Adjusting for highest vacuum???

[psycoreefer]

Ok, so I thought I was doing carb synchs and idle mix adjustments the right way but twice today, and at random other times I've seen people reference adjusting for the highest vacuum reading. Can someone explain to me the advantage of this method and how it works? Following are the two quotes that got me thinking....

The setup shown is used just to balance the carbs, not to adjust the (fuel mixture) vacuum level. I do that with something similar to the setup posted by Mr. Bill, except that I don't use an aquarium valve. I hook up the vacuum gauge to one carb at a time and adjust for the highest vacuum. After that I hook up the balance tubes to carbs 1/2 and 3/4 and synch. To synch carbs 1/2 TO carbs 3/4, I pick either 1 or 2 and synch it with either 3 or 4 with just one of the balance tubes. Sorry for the confusion.

Ok so this quote uses vacuum to adjust idle mix - seems like a way to "verify" the by ear method - the closer your mixture is to perfect the faster the idle and the faster the idle without opening the butterfly the higher the vacuum. This somewhat makes sense to me, and I might try it next time. For this method to work your carbs would have to close to balanced I think.

resynched the carbs... but... we started w/ the #3, got the highest vac reading we could get, (instead of a low #, then trying to synch the other 3 to #3...) This made all the difference, and that's been the end of that problem. Bike runs so much better now. So, synch can really be a major issue... didn't know if you'd tried to get your highest reading on the first carb, then setting the others to that...
We could get all 4 synched fine at the lower vac reading... but it wasn't right, and the bike's performance sure showed it. Just a suggestion, may not be your deal... but it sure sounds like something's screwed up in that carb.

This one gets me, #3 has no adjustment right? So how do you change the vacuum on the #3 carb? With the main idle screw? Won't the highest vacuum be with the idle screw backed out as far as possible without the bike stalling?

How do you maintain this "high" reading? every time I've synched my carbs a tweak to the balance screw causes the idle to change, which causes the vacuum on #3 to change.

I'm eagerly awaiting the knowledge that is sure to flow......

[XS1100 Newbie]

No 3 does have an air fuel mix screw. I does not nave one for synching.

[81xsproject]

The concept is simple and geared towards setting fuel-air mixtures, by adjusting for the most vacuum you are adjusting to the point where the cylinders are pulling the hardest, meaning that the mixture is providing to most energy to move the pistons to pull the vacuum...

The problem with this is that becomes convoluted when you have more than one carb because adjustments on one carb effect all cylinders and their associated vacuum because they are all powering the same crank. I though about trying this concept when I was super concerned about my idle mixture (around the same time I waisted $60 on the colortune). However, the more I thought about it the more I decided it was better suited for single carb engines.

[skids]

Here is the "thing":

If you adjust "synch" at higher rpms, you will be way off at idle. The butterfly adjustments should therefore be done near idle (1100 or 1200 rpms). Then you need to adjust the idle mixture screws. It isn't likely that after adjusting mixtures that you will need to readjust the vacuum synch much, but it is worth a check. If you did anything to readjust synch, then you need to readjust mixtures. This would be the process no matter which method you use to adjust both synch and mixture for final. I tend to use the colortune to get it close and then color a set of plugs for the reality check.
I hope that makes sense, 'cause it does to me.

[XJOK2PLAY]

Psycoreefer,

Newbie's got it right. That's what we did... we had the vac guage on the #3, adjusted the screw watching the reading... Vaccuum was barely present when we first started adjusting. As we adjusted #3, the vaccuum climbed, and so did engine rpms, and smoothness. When it was as high as we could get it, we then adjusted the other 3... and patiently adjusted until all 4 were as equal as we could get them. But it is the "By Ear" method.

By the time we had them where we wanted, we had lowered the idle speed screw... many times... finally got it to set on 1100 rpm... and always returns there now. That was 6 months ago... and it's still working.

We had tried to synch them twice before, but with the lower reading from #3to adjust the other 3. Bike would not idle or run right. It would stall on launch sometimes... almost got me run over a few times in city traffic too!
Everybody had always bragged about how much 'low-end grunt' these bikes had, but mine never did. It always bogged until 3000 rpm, then shot off like a bullit. now it has the grunt.

It is an XJ... and they're a little different... especially with timing & advance... which showed up dramatically when it wasn't synched well in the way the bike crazy idled. Once we synched this way... it was like a brand new bike.

Now, I've never put a colortune to it... I asked Cody about that once... he said he'd never done it to any of his bikes and didn't see much need in it, since his always ran well this way. Colortune might show something different.
But all 4 of my plugs are the perfect greyish color ya want... so....

Our way may not be text book... or even make much sense... all I can say is for mine it worked... great. And I've been relieved ever since.

Sorry if I confused ya earlier... hope I explained it better....

[DGXSER]

Personally, I have never heard of (with the exception of a couple post on here) or used vacuum to set idle mixture. I do believe the colortune is a good tool for setting mixture once you figure out its idiosynchrosies. But I also believe in checking plug color after the colortune and fine tuning for color as needed.

There is no other way I know of but vacuum readings to synchronize the carbs which is an entirely different process from setting the idel mixture. Although, they are intertwined and can effect each other.

Most folks I know, myslef included, will synch, then tune, then resynch, then recheck tune, and maybe check synch one more time. But then I hang with a very detail and anal retentive group that does the tuning. (Yeah Doug, you know I am correct.)

[WMarshy]

...Everybody had always bragged about how much 'low-end grunt' these bikes had, but mine never did. It always bogged until 3000 rpm, then shot off like a bullit. now it has the grunt.
...

Wow, exactly the way mine has been acting. On cold mornings its even worse so I think its leaned out too much in the cold air as it pops a little until under load trying to accelerate untill the rmp's get up. I think you just convinced me I need to buy some hose and do a synch!

[DGXSER]

Unsynched carbs can cause all sorts of goofy behavior. Hard starts on cold mornings even with full enricher might point to clogged enricher ports in the carb bowl though.

[WMarshy]

So whats the deal? I havent read the tech tips on synching carbs very close but, does it explain what were talkin about in the above posts? Adjust f/a mixture for each carb to match with highest vac possible?

[XJOK2PLAY]

Had something interesting happen last night w/ the bike...
Decided to fire it up and run downtown and grab a hamburger for dinner...

Lately, I've noticed the idle speed had dropped a couple hundred RPM, and when I'd first start it up w/ full choke, it always fired off instantly, but after a minute or so, I'd hear a POOF! sound coming from the front area of the motor... almost like exhaust. It would do it 2 or 3 times, then quit.

Last night it did it again... I've also noticed my fuel mileage slip a few MPG's..

One buddy that followed behind me not long ago said he saw occaisional black puffs of smoke from only my right side muffler at times.

Well... I removed my lowers from the faring for the warmer season, and could finally get the first look around the carb areas since Talimena...
There it was... staring at me! The rubber plug on the #2 vaccuum nipple had broken the top off, and behold! A really good vaccuum leak! The thing was placed on there new less than a year ago.

After a quick trip to the O'Reilly's...problem solved. RPM is back at 1100, no more strange noises... turn signals have resumed flashing at idle speed w/o my assistance... hopefully my mpg's will also return, and that also solves the mystery of the black smoke as well!

Amazing the trouble one little rubber cap can cause...

[TomRodgers]

Those pesky little rubber plugs...

Ive noticed on startup, while cold only, and first start of the day.....when bike cools off to the touch, 2nd start up doesnt do it.... will sputter a couple of times and #1 vacuum cap has 'popped' off. just the little cheapy caps from Autozone. they dont fit very tight.

[81xsproject]

I had one pop off at a stop sign once; went from running real good to real bad real quick. After that, I bought an M3 tap gently threaded the vacuum ports. Then I got some small M3 screws, put some fuel tape on the threads, and screwed them into the ports. Never had a problem after that.

[CatatonicBug]

There is supposed to be a clamp on those rubber caps. It keeps them from jumping off. I can't even pull mine off unless I remove the little clamp.

The thing to remember is that "Vacuum" and "Synch" are the same process, and "Mixture" and "Fuel/Air" are the same process, but the two sets are completely separate processes.

The vacuum is adjusted via the screws on the throttle linkage (the thing moved by the throttle cable). Once that is done, the fuel/air mixture is adjusted via the little screws in the top of the carb bodies (old style carbs have caps sticking up above the carb body, while the newer ones are buried in a hole, covered by an aluminum plug). Some folks use a Colortune plug to determine the correct setting, while others prefer to do it by ear. The two adjustments affect each other, so repeating them as DGXSER mentioned is a good idea.

[Yahman]

The carbs do need to be in the ballpark for this to work, by that I mean either bench synched or currently running.

By adjusting for the highest vacuum on each carb using the fuel mixture screws, with the rpm at the proper speed, you are setting the air/fuel mixture to its optimum for that carburetor, when it is running on its idle circuit. Then you balance the carbs (cylinders) by adjusting the throttle butterfly’s using the synch screws, so they are in synchronization at the proper rpm. Repeat as necessary until perfect.

All of this of course only affects the bottom end at idle and the transition off idle, like when taking off from a stop.

I've done it this way all of my life to many motorcycles and I always get the same reaction, it has never run so good!

Larry

[psycoreefer]

Ok, I get it now. Both quotes are talking about adjusting the idle mix, using vacuum as an indirect (but possibly more accurate) method of observing idle speed.

The part that got me was that quote #2 seemed to be talking about synching, starting with adjusting #3 which has no adjustment, but that's been explained, what was really being said is they adjusted the #3 idle mix using the vacuum method, then adjusted the synch between carbs as normal.

I think I'm going to try an experiment - I've adjusted both my bikes carbs via the ear method, then fine tuning based on plug color. Usually after the ear method I'm a bit rich and I end up turning the screws 1/8 to 1/4 turn leaner. When I get some time I'm going to throw my vac gauge on and see if adjusting from this point can make the vacuum rise. Maybe the vacuum method will allow me to "skip" the plug checking step's in the future.