Carbs off bike - Total disassembly or not?

[McCabe-Thiele (Ted)]

IIRC the carbs bodies on a 400 are numbered to get them in the right order - that said you could probably get the bowls on the wrong side - as you say they should point outwards on either side.

[Mikep328]

I've noticed that the choke lever, when moved to no-choke position will return slightly back due to the tension on the four choke butterfly springs.  It feels like there is a detent on the choke lever but the choke butterfly springs can pull the lever out of the detent.  Is there an adjustment for this?

[Nurse Julie]

I've noticed that the choke lever, when moved to no-choke position will return slightly back due to the tension on the four choke butterfly springs.  It feels like there is a detent on the choke lever but the choke butterfly springs can pull the lever out of the detent.  Is there an adjustment for this?

There should be a detent and a spring and a ball bearing to keep it in the closed position.

[Mikep328]

If I disconnect the link arm to the choke butterflies and operate the lever I can feel/hear it "click" into the detent but when connected, the butterfly springs can pull it out of the detent position.  Maybe the detent spring has weakened after 47 years!  I'll disassemble that lever and take a look, maybe some sort of shim behind the spring will do the job!

[McCabe-Thiele (Ted)]

Sounds more like an issue with the fast idle cam adjustment on the choke as they are separate links.
The throttle opening and closing is independent of the choke mechanism except when the cam engages with the throttle.

I would be removing the carbs to check all the links for alignment then recheck the bench synch for the slider settings.

Are all the sliders nice and free throughout their travel?
Are the card bodies in the right order - can't see how they could be fitted together back to front but the bowls are clearly not the right way round

Be careful you do not loose the little ball in the detent if you dismantle the end part - they can get a tad rusty in that area.

[Mikep328]

Thanks Ted!

I ended up separating the carbs.  All sliders are free and all linkages are OK.  The chokes tend to close a bit on their own because the detent is not strong enough to hold firmly against the choke butterfly springs.  I disassembled the detent mechanism - not much to be done there though cleaning out the detent itself, where the ball engages, helped - there was a lot of crud in there which kept the ball from seating as deeply as it should.  It's better now though still not as positive as I'd like.  I thought about drilling the detent "depression" deeper but I don't like modifying something that obviously worked fine when new.   But it does seem that those choke butterfly springs are much stronger than they need to be AS is the throttle return spring...Now that's a SERIOUS spring!  I'd be happy to find/install one that is a bit less aggressive. 

[Oddjob]

Find a stainless steel split ring (like the ring on a key ring), I found a pack of 10mm ones I use to have for sea fishing rigs, remove the spring from it's pin at the bottom of the carbs, I threaded the pin through the ring as the carbs were apart when I did this but you could thread it onto the pin like you thread a key onto the ring etc. When it's on clip the spring onto the ring

I tested this method to see if it actually made a difference and by using a fishing spring weight I can 100% say it does, the poundage needed to open the throttle decreased from around 14lbs to around 10lbs, I have the figures somewhere.

[Mikep328]

Good idea, I have some split rings laying around...

With the very generous help of Multiman, who came by the house/dropped off a section of tubing for the between-carb vents and a dollop of red rubber grease, I am reassembling the carbs with the intent of reinstalling them tomorrow.

I seem to recall reading in a post (of course now I can't find it) that the carbs could be installed with the airbox already mounted to them.  Of course it is quite possible that either I misread the post or that I mis-remember the post.  If that's possible it seems it would be a bit easier.   No idea if a new original airbox back in the day was somewhat flexible/rubbery or more hard/plasticy.  My airbox is quite pliable, maybe it's a replacement though its not listed on any of the numerous parts receipts that came with the bike.

[McCabe-Thiele (Ted)]

The trouble with the airbox is it is so pliable that when you try to fit it to all four inlets one always seems to pull partly off.

Only time mine fitted as it should was when there was no rear wheel, mudguard or battery box in place. It was easier to build backwards.

[Multiman]

I haven't tried fitting the airbox to the carbs before fitting the assembly to the engine so won't comment on that.

To help get the airbox onto each carb I shove the two centre fittings by pushing from through the air filter housing with my fingers or any other prodding device I have a to hand.

[Mikep328]

Well,  I'll give it a try with the airbox mounted to the carbs first, maybe it'll work (or not)!  In looking through my receipts again I found that it does have a new (reproduction) airbox from DSS in 2022.

[Matt_Harrington]

I'm pretty certain you will need to fit the airbox after fitting the carbs. There is a technique which I found but it is still a fiddle. You should be able to access the airbox from where the air filter would be to push the central parts home. I used a very thin smear of red rubber grease to ease things along. As Ted says, without the air filter housing, its nice and easy!

[McCabe-Thiele (Ted)]

Well,  I'll give it a try with the airbox mounted to the carbs first, maybe it'll work (or not)!  In looking through my receipts again I found that it does have a new (reproduction) airbox from DSS in 2022.

Pretty sure that will not work as there is not enough room if the air cleaner box is in place - it's a tight gap.

[Mikep328]

To follow up...

I did not attempt to refit the carbs with the airbox attached.  As has been stated, it didn't look possible at all so I did it the 'normal' way - airbox in place (more or less)/carbs slid in from the right side.   I lubricated all the rubbers and from the time I started to slide the carbs in to the time they were in position on the manifold and the airbox rubbers attached was maybe 5 minutes!  I've read about how difficult this can be so I figure I got lucky but I'm not complaining.   As they say, luck is better than skill!

No leaks or other unpleasantness, bike started immediately (choke on).  After warming up I adjusted the airscrews per the Honda instructions and went out for a ride.  The huge bog was gone and the bike took the throttle quite well though I think further tweaking could smooth it out even more.  I had bench synced the carbs and, TBH, I don't know if it's worth doing a vacuum sync since it runs very well.  I'm very pleased, neat little bike!!!!

[K2-K6]

Sounds like good progress and satisfaction in working through it yourself.

Carburettor synchronization, no difference between bench & gauge if carried out with careful diligence.  Obviously easy to set them on bench when they're not fitted, once installed of course you'd not consider taking them out to do this, hence gauges to facilitate.  Sync'ed is Sync'ed though either way, id not sweat it now, there's nothing magical in it as often expected on the great wide Internet view.

Sync hasn't the fine resolution to adjust and compensate for idle refinement anyway, thats always been through those idle circuit adjustment and optimisation to bring parity of burn for each cylinder with the main slides virtually closed.
This is mirrored in fuel injection system as follow on from carbs, the ecu monitors crankshaft pulse and bleeds different level of air into early systems to keep them stable with fairly crude injector control being insufficient.
Later, multipoint system more currently use crank monitors and trim cylinders with injector pulse length to keep them even. Same as these carb strategy wise, these just use you to monitor the crank rpm in trimming the mixture.

It's perfectly acceptable to have different carb to carb air screw settings as the cylinder burn takes precedent in smoothness, the screw adjustment just compensates for any variation in that carb, jet,  cylinder compression, spark plu etc and gives you the means to get equal firing pulse energy to make it all smooth.