question about carb slide springs

[Ivan]

What are the symptoms of weak springs on the carb slides? I was thinking about my bike still having a bit of an off idle hesitation, and wandering if the springs might be weak and not keeping the air speed velocity up to par with an unladen african swallow.

[bikerphil]

diaphram springs

I've heard that using the shorter springs intended for the 650 made the 1100 run worse. What I did was I carefully stretched the 1100 springs about 10mm longer and made sure all 4 were the same (they weren't before). Bike runs beautiful now, but hard to tell if that had anything to do with it, because I had other carb issues.

[XSPastor]

Someone a while back was experimenting with this and posting their results. I can't rememer who it was. Maybe try a search?

[prometheus578]

T'was T.C.... with the spring experiments.

[79XS11F]

Those springs are in constant motion when the engine is running so they are likely to get tired. I'm doubt if just stretching them to be the same length would return them to what the factory wanted but for sure it would help the engine run better. Regardless of their length the issue is how much downward Resistance they apply to the slide while it's installed in the carb body. They should all be the same. If $$ is not an issue I would replace them as a set. it would be on my list of things to do sooner or later.
Rob

[Ivan]

well, I did not find the postings by TC about them but I did see this.

My thinking is that softer springs will let the slides open too quickly, which will reduce the venturi effect, and result in less airspeed through the carbs. Since the emulsion tubes are perpendicular to the flow of air through the carbs, the only thing that draws fuel through them is Bernoulli's principle, the faster moving air causes a lower pressure than in the bowl's static fluid condition.

I think the theory that lighter springs will cause a richer mixture may be true when there is sufficient airflow through the carbs, as one poster reported about better response at 5k rpm. I suspect the slides are fully up at that time, and everything is relying on the main jets by then.

Now, at airflow values less than full open on the slides, a stiffer spring will keep the slide further down. This will make the air flow faster through the carb, and increase the amount of fuel drawn at any given rpm, when the slide isn't fully open.

CV carbs are neat in that they don't require an acceleration pump. Since the velocity of the air going past the emulsion tubes is constant, up to full slide opening, there is no need to dump extra fuel when the throttle plate opens up. On a regular carb like a holley or a Quadra jet, when the throttle is opened, there is a momentary drop in fuel flow during engine acceleration until the airflow catches up, so they need an extra squirt of fuel while the air velocity increases to match engine speed.

I have messed with my bike for quite a while, changing pilot jets and timing, trying to cure the off idle stumble. Just this last couple of days, I have been thinking about it intensely, and how it acts like an engine with acceleration pump gone south. After staring at the bike long enough I was declared a landmark suitable for navigation purposes, it occurred to my why these carbs don't need the acceleration pump, and the resultant question about the slide springs, since they are the only device that controls the velocity of the air through the carbs.

So, now i need to put theory into practice, and stretch out my springs. Or maybe I can find some stiffer springs. Time will tell.

[TopCatGr58]

Hey Ivan,

I wasn't doing experiments on the springs persae, but wanted to see how the diaphragms reacted under the presence of an OEM air filter vs. Indy filters, which are essentially like open carbs, no filters. There was a discussion about the need for the airbox to create a pressure difference which allegedly helped the vacuum slides to rise. I disagreed with this concept. The vacuum slides rise dependent on the air flow thru the arced port, and the pressure difference between the engine side and the intake side of the carbs, especially when you hit the throttle and open those butterflies.

So...I took my trusty video camera, and videotaped the carbs slides first showing the bike in neutral, and slowly raising the throttle up to 5-6k rpm, and you could barely see the slides move, yet the rpm reached a pretty good level. THEN from a low rpm I cracked the throttle open, and the slides popped up, and bounced with the pulsing of the engine but slowly lowered as the rpms raised, and the pressure differentials balanced back out, until again the rpms were sitting at 6krpm but the slides were essentially almost closed!

THEN I mounted the camera on the bike to view a couple of carbs while I rode it around a few blocks. The slides rise and fall as a response to engine load....without the presence of any air filter whatsoever! Regrettably, I had a sticking float, and so there was some fuel spraying going on, and I couldn't do a slower throttle roll on, but had to crank it fairly open to get it going down the road, but the important thing was that the slides did rise easily without the presence of the AIRBOX to create a restriction/pressure difference! The videos are on YOUTUBE, under my similar username Topcatgr, you can probably find them with a search for XS11 as well!

The guy at the end of the Blaine Hoopes carb FAQ tip also stretched his springs out, and it helped his off idle stumble, so I think the theory of overly compressed springs allowing the slides/needles to rise too soon is very plausible!
T.C.

[Bigleo]

weak springs.

I had a set of weak slide springs in my 1980. The symptoms were uneven running and hesitation below 5000 rpm. This was due to the weak spring letting the slide rise too high for the existing vacuum. This raises the main jet needle higher than it should be, resulting in a rich mixture, and crappy running. Above 5000 rpm and under load, you are mainly running on the main jet and the problem pretty much disappears. So I had some stronger springs lying around and in stalled them. The problem was mostly cured but not completely. The main issue is: I don't think any of the 30 year old springs have their original tension anymore. So I ditched the carbs and went to a round slide type carb. Way better.
Here is an interesting thought for some enterprising soul who doesn't want to discard their XS carbs. Block the throttle butterfly wide open, or best remove them entirely and plug the throttle shaft holes. Then drill a small hole in the diaphragm steel cover and thread them for throttle cables. Then drill a hole in the slide for the cable nipple. In other words, convert the XS CV style carbs to a round slide manually operated style. I would bet this mod would cure many of the carb problem threads on this site.
Leo

[Ivan]

I don't think the slides on these carbs sit low enough to get a slow idle. Interesting thought though. Would they work better with a turbo charger?

[TopCatGr58]

Hey Ivan,

I dug up the YouTube videos just for fun,
Here's the carbs under NO LOAD only about 1 minute long,

and here's the carbs under LOAD, about 4 minutes long,

Don't forget the SOUND ON.

The slides don't totally close off the throat, but we have a pilot circuit that provides the SLOW IDLE, so I don't quite understand your statement, but it might have been tongue-n-cheek?
T.C.

[Ivan]

no not tounge in cheek.

With the amount of air that would be let through at idle with our carb slides and the gap they leave, the idle mix would have to be terribly lean to idle at a sane speed, and that is usually not good on an engine. The slides would have to almost close the airflow entirely off to get an idle, with only just a crack to let in a very small amount of air.

I do see where bigelo was trying to go with his suggestion though.

[Ivan]

well, after reviewing the video footage, I would say that mine are definitely too loose, as they bounce around even at idle.

[Ivan]

Well, yesterday I pulled the carbs and stretched the springs. I found that they stretched about 3/4 inch VERY easily, from 5" to 5 3/4" without hardly any effort at all. The next 1/4" was much more difficult, stretching the spring to about a twice original length resulted in the 3/4", but it took about a two foot stretch to get the next 1/4 inch. This leads me to believe they had relaxed quite a bit.

The results are hard to determine, as it was very cold yesterday, and I wasn't able to really get on it, due to the fact that 25º and 80 mph isn't comfortable anyway you dress. It did seem to have a better off the line take off though, but this may be due to the cold and very dense air. Usually I have to rev the engine to about 2500 and modulate the clutch to take off, but this time it pulled through even with my fumbled starts due to cold fingers on the throttle and clutch. I have great hopes.

[79XS11F]

GREAT video of the carb slides in action TC!!!

It would appear that the 2 slides in plain view are in deed rising and falling together but I must wonder just how precisely matched they need to be for that "like new" operation. Carbs, even these older types, are made to very tight tolerances. We already know that making even small changes to jet sizes, like the main jet, has a big impact on carb and engine performance. So ... when we think about the slides needing to open and close the same amount, and do so at the same rate for smooth operation, we are likely looking at 1000s of an inch tolerance between them in rise rate. For smooth mid-range operation I would think that rise rate is more important then total lift so long as the main jet is fully open when it should be. I don't think stretching the springs to the same length will likely result in all the slides raising at the same rate. Too much resistance on any one or more of the slides after stretching the springs would have the affected carbs running lean unless you twist it hard and cause them to just fully "pop" open.
I would think that if the needle jet is slow coming out of the main jet during smooth acceleration it would cause that carb to function like it had a a smaller main jet then is installed and in effect cause the hesitation that is noticed?

Rob

[Ivan]

GREAT video of the carb slides in action TC!!!

It would appear that the 2 slides in plain view are in deed rising and falling together but I must wonder just how precisely matched they need to be for that "like new" operation. Carbs, even these older types, are made to very tight tolerances. We already know that making even small changes to jet sizes, like the main jet, has a big impact on carb and engine performance. So ... when we think about the slides needing to open and close the same amount, and do so at the same rate for smooth operation, we are likely looking at 1000s of an inch tolerance between them in rise rate. For smooth mid-range operation I would think that rise rate is more important then total lift so long as the main jet is fully open when it should be. I don't think stretching the springs to the same length will likely result in all the slides raising at the same rate. Too much resistance on any one or more of the slides after stretching the springs would have the affected carbs running lean unless you twist it hard and cause them to just fully "pop" open.
I would think that if the needle jet is slow coming out of the main jet during smooth acceleration it would cause that carb to function like it had a a smaller main jet then is installed and in effect cause the hesitation that is noticed?

Rob

These carbs are Constant Velocity carbs. The velocity of the air going past the fuel opening is controlled by the position of the carb slides. A stiffer spring results in a lower slide position, less air volume and more velocity. More velocity means more fuel, and combined with less volume means a richer mix. A weak spring lets the slide rise too quickly, decreasing the air velocity and increasing the volume. Less airflow past the fuel opening means less fuel, combined with more air volume means a lean mix.

Basically these carbs are a venturi that speeds up the air and causes a low pressure point at the fuel opening. making the middle of the venturi smaller with the same amount of pressure on either end (basically the engine's demand for air at any given point) will cause more suction.

I know it sounds counter intuitive since the needle is meant to give richer fuel at larger openings.

As far as the carbs being made to tight tolerances, I wouldn't set them on to high a pedestal there. I wish it weren't so, but they aren't exactly a poster child for casting and machining.