My Homemade Manometer

[kcorbin]

Thanks for the kind words all!

One other quick thing for VM carbs: when doing the actual synchronizing the direction I turned the adjustment screws above the carbs was a bit counter-intuitive until I thought about it for a bit.

The manometer is measuring the vacuum *after* the carbs (not at, say, the mouth of the carbs) so opening up the slides actually reduces vacuum in between the carb and the intake manifold. So, if a bottle/cylinder is pulling too strong (fluid level is rising) then turn the adjustment screw counter-clockwise to raise the slide, allowing more air to flow and reducing the pressure between the carb and intake mainfold. Likewise, if it is weak and getting its fluid sucked out, close the slide by turning clockwise and creating more vacuum in front of the carb.

[castlekin]

Rock on kc. I am thinking of using Salsa jars, form-a-gasketing in the tubes and lid seal. Creating a good tight seal and really killer jars.

I like the fact I don't have to have any restrictors in the lines. Also, I only got one problem with this setup, though I am going to use it. And that is I saw a video of someone using a store bought unit to adjust with and they were able to adjust at higher RPM's so as to make the bike run even more smooth at highway speed.

[Matchless]

AFAIK the vacuum generated in the vacuum tubes diminishes as the throttle is opened.
The highest vacuum is at idle, as the throttle opens this wider throttle opening starts feeding more air to the motor, thus negating the negative pressure or suction at the takeoff nipples.

Thus if the motor is running at high RPM's or low, with the vacuum equal on all four cylinders, the fluid level will stay in line regardless of it being a high or a low vacuum.

If may be wrong and it would not be the first time!

[kcorbin]

Matchless is exactly right and and I think the Bernoulli principle also goes into effect when you start opening up the throttles: as the *velocity* of air increases at the higher RPMs, the *pressure* actually decreases. At lower RPMs with the slides almost totally closed you would be creating more of a pulsing static vacuum like a vacuum pump.

This is the same principle that sucks the gas up out of the carb bowl: the lower pressure of the rapidly moving air above the jets attempts to equalize and pulls the gas through the jets.

I'm not a fluid dynamics engineer so I may have just completely munged the explanation though!

But, I see no reason why you couldn't run this manometer at higher RPMs either though. I ran it at 4~5K for a couple of seconds myself to make sure I was getting the same readings as at idle after I dialed it in and I didn't notice any change.

As for the salsa jars I think the form-a-gasket would work but if you have access to them, something like a tall liquor bottle would be more ideal I think. That would allow for a smaller, easier to create seal at the top with such a small opening yet have more volume than the smallish iced tea bottles I used. The greater volume would allow for more time to tune especially far-out-of-sync cylinders before you sucked a bottle dry.

[Guest]

Will definitely attempt a twin one for mine when I get to that stage.

[Steve]

I don't recommend trying to use the two bottle method and syncing cylinders in pairs because of the interaction between them all.

Thank you for noticing that and pointing it out. The downfall of many DIY manometer projects is that they try to do it in pairs and get frustrated when they can't get them right.

Like other methods however you really are only syncing against #3 so you don't need to adjust it. Leave #3 as it is and then adjust #1, #2, and #4 as necessary.

If you look at your adjusters a little closer, you will see that you are NOT adjusting 1, 2 and 4. You are adjusting the relationship of 1 to 2, 2 to 3 and 3 to 4. #3 is your "master" carb because it's the one that is 'wearing' the throttle cable. All the others have to be set to match it.

The manometer is measuring the vacuum pressure *before* the carbs so opening up the slides actually reduces vacuum in between the carb and the intake manifold.

First of all "vacuum" is the absence of "pressure", and you are NOT measuring it *before* the carbs.
Look where your hoses connect to the bike. I'll bet they are in the same location as mine (and all the others I have seen), and are between the carbs and the engine.

The rest of your description is not bad. Then engine is trying to breathe a lot of air, the small carb opening is restricting it. The more you open the carb, the less the restriction.

... I saw a video of someone using a store bought unit to adjust with and they were able to adjust at higher RPM's so as to make the bike run even more smooth at highway speed.

The whole purpose of synchronizing the carbs is to get all the throttle blades (or slides in the VM carbs) to open the same amount at the same time. If you get them right at idle speed, they will still be right at all the other speeds. However, the effect of mis-synchronized carbs is most noticeable at idle, and it's much easier to use the idle speed control as a repeatable setting for your adjustment.

... I think the Bernoulli principle also goes into effect when you start opening up the throttles: as the *velocity* of air increases at the higher RPMs, the *pressure* actually decreases. At lower RPMs with the slides almost totally closed you would be creating more of a pulsing static vacuum like a vacuum pump.

Yes, Bernoulli's principle is how carbs work. Yes, as velocity increases, pressure decreases.

The only problem with that is that your carbs are designed to minimize the change. The slides are before the jets and the throttle butterflies and are responsive ONLY to moving air. At idle speed, the slides are down, keeping the opening small and the velocity up to a point where there is some fuel flow action in the jets. As you open the throttle butterflies, more air is drawn through the space under the slide. This reduces the pressure a bit more. The hole in the bottom of the slide transfers that "vacuum" to the space above the diaphragm. If the vacuum is low enough, it will lift the slide. The lifted slide now allows more air to flow under it, but the velocity will remain rather constant, due to the variable size of the opening. I really do believe that is why they are called CONSTANT VELOCITY carburetors.

... The greater volume would allow for more time to tune especially far-out-of-sync cylinders before you sucked a bottle dry.

This is why you do a "bench sync" before putting the carbs back on the bike. If you do it right, your sync might be so close, you will be tempted to not mess it up by trying to get it perfect.

.

[kcorbin]

If you look at your adjusters a little closer, you will see that you are NOT adjusting 1, 2 and 4. You are adjusting the relationship of 1 to 2, 2 to 3 and 3 to 4. #3 is your "master" carb because it's the one that is 'wearing' the throttle cable. All the others have to be set to match it.

I have VM carbs and you actually *can* adjust #3. I should have clarified that in my descriptions as all my comments are based on my experiences with them. I'm fairly intimate with the VMs now but only know the CVs in theory since I don't own one.

First of all "vacuum" is the absence of "pressure", and you are NOT measuring it *before* the carbs.
Look where your hoses connect to the bike. I'll bet they are in the same location as mine (and all the others I have seen), and are between the carbs and the engine.

Well, yes, vacuum is the absence of pressure just as cold is the absence of heat and darkness is the absence of light.

On my carbs, you are measuring the 'absence of pressure' in the vacuum chamber created between the cylinder and the round slide of the carburetor. In the flow of air into the engine you are measuring *after* the carbs but looking at it from the left side of the bike as I was conceptually, the manifold ports are to the left of the carbs. Reading left to right like a book the ports are before the carbs. Duh, that was a dumb mistake. I get your confusion with what I said because I said it is *totally* backwards from the air flow. I'll correct my post to avoid confusion for others.

The only problem with that is that your carbs are designed to minimize the change. The slides are before the jets and the throttle butterflies and are responsive ONLY to moving air. At idle speed, the slides are down, keeping the opening small and the velocity up to a point where there is some fuel flow action in the jets. As you open the throttle butterflies, more air is drawn through the space under the slide. This reduces the pressure a bit more. The hole in the bottom of the slide transfers that "vacuum" to the space above the diaphragm. If the vacuum is low enough, it will lift the slide. The lifted slide now allows more air to flow under it, but the velocity will remain rather constant, due to the variable size of the opening. I really do believe that is why they are called CONSTANT VELOCITY carburetors.

Thanks for a good description of the CV carbs and that helps me understand them better. Again, I have the VMs so I apologize for not clarifying that in my posts since they are a bit different in overall operation.

This is why you do a "bench sync" before putting the carbs back on the bike. If you do it right, your sync might be so close, you will be tempted to not mess it up by trying to get it perfect.

I did a bench sync, and after using a manometer, I see how inadequate that can be for getting things 'right' IMO. Plus, you would never know without testing it. I ran into the same thing with setting the basic ignition timing. Setting the base timing left me with no power but after using a timing light and getting it perfect I had power again.

But, I'm just speaking from my own personal limited experience and haven't been doing this nearly as long as you so you probably can bench sync and bench time as good I can sync and time *with* specialized tools!

[Yogi]

This looks like a much easier set up than the one I'm currently trying to "get" to work. I am using 4 hoses connected together by at the bottom of a piece of wood with small restictors in the line near where they connect to the intake. My first attempt resulted in oil (2 cycle) beginning to be sucked up the #1 tube, (I think my bench synch was maybe not as good as I had initially thought). I would hate to have to take the carbs off just to re-bench synch them. I think I can turn the throttle plate screw in a bit on the #1 solving this problem. Any thoughts, or chastising for trying to take the easy way out? I have an '80 GS750 with BS carbs.

[Skateguy50]

pretty much like the one I made when I did my carbs, left it behind on one of the moves as it could have been done a bit cleaner.

think I used atf fluid or something like that but the idea is once you get them close it doesnt move that much.

[waterman]

You may want to check with lab suppliers for rubber stoppers, I can purchase a pound worth of two holed rubber stoppers for around $30 which is approximately $1 a piece.

My only concern is that all carbs will be synchonized with each other, but I was under the impression that the manual states that best tune is with the outer cylinders drawing slightly higher levels of vacuum at least for the CV carbs. I know when I have used my mercury sticks, I strive for a little bit higher draw on 1 and 4. Is there a way to accurately measure that with this device?

Please do not take this as criticism, equal synch has to be better than bench synch I am sure. Plus, I like the McGuyver factor. I may rig one up just for the heck of it to see how close it is to my sticks.

[kcorbin]

This looks like a much easier set up than the one I'm currently trying to "get" to work. I am using 4 hoses connected together by at the bottom of a piece of wood with small restictors in the line near where they connect to the intake. My first attempt resulted in oil (2 cycle) beginning to be sucked up the #1 tube, (I think my bench synch was maybe not as good as I had initially thought). I would hate to have to take the carbs off just to re-bench synch them. I think I can turn the throttle plate screw in a bit on the #1 solving this problem. Any thoughts, or chastising for trying to take the easy way out? I have an '80 GS750 with BS carbs.

I don't know the BS carbs, but I certainly wouldn't take them off again to try and bench sync; it would probably only get worse. If you are attempting this on a warm engine, I'd just adjust the major offender, start it up, then shut it off if it's still too strong and repeat until the response slows down enough to leave it running for fine tuning.

I think actual 'running' synchronization is important because not every piston/valve/carburetor is exactly identical and a bench sync assumes that. I mean, these are 30 year old machines and things will have worn slightly unevenly over that time.

[kcorbin]

You may want to check with lab suppliers for rubber stoppers, I can purchase a pound worth of two holed rubber stoppers for around $30 which is approximately $1 a piece.

My only concern is that all carbs will be synchonized with each other, but I was under the impression that the manual states that best tune is with the outer cylinders drawing slightly higher levels of vacuum at least for the CV carbs. I know when I have used my mercury sticks, I strive for a little bit higher draw on 1 and 4. Is there a way to accurately measure that with this device?

Please do not take this as criticism, equal synch has to be better than bench synch I am sure. Plus, I like the McGuyver factor. I may rig one up just for the heck of it to see how close it is to my sticks.

Heehee, $1 each is a good price but you only need 4! I swear I probably spent the dollar value in gas and time of a couple of the stoppers checking out prices at other stores. They were *mostly* on my way to other places but I still went out of my way; I end up kicking myself sometimes for spending $2 to save $1.

I'd be curious why it says to have a higher vacuum on 1 and 4 for your model. Mine says make them equal. I know there's always a reason for everything but I always want to know why so I can understand it all better. I hate doing things just because somebody/something says so. Is it to compensate for something?

You can certainly still set 1 and 4 stronger than 2/3: just adjust them until 1 and 4 are rising at some slow but equal rate and 2 and 3 are falling equally.

[Steve]

I have never figured out just why Suzuki calls for slightly higher vacuum levels on the outer cylinders.

I have done them "properly" and "equal" and have not noted any difference.

.

[Skateguy50]

I got them so they all stayed pretty even and called it a day, unless you are adjusting them like every few months I dont see the point to go overboard. Just wanted something better than bench synch only.

[Steve]

I got them so they all stayed pretty even and called it a day, unless you are adjusting them like every few months I dont see the point to go overboard. Just wanted something better than bench synch only.

You going on the Catskills Ride?

I have my "sticks" with me and can check your bike, if you like.

.