Topic: 74 CB350 FOUR Carb sync issue HHEELLPP

[craywm]

Hey guys new here. ive been trying to adjust my carbs to be properly syncd on my 74 cb350 four.


Things ive already done: adjusted timing, valve clearences, spark check, adjust air mixture screws to 7/8" +/- 1/8 OUT

so after my first synce, i realized that the choke was not opperating correctly, it did not help the engine warm up at all, so i figured i was too far up with the slides, i took em all the way down and adjusted up until 1200 rpm was in a reasonable setting, now my choke is overly useful and ive been getting weird smelling exaust. sometimes smoke (could be condesation) from exhaust. changes in air mixture screws cause significant exhaust changes. Im planning on resyncing again tomorrow and trying to get this totally right. thus far i havnt adjusted the screw that allows the choke to increase the idle speed (this is probably one problem) but my main concern is why am i getting smoke? i figure i must have synced the slides too far cclosed?

that brings be to my next question: HOW ARE YOU SUPPOSSED TO KNOW WHERE TO SYNC TO WITH NO BASE CARB!!? its killing me not knows where to adjust it too. depending on where you idle screw is, you have 100's of different areas where the sync is fine but the bike still runs poorly. i tried to follow my honda manual but its very unclear and ive never tried to sync vm carbs before. a good explanation of this would be great. it seems like in the manual you adjust the idle screw then leave that where it is and adjust the sync screws until the bike is idling and synced at 1200 rpm?? is this correct? seems like a pretty hefty goal to me after seeing the problems ive had already..

is anyone experienced in this that could explain this to me? i wouldnt be asking on a forum if i didnt already try a ton of online searching. Help me please!!

[Spanner 1]

Hey, 'Hello'........please explain how you 'sync'ed the carbs'...?  will help with more info...

[bender01]

You will need guages to get them perfect. The 350f is hard to get perfect since its smaller and needs to be in sync/tune to run properly. To start try a bench sync. Set all the slides to an object thats the same size. I use a wd-40 straw. You have cleaned the carbs already right?

[rboe]

Clymer manual states the vacuum must be between 16 and 24 cm-hg with a delta between carbs not to exceed 3cm-hg.

[craywm]

in what manor? i used a carb syng tool. i have both a gauge and a liquid tool but i used the gauges for this one. warmed up the engine, put the bike at 1200 rpm, read the gauges, adjusted one carb then tightened the bolt down then revved it 3-4 times, then i would move onto the next carb and adjust it. generally after adjusting 2 or 3 carbs i have to readjust the idle screw back down to 1200 rpm. is this enough info? i did do a bench sync but that was like 3 syncs ago so possible that its that far out.

do you know any procedures to sync a bike with no base carb running vm carbs?

and yes carbs are cleaned aswell and rebuilt in entirety. this isnt my first rodeo on these oldies. just my first FOUR rodeo with vm carbs. lol

[craywm]

i could get one cylinder to around 15 cm hg but then when i adjusted the others it would drop back down. im at 10 right now using my gauges but am unsure if thats accurate but it seems to run fine im just not quite sure the sync is in the correct possition (slide height positions vs idle possitions)

[craywm]

how do you know if a WD 40 straw is a good height to start the slides at? what if its too high or too low? is there a spec for this for the 74 cb350F?

[Spanner 1]

Can't 'tighten' the adjuster on one carb and move on to the next while vacuum sync'ing....
Takes many slight adj. of each carb to get balanced using a vac. tool. Each adj. will change the other carb settings.... needs patience 
BTW... Numbers on the gauges mean nothing.. the goal is to achieve exact readings on each carb..

[craywm]

The manual says to tighted the lock nut and rev the engine 3 times. If you dont lock the nut down you could get a false reading.

[Spanner 1]

See reply # 7... or maybe someone else can help ya ... 

[craywm]

okay let me explain again, i know how to sync a bike properly. what i dont understand is what the base line for a VM carb sync is. on newer bikes, you adjust the idle to coinside with a base carb, then you adjust the other 3 carbs to match your base carb sync.. so on these older vm carbs, where is my base measurement? or is this all just winging it to see where you get. I bench synced, then i fine tuned synced on the bike. it seems to run fine other then a funny smelling exhaust and over sensitive choke.. i just dont understand the concept of syncing without a base carbs because depending on where your idle screw is set and how open the slides are.. it can be synced at 1200 rpm either super rich, or super lean, or somewhere in the middle, and id rather leave the air screws at 7/8 turn out. so again, my question is, what is by BASE measurement to sync to with VM carbs? i dont quite buy the fact that the diameter of a WD 40 straw is a good base measurement for how open or closed your slides are to sync to.. i could shove a .500" JO block in there and od the same thing as a WD 40 straw theyd, slides, just be way more open...

[HondanutRider]

... depending on where your idle screw is set and how open the slides are.. it can be synced at 1200 rpm either super rich, or super lean, or somewhere in the middle, and id rather leave the air screws at 7/8 turn out. ...

I believe you set the air screws all the same (e.g. 7.8 turn out) and then only adjust them (all the same) if the mixture is off.  To synchronize these carbs, your adjustment is made to the throttle slide rest position, so that all carbs/cylinders draw an even amount.

That's why the bench synch gives you a good starting point, with all throttle slides cracked open the same small amount.  Then with the carburetors mounted and the motor running you synchronize them all to draw the same amount, compensating for small flow differences from cylinder-to-cylinder and carb-to-carb.  You could choose to make one carb's reading the "base" and leave it alone while you adjust the others to match, but be aware that the reading for the "base" carb you have chosen is going to fluctuate as you adjust the other three to match.  In practice, this "base" carb you have chosen is often adjusted as well, so that the readings of all four don't need to move so far.

[Gordon]

okay let me explain again, i know how to sync a bike properly. what i dont understand is what the base line for a VM carb sync is. on newer bikes, you adjust the idle to coinside with a base carb, then you adjust the other 3 carbs to match your base carb sync.. so on these older vm carbs, where is my base measurement? or is this all just winging it to see where you get. I bench synced, then i fine tuned synced on the bike. it seems to run fine other then a funny smelling exhaust and over sensitive choke.. i just dont understand the concept of syncing without a base carbs because depending on where your idle screw is set and how open the slides are.. it can be synced at 1200 rpm either super rich, or super lean, or somewhere in the middle, and id rather leave the air screws at 7/8 turn out. so again, my question is, what is by BASE measurement to sync to with VM carbs? i dont quite buy the fact that the diameter of a WD 40 straw is a good base measurement for how open or closed your slides are to sync to.. i could shove a .500" JO block in there and od the same thing as a WD 40 straw theyd, slides, just be way more open...

Choose one carb to be your base carb.  Leave that one set and adjust the others to match.  Like Spanner said, leave the lock nuts loose on all while you're making adjustments.  When you lock them down it will change the vacuum readings slightly, so you just have to compensate for that when making the adjustments.  Pay absolutely ZERO attention to the actual reading on the gauges.  The numbers mean nothing.  Just get them all equal. 

Keep adjusting the idle stop screw as you adjust the slides to keep the engine at a normal idle speed. 

[Spanner 1]

Ya don't mess with your idle air screws during a carb sync. Just leave them at the stock setting.... not part of the process.
The locknut and adjusting nut on top of each carb is all you adjust. Pick 1 carb and balance to that ,as Gordon explains 

[Bodi]

OK, the issue is that there's no "base" carb setting with racks that allow all four slides to be adjusted independently.
The choke cam is adjustable, the idle screw range is not so adjustable. After reassembling the carbs after cleaning, set the idle screw most of the way out and set the slide adjustments so the slides close all the way down. If you didn't disassemble the choke cam mechanism and its adjustment was not changed, you can set the idle screw so the choke cam clearance is just taken up to touch, then set all four slides to just close all the way.
The basic adjustments if you took everything apart and need to completely reset the rack can take a while. The most important settings: have the slides all close simultaneously with the idle screw most of the way out (to allow idle adjustment range) and the choke cam just disengaged with choke fully open.
Then set the full throttle stop on the bellcrank. At full throttle the slides should not quite leave the carb bores, sticking out (on the non-cutaway engine side) about a fingernail thickness (officially 0-1mm).
After that do not adjust all four slides, pick one to be "master" (2 or 3 are the obvious choices since they are less accessible) and lock down its adjuster lock nut. Start the motor and set a rough idle with the idle screw. Adjust the other three for balanced vacuum. You know how to do the balancing it seems, so no problem there. I set the airscrews for best idle and check the balance again before buttoning everything up.
Now after the balance is done the idle screw will have plenty of travel both ways. Readjust the choke cam for a bit (about 0.010") of clearance when the choke is open. The final choke cam adjustment is by RPM, the maximum RPM anywhere in the choke lever range as it lifts the slides and closes the chokes should be approx 4000 RPM but you absolutely need that .010 or so clearance with choke open.
last, check the throttle bellcrank overtravel. You should be able to turn the throttle past the "closed" point a bit, you will see the bellcrank with the cables attached turn a bit while the actual throttle assembly does not. This is not generally a required adjustment but should be checked. If you look at the mechanism, the stop that the cable bellcrank hits is an eccentric bolt, you adjust it by loosening the lock nut and turning the bolt for a bit of overtravel (Honda wants 2mm space to the stop) then retightening the lock nut.
That's it.

[craywm]

wow thats a really good manual. but whats the difference between having the sync screws fully in or fully out? like if i put it too far down with the slids hit the bottom of the carb before the throttle hits the idle screw? or should i back it out so that when the idle screw is all the way out, the throttle stop is hitting the rail. i have the carbs off the bike again (had a broken brass tube in a bowl. what im confused about is, i can sync the slides to all come up and down the same by screwing the sync screws in or out, down it matter how far in or how far out those sync screws are? because when i was synching them once, i backed my idle screw all the way out and adjust the sync screws to where the engine was running at 1200 rpm with NO idle screw touching the throttle... does anyone get this concept.. like i could have the idle screw mostly in but if my sync screws are adjusted properly, then i can get the engine to idle at 1200 still.. where is a good position to have the idle stop screw for adjustment? ive also tried to set the idle stop screws and adjust all the carbs down to 1200 rpm. in my head, theres so many ways to adjust the sync along with the idle screw.. does changing the carb sync change to mixture? like will it make it richer or leaner?

[HondanutRider]

Not sure what you are calling "idle screw"?  On the carb rack I have, the idle screw (called the "throttle stop") touches a stop for the throttle lever that acts on all four carbs.  It simply makes the idle at rest (i.e. no throttle applied) more or less.  There is an idle mixture screw on each carb (called the "air screw", but that only adjusts the mixture at idle - it should be turned out 7/8 +/- 1/8 turn according to the shop manual specs.  On some versions of these carbs, these screws may have limiters on them to prevent tampering - mine don't.  The air screw does not touch any ramp on the throttle slides, and hence only affects the air flow to the idle jets...which might affect the idle speed as it improves or worsens the mixture.  Adjusting the the synchronization of these carbs does NOT alter their mixture.

[Bodi]

The idle screw certainly interacts with the sync screws. You don't want the idle screw all the way out, adjusting the sync screws for idle speed, that means the bellcrank is at its stop and you have no idle adjustment except "higher"... you may need to lower it as well after all. The other concern is the choke throttle cam, it may not work properly with the sync screws set that way.
I suggested setting the idle screw a few turns in from where the bellcrank is at its limit, then adjusting the sync screw so each slide just closes all the way. Make sure the choke cam isn't holding the bellcrank up though.
Once this setting is achieved, pick one carb to be master and lock its sync adjuster with a bit of nail polish. Then proceed with carb syncing as usual. Use the idle screw to set rpm. Locking the other 3 sync screws with nail polish is a good plan.
Once it's all excellent you finally set the choke throttle cam.