Topic: Carb syncing question

[KC_Northstar]

I think I'm going to get a Morgan Carb tune to sync my carbs.   In the mean time I have been reading up
on how to do it.  The maintenance manual says to turn the idle adjustment screws in until they are
seated and then back out 1 full turn.  It then says to proceed with the synchronization.  After that it
does not talk about re-adjusting the idle screws.  Dint you have to adjust the air screws to get the
right idle mixture?  If you re-adjust them after the sync wont it mess up the sync?
 

[motosan]

I might be wrong, but my understanding is that the idle mixture set by idle mixture screw and idle speed have little to do with the vacuum uniformity controlled by the throttle plates, which is set by synchronizing carbs, AS LONG AS the carbs aren't wildly off synch....hence the needs for bench synch and THEN fine tune with gauges.....I think (gurus, please correct me if I'm wrong!)

Well at least I haven't seen any needs for readjusting idle mixture after syncing.  The only thing i had to do was to reset the idle speed if there was any change.

By the way, I like Morgan Carb tune.  Make sure to order the brass one and don't tighten too much

[lucky]

Don't get confused.
Syncing the carbs means getting the idle on all carbs set so that they all
are pulling the same amount of vacuum.

The mixture screws are different and should not be turned after you sync the idle.
Just set the mixture screws all exactly the same. Whatever the book says.
If there is any popping on decel then richen them all the same amount.

 THEN SYNC the idle on all carbs. LAST.

[phil71]

i have to respectfully disagree with lucky.  A little theory of operation: the idle circuit is a small passage that begins at the rear mouth of the carb and goes AROUND the slide (or slide and butterfly). It passes over a jet often called a 'pilot jet'. When that ever-so-small rush of air passes over top of the pilot jet, it picks up some fuel.. which then travels past a pointy tipped screw whose job is to meter the amount of air allowed to bypass the slide (or slide and butterfly). Some carbs meter the output of the jet, rather than the flow of air over it. It's good to know what you have first. An "air screw" will be richer the further you turn it inwards. A "fuel screw" is just the opposite. But in a way, it doesn't matter too much, as you will be tuning for best , smoothest , fastest idle . Then, you can use the idle stop screw to set it.
  Anyhow, the relationship between the idle air bypass circuit and how much air the throttle lets in is why it's good to sync before mixture settings
  It's better to sync at slightly higher rpm. Around 3k.
 On carbs with mechanical slides, you can sync at idle, but I have found it's a good rule-of thumb to crank the idle up a little bit and level them all. This is especially important on CV carbs, but if you just learn to do it this way, it'll work on most bikes you come across. The mixture screws then fine tune the idle after sync. It will take time for you to feel it out, but when you get the mixture screw right, the idle will be higher, and smoother. If it's  got more than one exhaust pipe, they should all feel about the same pressure at the tip, and similar in temp. A wet paper towel quickly dabbed on each header pipe will tell you if any one is burning much colder than another.
 
  Popping on decel is rare if all components are stock and in good health. Exhaust with holes will cause it, but you'd have to be pretty far off on your mixture settings for it to pop on decel, and then it would be a bear to idle and pull away from a stop sign.
  Listen to the engine, and go very slow with the screws... it'll start to make sense to you.

[TwoTired]

An "air screw" will be richer the further you turn it inwards. A "fuel screw" is just the opposite. But in a way, it doesn't matter too much, as you will be tuning for best , smoothest , fastest idle .

That may be true for CV carbs or carbs with an accelerator pump.  But, is just not so for the early Keihin mechanical slide carbs on the SOHC4.  These pilot screws are set for a rich idle, so when the slides are lifted, the extra air has some fuel available for power and RPM increase.  Unless you are fine tuning for off idle acceleration under load, set these to book value before doing the vacuum sync.  If you adjust air bleed pilot screws for max idle RPM, you will have extremely poor off idle acceleration under load.

  Anyhow, the relationship between the idle air bypass circuit and how much air the throttle lets in is why it's good to sync before mixture settings
  It's better to sync at slightly higher rpm. Around 3k.

Again, not true for the SOHC4 mechanical slide carbs.  The most critical vacuum balance point is at idle RPM when the slides are at their smallest opening point.  Small differences in slide opening make big differences in smooth, consistent idle operation, with each cylinder producing the same power pulse strength. 
 

On carbs with mechanical slides, you can sync at idle, but I have found it's a good rule-of thumb to crank the idle up a little bit and level them all.

That does make gauge needles steadier and easier to read.  But, it doesn't always give the smoothest idle operation.
 
I'll have to agree with Lucky and the shop manual on this matter.

[bjatwood]

When I synced mine KC, I used a Morgan Carb tune and just hooked it up to the carbs and balanced them out with the height of the slides..
This was after I  made sure the valves, timing and plugs, float heights and carb cleaning were all freshly done....as I had a bad float with carb #3 when I first got the bike.
Bike ran as smooth as a baby's behind after doing this carb sync...and has for 3 years now.

[phil71]

Having just sync'd and tweaked K4 750 20 mins ago, I stand by the suggestions. They aren't radically different than the shop manual, and I've discovered that once you've gotten a tight sync just off idle, and make one last fine tune of the mixtures at idle, you get better response at the crack of the grip when you're leaving a dead stop, compared with doing it to the letter of the shop manual.. Again,that's assuming all other things being healthy.
Of course they're suggestions, but the OP also has a few bikes that aren't one size fits all.

[TwoTired]

The reality of thirty five year old bikes, is that the cylinders, and cam lobes are no longer at "as new" specifications.  This means that the compression and intake draw among them is not identical.

When new, a vacuum sync, pretty much aligned the slides at all the same height.  On an older engine a vacuum sync aligned the slides slightly uneven so that each cylinder got enough breath to to produce the same strength power pulses.

At idle the engine need only produce enough power to sustain idle RPM against internal friction, but it is 100% of what is available.  Adjustments to even the power pulses are critical.  @ 3000 RPM and no load, each cylinder has excess capacity, more than what is needed to simply overcome internal friction, and each cylinder's individual volumetric efficiency is far less critical. 

If you can find an engine that has mechanically worn exactly the same for each cylinder, then vacuum sync the carbs at whatever RPM you want, and the idle will still be even.  I have found few used engines that respond that well.

I've already tried turning out the air screws for max idle speed (as well as minimal exhaust hydrocarbons).  Runs beautifully in the garage.  On the street with any load, acceleration is replaced by engine wheeze and the bike slowing down due to complete lack of power.  Turning the air screws inward improved the situation incrementally with the setting change.  When street driveability was eventually restored, lo and behold the air screws were at factory setting.  Imagine that!   
Lesson learned.

[toytuff]

You can find these on flea bay ever so often. I like the dial setup myself.

Just me.

tt

[BobbyR]

The reality of thirty five year old bikes, is that the cylinders, and cam lobes are no longer at "as new" specifications.  This means that the compression and intake draw among them is not identical.

When new, a vacuum sync, pretty much aligned the slides at all the same height.  On an older engine a vacuum sync aligned the slides slightly uneven so that each cylinder got enough breath to to produce the same strength power pulses.

At idle the engine need only produce enough power to sustain idle RPM against internal friction, but it is 100% of what is available.  Adjustments to even the power pulses are critical.  @ 3000 RPM and no load, each cylinder has excess capacity, more than what is needed to simply overcome internal friction, and each cylinder's individual volumetric efficiency is far less critical. 

If you can find an engine that has mechanically worn exactly the same for each cylinder, then vacuum sync the carbs at whatever RPM you want, and the idle will still be even.  I have found few used engines that respond that well.

I've already tried turning out the air screws for max idle speed (as well as minimal exhaust hydrocarbons).  Runs beautifully in the garage.  On the street with any load, acceleration is replaced by engine wheeze and the bike slowing down due to complete lack of power.  Turning the air screws inward improved the situation incrementally with the setting change.  When street driveability was eventually restored, lo and behold the air screws were at factory setting.  Imagine that!   
Lesson learned.

+1

[lucky]

i have to respectfully disagree with lucky.  A little theory of operation: the idle circuit is a small passage that begins at the rear mouth of the carb and goes AROUND the slide (or slide and butterfly). It passes over a jet often called a 'pilot jet'. When that ever-so-small rush of air passes over top of the pilot jet, it picks up some fuel.. which then travels past a pointy tipped screw whose job is to meter the amount of air allowed to bypass the slide (or slide and butterfly). Some carbs meter the output of the jet, rather than the flow of air over it. It's good to know what you have first. An "air screw" will be richer the further you turn it inwards. A "fuel screw" is just the opposite. But in a way, it doesn't matter too much, as you will be tuning for best , smoothest , fastest idle . Then, you can use the idle stop screw to set it.
  Anyhow, the relationship between the idle air bypass circuit and how much air the throttle lets in is why it's good to sync before mixture settings
  It's better to sync at slightly higher rpm. Around 3k.
 On carbs with mechanical slides, you can sync at idle, but I have found it's a good rule-of thumb to crank the idle up a little bit and level them all. This is especially important on CV carbs, but if you just learn to do it this way, it'll work on most bikes you come across. The mixture screws then fine tune the idle after sync. It will take time for you to feel it out, but when you get the mixture screw right, the idle will be higher, and smoother. If it's  got more than one exhaust pipe, they should all feel about the same pressure at the tip, and similar in temp. A wet paper towel quickly dabbed on each header pipe will tell you if any one is burning much colder than another.
 
  Popping on decel is rare if all components are stock and in good health. Exhaust with holes will cause it, but you'd have to be pretty far off on your mixture settings for it to pop on decel, and then it would be a bear to idle and pull away from a stop sign.
  Listen to the engine, and go very slow with the screws... it'll start to make sense to you.

Yes ...I agree. I just did not want to go into that much detail.

[lucky]

You can find these on flea bay ever so often. I like the dial setup myself.

Just me.

tt

   Yes ....very nice professional looking set.