Let's discuss vacuum sync'ing.

My Dad and I are having a heated discussion about vacuum sync'ing carbs vs. bench sync'ing.

He believes, with all his heart, that you can't get any better than a bench sync; that all butterflies, throttles, slides, etc. are mechanically, physically exact and that anything you do after that will disrupt the equalization.

I believe that vacuum sync'ing will account for differences in the engine from cylinder to cylinder. Low or different compression, valves, etc.

Please educate BOTH of us on which one is better (vacuum... 8) ) and why. Please provide plenty of evidence, documentation, proof, videos, links, etc. My Dad is very skeptical of information on the internet. He's from the school of thought that "Anyone can put anything on the internet...doesn't mean it's right. I can type in that bench sync'ing is best and vacuum is wrong".

I personally have had very good luck with bench sync'ing. I've had a few bikes that were perfect right off the bat once I put vacuum gauges on them, or so close that it wasn't worth messing with.
I've also seen where all four carbs read differently.

We're currently working on a set of CV carbs. I tried to explain to him that, especially with diaphragm carbs, unlike mechanical slides, even a bench sync won't make the diaphragms/slides rise uniformly. When I asked him "OK, I bench sync the butterflies and put the gauges on it and one carb/cylinder has 10 inches less vacuum.....what do you do?" His reply? "Take off the gauges and throw 'em away."

Help me out here. or, if I'm wrong, say so and explain why.
I'll be cross posting this on several forums to get a wide range of opinions.

 

I always do the bench sync first, just to make sure that they don't leak before i put them on the bike 'cos i don't want to take them off again, then i'll do the vacuum gauge sync just to get them tweaked to as close as it gets.
But yeah, a bench sync will get them all pretty close, it's surprising.

 

I will leave the full explanation to experts here but I'd say that diaphragms and slides don't havemuch to do with vacuum sync because at low RPM the slides barely move if not at all.

The most accurate bench sync will save you time and adjustments when vacuum syncing but it will not replace it. If you do a rough bench sync, be prepared to spend more time on the vacuum sync.

Actually, there is no way to be very accurate when bench sync'ing because there is no instruments involved.

But your dad is right in a way: if you use cheap gauges after a very good bench sync, you could easily go wrong. That's why I use an oil-filled homemade manometer.

 

THAT IS your answer. And the vacuum-operated nature of CV carbs makes it even more important.

That being said, my Norton has (only 2) fully mechanical carbs and they can be gotten VERY close physically but still be just a tad "off" when checked with a manometer.

Bench sync gets you close. Vac sync makes it accurate, because of those very differences you outlined above.

 

You want all those vacuum operated slides to rise at the same time and rate right? Well how are they going to do that if the vacuum is different for each one?

 

FWIW, the bike that started this discussion (well...got it rolling...we had already been in disagreement over it) is an '85 FJ600. Always been cold natured and finicky. Sat all last year. It has that "clunky" sound at low RPM's that I know out of sync carbs can cause. It has four CV type carbs.
Pulled the carbs to make sure no slow jets, etc. were plugged that would cause the hard start issue. Bench sync'ed. Will adjust valves and cam chain next, and then I mentioned sync'ing the carbs. That's what got the ball rolling.
This is a high revving sport bike (Fastest production 600cc bike in 1985) and I can't help but think that the diaphragm slides need to be sync'ed with the engine tuned, warm and running.

BTW...EVERY manual I have says to sync using gauges, be it vacuum, manometers, etc. The factory Honda manual for my 500/550's, Chilton, Haynes, etc. Be it Yamaha, Honda or even the Weber/Dell'Orto manuals I have for my Hi-Po VW's...they ALL use some form of vacuum to sync.

 

Conduct a "Clunk-Test".

If the Carb's Diaphragm Bores are oxidized, ... the Pistons don't Rise and Fall as they should.

Vigorously scrubbing the Bores with ScotchBrite (Gray / Med.) will rejuvenate the Bores and contribute to resolving an ill-running issue.

Polishing them is a Performance prep.



"Clunk Test": http://xjbikes.com/Forums/viewtopic/t=1 ... clunk.html

 

There'll be a good reason for that

And you're right, the engine does need to be warm as well. Basically you're setting it up so that under normal running conditions each cylinder is doing an equal amount of work & not fighting the others.

A VERY good bench sync can get very close on an engine that's totally even on compressions/valve clearances etc (and indeed on my 21,000miler I had barely any adjusting to do on my bike after bench syncing using a bright light) but I defy your dad to find any 3 year old let alone 30 year old engine with comps within 1psi and valves all within .01mm of each other.

The closer you can get to each cylinder doing exactly the same as the rest, the smoother the motor will be, and the only way to get that last couple or maybe five % is to vac sync under normal running conditions...

BTW, if it's still acting "cold blooded", did you ensure the enrichment("choke") jets in the float bowls were clear? that's a common fault that's easily overlooked, they need to be able to shoot carb cleaner out in a jet 15foot (or into an eye if you're not careful) clear across the room otherwise they're not clean enough...

 

That's the hole in the edge of the bowl that pulls fuel from the bottom, right? Yes, they're clean, but now you have me wanting to double check.

And I'm curious about the "clunk test", too. The slides seem to move freely and I tested the diaphragms.

These are to easy things to check that won't affect anything else I've done so far. The carbs are still off the bike.

 

Refurbish and Polish the Bores. Win-win.

 

If I can make you unsure with mere typed words, you didn't truly believe the carbs were "church" clean anyway lol Better to be 200% sure once than 99% sure 2 times if they're anything like as bad to get on/off as an XJ!

And while they're in bits you may as well go ahead & follow Rick's advice on polishing the slide bores, like I say 200% X1 is greater than 99% X2

 

Right. I know I sprayed cleaner through those holes, but it's just a few screws to double and triple check since the carbs are off the bike.

The slides went in and out easily and I tested the diaphragms with the finger-over-the-kidney bean-hole method but, again, just a few screws to make sure they're perfect while they're still sitting on the bench.

 

The clunk test will tell you more about the slides, you let the slide drop with the spring & hat off. It should drop like a stone & go clunk in the bottom of it's hole, any chattering as it falls or acting like it's oil-damped means it's not right.

Have you seen how in-depth the church of clean is? Seems really anal and unnecessary but when you think how old these carbs are now & how much of a precision instrument they are, they need all the help they can get to do their job properly.

 

I think it has been outlined pretty well above, but just in case, here it is in a 'syncing for dummies' format:

Why do we sync the carbs? This is really the question you need to answer to resolve the debate on which is really necessary: vacuum sync or bench sync. What are we really trying to achieve here? What variables are we trying to eliminate by syncing the carbs? Let's answer these questions one by one.

First, how do these engines run?
This is clearly the first question to be addressed. The motorcycle engine (well, any engine for that matter) is a precise machine designed to convert chemical energy into mechanical energy in a smooth and controllable way. The engines in the bikes we are riding tend to be 4 cylinder, 4 stroke bikes. What does this mean? Each cylinder requires 4 strokes (or two revolutions of the crank) to complete one combustion cycle (intake, compression, combustion, and exhaust). The reason we use 4 cylinders is to smooth out the power and provide even propulsion through a full revolution of the crankshaft. With a 4 cylinder engine, we have 2 cylinders firing per revolution. Since each combustion stroke provides power for approximately 1/2 revolution of the crank, the bikes are timed so that there is always combustion happening in one cylinder of the engine and thus always pushing power into the crank and ultimately out to the rear wheel. So in a nutshell, we want to have power through every revolution of the crank to maximize output to the rear wheel.

So what are we trying to achieve here?
Since each cylinder has its own carburetor to provide fuel and air (as well as its own spark, everything needed to create combustion) we can view each cylinder as its own individual engine. So now we have one bike, four engines, and all output to the same transmission. We need each engine to rotate at the same speed and provide an equal amount of power for the engine to run smoothly. Smooth engines provide even and controllable power.

What variables are we trying to eliminate?
Let's discuss this in terms of power. Assuming your motorcycle is an 80hp, 4 cylinder engine you would want each cylinder to output 20hp for a cumulative power of 80hp. Now due to imperfections of manufacturing processes (casting limitations and inequalities), environmental variables (such as cylinders 2 and 3 theoretically having less cooling than 1 and 4 due to position relative to airflow), and engine compression variations, these 4 cylinders may cumulatively produce 80hp, but that may not be exactly 20hp per cylinder.
Consider this: cylinder 1 might provide 20.5hp, 2 may provide 21hp, 3 may provide 18.3hp and 4 may provide 22hp. Horsepower is a RATE of work, meaning that cylinder 4 can work faster than the rest of the engine, and cylinder 3 is working slower than the rest of the engine. These differences can be due to minute differences in valve clearances, engine compression, cylinder temperature, or exhaust backpressure - all variables that are unique to each cylinder and completely independent of the carburetors (this should be an A-HA! moment).

So how can these variables, external to the carbs, possibly have any impact on carb sync?
Let's go back to the idea of the 4 1-cylinder engines connected to the same transmission. If each engine is providing different amounts of power, then you have the more powerful engines being held back by the less powerful engines and vice versa. The end result is now you are using extra fuel just so these engines (or individual pistons) can waste energy fighting each other (and building damaging heat) instead of putting that power to the rear wheel or saving fuel so you can go farther on less fuel.

Now what is the difference between a bench sync and a vacuum sync? A bench sync is a ROUGH set of the carbs. Typically this is done with a sliver of film, plastic, or paper. This is a very imprecise and rough setting since paper, film, and plastic vary in thickness (the same kind of manufacturing differences that impact the engine outputs).
A vacuum sync on the other hand does not maximize power output of each cylinder, instead it SYNCHRONIZES the more powerful engine cylinder power outputs to the least powerful cylinder output. Now instead of engine power varying from cylinder to cylinder by up to 2-3 horsepower, you have all engine cylinders running at the same output as the least powerful cylinder, which by the example above may be only 18.5 horsepower per cylinder.

So what is the end result?
This synchronized engine provides less power than expected, now only 74hp, however the power is delivered more smoothly and evenly, meaning that YOU as the rider are in more control of the bike, get better fuel mileage, and are more confident in the bike accelerating and handling in a predictable manner instead of haphazardly gaining and losing power as the cylinders fight or work with each other as they would in an asynchronous motor.


Now I am sure I have made some mistakes or overgeneralizations in this description and explanation of the importance of vacuum syncing versus bench syncing, and I am sure others will be along to correct me shortly, but the principles are sound and the logic is true.
For comparison, I started with one bike and that was enough to justify my expense of a large sum of money on a vacuum sync tool to keep my bike running in perfect tune - better, as some argue, than when it rolled off the showroom floor some 32 years ago.

I should also note, this is all just relating to vacuum sync - the other variable NOT discussed here is fuel mixture setting, which can only truly be accurately adjusted with a ColorTune Spark Plug or many, MANY years of experience behind a solid set of ears (of which I am only in possession of the former described here).

 

im going to start listening to BluesBass. he broke it down quite nicely imo

 

Yeah, good wrap up!

 

Thanks, I just tried to sum up the information I learned here through my research. I hope my input can be as helpful for novice owners/riders as the information here has been for me!

 

I have two XJ550 Secas, and '81 and an '83. Not very different at all from your FJ. And I can tell you from experience, if you want them to run right, you MUST do the following:

-Get your valve clearances in spec, first and foremost.

-Make sure your carbs are indeed truly clean, here's an exploded Mikuni for you: http://xjbikes.com/Forums/viewtopic/t=31061.html

-ACCURATELY, individually "wet-set" the float levels, and be precise. The Mikunis reward precision in this respect.

-Set your pilots at 2.75 turns out to start.

-Then do an equally accurate running vacuum sync.

Then you can ColorTune/fiddle with the pilot mixtures, etc. Once you get it right there will be no "lump, lump, lump" at idle, it will sound like a sewing machine instead. And quickly pull from 2K to redline in all gears except 6th.

Both of my 550s were "finicky" and "cold blooded" until I did the above-listed steps.

 

Fitz is absolutely right on the importance of those steps IN THAT ORDER. Remember that these bikes and their fuel delivery systems are not the computerized, electronically sophisticated machines of fuel injection, they use carburetors which, to the untrained eye, seem to be old clunky and inaccurate dinosaurs. Just as a computerized fuel injection system requires a specific voltage range, fuel pressure, engine timing, and a highly accurate throttle position sensor our bikes replace these 'modern' systems with just as picky carburetors. They require specific fuel bowl levels, carburetor synchronization and fuel mixture settings.

ALL engines - and I repeat again ALL ENGINES require proper engine timing and valve clearances. Improper clearances mean valves stay open too long or don't open enough and reduce power output, compression, performance, and cause excessive and sometimes dangerous wear on the engine. It doesn't matter how clean your carbs are, how perfect your ignition timing is, or how good your fuel injection system may be, if the valve clearances are off, the engine will not run right and tuning becomes practically impossible.

 

bigfitz52 please explain your >>> "Set your pilots at 2.75 turns out to start." If ya' have the time.

 

The bike being discussed has Mikuni carbs. I recommended an initial pilot mixture screw setting of 2.75 turns "out" (from bottomed) as a good starting point.

That doesn't apply to Hitachis. I believe the recommended spec for Hitachis is 2.5 turns, at least on the "early" ones.

 

After the fiasco of the Yamaha-supplied, EGA Testing Machines blew-over; Dealerships all but abandoned the sniffer. They usually were "Present" at the Dealerships for some time. Eventually most (if not all) became coat-racks, bulletin boards or a skeleton in the corner. Today, I'd say anyone who has one would let you haul it out for them.

Most Bikes had EGA Test Bungs.
If yours aren't seized; consider yourself lucky!

Tuning pretty-much got handed-over to the Tech with your W/O.
The era of Customer Satisfaction to end the come-backs of more tuning
evolved.

There seemed to be a consensus that if the Pilot Air Screws (Mixture screw)
were set to whatever kept the Customer from needing to come back for a lost time tweak be used.

This got determined by Off-idle response, ... (most cases).

This Preset, intended to make people smile, ... usually did the trick.

"Between 2.75 -and- 3.0 Turns OUT.
(Closer to 3.0 splitting the distance between (a) & (b).

 

ABSOLUTELY true. There are Yamaha tech bulletins to that effect. ("Tweak 'em rich until the customer is happy.") Not bad advice, actually.

BUT this is another reason to not trust the "as found" pilot screw settings for anything other than maybe reference at some point, even if they were capped.

If the carbs are properly serviced, the pilot screws will have been out and their o-rings replaced.

So ya gotta start somewhere. The factory was really cagey about "admitting" to even the very existence of the pilot screws; and only published specs for some bikes (mainly the pre-YICS 650s.)

The 2.75 turns to start recommendation comes from my personal experience with Mikunis and on multiple bikes.

Oh, and I have a copy of the Yamaha EGA manual for our era bikes. It is quite honestly, a ring-tailed hoot to read through. They were very afraid of the EPA.

 

Now y'all have me intigued. I am currently going through the above mentioned steps but was not aware of the Pilot Screw step. I have my carb rack off right now and can't seem to locate these Pilot Screws. I know that they haven't been messed with or serviced because *cough cough* I'm the original owner.
Where are they and should I mess with them since I know they were last set at the factory?

 

Follow the arrow for the mixture screw:



and after a bit of cleaning:

 

Would you PLEASE POST THIS I would LOVE TO READ THAT. 8O

 

There is a chance ... but, thanks to the many great "Tuning Tips" from our Forum's Wizard's, a slim chance your Pilot Air Screws will be Plugged.

If they're still in there, ... call the FBI and ask one of their Top Agent's, "What's the best drill size, for yanking-out the EPA Plugs on Mic and Hi-tots?"
Don't be surprised if he tells you and warns you about how close the Top of the Air Screw is to the Bottom of the Plug.

If you need to drill; don't lean into it.
Cause no harm to the very closely underlying Screw Head.
They need to be removed for you to get those Felonious Air Screws pulled, O-rings removed, and top-end flushed before the FED's bust-down your door, and Tazer you; then hall you away for testimony before an EPA Regional Sub-committee.


Clean the Hole.
Oil a M6x0,5 *Bottom Tap* -- Chase the Threads.

If you are lucky enough to get a Chewed-up Air Screw ... OUT, great.
Don't put-it back in.
Toss it.
Get new ones from Len.

Put the old ones out with the Recycling.

 

You are 100% correct in your thinking You are syncing which means to make the same( sync your watches on the count of 3 its 0500 we all set our watches to 0500 right) The slight differences (this is what is known as tolerances) compression, valves clearances, etc whatever is why we sync the carbs we are trying to get all the cylinders to perform the exact same as much as we can this makes for a balance of power and a smooth running engine. We bench sync to get the carbs close enough for the engine to run so we can tune it out thats all Bench syncing should get it within the paramaters that the cylinder should at least fire . If your gages have weird readings just switch them to a different cylinder if the reading matches the previous readings your gages are good dont toss them. Start the process with carb #3 its the kingpin all the other carbs sync to it

 

PS the clunk test has no effect on the carb sync if the slides do not work correctly thats a performance problem has nothing to do with carb syncing

 

Absolutely you should check them out and replace the o-rings if you are having poor fuel milage and performance problems

 

Ahhh okay! I do remember seeing those screw heads now, so mine must not be capped. Thankfully. I'll have to check them out when I get back home this evening.
Thanks for the pics! Like they say, "A picture is worth a thousand words."

 

Don't try to move them until you buy or make a Special Tool Screwdriver.
The Screwdriver Tip MUST fit the Air Screw's SLOT with NO side-to-side free play.

I bought New Ones.
I'd like a set with External Tweak Knurl.

If you find one that has a look of being Hacked-at; evoke Rule No.1!
"Don't do anything that will make your Bad Situation, ... worse!"

Whatever you decide to do if you find yourself behind the Eight Ball with one or more of those Damaged or Seized Air Screws, ... (after discussing the matter with the New Pope, extensively) ... should NOT be without the aid of a DRILL PRESS!

My advice from now-on, ... If you ain't got a drill press; find a Machinist to bail you out.

 

Yeah today I've been contemplating what to do. I could have sworn I read somewhere that they were factory set and not to be messed with. Now I know that's just an invite to mess with them for most of you renegades on this site. What exactly am I going to gain by tightening them down, then backing them off 2.75~3 turns? If they are "stuck" wouldn't they be stuck at the proper setting as I know for a fact that they have never been turned by a tool?

 

You need to remove the Air Screws to replace the TINY O-ring seated at the Bottom of the Screw Port.
Removing them and Flushing the Pilot Passages is important after over 20 years of neglect.

After the Screws are OUT, ... make sure you will never have to fight to move those Screws again.

Chase the Bore with a M6x0,5 Bottom Tap
Flush clean.
Flush Passages by douching through the Pilot AIR Jet, flushing the Pilot FUEL Jet and Air Screw Port passages.

Prep the Air Screws for Fine-Tuning.
Anti-seize the Threads in the Hole.
Teflon-wrap the Threads on the Air Screw.
One Layer.

Preset for future Fine-tuning.
Darken One Wing of the Air Screw's two Slot Wings with a Magic Marker.
It makes the Observation of the Adjustments less confusing and less likely to lose track.

 

That's what they wanted you to believe, so as not to mess up their emissions. As we discussed above, the dealers were told to go ahead and mess with them to keep the customers happy (and about half of the dealers re-capped them.)

The very first time the motor had its valves adjusted, the original settings became invalid. But it doesn't matter, because you're going to be removing them anyway.

Before you REMOVE THEM to clean the passages and REPLACE the tiny o-rings, carefully turn them in until they gently bottom out, and record the original settings. For posterity.

If you're working with (early, pre-YICS) Hitachis the factory recommended setting is 2.5 turns out.

 

Okay I can do that Rick. Once I've followed those steps and have backed out the screws 2.75~3 turns. What is it I'm listening, feeling, tweaking for?
Or just back them out that amount and it's good?

 

"Off-Idle Performance"

You want to dial-in a STEADY reliable IDLE.
Plus, ... a smooth, uninterrupted ... "Transition" from IDLE to OFF-Idle.

When you open the Throttles to take-off, ...
NO Bogging-out or Stall
NO Hesitation in the least.

The Bike should IDLE without Hunt or, what I call "The Poppities".
You should Idle: "Rummmmmmmmmmmmmmmmm"
Not: "Rummmm-popity-pop, rummmm, pop, rummmm, poppity-poppity".

You'll hear it at 2.5
Maybe even stall.
As you add Richness, ... Idle will sound like a Sewing Machine.

The ColorTune Plug allows you to "Find" this accuracy, ... Visually.
The Plug shows you the Burn inside the Combustion Chamber.
Adjust OUT until you achieve: "Bunsen Burner Blue".
Perfect Idle.

Tweak the Screw a bit more Rich, ... until the Blue fades-away.
Utopia.

 

Utopia I shall seek!

 

Now THAT is my nomination for "tuning tip of the month"!