Author Topic: cb400f - carb sync  (Read 2842 times)

Offline blackeagles

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cb400f - carb sync
« on: February 13, 2019, 11:31:26 AM »
finally got the carbs set up something like it says in the book - bit difficult judging 1mm for the throttle slide even if you do it with the slide down - anyway its near enought but with the air pilot screw out 2 3/4 turns it runs (a bit rough) but its very rich - when i take the plugs out they are black.

when i get the sync guages and start resetting the hight of the slides to get the 16-24 cm hg +- 3cm hg reading it is going to move the slide away from the 1mm setting which i assume is just to allow you to get the bike started...which brings me onto the question

most of the youtube carb sync videos seem to say that the cm/hg reading is not important just get them all the same so how many of you folks set them between 16/24 or do you all just 'get them the same' and forget heo reading?.

i'm asking this 'cos since i'm already running very rich then the more i raise the slide (to reach the correct reading) the richer it will become (and the more the primary chain is going to 'rattle')

Offline paul G

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Re: cb400f - carb sync
« Reply #1 on: February 13, 2019, 12:14:51 PM »
Raising the slides will not make it richer that is to do with the float height and the air screw ;)
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Re: cb400f - carb sync
« Reply #2 on: February 13, 2019, 12:17:32 PM »
The synchro routine has virtually nothing at all to do with jetting. It's just to get parity across the four carbs,  and hence cylinders, to make sure that during use the engine is running with all four pulling equally.

Mixture is controlled by the jets, needle position and trimming pilot circuit with screws to get each part of the range correct.

Yes to just getting all vac levels to match and don't worry about absolute reading. If absolute is really far out when running then it would bring up different questions about engine condition, possibly. But certainly wouldn't worry about it at this stage.

Offline Rob62

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Re: cb400f - carb sync
« Reply #3 on: February 13, 2019, 01:14:05 PM »
To answer the original question "do you set to a specific vac level of just get them all the same". You will find that as you adjust the individual slides the idle speed will go up and down and you will have to keep adjusting all the slides up or down until you reach a point where they are all showing equal vacuum and the motor is at the correct idle speed. At that point the level of vacuum will totally depend on the idle speed i.e. the lower the idle the higher the vacuum (as the motor is being throttled)… the higher the idle the lower the vacuum.. So the short answer is just balance them out and the level of vacuum takes care of itself.

Offline billywingnut

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Re: cb400f - carb sync
« Reply #4 on: February 13, 2019, 04:02:50 PM »
I bought a set of cheap gauges, before using them it's best to check there calibration. I rigged up some plastic tubing ot connect all four gauges together then connected them to one of the carb inlets. I was then able to check all four gauges gave the same readings.
I tweaked one to get them all the same before using them.
It's true that as long as all for carbs read the same the levels are not that important. Dont adjust all four though, you need one carb as a reference.
two and a half turns out of the airscrew seems to be a good starting point.
good luck

Offline Bryanj

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Re: cb400f - carb sync
« Reply #5 on: February 13, 2019, 04:24:01 PM »
Dont just set at tickover but check at about 3000 rpm as well

Offline blackeagles

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Re: cb400f - carb sync
« Reply #6 on: February 14, 2019, 12:24:18 PM »
thanks for the info.

the gauges arrived but i'm a bit busy at the moment so will try later.

still don't understand why it running sooo rich.  i know i'm only looking at it after it's ticked over for 5 or 10 mins with the screw out 2 turns turns its so black i can wipe the soot off with my finger - along with acrid fumes!  it seems to prefer at least 2 3/4 turns out with little fumes and any more and i recon the needle will fall out.   i did try to set the floats at 21mm and the slow running jet looked ok (but who knows) anyway lets see how it is after the sync.

Offline Greg65

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Re: cb400f - carb sync
« Reply #7 on: February 15, 2019, 08:03:09 AM »
Just a thought. As one part affects another what state is the air filter in? If it’s partially blocked it will create a greater vacuum and therefore more fuel will be pulled from the carbs. Good luck.
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Offline Lobo

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Re: cb400f - carb sync
« Reply #8 on: February 15, 2019, 12:00:19 PM »
... an aside, I phaffed & farted so much balancing my ‘750 carbs (hot summer’s day) and had a coil fail on the next ride out. Coincidence maybe, but looking back the engine got overly warm...shoulda perhaps had a fan across it.

Offline Clem2112

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Re: cb400f - carb sync
« Reply #9 on: February 15, 2019, 03:43:43 PM »
Cheap household fan and all doors wide open !

Offline blackeagles

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Re: cb400f - carb sync
« Reply #10 on: February 15, 2019, 04:04:30 PM »
filter is new.

i have one of those exotic dancer fans i got from a night out, maybe i should fan the engine and make the adjustments ;D

ok, used the gauges and only 3 worked - one hardly moved - but when i swapped the tube over i managed to adjust it ok so now all readig same (115ish mmhg).

even with the air screw out 2 3/4 turns out its still far too rich to the point that there is a smoke screen from the pipe.  interesting that when i took the pipes off to swap over the gauges to get a reading the engine perked up.

looks like its down to taking the carbs off yet again and changing the 'brass' - i can only guess that when i cleared the blockage from all 4 slow running jets using an ultra fine drill (23thou) i must have altered them or maybe i set the float level wrongly ( i set it to 21mm) - also the origional needle setting was 2 down and i reset it p to 3 down but the needle position should not make any real difference!!??

the kit i'm thinking of using is damineding
« Last Edit: February 15, 2019, 05:57:34 PM by blackeagles »

Offline blackeagles

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Re: cb400f - carb sync
« Reply #11 on: February 25, 2019, 10:54:07 AM »
quick question.

there are very small 'cut aways' on the lower lip of each on the throttle slides (pic of two below).  they look like a previous owner has put a drill or something under the slide to get the initial gap but either ramed it in or banged the slide up and down causing a small chip in the slide skirt - anything i need to worry about/replace the slides as the damage is very small?

Offline Nurse Julie

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Re: cb400f - carb sync
« Reply #12 on: February 25, 2019, 11:05:33 AM »
Worry not, it is very common.
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Offline Deano400

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Re: cb400f - carb sync
« Reply #13 on: February 25, 2019, 11:11:10 AM »
Mine are exactly the same and it runs just fine. Don't think it is PO damage.

Offline blackeagles

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Re: cb400f - carb sync
« Reply #14 on: February 25, 2019, 01:50:51 PM »
thanks, one less thing to worry about!

 

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