Quote from: herennow on June 04, 2018, 06:38:58 AM
In classic slide carbs, which I am much more comfortable with, this means that the main is too small  (running lean up top), does this also hold for CV tye carbs?

Not exactly, but maybe coincidentally.

The following is all just theory, IDK precisely about what can be done to tune your carbs.

In a CV carb, it's VACUUM AND VACUUM ALONE that opens the main jet by virtue of pulling the slide up. The needle is attached to the slide. So to get more fuel, the slide has to move up and pull the needle out of the jet. The position of the slide at a given vacuum level is determined by the diaphragm and spring tension. The amount of opening of the main jet at a given slide position is determined by the needle profile and needle position (shimming). Shimming the needle pulls it "up" more giving more fuel at lower slide position, making it run richer at lower vacuum levels.

The issue with the K&N drop in filter is that it will reduce vacuum and therefore cause the slide position to be a little lower, so you might have to shim the needle to compensate. It may also reduce velocity of air enough to require a bigger jet so you will get more fuel per unit volume of air since the air is moving through more slowly. This air velocity issue will also affect the mid jet to some degree but not the slide/needle/vacuum issue.

There's also the effect of how quickly the slide moves up, which is a result both of the spring tension and also there are holes in the slide that allow air to pass through sort of like a poor man's damping and change the rate at which the slide goes up. In my old SU carb tuning days there was oil in the damper on the carb and we could change the oil to change this rate of enrichment under transient vacuum conditions. Anyway, if the slide is going up too fast then it will be too rich too quick. I think this is unlikely. A wideband O2 sensor would tell the tale and is about the only way to reliably know for sure but my guess is the slide coming up too slow is a more common complaint especially if you've put in a less restrictive air filter so there's less total vacuum.

If slightly less throttle results in more power then that sounds to me like it's lean on the main jet; less air = richer = more power, more air = more lean = less power. I'd say put the paper air filter back in and see if this situation improves. If so, be done with it. If not, then I'd look really hard at the condition of the diaphragms and springs on the slides, and if you are certain they are good to go, then look real hard for vacuum leaks. If you're absolutely sure there are no vacuum leaks then shim the needle and see if it gets better. Last resort: bigger main jet. My point here is that I think changing the vacuum condition likely led to this condition, and changing it back could remedy it.

BTW I have the opposite problem ... 3K rpm very small throttle opening I get a consistent stumble which I am quite confident is lean condition. I think the solution to my problem is going to be shimming the needles, but it only shows up when riding around parking lots looking for a space, so it's not really bugging me. Actually I have likely a too-big main jet (127.5) and could probably put 1-2 washers on the needle and go back to a 125 and it would work better. Next time I have the tank off I might actually do all that. By then maybe I'll need another round of carb cleaning. It has been about 3K miles.

Very good points.

Any modifications often require extra tuning for best performance. For example, I use 127.5 mains on my liter bike. Everyone says, "Oh, they're too small, blah blah blah." Throttle response is crisp and she flat hauls a$$; the front wheel comes up when shifting into 4th at speed. It took a bit of tuning to get her there.

I found this to be a great resource when tuning CV carbs.

That looks like a really good resource. Simple, easy to understand, makes a ton of sense.

The big key facts that are easy to miss are: 1) high flow air filter doesn't mean more air, it means slower air and less vacuum; 2) opening the throttle does not directly cause the slide to come up, that is caused by vacuum ... then it's easy to understand why a higher flow air filter may result in lean condition because of less vacuum and lower air velocity.

Thanks for the inputs folks. Interesting - yesterday I bought this for another bike:

https://www.plxdevices.com/ProductDetails.asp?ProductCode=897346002870

Think it's going to come in handy here too.

I'd like to check a couple of logic items with you though:

The first is the effect of a free-er filter (I'm not trying to defend the K&N, it came on the bike and I might well buy a new one to test, and I also don't want to argue at all- just trying to understand!).

I understand that the K&N will let more air through (less resistance). This lower resistance will lead to a lower pressure differential between atmosphere and intake manifold.
However, this will also mean that more air will pass per given amount of time (lower resistance = higher current). This greater amount of air will lead to a greater velocity, not lower as stated before?.

Secondly, Assuming the carb respects Bernoulli (yeah right...) would that not mean that at higher throttle openings (very little constriction for a venturi effect) the greater airspeed from a free flow filter would obviously mean more air, but also more gas sucked by the Bernoulli vacuum effect due to the higher velocity? Of course its hard it say which would win out at any given point. I know CV's are designed to "not rise too fast" as a slide type carb would but it can obviously happen. Bottom line is I have a hard time deciding whether a slide rising too fast would;
a. Momentarily lean out the mixture too much by letting in too much air before the gas delivery can catch up, or
b. Momentarily richen the mixture by letting in too much gas from the needle as it opens quickly.

My gut feel tells me "a", as I think the vacuum will reduce momentarily (less restriction), and the velocity will decrease momentarily at the main jet (greater area) until the engine speeds up and velocity increases. This would lead to less gas for a short time.

This is hard to explain but I hope you can understand my questions.

Wow, carbs are complicated

EDIT - just realised that CV carbs mean "constant velocity" of airflow,  will have to rethink the above in respect of this realisation.

cheers.

This sound exactly like what I was discovering.

http://gstwins.com/gsboard/index.php?topic=72399.0

I think Buddha's suggestion for better throttle control is the right answer

Quote from: herennow on June 04, 2018, 01:33:02 PM
I understand that the K&N will let more air through (less resistance). This lower resistance will lead to a lower pressure differential between atmosphere and intake manifold.

Well sort of. The K&N could potentially let more air through, if such a need were to arise. On a GS500, it will not need more air. But the vacuum will be reduced as you point out.

Quote
However, this will also mean that more air will pass per given amount of time (lower resistance = higher current). This greater amount of air will lead to a greater velocity, not lower as stated before?.

No, this is not correct. There will be no more air. The engine still displaces 487cc, so it will still draw in that amount of air (and fuel) with each cycle while the intake valves are open. Since the total rate of flow (volume over time) remains the same but the pressure is lower, the velocity has to necessarily be lower. Since the velocity is lower, the amount of fuel picked up by the air as it passes over the jet would be lower, which is half of the reason you might need a larger jet. The other half of the reason is since there is less vacuum the slide will be lifted less and result in the needle being at a lower position than it was designed to be so the jet is more closed.

Quote
Secondly, Assuming the carb respects Bernoulli (yeah right...) would that not mean that at higher throttle openings (very little constriction for a venturi effect) the greater airspeed from a free flow filter would obviously mean more air, but also more gas sucked by the Bernoulli vacuum effect due to the higher velocity?

It's just like that only opposite because it is lower velocity.

QuoteBottom line is I have a hard time deciding whether a slide rising too fast would;
a. Momentarily lean out the mixture too much by letting in too much air before the gas delivery can catch up, or
b. Momentarily richen the mixture by letting in too much gas from the needle as it opens quickly.

It's b. The throttle position meters the amount of air, the needle (and slide) position meters the amount of fuel, generally speaking. I mean, the slide does restrict the air flow a little bit, but not nearly as much as the throttle plate.

Quote
My gut feel tells me "a",

No, it's b. And that's because as you mention in a moment, in a CV carburetor, the whole objective is that the air velocity is constant. The only thing that changes is the position of the throttle plate and slide which together limit the cross-sectional area through which the air can flow so a high throttle opening and low slide results in more air restriction (low rpm) and a high throttle and high slide results in less air restriction (high rpm) but in both cases the velocity of air remains approximately the same. If it's not working this way, something is wrong, and it will be difficult to tune. When you make a change to the vacuum behavior you wind up imbalancing this system and you have to compensate somehow. That's why vacuum leaks are a tuning killer and a lower-restriction air filter is much like a vacuum leak in terms of the effect on the engine.

The thing to remember here is that it's the engine's demand for air that causes the volume and rate of flow, not the air filter's restriction. In other words, your bike will make the same power and consume the same amount of air with no air filter or with a Briggs & Stratton air filter. Only difference will be the velocity and vacuum so you will have to tune the carbs accordingly. To actually consume (or demand or use) more air per cycle you would have to change the cams and/or valves, add displacement or add forced induction. To add to the total rate of airflow you could do those things plus increase operating rpm. You know an engine operating at 12,000 rpm will use 33% more air than one at 9,000 rpm and will also make 33% more power all else being equal, and it would require an air filter (as well as valvetrain and head and intake tract and exhaust etc.) capable of flowing that 33% more air.

Quote
EDIT - just realised that CV carbs mean "constant velocity" of airflow,  will have to rethink the above in respect of this realisation.

Yep!

Quote from: herennow on June 04, 2018, 01:33:02 PM
b. Momentarily richen the mixture by letting in too much gas from the needle as it opens quickly.

It does this, but in simpler terms a big inrush of air will get fuel added to it, the engine can only eat up so much cos its flow is limited by cams and exhaust etc. The air column hits the back of the closed intake valve and rebounds which results in more fuel getting added as it flies past the venturi this time going toward the air filter. Then ofcourse the intake open and the air flies to the engine, making it add fuel again etc etc

This is the single biggest reason a K&N lunch box without pipe can likely never be jetted right. Of course training your wrist and closing off 1 hole in the slide could help, but I have never experimented with that and I am not about to mangle a few $100 worth of unobtanium parts to find out nor do I have an O2 sensor fitted stock exhaust pipe. And besides O2 sensor will just read rich, I guess that fall on its face feeling combined with rich reading can indicate that while a steady state at that same throttle position may be much leaner indicating pretty much the slide rising problem.

Cool.
Buddha.

Interesting Stuff, I hope that I will be able to shove the O2 sensor up the GS500's bum, I mean the exhaust pipe when it arrives. That hole is pretty small and I am not really keen to drill and weld a bung on the exhaust itself (I bought this for an older carbed bike with almost a 1" exhaust that I can shove the sensor up).  I know this is not the most accurate way to install the sensor but I'm looking more for relative AF ratios than exact ones.

Buddha, did you use a logging recorder or look at the dial in real-time? How fast does the reading change to reflect instantaneous conditions?


Mr72, I'd like to unpack your comment a bit:

QuoteSince the total rate of flow (volume over time) remains the same but the pressure is lower, the velocity has to necessarily be lower.

You are of course correct that the bike always sucks the same 487 cc volume. The throttle butterfly dictates how many "gasps of air per minute/RPM" the engine can breathe. That got me thinking about how the different air filters work, because, if one can never flow more air at a steady state, what exactly do they do? The only answer I could come up with is that more restrictive filters create higher pressure drop resulting in less dense air and hence less oxygen and thus less power? Free-er filters would conversely have less pressure drop, so more O2 molecules per given volume? Or does it just lead to lower rpm, like closing the throttle a teeny bit?

Obviously, this density change would be very small - assuming that there is none for argument's sake, my understanding is that the volume and the area that the given volume flows through, would define the velocity of the gas.  (m/s = (m3/s) /m2 ). Would that not indicate that even with the pressure being lower, the velocity is still the same (going back to the CV name)?. if this is the case velocity vacuum (Bernoulli) effect would be minimal and most effect would come from the slightly lower pressure differential not raising the needle as much  = less gas?

Finally, I might one day test out closing one hole temporarily with a plug of silicone to see the effect. For now its "right-hand modulation" for me.

Thanks for this interesting discussion folks.

Its arrived!! - my air-fuel ratio sensor (works with a smartphone app via Bluetooth). See picture below

I look forward to finding out what is causing the 6 k RPM flat spot.

While waiting I had a look at how the slides work when the bike is running with an endoscope end stuffed in the airbox. I was quite surprised to see how much the slides are moving all the time, even at steady revs. Probably due to the time taken for a cycle on each cylinder, when the intake valve is closed the pressure will drop. This happens very fast and my cheap endoscope could not capture the movement at revs much above idle.

This got me thinking that (seeing as the movement of the slide is so fast), and the fact that blocking one of the slide holes will not change the end point of the slide but only the speed that it takes to open to the point when the pressures are equal -  maybe it is better changing spring tension rather than the holes of the slide? Changing spring tension will make the valve open earlier, if weaker (or later if spring is stronger) and might be a more beneficial modification than blocking holes.  I might buy some spare springs and play with stretching them or shortening them after I've understood what my A/F sensor is telling me.

Devilishly complex, these simple carburettor things!!!

Quote from: herennow on June 25, 2018, 06:22:26 AM
Its arrived!! - my air-fuel ratio sensor (works with a smartphone app via Bluetooth). See picture below

That's a really cool tool. I kinda want one.

Quotemaybe it is better changing spring tension rather than the holes of the slide?...
Devilishly complex, these simple carburettor things!!!

Better not to change any of it. As you say, it's a carefully balanced system and seemingly simple changes have side effects that are difficult to predict. The engineers of these bikes are near magicians for making them work as well as they do. I find it hilarious the arrogance of most of us shadetree mechanics who think we know so much more than those engineers!

The diaphragm also offers both some spring action and damping, as well as the flexibility of the diaphragm affects how quickly the slide will open or how much vacuum it takes to pull it up. That's a wear item and a small pinhole or tear will make it useless. So that's one thing that can be replaced to perhaps restore original function. But the springs don't wear and while the sides may wear I don't think it's significant enough to warrant changing anything unless they stick.

It's just funny, we're so simple-minded about this stuff, we think "more airflow through air filter means more air means MORE POWER better get bigger jets so I can make MORE POWER" and it's just wrong. Then we make this one wrongheaded change (not you, but in general) and wind up chasing all kinds of other "problems" to try and compensate for something that worked just fine if we just undid our first change.

Without screwing it into the headers you will never be able to get a real world 6K or 4K or - more precisely 1/2 throttle or 3/4 throttle reading without a dyno.
If its bolted in the exhaust, you can just ride it and you can read it if its sitting on the tank or dash.
Also it needs heat, it warms up with the exhaust gasses flowing through it and then starts reading accurately. Till then it is likely off to the lean side. That's a 20min time frame at a minimum @ rpm and load. My dyno guy way back when I put it on the dyno had to let it idle and rev and idle and rev and still wasn't getting good temp in it cos a bike that's jetted rich warms up much slower cos its exhaust is lower temp and it also has liquid gasoline cooling down the sensor if you were that rich.

Cool.
Buddha.

Hey Buddha,

The manual states to install the unit at more than 24 inches from the exhaust port. I plan to shove it up the exhaust hole to around that distance (attached to a spring steel guide rod which will be clamped to the exhaust) would that prevent me from testing at 6k RPM?

Interestingly, the newer sensors (LSU 4.9 +) have integrated heaters built in to avoid the heating up issue.

" reaches operating temperature of 700 to 800 degree Centigrade (1,400 degree F) within 20 seconds or less using its internal heater"

Cheers

Brett