The battle to get carbs synced

[AndyH]

After cleaning the carbs and replacing the aftermarket jets/needle with Keihin originals, I then bench synced using the method espoused by many (back off the idle screw completely, then set using light gap.)

I selected carb #2 as master and calibrated the Motion Pro sync tool on this carb.  The motor was previously warmed up and was running OK but somewhat "rough".  The idle screw was adjusted out to get near 1000rpm.
Initial indications showed #4 out by quite a bit.  Anyway, after some adjusting across #1,3 and 4 the idle could not be reduced below ~2200rpm (idle screw not contacting the throttle bar).  I did not adjust #2 as it was the master.

As per the 3000 service .....timing, tappets and cam chain adjusts have been specced.

Jet needle is at 4th position from top, air screws at 2 turns out.  (DeltaRider I'm setting this bike up to US specs!  EDIT:  DR knows this! )

What is my next troubleshooting step folks? 

[K2-K6]

By individually adjusting the air screws,  while it's running you can determine if all of the idle circuits are clear and working something close to correct.

You probably need to go back to a bench sync start point though to make sure you are looking at the idle circuit in isolation.

I'd not sync them with a gauge until you understand if the idle circuits are functioning else you risk compensating for impaired idle circuit by raising a throttle slide,  which is back to front logic wise.

[Seabeowner]

Logic says that if you started with 1000 rpm and have not altered anything else except the throttle slides by means of the adjusters between 1/2 and 3/4 and they are now balanced and you have 2000 rpm min, then the master that you chose to start (2) was already open too wide.
If you are confident of the idle circuits then you could try closing the slide on 2 a little and repeating.

[deltarider]

By individually adjusting the air screws,  while it's running you can determine if all of the idle circuits are clear and working something close to correct.
You probably need to go back to a bench sync start point though to make sure you are looking at the idle circuit in isolation.

I'd not sync them with a gauge until you understand if the idle circuits are functioning else you risk compensating for impaired idle circuit by raising a throttle slide,  which is back to front logic wise.

Although that method is OK to determine an individual idle circuit is clear or not*, I don't see how the airscrew can have an effect on the vacuum. It's the piston going down in combination with the almost closed slide that creates the underpressure, not the A/F mixture.
* If idle RPM doesn't change when an airscrew is turned to fully closed or to say 2 turns out, that particular carb probably has its idle circuit blocked by dirt somewhere.

[K2-K6]

I'll not disagree with you on that delta. Although a cylinder that's not running correctly because of an inop idle circuit I'd expect to have a different vacuum correlation to a good one because of inlet and exhaust valve timing overlap. You'd expect a properly combusted mixture exiting the exhaust to influence the intake vacuum compared to a partially cold cylinder that's not firing well.

I think carb syncing is viewed as a hopeful fix for many things on bikes when in reality it only fixes........out of sync carbs.

As they've been bench synced first then it does suggest something else is out of range which will need to be assessed and sorted anyway,  synchronisation is just the icing on the cake after everything else checks out as good.

There are numerous examples on here of the idle circuit not being up to scratch regarding cleanliness and flow,  so it would seem reasonable to establish that first prior to chasing information circles.

[Bryanj]

If the idle is high with the throttle stop screw not touching all the slide adjusters are too high. When using the bench settup the throttle stop needs to be screwed in then use the smallest drill bit shank you have as a feeler gauge to set the slides even. unadjust the throttle stop to make sure that all slides bottom out

[AndyH]

Thanks everyone for your comments and suggestions.  I have removed the carbs from the bike and will check the idle circuit passages and airscrews for any contamination/blockage that somehow may have found its way into the area even after I gave everything a thorough clean beforehand.

I'll re-do the bench test as Bryan suggested and report back.

[K2-K6]

I think you can leave them set as Bryan explains,  certainly won't be anything much away from ideal depending on how diligent they are set initially.

I think the whole carb sync thing with gauges has been misaligned over the years as something that is part of tuning the engine.
I can't construct any competent case for altering the carb's slides away from mechanical parity to run the engine correctly under load.

Honda provide in their manuals the method for adjusting the air/idle screws at tickover to produce an even running engine. This is the correct device for equalising the cylinders at very very low engine speeds as they are effectively micrometer adjustment throttles for each cylinder..... Not the main throttle slides.
As Bryan has posted over the years on here,  you need all the setup done, verified and complete before you put any gauges near them.

The gauges in my view are just a device for a workshop to get the carb's slides something close to parity without having to dismantle the bike. That assumes all other things are working correctly else it'll more likely put an error on the bike.
Bench setting should hold sway if done properly,  the confidence should go with that as an engineering principle,  not the other way round.

[AndyH]

Fair enough mate...duly noted! This is the 1st bike restoration for me so I will remain for the foreseeable future on an ever upward learning curve!!    It is loads of fun though! 

Anyway I’ve pulled the carbs again, double-checked the idle ccts for cleanliness, and took a slightly different approach to the bench sync.  Carb throttle valves now completely close off when I back the throttle stop (idle adjust) screw right out.  As others have suggested, I may have initially set my “master” carb throttle valve too high which then gave me the issues.

On a separate note, I checked the static timing and discovered I didn’t read the document that came with the electronic ignition properly!!    So I was a few degrees retarded which would not have helped matters.
All correct now.

I haven’t re-installed the carbs yet as I’m still mulling over some conflicting data from the various Honda manuals.  In Europe it seems that the std needle clip setting is in the 3rd groove from the top, with idle mixture screw 1 ± 1/8.  In the US it’s 4th groove down and  1½ ±½.  In the genuine Honda Shop Manual it states  1±3/8 for the air screw. (No needle groove setting mentioned).  I should mention that the needle settings on the jets I originally pulled were on the 2nd groove down and IMSs were 2½ turns out!!!   And yes, close scrutiny shows the genuine Keihin needles have a slightly different profile to the aftermarket ones.

I realise that with a non-std exhaust the std settings may not be 100% but I should be close.

Stay tuned.....

[deltarider]

I think you can leave them set as Bryan explains,  certainly won't be anything much away from ideal depending on how diligent they are set initially.

I think the whole carb sync thing with gauges has been misaligned over the years as something that is part of tuning the engine.
I can't construct any competent case for altering the carb's slides away from mechanical parity to run the engine correctly under load.

Couldn't agree more. Honda did an excellent job to design the rack so that synchronisation is seldomly needed. Doing this with a 4 gauges set is only to save time (= labour costs) in the workshop. Yet a lot of us think they also need a set as if they also have to save on labour (=costs). But... from what I read, it's much more common they have the carbrack already removed and on the bench or kitchen table for that matter. Then you can do a bench sync. The way I do it, I've managed, on all occasions, to sync them within 2cm Hg when tested afterwards using the vacuum set.

Honda provide in their manuals the method for adjusting the air/idle screws at tickover to produce an even running engine. This is the correct device for equalising the cylinders at very very low engine speeds as they are effectively micrometer adjustment throttles for each cylinder..... Not the main throttle slides.

Can you direct us to where we can find that? I remember having read something on this, but I doubt it will work on a 4 cylinder bike, each cylinder having its own carburetor. Let me put it this way: I myself have never managed.

As Bryan has posted over the years on here,  you need all the setup done, verified and complete before you put any gauges near them.
The gauges in my view are just a device for a workshop to get the carb's slides something close to parity without having to dismantle the bike. That assumes all other things are working correctly else it'll more likely put an error on the bike.
Bench setting should hold sway if done properly,  the confidence should go with that as an engineering principle,  not the other way round.

Again, couldn't agree more.

[K2-K6]

The idle adjust routine is in a Honda workshop manual for 750 SOHC,  section fuel system,  it describes carb strip and inspection then goes through reassembly onto setup.
It gets to idleness screw initial setting then a further setting on idle mixture that contains the fine tuning routine. ( this I find repeated in later manual for CV carbs with changes for type).

The thing is,  it looks like a very benign little routine that is easily overlooked for application and ultimate effect as it doesn't lay out its aims for the reader to see.

I've come back to this from more modern petrol any diesel cars which are in effect using this routine on the fly to stabilise tickover. They use a flywheel sensor with a trigger for each cylinder,  as the trigger passes the sensor the pulse time is checked by the ecu and if it has arrived late or early for each cylinder,  then it alters mixture and /or timing of fuel injection pulse to accelerate or reduce speed for that one cylinder. It's why when you start them they just go into a flat rock stable idle with apparently no wandering or variation.

This I see as an extension of idle adjust screws,  and with that information it's easier to see what the original Honda routine is aimed at achieving. That is each cylinder combusting as close as the same to each other that the design will allow. As previous post,  you've effectively got a micrometer adjustment for each cylinder at tickover.

It appears to have become established for most people setting up multiple idle circuits to fix all of the screws at the same nominal mechanical position,  that just discards a big part of their capability in facilitating a even and smooth idle speed.

[deltarider]

I'd love to see a vid of someone practicing this on youtube or similar.

[AndyH]

OK back to my 627B carbs  ! Carbs are back in the bike.  All Keihin brass except for the float valve & needle.
#40 slow, #100 main, IMS 1½ turns out, jet needle 3rd groove as a starting point.   My exhaust is a 4 into 1 (I think Delkevic, dunno how to confirm?) but I'm saving my shekels for a repro 4 into 4 from a bloke in Preston, VIC, Australia.
https://www.ebay.com.au/itm/Honda-CB500-Four-CB550-Four-Mufflers-Exhaust-System-Reproduction-/182588936538

Other commitments today have precluded me firing up the bike but I'll do so tomorrow.

Stay tuned....again!

[deltarider]

Oldstyle Keihin carbs like the 627B had airscrews that metered air to the slow jet. The term IMS or pilot screw (Hondaspeak) we reserve for the screw engineside that meters the already mixed aerated fuel. The latter is found on the PD carbs that for instance the CB500/550K3 models had.

[AndyH]

Mate, you can call it what you like, makes no difference to me.  I'm sure most folk on here will recognise that IMS refers to Idle Mixture Screw.  Call it an Elephant if you want, it still does the same thing FFS!

Please stop steering topics sideways with specious comments which add nothing.  Thanks.