Author Topic: cb400f - carb sync  (Read 569 times)

Offline blackeagles

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Re: cb400f - carb sync
« Reply #15 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!!
« Last Edit: March 03, 2019, 02:29:18 PM by blackeagles »

Offline PatM

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Re: cb400f - carb sync
« Reply #16 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 ....

Offline Rob62

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Re: cb400f - carb sync
« Reply #17 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..

Offline blackeagles

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Re: cb400f - carb sync
« Reply #18 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.

Offline Rob62

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Re: cb400f - carb sync
« Reply #19 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.