Diaphram spring length

[81xsproject]

In the tech tips, I was reading Blaines Carb FAQ and saw this:

Q: What about the springs that push the slides down? Do they wear out? A: The short answer is that stiffer springs seem to cause the mixture to be leaner and softer springs seem to be richer. I believe they do soften up over time and I think it's important that all four carbs have matching springs. Here is my experiment:
I have several XS650 carbs as well as XS11 carbs. Many of the parts are interchangable. I noticed that the XS650 slide springs are about 10mm shorter than the XS11 springs (both from '80 models). I have been fighting an "off idle" stumble on the XS11 since I've owned the bike. I got to thinking that shorter springs would let the slides open quicker and probably improve the performance (WRONG!) -- it's a good thing I had the shorter 650 springs to try instead of cutting my XS11 springs shorter. I discovered, to my surprise, that the engine didn't run well at all with the shorter springs - it wouldn't take any throttle, just sort of gagged on it unless opened very gradually - it seemed to exaggerate the problems I had been having before. So, I decided to try lengthening the springs instead and started wondering how much tension they might have lost being compressed for 20 years. I carefully stretched the stock springs out about 8mm longer and reassembled the carbs. It accepts the throttle much better than it ever has. NOTE: my bike has low restriction air filters and exhaust and perhaps this is unique to my setup and not something required on a stock system. My impression is that the springs had softened over time and were allowing the slides to lift too quickly, causing an overly rich mixture at low rpms - but it may just mean my exhaust setup requires slightly stiffer springs than stock.

What I am wondering, is if anyone knew what the correct measurement of the spring was supposed to be when new. I was thinking of buying one brand new (still available at around $14) and posting the info. Money is tight right now though and SWMBO has forbid the purchasing of moto-parts . If no one knows, I will do it in the not too distant future.

[79XS11F]

All springs get "tired" over time if they are under a load which would be the case for the springs in our carbs. It would likely not be a bad idea to change them out after 28 or more years. It would be interesting to see if the free length of new springs would be different then old ones and if the engine ran better after a change out of those springs.
Rob

[81xsproject]

That is kinda what I was thinking. It would only take one new one to measure, and then we would have a resting length to stretch our old saggy ones out to. I had some old ones on the bench today and was able to notice a difference in lengths between them. That alone is yielding different mixtures among the different carbs in the mid and upper rpm.

[olebiker]

Something about this is not making sense to me. The slides will still open as fast as you can roll your wrist, all the springs do is drop the slides back down. Now as you ease of the throttle the springs may cause the carbs to go out of snyc. if the springs are old and tired. Aw well I am felling old and tired as well, maybe a younger brain can pick this one apart.

[pggg]

You snip the springs shorter for less resistance therefore quicker slide response, some claim it affects other throttle positions but I never noticed that, you do it in conjunction with drilling the vac slide holes larger too, combine both to feel the real effect, that guy was stating an opposite effect - stronger springs and better response, air filters, pods, exhaust, air jets, fuel jets, fuel height, carb design all involved, no wonder 99% of owners will freak out and leave as stock standard..

[petejw]

I cut the spring down,
firstly I cut off 3 coils, then went to a total of 8,
I did notice a difference both good and bad,
it had a quicker response when the revs were
fairly high to start with over 5000, but i noticed
a big bog down from WOT, also depended on wat gear
i was in, better difference with the lower gears,
overall i didnt like the change, the standard length
had better overall characteristics.

would be very interested to c how u go with slightly
stiffer springs.

quote: olebiker
Something about this is not making sense to me. The slides will still open as fast as you can roll your wrist, all the springs do is drop the slides back down. Now as you ease of the throttle the springs may cause the carbs to go out of snyc. if the springs are old and tired. Aw well I am felling old and tired as well, maybe a younger brain can pick this one apart.

The spring tension will make a difference, by wether how fast/slow they raise, resulting in how rich/lean the mixture becomes.

If you want to check if the slides are raising equally, connect
ur vac guages as u would to sync the carbs, then open close
throttle, if good all gauges will move exactly the same for each carb, unfortunately mine doesnt, :-( but my diaghrams are pretty buggered.

[AlanB]

Hi Guys,

To go right back to the original post which was quoted, the problem he was trying to deal with was an "off-idle stumble".

That should be dealt with by adjustment of the pilot jet, and has nothing to do with needles or slides, so his conclusions may be flawed!

AlanB

[pggg]

If you overdo it its no good, the springs only snip back a 1/2 inch, about 2 or 3 coils, the slide holes, only enlarge by 0.5mm

[olebiker]

Originally posted by petejw
I cut the spring down,
firstly I cut off 3 coils, then went to a total of 8,
I did notice a difference both good and bad,
it had a quicker response when the revs were
fairly high to start with over 5000, but i noticed
a big bog down from WOT, also depended on wat gear
i was in, better difference with the lower gears,
overall i didnt like the change, the standard length
had better overall characteristics.

would be very interested to c how u go with slightly
stiffer springs.

quote: olebiker
Something about this is not making sense to me. The slides will still open as fast as you can roll your wrist, all the springs do is drop the slides back down. Now as you ease of the throttle the springs may cause the carbs to go out of snyc. if the springs are old and tired. Aw well I am felling old and tired as well, maybe a younger brain can pick this one apart.

The spring tension will make a difference, by wether how fast/slow they raise, resulting in how rich/lean the mixture becomes.

If you want to check if the slides are raising equally, connect
ur vac guages as u would to sync the carbs, then open close
throttle, if good all gauges will move exactly the same for each carb, unfortunately mine doesnt, :-( but my diaghrams are pretty buggered.

Sorry but the slide speed upon opening the throttle is dependent on how fast your turn the grip not on the springs. The ONLY thing the sprigs do is push the slides down.

[olebiker]

Oops disregard previous post as I was dead wrong. Need more coffee. Sorry.

[79XS11F]

Here's yet another thing to take into account... Does syncing the carbs not correct for any difference in spring tension in any case ?? Maybe that's part of the reason why syncing is required as often as it is on these carbs. I think it's every 5000 KM.
Rob

[81xsproject]

I would think that since you sync the carbs at fairly low rpm, it wouldn't compensate for the mid range being affected by the slides. However, I am NO expert.

[petejw]

quote,
"Here's yet another thing to take into account... Does syncing the carbs not correct for any difference in spring tension in any case ?? Maybe that's part of the reason why syncing is required as often as it is on these carbs. I think it's every 5000 KM."
Rob

The way i look at it When ur syncing the carbs, the revs are consistant, as a result the slides are sitting in the one particular position,

the spring tension comes into effect when the slides are moving up and down with the throttle response, different tensions different rate of rise and fall.

i hope the above makes sense. :-)

but the way i checked my slides for consistancy was after the carbs are sync, rev the engine up and down, and watch the vac guages move up and down and see if theyre all consistant.

[81xsproject]

Maybe I have the concept of the slides all wrong. I always thought that at different rpm, the difference in air speed through the carbs would create more or less force to push against the resistance of the spring resulting in different slide heights at different rpm and more or less fuel coming through the emulsion tube. From what you all seem to be saying, the slides are simply on or off (up or down). So then, if the slides don't maintain intermediate positions at mid throttle/rpm ranges, then why is the taper of the needle so important?

[petejw]

I didnt mean it 2 come out that way,
if that is wat were u thinking.
You are correct in that the slides (all 4) are all over the place not just top or bottom, thru out different rpm and throttle response, wat i meant was when syncing at a constant rpm, the slides would be in the one position so you wouldnt b able to tell anything much about the spring.