Topic: Carb Synching Advice

[Rosinante]

I am in Olympia, and needing carbs synch'd on my 1978 CB750K.  Perhaps someone would recommend a shop that can do this for me.  Or, perhaps someone can recommend a carb synch tool so that I can do it myself.

[denward17]

I just did this on a '78 CB750 using the Carb tool shown below.

I had already bench synced mine, so they were kinda close anyway.  Remember to only adjust #1, 3 and 4.  #2 is reference and cannot be adjusted.

I recommend getting the ignition timing setup good before starting the sync.

[dave500]

get that ignition squared away perfect and the valves and cam chain adjustment done before you #$%* with the carbs!,youll chase your tail like a dog otherwise!

[Rosinante]

I adjusted the valves two weeks ago and installed new spark plugs.  Ignition is a Hall sender, not breaker points, and ignition timing is perfect each time I set it.  Those things are already addressed.  Idle is rough, and I'd like to fix that.  I think these carbs are way out of synch.  Carbs were rebuild professionally, several years ago, and synched at that time unprofessionally.  I think the PO synched the carbs with the valves out of adjustment.  So yeah, thanks for the suggestions and the carbs now need to be synched.

[newday777]

I adjusted the valves two weeks ago and installed new spark plugs.  Ignition is a Hall sender, not breaker points, and ignition timing is perfect each time I set it.  Those things are already addressed.  Idle is rough, and I'd like to fix that.  I think these carbs are way out of synch.  Carbs were rebuild professionally, several years ago, and synched at that time unprofessionally.  I think the PO synched the carbs with the valves out of adjustment.  So yeah, thanks for the suggestions and the carbs now need to be synched.

Has gas been left in the carbs to set unused for a period of time? If so your emulsion tubes are probably plugged again by now.

[TwoTired]

Look at the spark plug tips.  If the carbs are too rich, it will soot the plugs.  If the center electrode insulator has carbon on it, that will shunt spark energy away from the gap, making for erratic running.  This problem will remain after the carb mixture is corrected until the plugs are replaced or cleaned of carbon deposits.

[Rosinante]

The plugs are new.  The outgoing plugs looked just fine.  I understand that everything (ignition, valve clearance, linkage adjustments, etc.) must be in good order and that the last step in the process is the synching.  I appreciate all these suggestions, and am looking mostly for suggestions about this last step. 

Let me ask this:  The Carbtune Pro looks like a good tool.  Is this what most folks here would recommend?  It comes with the correct fittings for these Honda carbs, right?

[carnivorous chicken]

Let me ask this:  The Carbtune Pro looks like a good tool.  Is this what most folks here would recommend?  It comes with the correct fittings for these Honda carbs, right?

Yes. I like mine, seems like a lot of others like theirs too. For less than what a shop will charge you to sync carbs you can get one and do it yourself. It's pretty easy. And rate of the GBP is low (although there are some distributors stateside from what I understand).

[denward17]

Recent thread:

http://forums.sohc4.net/index.php/topic,192037.0.html

Good tool, but I have only used it once, and comes with correct fittings.

[Tracksnblades1]

If you’re planning on syncing with the dial type gauges, I like to hook all four gauges to the same vacuum source at the same time. This will verify all four gauges read the same vacuum. Most of the older gauges had an adjustment on the dial so you could make sure your gauges were “synced” before synchronizing your carbs.

+1 on making sure your carbs, engine, ignition, etc are spot on first. Before what Dave 500 indicates 😁

[Bodi]

The mercury tube ones are super nice but now mercury is outlawed, so... Anyway I lost my mercury set (left hanging in the basement of a sold house dammit) and haven't tried any other fluid type, I don't understand how they work when the column height for water is 30 feet or so at full vacuum.
Gauges are good but as mentioned, connect all four to a single vacuum source (big syringe?) and adjust to have the readings match.
Carbtune is great but not cheap, main advantage is that it's not easy to break and can't spill as there's no fluid.
If you do use a fluid type, avoid revving the engine then shutting throttle, the fluid tends to be sucked out.
It sounds like you're ready to sync. Just need a remote fuel supply and you should have a window fan or similar blowing on the engine. I don't know if yours have one unadjustable carb - some do some don't - if not, pick one and do not adjust it.
They interact... adjusting one changes the other three.
Have a low steady idle, not so low it's hunting though. As the idle smooths out you can lower the rpm a bit.
I would do a leak test using propane (unlit torch) around the carb spacers, any vacuum leaks will make syncing pointless. Leaks will cause rpm to rise if extra fuel (the propane) is sucked in between carb and head.

[dave500]

great you got all the adjustments up to par,you say you have a hall sensor ignition?what brand is it?

[Deltarider]

When I had all 4 gauges on one port simultaneously, I experienced interference between the chambers (drums?) inside them, causing the dials to show different pressures. When I later tested my set with a stabil, not pulsating vacuum pump, they all 4 proved perfectly calibrated. Hence my advice not to test them simultaneously, not dynamically, on one port, but one after the other. Or joined on one stabil, not pulsating vacuum pump.

[Rosinante]

I do not recall the brand of the electronic ignition, but it is a non-contact technology.  It "knows" the position of the crank lobe via optical or magnetic or whatever.  It is rock steady, and the coils are new  NGK, I think.  At any rate, ignition is no longer a source of potential problems.  And, as I say, valves are adjusted as accurately as I can manage.  I have adjusted these kinds of valves many time.  Candidly, I prefer to use a dial indicator and holding fixture, but I don't see how that method can be used on these engines.  I adjust valves on a vintage Porsche 911 sometimes using the dial indicator method. 

I'll use the propane trick.  Thank you.  I have renewed the rubber things on both sides of the carbs.  And, as I say, the carbs were professionally rebuilt including accelerator pump.

Idle is troubling on this bike.  I really do think the carbs are way out of synch.  Idle is all over the place, depending on air temp and engine temp and other factors.  Part of what I will be doing is to make sure the cables connected to the carbs are all in adjustment, and all other exterior carb adjustments are proper.  Even just adjusting the idle air is difficult or impossible because this engine just does not idle smoothly.

Only three carbs are adjustable.  They all get adjusted to match #2.

CarbTune is a non-fluid tool, correct?  I do want to avoid damaging the tool itself and if CarTune is a non-fluid type of tool, then my mind would be eased somewhat.

Thanks to all!

[Rosinante]

Also, another thing:  I know that if other stuff is not properly, adjusted then carb synching will not be optimized.  But....certain adjustments and stuff, like idle air and the propane test for leaks, require a smooth idle.  I figure that having the CarbTune tool gives me the ability to zero-in on those things iteratively.  I can get everything adjusted as well as possible, then synch the carbs, then perhaps do some more adjusting and testing, then synch them again if needed.

[carnivorous chicken]

CarbTune is a non-fluid tool, correct?  I do want to avoid damaging the tool itself and if CarTune is a non-fluid type of tool, then my mind would be eased somewhat.

Yes, that's right. Metal rods, and it's one of the reasons it's easily portable.

Idle can vary on these bikes in terms of the things you describe -- air and engine temps -- and idles can rise as the bike gets warmed up.