Author Topic: CB750 carb air screws - which way to turn?  (Read 270 times)

Offline Phil9886

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CB750 carb air screws - which way to turn?
« on: May 26, 2022, 09:53:47 AM »
Hi. I recently got my CB750 K6 back on the road again after a full carb strip / clean / carb kit and rebuild. The bike runs fine and out on the open road I have no problems.  However I let it run at about 4000 rpm for about 15 mins stationary and whereas the two left hand pipes were fine the two right hand started to blue slightly (first 6”). So that means 3 & 4 running lean correct?  So which way to turn the screw to make the mixture stronger?  Thanks. Phil. (PS. I realise was probably a mistake to run at a standstill at higher revs but what is done is done - won’t do it again).

Offline K2-K6

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Re: CB750 carb air screws - which way to turn?
« Reply #1 on: May 26, 2022, 10:06:06 AM »
We are currently having a little debate in another thread about this  :)

Turning inwards with screw will give more fuel to the cylinder.

There's a very good routine in the Honda manual about setting these though which I feel is advantageous to follow (we can get that to you if you've not got access) an innocuous looking instruction that has far reaching effects with these carburettor type. Essentially it sets the correct mixture for the hardware and fuel in use for you, along with the element that this is the slow speed carb balance too that will give smooth running and idle characteristics. 

Offline Phil9886

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Re: CB750 carb air screws - which way to turn?
« Reply #2 on: May 26, 2022, 10:09:37 AM »
Many thanks for the quick reply. If you can give me access to the Honda manual wording that would be really helpful. Thanks. Phil

Offline K2-K6

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Re: CB750 carb air screws - which way to turn?
« Reply #3 on: May 26, 2022, 12:13:37 PM »
Top line on forum selection go to either "Aladdin's cave" or "Ash's dropbox" for manuals. Should all be there for you.

If difficulty, the routine sets all the screws at baseline about 1 1/2 turns out to start, warm up to get off choke, then same routine for each carb successively,  turn the screw very slowly outward and as revs rise it should start to just falter in firing that cylinder (it's the beginning of running too lean at this point) then concisely turn the screw back inward to drop the revs by 100 rpm (this sets it at optimum for the fuel in use on that cylinder, and with just enough richness) then adjust overall tickover with master idle control to reasonable level and work through each successive cylinder until all all four are completed.

The screws may end in slightly different places one to another,  but should be reasonably close. This correct and not a blanket turns out setting for all cylinder. 

Offline philward

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Re: CB750 carb air screws - which way to turn?
« Reply #4 on: May 26, 2022, 11:04:24 PM »
Just a cautionary thought, I wouldn't want to run the engine at 4000 rpm stationary for 15 mins without some good cooling fans - don't know what others think?

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Current Bikes:-
Honda CB750K2 (1975)
Honda CB500K2 (12/1972)
Honda CR750 Replica (1972)
Kawasaki ZZR1100D3 (1995)
Kawasaki ZZR250 (1990) Project (Not started)

Offline Lobo

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Re: CB750 carb air screws - which way to turn?
« Reply #5 on: May 28, 2022, 01:14:49 PM »
Agree Phil…

I once carb balanced my CB750 on a summer’s day (no fans) and <24hrs later a coil died.

EBay fans are MUCH cheaper than Honda OEM Ignition coils…. why wouldn’t ya?

 

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