#4 cylinder running lean

[cobrajet]

Hello everyone, I have an issue that's driving me nuts. My bike is a 1981 cb750c. I just purchased it, so I don't know much of its history. My problem is #4 cylinder running very lean if firing at all. Will not idle and on accel it bogs down and backfires. I have it running with airbox off and I can hold my hand over #4 carburetor inlet and the engine rpm's speed up greatly(about 500rpm). Spraying carb cleaner into it does the same thing. This is the only cylinder that is effected this way . The others cause the engine to die out when I do this to carbs 1-2-3. I have replaced battery, verified charging system operation. Cleaned fuel tank and replace fuel filter. Removed and cleaned all carburetors including all jets. #4 carb disassembled several times looking for an issue but cannot find anything wrong there. Pushed thin wire through jets. #4 fuel bowl removed to verify it is full of fuel. Cranked engine with bowl off and verify fuel flow through float needle. Sprayed around carb and boot with carb cleaner but find no vacuum leaks(rpm increase). Verified cv piston in carb moves up and down freely. Intake boots also replaced. Spark plugs replaced. Ignition coils checked. Plug wires and resistors checked. Compression checked and reads 130psi. These are keihin cv carbs by the way. Any suggestions would be greatly appreciated. I don't know what I could be overlooking.

[TOOLS1]

Sounds like a vacuum leak, have you checked the aircut valves?
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[cobrajet]

No I have not checked those but I have read they would only cause lean backfire condition on decel only if they were bad. Is that wrong?Do Carbs have to be separated to get the covers off? Thanks for the response, this problems driving me crazy.

[shinyribs]

Could be a air cutoff valve. Their purpose is to prevent a lean pop on decel when you jump off the throttle quickly.

Hand over the carb helping sounds like increasing the vacuum signal is helping you out. Sounds like you checked the boots well for leaks. Did you remember to reinstall the crush washer for the vac port when you synched? Also, starting fluid will expose a lot of vac leaks that visual inspection never can. And carb cleaner is violent enough. Needs to be starting fluid.

Also, could be valve lash too tight causing low vac in the cylinder.  

[Maz]

Just a thought, have you checked the idle mixture screw? Is it screwed right in? Maybe worth trying to screw it out slowly whilst the engine is running to see if #4 starts firing.

[cobrajet]

I have tried turning the mixture screw out. It has no effect on running no matter how far out I turn it.

[Maz]

If it makes no difference when the mixture screw is screwed right out, I would suggest either the pilot circuit is still blocked/plugged or there is a major air leak. I would agree that the Air cut-off valve could be faulty. Pig of a job to check on #4 as you have to split the carbs to get between #3 and #4.

Maz

[cobrajet]

Thanks Maz, Well sure enough the idle circuit was restricted. Boy do I fell like a dumb a. Was sure I had it all cleaned out. Lots of spray cleaner and blow gun cleared it out. Now to get some pod air filters---juuuust kidding.

[Maz]

Thank god you're kidding. We don't want to open that can of worms, do we?!

[TOOLS1]

Filter pods.

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[cobrajet]

No, I think that topic's been beat to death already.