My Homemade Manometer

[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.

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[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. :-k

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

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[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? :-k

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

.

 

[castlekin]

So, I took a 3/16 x 12 brake line I had laying around, or also known as 4.7mm X 30.5mm. I cut off about 3 inches of it, threaded it though a M5 thread die, and put on a 007 o-ring on that. I need to get a few nuts on those, but they thread into the carb boots very well and I am able to make them the length I need.

Hope that helps others to make those adapters a little cheaper.

 

[kcorbin]

So, I took a 3/16 x 12 brake line I had laying around, or also known as 4.7mm X 30.5mm. I cut off about 3 inches of it, threaded it though a M5 thread die, and put on a 007 o-ring on that. I need to get a few nuts on those, but they thread into the carb boots very well and I am able to make them the length I need.

Hope that helps others to make those adapters a little cheaper.

I tried that same thing with the brass tubing as well as some stainless steel I had and couldn't get it to thread. But, that was more of a factor of my poorly designed Harbor Freight tap & die set. The taps seemed to be just fine but the dies didn't have enough 'slope' or 'graduation' at the mouth to actually accommodate the stock and start cutting new threads. It works fine for chasing threads to clean them up but not actually creating new ones.

I tried it first because I thought drilling all the way through center of the 5mm socket head screws would not be easy but it ended up being no big deal. No drill press and it still worked out great.

 

[850GT_Rider]

What is the actual measurement

What is the actual measurement

I may have access to some guages and build my own manometer. Anyone know what the actual suction measurement is? Either in inches of water or inches of mercury?

 

[MAdams450]

I am not positive if this is correct, but I was once told by a suzuki mechanic that the reason for the unbalance when syncing has to do with cylinder cooling.(inner 2 cylinders dont run as cool as outer 2).

 

[Steve]

I am not positive if this is correct, but I was once told by a suzuki mechanic that the reason for the unbalance when syncing has to do with cylinder cooling.(inner 2 cylinders dont run as cool as outer 2).

Then why was it specified for the bikes that had crossovers in the exhaust?

Or was it just coincidence that the specified imbalance just happened to come at the same time as the crossovers?

.

 

[850GT_Rider]

I may have access to some guages and build my own manometer. Anyone know what the actual suction measurement is? Either in inches of water or inches of mercury?

Anyone? Bueller??

 

[BassCliff]

Hi,

I may have access to some guages and build my own manometer. Anyone know what the actual suction measurement is? Either in inches of water or inches of mercury?

The actual vacuum measurements will vary with the rpm speed. The factory manual states no hard and fast numbers and the factory tool has no numbers on it at all, only balls in cylinders to represent the vacuum balance between the cylinders.

I have a few pictures of my Morgan Carbtune from which you can get a general idea of how much vacuum we're talking about. These pictures were taken at 2000-3000 rpm.

This is at idle, before synchronization:

You see the vacuum varies between 14 cm/Hg and 18 cm/Hg.

This is at ~2000 rpm:

You see the variance is between ~21 cm/Hg and 26 cm/Hg.

This is after synchronization, again about 2000 rpm:

You'll notice the vacuum between 22 and 24 cm/Hg.

Here are a couple of illustrations from the factory manual. Notice, no numbers.

For the bikes with exhaust crossovers:

For bikes without exhaust crossovers:

I would think that gauges that read 0 - 50 cm/Hg would suffice. It seems that the Morgan Carbtune reads from 8 - 42 cm/Hg. I'll let you do the math to convert to inches.

Thank you for your indulgence,

BassCliff

 

[850GT_Rider]

That's perfect Cliff - Thanks!

Edit: I know we're more interested in the difference (delta p) than in the actual number, and now I know that 0-60" H20 gauge will be perfect. I can get those fairly easily - we use them all the time. I'll put snubbers in the tubing to quiet down the oscillations and be good to go!

 

[kcorbin]

Then why was it specified for the bikes that had crossovers in the exhaust?

Or was it just coincidence that the specified imbalance just happened to come at the same time as the crossovers?

.

I don't think it has to do with the crossovers. The 550 has crossovers for both the 79 and 80 model years yet the 79 factory manual says to sync straight level across all the balls. In the 80 manual it says to sync 1 and 4 even, and 2 and 3 even, but slightly lower than 1 and 4. The major change in 80 was moving from the VM22 carbs to the BS32 (constant velocity) carbs. It would seem that the carb type is probably the overriding factor and not the exhaust type.

Interestingly, the Haynes manual states the VM22 carb is actually slightly undersized for that engine at higher RPMs and recommends swapping for the larger BS32 for performance. So then, does cooling the inner two cylinders more come into play when you are pushing the engine harder with the larger carbs? Or, is it just a function of how the new CV carbs work? I can't tell *for sure* in BassCliff's pictures, but I bet the first picture showing the un-aligned balls is with a BS/CV carb.

 

[hampshirehog]

I tend to agree that the 'unbalanced' carbs are required because of the airbox rather than the exhaust. If you balance the carbs with the airbox fitted as per the handbook and then without the box on the gauges become level ( or more level - is 'leveller' a word...?). And this is for the CV carbs - the VMs are in comparison crude old things that are pretty insensitive.

Is the Suzuki manual wrong (again)?

In practice I have never found one iota of difference between carbs balanced evenly across all 4 or with the half a ball difference so I just set them all level now.