electronic carb synch?? how many mm's of mercury do we pull?

[snowbeard]

so I have a crazy idea, I know there really wouldn't be much to beat the exact specificity of comparing two cylinders directly with a mercury stick setup, but how about an electronic pressure meter?

so here's the idea, I've got a pressure guage that reads in mmHg, atmosphere being about 760mm, down to 0, or full vacuum. what if I set it up to read my carbs? hoses etc would be well within my means, so here's the clinch.,


what kind of vacuum do these engines pull on one cylinder?? when you hook it up to the mercury sticks how many mm's of mercury do they pull?

[bwringer]

The actual amount of vacuum doesn't matter much -- it's the difference between the cylinders that matters.

FWIW, my bike usually reads something between 10CM and 20CM Hg on my el cheapo mercury carb stick:

http://motionpro.com/images/carbbalancer.gif

There's an electronic dealy that simply compares the vacuum between any two sources. Hook it up to any two ports, and when the light in the center is on, those two are balanced. You could add some sort of valve setup from a fish tank or something so you could easily switch which two cylinders are being balanced.

However, that doesn't help when you need to set the inner two cylinders at a slightly lower vacuum (if you have the stock airbox).

Personally, I'm saving my pennies for a Morgan Carbtune:

http://www.carbtune.com/

They're the cat's patoot. The finest synchronizing tool ever created -- the stainless steel rods can't spill like mercury and the weight of the rods damps out all the annoying pulses that make mercury sticks so $%#@! hard to read.

I've got a set of mercury sticks that cost about $60 several years ago, and I really wish I had known about the $100 Morgan carbtune at the time.

Or if you've got a few extra bucks lying around:

http://www.vacuummate.com/

[bwringer]

To actually answer your question.... :roll:

The amount of vacuum should be well within your electronic guage's range.

And it should be easy to come up with some sort of contraption you can use to switch between cylinders and get readings on each one. Fish tank air valves wouldn't be too far-fetched.

The problem is that changing a setting on one carb affects the others, too. You balance in pairs, so it would be best to at least be able to compare two carbs at once.

You could tune using one vacuum guage, but you'd need an easy way to keep switching back and forth between carbs. After you got the feel of it, you could probably get it done pretty quickly.

You'd also need to come up with some way to damp out or average the pulses so the reading doesn't jump all over the place. Whether this is best done electronically or with some sort of accumulator and/or restriction... I dunno.

My dad is an instrument engineer. I wonder how hard/expensive it is to find electronic vacuum guages? Hmmmmmmm...

[snowbeard]

so it sounds like I'd spend a lot of time going back and forth, as I need to synch to one carb (say 3) but that carb will change with each synch?

so two lines with a valve to switch between shouldn't be too bad, but four all at once would be the best, right?

with two I'd get the primary and first synch done, then change the second synch, which thereby changed the primary, making it necessary to then return to the first synch...

I could also find a flowmeter type deal, with a weighted ball that rises with the rate of gas flow... would need a restrictor on the far end so I didn't just suck the ball into the engine (really so I could use a smaller weight and just lessen the flow..)


hmmm........

[snowbeard]

hmmm indeed.

went to your site and re-read the carb synch piece, what would I do if, say, I didn't have a colortune? just go by sparkplug color? guess?

I thought it was all about flow, less about mixture at this point...

[Poot]

i just bought a synching device from the parts department at a local cycle dealer. paid 90 bucks (cdn) for it.


it has 4 individual gauges (non mercury) as well as hoses for each carb, and I'm wondering if it is any good. it should be for what i paid!

[Anonymous]

I just got fed up with a handful of do-it-yourself manometer experiments. Here is the good and the bad...

I have the weird dual-throat carbs. This means that even with four cylinders, I only need to compare two vaccum measurements, and I only need to make one adjustment. It also means parts are a bear...

For the threaded nipples for the side of the carbs, I bought a few M6 grease zerks. The thread was perfect. Then, I threw them in my mini-lathe (you could do it with a vice & a drill, or even a file, if you are patient), and cut out the spring retaining flange on the (threaded) end of the zerk. They turned out beautifully.

Because I only need to compare two pressures, I decided to go with a differential measure (between the two) instead of comparing to ambient. That way, I wouldn't have to measure much pressure (22 vs 18 mmHg; I only have to measure 4mmHg). With less to measure, I figured I could use water as the working fluid, and not have to worry about spilling or sucking it through the engine. Also, I figured that with the carbs pulling against one another, I could probably balance them at speed (as well as idle) without any problem.

So, with 8 ft of 3/16 clear tube, with a bit of water in the middle, I set out to do the job.

As most of you have guessed by now, the big problem with this setup was the damping. It was absolutely worthless, I sucked a lot of water through the engine trying to get it to work. No dice.

I debated going with an absolute measure (run it down a steep driveway), figuring the bucket at the end would add some damping. I thought about using a larger hose, figuring more mass would yield more damping. I thought about a flow restrictor, or a large volume of air ahead of the water. I even thought of floating some oil on top of the water to add viscous damping.

The next iteration was to try hooking up a stiff plastic bottle full of water to act as a mass damper. Two more makeshift nipples, a couple of nuts and rubber washers, and I had an air tight setup. Well, it did a great job of damping the system, but the vacuum crushed the bottle in no time. Oh well...

And from there, I just ordered a $50 4-tube mercury manometer.

If your vacuum gague can measure relative to another source, you can do two at once by going differentially. If you set up a much more rigid reservoir, you could probably make the water mass damper work. The next thing that would probably happen is the clear tube may collapse under the vacuum. Certainly if you go with a large diameter (thin wall) hose...

Good Luck,

Rob

[Anonymous]

dual throat carbs

Is there a drawing that shows where the air screws are on the dual throat carbs? I've been working on (I think it is) an 82' GS550 and it's running lean at idle. I wanted to change the pilot jets and adjust the idle air screws but I don't know where they are. This is not my bike so I'd like to know before I just go around drilling what I think are air screw caps.

Thanks!

[Anonymous]

Yes, there are a few pictures.... I don't have the links handy right now, but if you do a search for messages from me, I've been fighting these carbs for a while now; I often include a link to an exploded diagram, and have been known to describe the position of the idle needle on the diagram.

If you think you've found the plug, you probably have. I did a lousy job taking mine out and caused a lot of problems, good luck.

Also, just a few days ago, someone posted a link to a .cz site with a bunch of downloadable Haynes manuals. The diagram included therein is modified from the fiche. It has the needle labled (and drawn horribly).

Let me know if you have any questions,

Rob

[mcycle-nut]

i just bought a synching device from the parts department at a local cycle dealer. paid 90 bucks (cdn) for it.


it has 4 individual gauges (non mercury) as well as hoses for each carb, and I'm wondering if it is any good. it should be for what i paid!

I've got one of those too. I assume it's half decent, my bike runs fine.

[snowbeard]

from the front page there is a carb cleaning guide link, pics, total walk-thru, haven't used it yet, but it looks really solid!