synch then tune?

[Guest]

ok, I did not disassmble the quartet of carbs and they are still factory set as far as synch.

I did a bench rebuild and the carbs were very clean inside not warranting a further disassembly. I did remove the EPA caps and remove the idle screws for a proper cleaning.

Should I attempt to adjust the the fuel mixture screws with the 4 vacuum gauges and use the old school method for vacuum setting carbs for that sweat dial in:Adj to highest vacuum then richen a tad?

Bike currently idles great , revs great, but when riding it has the slightest stumble from about 2-4g rpm

I was looking for some advice before I dropped 60 bucks on the vacuum quartet.

bike just turned 9700 miles and I have not adjust valve clearance yet, but for the sake of this story, lets say they are in spec.

[Baatfam]

Actually, you'll get all the help you need here...and probably without the rudness...:-D

I'm fairly new here myself and these guys are great...

First...do a search on carb sync. You'll get lots of reading material. Then ask your questions....

You'll likely find out that there are choices in sync tools...
You'll also find out that the valves need to correctly adjusted to make it all worthwhile...

Personally, I'm going to order the one from England...forget the name right now. But it's not that expensive and a lot of people swear by it...
Of course I can use it, as I have three bikes to work on.

Later,
Bob T.

[82Shafty]

I am sure there are lots of nice riders on here and then there are those who like to call out the search button because that's what they know how to do.

Yes, I know how to use the search button and there are ten different topics that pop up all asking something in the same direction and all have the same answer. It's like reading a clymers manaul. I get tired of general questions or the same questions constantly popping up over an over again. It's still no reason to be ignorant. Perhaps it was my ignorance in not explaining what it was that I was truly after.

I know that you check the valves first... made a big difference on my 750F2 so I am saying that once you get past that point; I saw many posts that kind of tell you what you should be looking at.

http://www.thegsresources.com/_forum/showthread.php?t=97620&highlight=carb+synch

http://www.thegsresources.com/_forum/showthread.php?t=98077&highlight=carb+synch

http://www.thegsresources.com/_forum/showthread.php?t=96763&highlight=carb+synch

I have been on message boards for about 10 years now and am not a noob by any means and I also wasn't looking for any sort of e-butt hurt.


Here's my reasoning:

If the vavles have been adjusted and I synch the carbs BUT the air/fuel ratio isn't correct, how the heck do you know that the synch is going to be correct beyond bench synching and then when you go to dial/synch with vacuum gauges or you think you have it, but then you go to adjust the air fuel ratio, unfortunately it won't be stoichiometric, which of course is not what you want at WOT, but cruising and anyways, I see a total chasing of the tail.

If I had an option to run a wide band on each cylinder to set the idle air/ fuel after bench synch, then synch, and possibly re-adjust idle air/fuel I wouldn't be on here asking. Reading plugs can get you awfully close but several attempts are made until it is truley set to most riders liking.

My concern is the percentage of error that could occur:

If the throttle blade, not the variable venturi is open a hair more than the others, but the fuel maybe compensating on that cylinder more than the others, beyond bench synching how is anyone really know that they are dead nuts on.

I am looking for a response from a senior member, one who perhaps has tuned a few bikes in his or her day and can vouch with real-world experience. Theories are great but I am asking for practice and not the practice that most of you may think that I am referring to, but the sorts of which would be considered medical termonology.

I have read about 9-10" of merc but still, do we take for granted that the air/fuel is appropriate before synch, then synch, the reset air/fuel?

If it is simply trial and error, then by all means then answer with just that.

Thank you for your time.

[nert]

A successful sync of the carbuetor bank has little or nothing to do with each cylinders fuel mixture or cumbustion efficiency. What you are attempting to adjust and equalize between all carburetors is that the same volume of air is passing throgh the individual intake tracks as in every other carb on the bank at any given throttle position. This is by and far controlled by the opening of each throttle plate. Cylinder vacuum is not being measured. Air flow, venturi affect, produces a negative pressure on the each of the test gauges. By adjusting the throttle plate openings, you can pass the same volume of air through each carb by obtaining equal gauge settings among the gauges. Like one BIG carburetor, with one big throttle plate, providing all cylinder with an equal amount of air.

You can get close with a drill bit and a manual sync. However, that adjustment does not compensate for differances in carb bore, bore surface, throttle plate, shafts, or any other factor that can affect the actual air flow.

To adjust for equal air flow, you need to measure the air flow.
You could build a manifold hook to a powerful shop vac, mount your carburetors, (no fuel, no combustion, no heat, or vibration) and do a perfect job of synchronizing the bank. (labratory perfection)

Once the carbs are in sync, than you can adjust fuel mixture per each individual carburetor, feeling confident that each carb is passing an equal amount of air per given throttle opening. An old analog tachometer with an upper scale of 1500 rpm, would be a better, more sensative tool for adjusting fuel mixture at idle.

just a theory!

[82Shafty]

As in any theory Nert, it can be augmented.

I, in a mechanical engineering aspect, believe very strongly in a cause/effect relationship.

I am really fond of your air flow bench idea, been there with heads, never with carbs; Props to Nert!

Now, in order to achieve the pull/vacuum in the intake track which is the direct effect of the piston traveling down in the cylinder, air rushes over the venture creating a low pressure area in the throat and because atmospheric pressure is inside the bowl that then in turn strives to meet balance and pushes fuel up the idle circuit where it is atomized (boiled due to low pressure) into the air stream. This has been proven; not my theory and therefore is a fact.

So when you told me that I am not measuring cylinder vacuum, well actually I am because if it were not for that cylinder vacuum in the intake runner, I would not be able to measure the air being pulled (due to the vacuum) through the carb.

I think what we do have is a given percentage of what I like to call fluff.

Let’s say that I ran with the flow bench idea and set the carbs dead nuts on CFM wise at 28” of merc. Great! Well but that excludes all real world factors such as all cylinders may not pull the exact same volume…should in theory, but not always true in the real world. Granted the leak down is performed and comes back less than 5% (more accurate than compression testing.) Then this also accounts for the induction of atomized fuel in which, as you mentioned before, there could be tolerance issues in the carbs’ bores so even if I have them exact (dry air run), when fuel is entered it may not swirl exactly the same and therefore decreasing the combustion efficiency which leads to more guessing. This doesn’t even account for runner length being tuned. I suppose this is why they went to fuel injection measuring crank speed to adjust for any inconsistencies.

I suppose that the situation truly is what it is and it is a method of constant fine tuning in order to achieve the median throughout the operating range.

I suppose the best method would be to air flow the carbs to precisely synch them, use fuel to compensate for any inconsistencies, achieve maximum rpm per given throttle opening, then slightly richen the mixture to keep the air cooled four happy.

Funny thing is the Clymer manual says to set 2&3 a tad more open on the t-blades by what is shown on the manometer.

Hmm now to build the flow bench.


Thanks for the idea Nert. Perhaps in the dead of winter I will have some leisure time to try this out.

BTW I am not trying to argue anyone, just trying to figure out the best (most accurate method) for all riders to dial these old girls in.

[mark]

I've had my mixture screw settings all over the place and it hasn't affected my synch much if at all in my experience.

As previously stated, valve clearance is important so that the valves are opening and closing properly. Assumed is adaquate compression, that you don't have vaccum leaks and your air box is hooked up. Assumed also is that your vacuum line to the carbs is plugged.

Set mixture screws to a reasonably close setting....then synch (to whatever rpm you prefer, I use about 3,000 rpm....then finish it off. Highest idle on air screws etc. Having a couple of good fans is critical.

Its possible I guess but I've never heard of using a sych tool to adjust mixture or air screws. I've certainly never needed to, and adjustments I've made post-synch have not affected synchronization at all.

You're right on the slightly different setting between 1-4, and 2-3 on your model as you manual describes.

Most people here use a Motion Pro synchronizer. I use vacuum guages.

[nert]

Ahhhh
perfect, you are trapped in the same quagmire i have been in for years.
I agree with all you say. Even downstream mechanics (dent in exhaust pipe, carbon, etc) can affect inlet flow as well. Synchronizing the carbs to an engine with something less than (4) perfectly matched, performing air pumps can only compromise maximum performance. Thus, the vacuum bench idea for the carburetor. Each component needs to be operating to max capacity and equal to same components for maximum performance.

I read on this forum the difference in reading for cylinders 2&3 and 1&4 is required due to air inlet air box effects. (have not formed an opinion myself)

I also find that adjusting sync at lower RPM is more accurate as the percentage of difference for a smaller throttle opening, becomes less as the throttle area opens.

Some what like how a drop of water affects a tea spoon of water, compared to how a drop of water affects a cup of water. The drop being the sync error or difference, the tea spoon being a small throttle opening, and cup being a large throttle opening. Fine tune the drop to the teaspoon, and the cup will notice even less that difference.

[82Shafty]

It's madness I tell ya. Sheer madness! So many barely measurable factors to consider for those anal at heart, and nothing but a calibrated ear to rely on.

I am too spoiled with having a piggy back and data logger on my Mustang.

Trace the wide band, look at where you want the a/f ratio to be throughout the range and what given tps angle/maf volt and go into that cell and change the ratio.

This 4 carb synching and tuining is the equivalent of taking my calculator away and making me utilize pythagoras's theorem. Hey what's only 64 proof's? Doable yes, favorable; depends on my mood.

Nert.. BTW I thought your idea with the cotters was pretty crafty in regards to synching. I was especially fond of the don't do's because I could very easily see some peoples attempting some pretty interesting angles.

[chef1366]

Throw away your caculator and try this for the fuel air ratio

http://www.carbtune.com/colortune.html

I personally don't own one but I don't think I'm as into detail as you. The difference between the inner and outer cylinders are the inner run hotter because they don't have outer cooling fins. I sync with a mercury manometer and my bike runs great. I might be able to gain a nanosecond with mathematical perfection but I'm very happy with the way it runs. Well, least until I but my 1166 kit in my bike. Faster faster faster

[KEITH KRAUSE]

I've tuned and re-jetted a lot of these old bikes over the years. I don't try to get too scientific about it. Hurts the head.
I believe in checking/adjusting all the basics first, such as compression, valve clearances, ignition timing/spark quality, clean parts throughout...only after I KNOW the bike is ready for carb work do I tune/re-jet the carbs. This may seem like more work to some but it saves time and frustration down the road.
I then bench synch as best I can, start the bike, warm up completely and set the mixture or separate air/fuel screws for best idle using 1,000 rpm's (or factory recommended idle rpm) as a base idle, vacuum synch at approx' 3,000 rpm's, double check the vacuum synch, then double check the mixture or air/fuel screw adjustments and fine tune if necessary. Works for me.

[jgordon]

I just do as Suzuki's manual says to do and my bike is Veerrry happy!!!!

[chef1366]

I've tuned and re-jetted a lot of these old bikes over the years. I don't try to get too scientific about it. Hurts the head.
I believe in checking/adjusting all the basics first, such as compression, valve clearances, ignition timing/spark quality, clean parts throughout...only after I KNOW the bike is ready for carb work do I tune/re-jet the carbs. This may seem like more work to some but it saves time and frustration down the road.
I then bench synch as best I can, start the bike, warm up completely and set the mixture or separate air/fuel screws for best idle using 1,000 rpm's (or factory recommended idle rpm) as a base idle, vacuum synch at approx' 3,000 rpm's, double check the vacuum synch, then double check the mixture or air/fuel screw adjustments and fine tune if necessary. Works for me.

I just do as Suzuki's manual says to do and my bike is Veerrry happy!!!!

My two teachers:-D :-D \\/

[82Shafty]

Well I synch'd the carbs today @ 1100 rpm with a vacuum gauge set I bought off of ebay.

Set the inners to 9 and the outter to a hair over 10"

When you set the carbs to the merc meter such as the Clymer's manual shows, what equivalent is that in " of merc on a Vacuum gauge? The drawing is very nebulous.


I believe, many years ago, this bike was cold blooded and took a good 5 mins to get up to operating temps. Once you ride the old girl for about 5 mins it runs great but anything before those 5 mins and it falls all over itself.

I have the mixture set to 2.5 out is where it idles the best; bike is stone stock.

Like I said that once the bike is FULLY hot, it revs happy and runs awesome, but it's like a bear coming out of hibernation when I first fire it up.

[duaneage]

These bikes are cold natured. If your having problems when it is cold maybe your choke circuits are not working in the carbs. These are real small passages that richen the mixture when the choke is pulled on. They introduce fuel near the butterfly valves.

But the GS does take a while to warm up. On a cold day I can drive 5 miles before it behaves itself. Worth the wait!

[rustybronco]

12qwew
When you set the carbs to the merc meter such as the Clymer's manual shows, what equivalent is that in " of merc on a Vacuum gauge? The drawing is very nebulous.

10 in vacuum =25.4cm of Hg is that what you need to know?