need some vac gauges to balance them.
There will be a specific order to this
(say all with reference to cylinder 1 mebbe)
so you'll have to RTFM or ask someone who knows.
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bikers smell of wee

[stinkwheel]

Replace all the jets you can remove with stock standard ones. Replace the needle valves with viton tipped ones.

It'll be quicker in the end.

Drop Allens performance a line, they stock most genuine kehin jets and at a fraction of the cost of OEM ones. Most jets are less than a fiver each.
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“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.