Honda-SOHC

SOHC.co.uk Forums => CB350/400 => Topic started by: blackeagles on February 13, 2019, 11:31:26 AM

Title: cb400f - carb sync
Post by: blackeagles 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')
Title: Re: cb400f - carb sync
Post by: paul G 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 ;)
Title: Re: cb400f - carb sync
Post by: K2-K6 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.
Title: Re: cb400f - carb sync
Post by: Rob62 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.
Title: Re: cb400f - carb sync
Post by: billywingnut 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
Title: Re: cb400f - carb sync
Post by: Bryanj on February 13, 2019, 04:24:01 PM
Dont just set at tickover but check at about 3000 rpm as well
Title: Re: cb400f - carb sync
Post by: blackeagles 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.
Title: Re: cb400f - carb sync
Post by: Greg65 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.
Title: Re: cb400f - carb sync
Post by: Lobo 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.
Title: Re: cb400f - carb sync
Post by: Clem2112 on February 15, 2019, 03:43:43 PM
Cheap household fan and all doors wide open !
Title: Re: cb400f - carb sync
Post by: blackeagles 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
Title: Re: cb400f - carb sync
Post by: blackeagles 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?
Title: Re: cb400f - carb sync
Post by: Nurse Julie on February 25, 2019, 11:05:33 AM
Worry not, it is very common.
Title: Re: cb400f - carb sync
Post by: Deano400 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.
Title: Re: cb400f - carb sync
Post by: blackeagles on February 25, 2019, 01:50:51 PM
thanks, one less thing to worry about!
Title: Re: cb400f - carb sync
Post by: blackeagles on March 02, 2019, 11:03:58 AM
just to wind the "running too rich" bit up - the replacement carb kits arrived (only 25ish the four) and i fitted all the brass other then the needles and it now runs just fine.  i am going to guess i did in fact open out the pilot jets (all were blocked) a bit too much when i cleaned them out.

and

although i don't think i have sync'ed them quite right they are all near 250 mmhg (10inhg) at 3k rpm so i will ride it a bit and redo it later this year if i think it needs it.

almost there now!!
Title: Re: cb400f - carb sync
Post by: PatM on March 11, 2019, 05:18:01 PM
Ive done the 400 and its predecessor many times with a set of £70 gauges-The first thing to do is re-build the cabs with a jet-set and O rings. Oh and make sure they are clean!
As you have eluded to-the gauges measure the air-flow/suction-so have nothing to do with mixture setting-thats another black art if you have the correct jets fitted and seals.
From my experience, the most common-fault with those carbs is blockages and leaking main-jet seal-the later causes too much fuel to be sucked up the emus-ion tube - leaving you with a poor metered fuel mixture.

Get all gauges reading the 'same' and you are 80% there. The general 'noise' is an indication of where all 4 'motors' are sharing the load-as said, do the fast-idle and 3k-if one works, the other should. It does take a bit of trail and error- and make sure the tubes (usually brass) are seated and air-tight on the take-off points. Remember, as the engine heats, the readings will change-so just aim for equilibrium-then take it out for a run and re-check.

Just as a side-note- Rich running? How do you know this- plugs back? My first point of call on rich is to remove the air filter- long story short, my sea stand-off bungs where too low and blocking the air-intake. it was fine on the guages and tickover in the garage- then i stuck my fat-arse on the bike and it would pull ....
Title: Re: cb400f - carb sync
Post by: Rob62 on March 12, 2019, 01:14:27 PM
Yep I think you created the rich problem when you drilled out the idle jets.... that's definitely a no-no, the orifice is very fine and any increase in diameter will make a big difference to the fueling, glad you got it sorted with new jets. Regarding the carb balancing, there is only one setting required and that is at idle... all you are doing is adjusting the different heights of the throttle slides in relation to the rotating bar that lifts them all simultaneously via the throttle cable, there is no other adjustment to be made at various speeds. It is advised to blip the throttle between making adjustments purely to let the slides settle back after each adjustment...but other than that the process takes place at idle speed. A decent set of vacuum gauges is a big help as they have quality dampers and make the reading more accurate, plus they are better quality in general. I have a set of Sealey gauges, they are pretty good..
Title: Re: cb400f - carb sync
Post by: blackeagles on March 14, 2019, 12:05:09 PM
thanks.

i have to admit i was begining to get really annoyed with the 'rich' running (you could hear the engine choaking!!) but that has now gone.

i think the suggestion of " a good set of guages " is central as the ones i bought were 'cheap' and i bought them because many had been sold (according to ebay counter).  dont get me wrong, they worked after a fashion but it was very difficult to stop them bouncing all over the place - even with the plastic 'restrictor' which ment that the needle was either bouncing, sort of slightly flickering or stopped dead.

in the back of my mind was the fact that an expensive set only used a couple of times was not a reasonable outlay  -  may have been a mistake.

anyway, just waiting for the road tax and a bit of sun/dry day  8) and i can give it a bit of a shakedown and make any last adjustments.

ever onwards.
Title: Re: cb400f - carb sync
Post by: Rob62 on March 14, 2019, 12:49:22 PM
Either way, cheap or more expensive, has valid merits... cheap has an obvious benefit and if you can get them to work in a satisfactory manner and achieve the objective then why not. My reasoning was that I believe the CB will require regular adjustment to the balance in order to keep it running smoothly plus I have other bikes that I can use them on too, so buying a decent set made sense to me. Plus I have always wanted a set of gauges and never got round to buying any... But there is no right or wrong.
SimplePortal 2.3.5 © 2008-2012, SimplePortal