XJ550 Throttle Slide Spring Length - that's a natty title...

I've got a little time to work through some of my long term projects and recently I put my old XJ550 back on the road properly with tax and mot.

Ages ago I cleaned out the carbs, pretty thoroughly I thought at the time, but now she is running, she is hard to start and won't tick over. When running she is reluctant to pull to high revs or generally go above 50-60mph. I suspect that one of the throttle slides is sticking.

The history from some years ago was that I blew a hole in a piston because a throttle slide stuck OPEN on the motorway. I put another top end on over a weekend and kept using it until it went into the garage a few years back (ahem), but when I rebuilt the carbs, I think I tugged on each of the springs to stretch them slightly so that they would be a little stronger and would more definitely return the slides - I didn't want another holed piston.

I'm surprised though that the spring would have such a great effect on the carburation as it's having. I've just measured a spring on the bike as 12cm and one from an old set of carbs as 10cm - that's nearly 5" versus 4" and I'm going to check that they all slide smoothly.

I'm just wondering if anyone knows a 'right' figure for the spring free length - and thought I'd see if anyone had any thoughts on how great the effect could be.

 

I'm not surprised at all that stretching those springs has a great effect.

Those springs aren't in there simply to push the slides back down. There exists a balancing act in those carbs between engine load, RPM, airflow, pressure drop, and spring compression force.

Stretching them will add more fuel at all operating conditions except idle, and you just increased the spring force by 20 percent. 8O

 

Re: XJ550 Throttle Slide Spring Length - that's a natty titl

As I tried the other springs, they weren't all that long. At least one has small holes in the diaphragm, so I've stopped while I search for the liquid tape that vanished just when I wanted it. Grrr...

As you say, carbs operate on a delicate balancing act and at the time, my thinking was that they might have weakened over time (leading to the holed piston) and that the diaphragm balances pressure above as well as below, so the spring is not the only downward force. The spring getting compressed from 120 to 40mm (rather than 100 to 40) is still a 25-30% increase in pressure, so I have to say that this is one 'bright idea' of mine that I shan't be recommending to people.

I wonder why they don't list the spring free length though ? They do for valve springs, suspension springs, clutch springs etc.

 

My experience with holes in pistons has been the result of running "HOT" or as they say now days running to lean a mixture and burning a hole in the top of the piston. At least that was the cause back in the day when I raced.

 

Re: XJ550 Throttle Slide Spring Length - that's a natty titl

Which is generally the result of either tuning or of a fault. I had made no changes apart from checking shims occasionally etc for the past fifty thousand miles. When I opened the engine up and looked at the carbs, a slide was stuck open. I made an assumption that an open throttle on the cylinder with a holed piston just might have led to weak running and overheating.

Cruising at 70-80mph had not generally been a problem previously.

It was a shame as I had put a new engine in about five years previous to that which had been low mileage (25K ?) and I had only put another 30-40K on it.

 

Re: XJ550 Throttle Slide Spring Length - that's a natty titl

In theory, springs should not change characteristics over time. Assuming they don't corrode, aren't asked to travel beyond their deformation stress, and aren't subjected to temperature extremes beyond which the metallurgy changes, they should be the same today and 1000 years from today. Of course, that's theory only.

I have no idea how much of a "real" change tweaking those springs an inch longer would do, but it will certainly result in a richer mixture off idle. I can tell you, however, that if a slide was sticking open, it wasn't the springs fault. I would also surmise that if the slide was sticking open, it would most likely result in a lean mixture on that carb, and lean can burn pistons, right?

I've got a spare rack of 550 Mikunis here, and I believe the springs are all OK. I'll take a look and take some free length measurements for you next chance I get. If they are all the same length, I would have confidence that they are unadulterated.

Haha. Good plan.

 

Stretching the springs will make it leaner

 

You know what? I understand both side of it, and the more I think about it, the less comfortable I am with my blanket statement that it would run richer. However, I'm not comfortable with your statement that it would run leaner either.

On one side, the smaller venturi created by a stronger spring provides more vacuum, and hence, more fuel.

On the other side, with a smaller venturi, you're running at a wider part of the needle taper, and hence, less fuel.

Which one wins? I think it might depend on the carb. What makes you so sure that the needle taper is the more dominant factor? :?:

 

Stretching the spring will make it leaner.

The whole reason the slide raises is to lift the needle out of the top of the emulsion tube, allowing fuel to be drawn out. If you make it more difficult for the diaphragm to lift the slide, it will provide less fuel.

A stuck slide will not cause a holed piston, that cylinder would if anything, be getting too much fuel.

When I get home, I'll measure a couple of Mikuni diaphragm springs for you.

 

Changing the LENGTH of the spring, without changing any other characteristics of the spring DOES NOT change it's SPRING CONSTANT. You have only PRE-LOADED the spring, meaning it'll take a GREATER pressure differential to begin opening the slide, but it will move at the samre "rate."

See the following wikipedia articles:
http://en.wikipedia.org/wiki/Spring_(device)#Theory
http://en.wikipedia.org/wiki/Deformation_(engineering)

Note that the number of coils hasn't changed, and neither has the Young's modulus through Plastic Deformation. And don't even THINK about cutting the spring to shorten it, that only stiffens the springs (number of coils is in the demoninator).

 

holding the slide down farther in the thicker part of the needle would be like using a fatter needle, leaner.
if you take it a little farther and stretch the spring more it may never get fully on the main jet, yet the throttle plate and engine want the full main jet mixture, lean
this is good though, i think we finally got a question that's never been asked before

 

Exactly. Which will require more AIRFLOW to produce, meaning a leaner condition.

The bottom line is: Stretching diaphragm springs is NOT a good idea.

 

I terms of stretching the spring no, but I have seen posts of someone trying to "soften" the carb springs by cutting them (which of course makes them STIFFER, but does reduce preload)

If your slides are sticking, the cure is to refinish the bore of the slide so they go CLUNK! Maintain the delicate balancing act of the spring/diaphram/venturi.

 

Re: XJ550 Throttle Slide Spring Length - that's a natty titl

Mr. Manbot, run this a few times then tell me how cutting coils from a spring makes it stiffer
calculator

 

because k goes up when the number of free coils goes down. I know, it's counter-intuitive, but it's true. That calculator assumes a fixed k.

Try this one ;-)
http://www.engineersedge.com/calculator ... _k_pop.htm

A spring is basically a helical rod, and you are asking it to elastically deflect by a certain amount when a force is applied.

Imagine the spring is actually a rod. It has a consistent diameter and length. If you put a force on the end, it will deflect by a certain amount. Shorten the rod, and you'd expect it to deflect less for the same amount of force. <- That is what you are doing when you shorten a spring.

Of course, you might be reducing the preload of the spring when cutting, but you are increasing the spring rate k.

EDIT: I did the calculations for shortening a spring in a set up like the carb slide (preload distance is fixed). The slide will raise higher for a given force (pressure differential), even though it is stiffer)

 

try this, take any spring cut it in half, your theory says it gets stiffer,
now put one half on top of the other half, are the 2 half's together stiffer than the original whole?
i did run the numbers, on both calculators, the cut off spring has less force...........then they threw me out !, the nerve

 

We had a spring expert on here once, what happened to that doughnut ?

 

Re: XJ550 Throttle Slide Spring Length - that's a natty titl

I think Manbot is on the right track theory-wise. If you took a spring and cut it in half, then the two halves together would compress the same amount; BUT if you stretched the two springs to each be the original length, then the strength of those springs would be different (lambda was the name of the coefficient when I had to learn about Hooke's law). You would expect them to be STIFFER springs - except with the level of compression of these springs, the theory is pushed to its limit.

Certainly when I have had shock absorber springs made, there were stiffer springs which had fewer coils, each the same diameter (cross section).

I HAVE polished the bores. At the time, the carbs were the originals on the bike and had done 150-200,000 miles so I thought they might have weakened, but just stretching them to make them stiffer was not exactly 'brilliant.' Whether that has caused a problem I'll know when I've run the engine again - I just want to sort the diaphragms first as well.

I'm using very old parts though, so confirmation that the springs are okay would reassure me somewhat.

 

Re: XJ550 Throttle Slide Spring Length - that's a natty titl

I think Manbot is on the right track theory-wise. If you took a spring and cut it in half, then the two halves together would compress the same amount; BUT if you stretched the two springs to each be the original length, then the strength of those springs would be different (lambda was the name of the coefficient when I had to learn about Hooke's law). You would expect them to be STIFFER springs - except with the level of compression of these springs, the theory is pushed to its limit.

Certainly when I have had shock absorber springs made, there were stiffer springs which had fewer coils, each the same diameter (cross section).

I HAVE polished the bores. At the time, the carbs were the originals on the bike and had done 150-200,000 miles so I thought they might have weakened, but just stretching them to make them stiffer was not exactly 'brilliant.' Whether that has caused a problem I'll know when I've run the engine again - I just want to sort the diaphragms first as well.

I'm using very old parts though, so confirmation that the springs are okay would reassure me somewhat.

 

Wait whoa hold the phone.

You've got an XJ550 with "150-200,000 miles" on it? REEAAALLLYY?????

Damn.

I'll go measure those springs for you.

 

it's about time, Fitz

 

Greg, maybe you could use this guy too
http://jbmindustries.com/Yamaha650.html

 

C'mon dude I'm old. And live 58 miles from work, in Detroit of all places.

SOOO... surprising results: 96mm.

I have a brand new NOS Mikuni BS28 (that I used for this: http://xjbikes.com/Forums/viewtopic/t=31061.html) so I cracked that puppy open and measured the spring. 96mm on the nut, measured with two different tools (digital caliper and metric ruler.)

So then I dug around and found the springs out of my ratty nasty $20 "parts" rack of unknown but obviously unkind history, and measured two of those.

They were also exactly 96mm, on the nut. I would have expected a few thou of "sag" but couldnt' detect ANY. Stood all three of 'em up on the workbench; was gonna take a pic but the camera battery's dead and I'm outta AAs.

So there you have it; some mighty durable springs, those. 96mm.

 

Re: XJ550 Throttle Slide Spring Length - that's a natty titl

You guys are getting caught up mixing relative and absolute measurements.

Take a ten inch spring with twenty effective coils.
Measure the force it takes to compress it to six inches.
Then measure the force it takes to compress it to five inches.
Subtract those two numbers and you'll know how much change in force it takes to compress it from six inches down to five inches.

Now take that same spring, cut off one coil, and take the same force measurements at six and five inches.
You will find that it takes less force to compress it to six inches than it used to.
You will also find that it takes less force to compress it to five inches than it used to.

But... When you subtract the two new force numbers from six and five, you will find that it now takes more change in force to compress it from six down to five inches.

And that's what Manbot means by "stiffer".

 

I would call them "well designed and applied".

 

Re: XJ550 Throttle Slide Spring Length - that's a natty titl

About the controversary about whether you will run rich or lean when you stretch the spring... I offer only the following to my defense.

The carbs on our bikes are all offspring of the original Skinner's Union (SU) "Constant Velocity" or "Constant Depression" carbs originally created in England in the early 1900's. The traditional SU carb includes a damper (one way shock absorber) on the "slide" (they call it a "suction piston"). That damper acts as a sort of "accelerator pump" by slowing down the upward travel of the slide, thereby providing a burst of extra fuel when the throttle position is increased regardless of the needle position.

We don't have dampers on our slides, but increasing the spring force will decrease the venturi area just like the damper would. Is that decrease in venturi area overshadowed by a needle diameter increase? I don't know for sure, but I would love to see the data.

Want to learn more?

http://www.sucarb.co.uk/

From: http://en.wikipedia.org/wiki/SU_carburetor
"To prevent erratic and sudden movements of the piston it is damped by light
oil in a dashpot, which requires periodic replenishment. The dampening is
asymmetrical: it heavily resists upwards movement of the piston. This serves
as the equivalent of an "accelerator pump" on traditional carburettors by
temporarily increasing the speed of air through the venturi, thus increasing
the richness of the mixture."

From: http://www.zparts.com/zptech/articles/m ... 11601d.htm
"PISTON SPRING
§ If the spring is too weak, the piston will be elevated to a level
higher than its optimum position causing the engine to run too lean.
§ If the spring is too strong, the piston will not rise to its optimum
position causing the engine to run too rich."

 

roll this around, when you adjust the idle mixture, it has to be slightly rich so it doesn't go lean and hesitate till the midrange needle jet and needle circuit take over.
now my head hurts

 

Re: XJ550 Throttle Slide Spring Length - that's a natty titl

At least two thread going on here - yes, stretching the spring will increase the modulus of elasticity, making it stiffer. As it is now longer, it will also need to be compressed more to get to operating length. If that is 40mm with the slide part way opened, then it has been compressed 80 mm instead of 60mm (or 56mm as the spring has been accurately measured as 96mm)
**thanks bigfitz52 - much appreciated**
and so with the stiffer spring the loading will be 30-50% higher. Not surprising if the slides aren't opening fully, then, although I had it in my head that there was an equilibrium between the venturi pressure and the pressure above the diaphragm. I reckon I exaggerated the effect of that.

As to whether the slide is richer or weaker with the slide open, I have always taken a similarly simplistic approach and assumed that the principle of the vacuum slide is similar to the slide in an old fashioned slide carb.
That is, that almost regardless of air flow, lifting the slide allows more petrol past the needle to be whipped up with the air flow. Keeping the slide closed means only the pilot system can be used. the carburettor keeps the air/fuel ratio somewhat near the right mix, but when the engine is pulling hard, the air gets an advantage and when you're lugging, max petrol can flow, but air flow is down.
That makes me realise how garbled thinking can be, BUT - as long as there is air flow, opening the slide richens the mixture and I have assumed it is the same for vacuum carbs.

If the bike is being ridden gently, with a very partially open throttle, and one slide is way open though, MAYBE the air flow is not sufficient to draw the fuel, so it ACTUALLY runs weak - and possibly it ends up working much harder than the other cylinders too and therefore gets hotter. Either one cylinder must have been running lean, or else it was running full bore when the others were not.
I do know that I dismantled it and had a holed piston and then quickly put a spare top end over a weekend to commute on the Monday, so having found a sticky carb slide, was happy with that as a conclusion.

As to mileage, it is actually an XJ400 frame, and XJ550 engine and an XJ650 front end, which I bought, crash damaged from a friend just over twenty years ago. He had been a courier and the mileage was at least 60,000 then. I then used it for courier work and recreational use until I gave up dispatch work, but still did relatively high mileage. While dispatching I was doing 500-1,000 miles per week, so the mileage added up quickly.
About fifteen-sixteen years ago I replaced the engine, about ten-eleven years ago I blew the piston and seven-eight years ago it was semi-retired until I could replace some parts. The bike is on the third speedo that I know of, so I'm hazy about the exact mileage, though as I still have old MOTs I guess I could work it out.

 

Yeah, as simple as the operation is, the whole CV thing can be very counter-intuitive. Good for making your head hurt.

What you described above is the exact problem that the dashpot dampers on top of the SU's are intended to prevent. The dampers slow down the upwards travel of the slide. The slide will eventually reach the same equilibrium point that it would reach even if there were no dampers at all, but in the interim, the increased vacuum at the venturi provides a temporary additional fuel burst.

You want a stronger fuel burst? Use a thicker oil. Since we don't have that option, we resort to other means.

Like I said before, I can see both sides of this spring length discussion, and to be honest, I don't know which side is more dominant. I can tell you, however, that there is more to it than "it will run leaner because the needle is down further". That statement is no more clear cut correct than "it will run richer because the venturi is smaller".

I'm saying that I can't prove it one way or the other, and guess what... Neither can you. :wink:

 

Re: XJ550 Throttle Slide Spring Length - that's a natty titl

Heh. At least!

And getting back to your original thread... Replace the tweaked spring if you can, figure out what is binding and preventing the slide from moving smoothly and congratulations for getting an incredible amount of use out of your bike. Or Frankenbike?

 

You don't change the modulus of elasticity when you go through plastic deformation. You only shift the stress/strain curve, as shown in this diagram:
http://en.wikipedia.org/wiki/File:Metal_yield.svg

Plastic deformation shifts the atomic bonds to fill voids that existed. The Youngs modulus (slope of the linear region of the stress strain curve) remains constant, but the elastic limit increases and approaches the fracture toughness.

But you've got the right idea about preload. (going to pick numbers out of the air) If your spring is compressed from 96 mm to 40mm when assembling, and you deform it to be 100 mm, you still have to cram it into the same 40mm space. Since the spring constant is the same, the preload goes up because the spring is now displaced 60mm instead of 56mm. It will open at the same rate, only later.

The spring probably doesn't change it's properties because it's coils probably bind before it can reach a yield stress in compression, and it doesn't go through extreme heat cycling, to is doesn't get tempered, like say a valve or clutch spring..

RiceBurnarr, you've got me when it comes to absolute vs relative measurements. I intuitively thought there would be a "cross-over" point if you cut the spring down, but then I sat down and did the calculations and figured out that there wasn't.

 

Re: XJ550 Throttle Slide Spring Length - that's a natty titl

You're more than welcome.

So the 550 is like "Old Ollie's Axe," after nine handles and three heads, is it still the same axe?

Still it bodes well for the longevity of the XJ550 series. Good to hear you're resurrecting it yet again.

 

As an eventual update, changing the springs made hardly any difference and the bike continued to refuse to pull properly.

I kept an eye out for XJ spares and picked up an engine in bits with a spare set of carbs. They'd been cleaned very well and after fitting them to my 550 a couple of weeks ago it has been running just fine (better after a balance).

I still have no idea what was wrong with the original carbs - the slides seem to move alright and nothing appears blocked - but now they're just spares, diagnosis doesn't seem so important...