Topic: Setting sync without a sync gauge 1977-78 CB750

[lucky]

I recently ran into a problem with my Motion Pro sync gauge.
I started the engine after getting the sync gauge calibrated on the #2 carb.
On the second start up to actually sync all the carbs the engine rpm raced upwards and sucked the blue fluid into the engine. Most of it anyway.
I took the sync gauge apart and found out very little of the fluid was left.
It was my 4 or 5th time using the sync gauge.
From no on I will always store it hanging up so it cannot loose fluid.

Any way I still needed to sync the (keyhole) PD style carbs so this is what I did,
I had the tops off all of the carbs (1978 PD carbs) and I backed the adjustments on the three adjustable slides off completely. #2 is not adjustable.

Then I took a vernier caliper and using the depth gauge function I measured the top edge of that slide (#2) when it was all the way down. Closed position.
It was 1.205 thousandths. Then I just adjusted the other three slides and made them
all the same. (1.205)
All of the slides will be within .001 of each other!
No need to sync with vacuum manometer.

[ADW]

That's a good start for a bench sync but you'll still need to vacuum sync because each cylinder will be a little different in how much fuel/air it's pulling in. Internal engine differences such as quality of valve sealing, piston ring sealing (compression) will make each cylinder a little different and hence, require that they all be slightly fiddled with to draw the same vacuum. You want to have an equal vacuum reading on all 4 carbs because then each carb will be feeding each cylinder the correct mix to compensate for that cylinder's internal differences.

[scunny]

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+1, carbs still should be synced for smooth running.

[70CB750]

What ADW says. You may call it static versus dynamic synchronization.

[lucky]

Deleted for technical reasons.

[lucky]

My method got the slides within .001 of each other!

[BobbyR]

You have probably been very lucky. The vacum synch allows for the inevitable variations that can be found between cylinders and carbs. The dial gauges prevent sucking liquid into the cylinders. When you go through the synching process is that as you synch a carb the carbs around it change. So you finally tweak them all to  get 1 3 4 even with 2. Your method does not allow for this.

You may be close enough so the differences are negligible. It is not microsurgery.

[TwoTired]

Mechanical sync of the carbs is only accurate if the volumetric efficiency of each of the 4 cylinder is exactly the same.
In reality they seldom are unless the engine was recently blueprinted.

Vacuum sync is best for all other engines.

In the video, the engine seems to have an uneven idle pulses, (where vacuum sync helps the most).

[lucky]

The volumetric differences were minimal.
The engine was just rebuilt.
All of the seals on the throttle shafts were ok.
There were no air leaks.
I got better results with my method.
Most of my life I have always used a manometer.
This time i decided to do it in a more precision way.
Listen to my video.

Try my method.
If you do not get an even idle it means you must have
air leaks in the throttle shafts. Something to investigate.

Many forum members said they have boiled the carbs, soda blasted, and worse.
That could not be good for the throttle shaft seals.

[lucky]

You have probably been very lucky. The vacum synch allows for the inevitable variations that can be found between cylinders and carbs. The dial gauges prevent sucking liquid into the cylinders. When you go through the synching process is that as you synch a carb the carbs around it change. So you finally tweak them all to  get 1 3 4 even with 2. Your method does not allow for this.

You may be close enough so the differences are negligible. It is not microsurgery.

I would like to take the time and answer this fully.
Although there are variations you would still not want to look into the intake and find out that one of the slides is 1/4 higher than any of the other slides even if the manometer showed it was right.

It would mean that that carb has something very wrong or different going on with it and that it must have an air leak in the intake boot or the throttle shaft bushings.

When you look into the intakes you want to see all of the slides going up and down at the same time. Also when the throttle is closed, you want all the slides bottomed out
other wise it means that one of the slide needles would be raised up and causing more gas to be used on that carb. Because when a slide moves upwards the needle moves upward in the main jet.

If you have a old engine you may have to use the manometer until some of the problems can get corrected. New pistons,rings, and carbs rebuilt etc.,.

But I do understand what you are saying.

When you use the manometer on the 1977 or 1978 CB750 carbs you making the adjustments that directly raise or lower the slides on #1,#3, and #4 cylinders.
WHY NOT just measure them? Especially if the carbs are off of the motorcycle.
Then if you use the manometer and it says one of the carbs is WAY off you know that particular carb or cylinder has some kind of problem. Air leak bad rings ,throttle shaft leak to look for. Then if a compression check shows all cylinders are the same then you know it is the carb or rubber intake boots.

Just another way to do things.

[BobbyR]

You have probably been very lucky. The vacum synch allows for the inevitable variations that can be found between cylinders and carbs. The dial gauges prevent sucking liquid into the cylinders. When you go through the synching process is that as you synch a carb the carbs around it change. So you finally tweak them all to  get 1 3 4 even with 2. Your method does not allow for this.

You may be close enough so the differences are negligible. It is not microsurgery.

I would like to take the time and answer this fully.
Although there are variations you would still not want to look into the intake and find out that one of the slides is 1/4 higher than any of the other slides even if the manometer showed it was right.

It would mean that that carb has something very wrong or different going on with it and that it must have an air leak in the intake boot or the throttle shaft bushings.

When you look into the intakes you want to see all of the slides going up and down at the same time. Also when the throttle is closed, you want all the slides bottomed out
other wise it means that one of the slide needles would be raised up and causing more gas to be used on that carb. Because when a slide moves upwards the needle moves upward in the main jet.

If you have a old engine you may have to use the manometer until some of the problems can get corrected. New pistons,rings, and carbs rebuilt etc.,.

But I do understand what you are saying.

When you use the manometer on the 1977 or 1978 CB750 carbs you making the adjustments that directly raise or lower the slides on #1,#3, and #4 cylinders.
WHY NOT just measure them? Especially if the carbs are off of the motorcycle.
Then if you use the manometer and it says one of the carbs is WAY off you know that particular carb or cylinder has some kind of problem. Air leak bad rings ,throttle shaft leak to look for. Then if a compression check shows all cylinders are the same then you know it is the carb or rubber intake boots.

Just another way to do things.

Great explaination. Now getting real. Once the bike is toghether how would you do this again? I have to tell you a carb synch is not a one time occurance. I am sure some has a bike that was synched in 1975 an they can balance a quarter on the motor. I am not so blessed, I get about 4 years and I am pushing it.

[lucky]

You have probably been very lucky. The vacum synch allows for the inevitable variations that can be found between cylinders and carbs. The dial gauges prevent sucking liquid into the cylinders. When you go through the synching process is that as you synch a carb the carbs around it change. So you finally tweak them all to  get 1 3 4 even with 2. Your method does not allow for this.

You may be close enough so the differences are negligible. It is not microsurgery.

I would like to take the time and answer this fully.
Although there are variations you would still not want to look into the intake and find out that one of the slides is 1/4 higher than any of the other slides even if the manometer showed it was right.

It would mean that that carb has something very wrong or different going on with it and that it must have an air leak in the intake boot or the throttle shaft bushings.

When you look into the intakes you want to see all of the slides going up and down at the same time. Also when the throttle is closed, you want all the slides bottomed out
other wise it means that one of the slide needles would be raised up and causing more gas to be used on that carb. Because when a slide moves upwards the needle moves upward in the main jet.

If you have a old engine you may have to use the manometer until some of the problems can get corrected. New pistons,rings, and carbs rebuilt etc.,.

But I do understand what you are saying.

When you use the manometer on the 1977 or 1978 CB750 carbs you making the adjustments that directly raise or lower the slides on #1,#3, and #4 cylinders.
WHY NOT just measure them? Especially if the carbs are off of the motorcycle.
Then if you use the manometer and it says one of the carbs is WAY off you know that particular carb or cylinder has some kind of problem. Air leak bad rings ,throttle shaft leak to look for. Then if a compression check shows all cylinders are the same then you know it is the carb or rubber intake boots.

Just another way to do things.

Great explaination. Now getting real. Once the bike is toghether how would you do this again? I have to tell you a carb synch is not a one time occurance. I am sure some has a bike that was synched in 1975 an they can balance a quarter on the motor. I am not so blessed, I get about 4 years and I am pushing it.

Is that as condescending as you can be? LOL...lol
BTW I would not do this procedure again.
IF the idle gets off it is because I have an air leak. I would find and fix that air leak.

[BobbyR]

No not condescending. I have read your posts on other topics and find you to be very clever. As time goes on things change in an engine, so it will need a touch up. Air leaks should be dealt with, but not everything is an air leak. Stuff starts wearing from the first stroke of a piston.

[TwoTired]

All of the slides will be within .001 of each other!
No need to sync with vacuum manometer.

I believe this to be untrue and unwise for the general populace of SOHC4s.  Here is why...

The source of the vacuum is the four individual engine cylinders, not the carbs.  The carb slides only provide a variable resistance to the pressure equalization between engine cylinder and outside atmospheric pressure.  When all the pressures at the intake runners are the same (or within spec), then the cylinders are getting an equal a/f mixture charge with which to fire, as they would if only one carburetor was attached. This is the purpose of a vacuum synchronization, too make all four carb units and connecting ducts behave as one, even though they have separate slides and unequalized intake runners between carb and cylinder.
In theory, if all the cylinders operate with the exact same efficiency, then all the slides will be in the same relative position to provide equal mixtures among the carbs/cylinders.  However, only angels can make all the cylinders operate exactly equal.  In the world humans live in, perfection is not possible.  You can get cam lobes close to the same, but not exactly the same.  You can get the valve clearances close to the same, but not exactly the same.  You can get piston rings to seal close to the same, but not exactly the same.  You can get intake runners to flow close to equally, but not exactly equal.
Since the vacuum source is not equal among the four cylinders and cannot remain so over time while the engine wears (unevenly among cylinders is a definite possibility), mechanically aligning carburetor slides cannot guarantee all cylinders will receive equal vacuum pressures or equal air/fuel mixture charges.

Bench or mechanical sync will get you close, and if you happen to roll 16 dice and they all come up seven, the engine may actually have good enough vacuum balance.  Chaos is like that, things may or may not fall into alignment.  But, until you check your mechanical alignment and verify that the vacuum is balanced with such an alignment.  The job is not done, and certainly not proven to be a effective alternate procedure on anything but that one specific engine.

I don't care what your engine sounds like, I attend to how it performs.  And, I already noted that Lucky's engine sounds like it is favoring some cylinders over others during idle, where vacuum sync is most critical.

If the mechanical adjustment is being used to eliminate/avoid the purchase and use of vacuum check/adjustment equipment established by Honda and the entire motorcycling community, I believe it has a long way to go prove it's effectiveness, and certainly NOT on a test case of one without corroborated vacuum balance indication, or a casual listen of a youtube sound track on a million different sound systems.

You know, I've had this mechanical sync vs vacuum sync debate before.  But, never with an actual angel. 

Cheers,

[lucky]

I understand your concerns about all things not being perfect or equal.
If a motorcycle engine has air leaks in its intake system this procedure will not work.

I was a Honda dealership mechanic from the first Honda 50 to the introduction of the CB750. In Tucson Arizona.
I was one of the first mechanics to put together a set of vacuum gauges. They did not sell the manometers to the public at that time.

I would never judge how well an engine is running from seeing a online video.

I understand what you are saying, but carbs have changed a lot from the first mechanically actuated cable style carbs. A new procedure can be introduced at any time by any shop or any mechanic.

You may even make a discovery some time too.

BTW....I have won over 67 motorcycle races during my life. AMA sanctioned races.

[markb]

Just subscribing.

[lucky]

If the reader of this thread does not like my method they do not have to use it.
You still need to set your slides and get them even.

If a beginner uses my method to set up their carbs they will not suffer in any way.
If they are rebuilding their carbs and they measure the height of the slides as I have described, they will at least start off the tuning of their engine with all of the slides within .001 thousands of each other and then if they want to use the Manometer/vacuum gauges to set their carb slides, they will still not have suffered any setbacks. But if the engine runs very poorly after using my method, and all other things being correct ie, mixture etc. , they should be looking for air leaks and a do compression check.

[dave500]

bench sync gets the motor running good enough to run and use the vac sync period.,get a perfect vac sync then measure the slide heights,they will differ.,a bit like points,,you can set them by eye and get it running,then use a dwell meter and timing light if you want.

[lucky]

bench sync gets the motor running good enough to run and use the vac sync period.,get a perfect vac sync then measure the slide heights,they will differ.,a bit like points,,you can set them by eye and get it running,then use a dwell meter and timing light if you want.

If the dwell is over the limit, then you need new points because it means the plastic block that rubs on the points cam is worn down. The dwell and the points gap are related.

[dave500]

yeah,,im just saying that a bench sync is a bit like just eyeballing points,,youll get it running but then you can use dwell meter and timing light to square it off.

[lucky]

yeah,,im just saying that a bench sync is a bit like just eyeballing points,,youll get it running but then you can use dwell meter and timing light to square it off.

Yes, it is good to check the timing with a light after you know the dwell and points gap is within limits.

But setting the slide heights with vernier calipers is not "eyeballing".
A precision tool is used.

[dave500]

and you look at this tool with your eye,,i think its been mentioned above that the idea of carb syncing is to balance the actual vacuum between the cylinders not actually get the slides at the same height,which will get the bike running ok then use a vacuum method,leaving them set where the precision instrument aligns them is precisely wrong.

[lucky]

and you look at this tool with your eye,,i think its been mentioned above that the idea of carb syncing is to balance the actual vacuum between the cylinders not actually get the slides at the same height,which will get the bike running ok then use a vacuum method,leaving them set where the precision instrument aligns them is precisely wrong.

Your eyes and a visual inspection is the first step to any mechanical or welding procedure.
I do understand your concerns about the amount of vacuum being created in each cylinder.

[apehanger 550]

  This thread hit at just the right time. I never wanted to remove the carbs from the rack, but as it turned out,I had to.
  Now they need a bench synch,and it makes sense to me that this method would be easier than a drill bit-- at least it is easier to understand with this type of carb.

[dave500]

they still wont be synced unless you vacuum sync them,full stop.