Cut Slide Springs with Pods and Straight Pipes

[D0wn5h1ft]

Decide to do an experiment this morning. I had an extra set of slide springs sitting around so I wanted to play with their length to see if I could get an increase in performance with my setup.

Long story short, with the pods the vacuum seen inside the carb bodies is going to be significantly less due to the less restrictive filters. This means that it drastically affects the RPM range where your slides start to open up. I thought that maybe with shorter springs (less force down on the slide) that they would open up sooner with less vacuum. So since I don't really care about these extra set of springs I cut about 6 coils off each and made sure they were the same length.




I took her out for a spin and obviously didn't notice any difference at lower speeds. Got into a straight and opened it up. Let's just say for the first time I was almost thrown off the back of the bike. In second gear at ~4200 RPMs it picked the front tire up off the ground at 3/4 throttle opening after dropping the clutch.

A little background about my bike, though. First, I have a bad flat spot at 3k RPMs because of something I don't understand (reversion, I've been told due to the straight pipes). So it's always going to be cruddy there. Second, I'm running 45 pilots and 120 mains (stock is 42.5 and 110). It's not that performance was bad before the slide spring cut but it was lack luster to say the least. Third, I have a 10 degree initial advance. Fourth, I have exhaust wrap (not sure if that matters but people say it does).

I'm curious to see what further modification would do but at 4k RPMs it pulls really hard now and I kind of don't want to mess with it anymore. I assume with a little more restrictive exhaust I could probably get rid of the flat spot and I'd notice a lot better performance.

[natemoen]

I would suggest if anyone else wants to try this, use a small pair of zip ties and use those to shorten the springs before doing any cutting.

[D0wn5h1ft]

I would suggest if anyone else wants to try this, use a small pair of zip ties and use those to shorten the springs before doing any cutting.

It would achieve a similar effect but not exactly the same. Cutting the spring changes the spring dynamics and is quite different from just adding spring preload (adding zip ties). It's a good starting place especially if you don't have extra springs to hack up like I did.

Good point, Nate.

[natemoen]

It would achieve a similar effect but not exactly the same. Cutting the spring changes the spring dynamics and is quite different from just adding spring preload (adding zip ties). It's a good starting place especially if you don't have extra springs to hack up like I did.

Good point, Nate.

You are not really adding preload if you are 100% compressing a portion of the spring and leaving the rest alone. It is essentially the same as cutting it except you have a thick base.

[D0wn5h1ft]

You are not really adding preload if you are 100% compressing a portion of the spring and leaving the rest alone. It is essentially the same as cutting it except you have a thick base.

OH! I thought you meant compressing the whole spring with a zip tie. I was thinking...uuuhhh

That makes MUCH more sense. I kinda wish I would've done that now so I could have played with it more. :/

[Tmycal]

OH! I thought you meant compressing the whole spring with a zip tie. I was thinking...uuuhhh

That makes MUCH more sense. I kinda wish I would've done that now so I could have played with it more. :/

Interesting. . .. I did a dynojet kit on my bike, which included a drill bit to
bore out the slide lift holes a tad more. Well. . . being chicken &^%$ I didn't do that portion of the install. Shortening up the springs, might have the same effect?? Ideas?

I might play with it over the weekend. . .clipping them shorter that is.

[GLoweVA]

Long story short, with the pods the vacuum seen inside the carb bodies is going to be significantly less due to the less restrictive filters. This means that it drastically affects the RPM range where your slides start to open up..

I think this is the key statement that needs to be talked about.
I agree that with the stock airbox and filter there is a certain amount of 'restriction' that helps the vacuum from the cylinders that is felt on the diaphrams when the butterflies open.
With pods (especially the ones that have very little filter mesh 'restriction' in them) then the inlet to the carbs see very little vacuum except at almost wide open thottle (when it's really sucking hard)
I think that trial and error here is going to be your best bet on finding what spring length will give you the best response for your particular setup.

On a side note, what would happen if one would shorten their springs with the normal airbox setup. Would that cause the slides to open sooner on throttle up? would it be too rich then?

Here's a video that TC put on U-Tube a long time ago showing how the slides worked with the bike running but it's with the inlets wide open.
http://www.youtube.com/watch?v=tR63vrfhwr8


So how does that affect the slide operation?

I guess I've got more questions than answers, but that's what this forum is all about. SHARING IDEAS and BEEN THERE, DONE THAT. ...etc.

[D0wn5h1ft]

I think this is the key statement that needs to be talked about.
I agree that with the stock airbox and filter there is a certain amount of 'restriction' that helps the vacuum from the cylinders that is felt on the diaphrams when the butterflies open.
With pods (especially the ones that have very little filter mesh 'restriction' in them) then the inlet to the carbs see very little vacuum except at almost wide open thottle (when it's really sucking hard)
I think that trial and error here is going to be your best bet on finding what spring length will give you the best response for your particular setup.

On a side note, what would happen if one would shorten their springs with the normal airbox setup. Would that cause the slides to open sooner on throttle up? would it be too rich then?

Here's a video that TC put on U-Tube a long time ago showing how the slides worked with the bike running but it's with the inlets wide open.
http://www.youtube.com/watch?v=tR63vrfhwr8


So how does that affect the slide operation?

I guess I've got more questions than answers, but that's what this forum is all about. SHARING IDEAS and BEEN THERE, DONE THAT. ...etc.

When you're talking about slide openings it's more about RPMs than it is about throttle opening. The two are often confused because obviously when you open up the throttle more you have a resulting increase in RPMs. The lean condition that you get when you switch to pods isn't just because you're introducing more air into the system it's actually because the inside of the carb sees less vacuum and thus is unable to pull enough fuel out of the bowl to match the incoming air. Run your stock setup without the airbox on and you'll see what I'm talking about. Even when i run without pods on for testing it performs like garbage too. Not because of jetting but because of the lack of vacuum. While jetting makes it easier for fuel to get pulled up (by creating a bigger opening) it doesn't really solve the problem which is lack of vacuum. A true solution to this would be velocity stacks which keeps air velocity high and thus air pressure low thus helping this lack of vacuum issue. However, the length of the stack required to properly compensate for the removal of the airbox would be quite ridiculous even if you went with a smaller diameter than the intake bell.

I'm getting a little off track here.

To answer your question, shorter springs would lower the RPM threshold at which your slides start opening. Also, less stiff springs would achieve the same thing. (F = k*x) We could either change the stiffness (spring constant, k) or shorten the length of the compression (x). Since it's difficult to consistently modify the spring constant, I just shortened the springs to achieve the same effect. Note, though, your metering will still be the same (fuel amount once the slides open) but it will happen at a lower vacuum (corresponding to a lower RPM/throttle opening) and thus with less air making it run rich-er in that RPM range than normal. Once you get into the RPMs where your slides would be open anyways it doesn't matter because you're all on mains then.

Here's a graph!



Changing your needle taper (putting in new needles) does sort of the same thing. Rather than changing when your slides open up it just enriches the mixture when the slides do open up at the stock RPM range. By modifying both the needle taper and your springs you could control when the slides opened up and how much fuel is being introduce when they do open.

Weeeee...CV carbs science.

[D0wn5h1ft]

Correction:

I was wrong when I said "A true solution to this would be velocity stacks which keeps air velocity high and thus air pressure low thus helping this lack of vacuum issue". This does not apply for CV carbs...(constant velocity). It is still and always will be a vacuum issue because the slides are vacuum operated and that's what keeps the velocity constant! So a jacked up vacuum means the slides don't operate as intended and thus you get poor throttle response and poor performance due to improper metering.

[motoman]

Correction:

I was wrong when I said "A true solution to this would be velocity stacks which keeps air velocity high and thus air pressure low thus helping this lack of vacuum issue". This does not apply for CV carbs...(constant velocity). It is still and always will be a vacuum issue because the slides are vacuum operated and that's what keeps the velocity constant! So a jacked up vacuum means the slides don't operate as intended and thus you get poor throttle response and poor performance due to improper metering.

In a nutshell, the intake vacuum drop IS controlled by intake set-up, Stock verses pods for example. The slide springs ARE designed(lenght/tension) to work with a stock airbox set-up, dampening piston pulses and a more controlled rise of the slides. More, not less tension or longer would help slow the rising of the slides, which IMO would be a bit helpful for those that choose to combine straight pipes with pods.
Least maybe make it more ridable in the under 4K rpm range.

All else equal, that improper metering you refer to is due to improper intake restriction and dampening.
I would imagine it did feel like jumpin' out from under you with less tension holding slides when wacking the throttle @4K!

[D0wn5h1ft]

In a nutshell, the intake vacuum drop IS controlled by intake set-up, Stock verses pods for example. The slide springs ARE designed(lenght/tension) to work with a stock airbox set-up, dampening piston pulses and a more controlled rise of the slides. More, not less tension or longer would help slow the rising of the slides, which IMO would be a bit helpful for those that choose to combine straight pipes with pods.
Least maybe make it more ridable in the under 4K rpm range.

All else equal, that improper metering you refer to is due to improper intake restriction and dampening.
I would imagine it did feel like jumpin' out from under you with less tension holding slides when wacking the throttle @4K!

The lower spring force is to encourage the slides to rise faster because of the lean condition created by the "improper" intake. Basically just another band-aid, I'll admit it but it does help quite a bit and I'm impressed with the results.

[motoman]

By modifying both the needle taper and your springs you could control when the slides opened up and how much fuel is being introduce when they do open.

Mikuni engineers already figured that all out for us with the four different mettering rod applications of the 78-79's, 80-81Standards, 80-81Specials, and XJ's

[D0wn5h1ft]

By modifying both the needle taper and your springs you could control when the slides opened up and how much fuel is being introduce when they do open.

Mikuni engineers already figured that all out for us with the four different mettering rod applications of the 78-79's, 80-81Standards, 80-81Specials, and XJ's

IF you're stock. This is only for those of us who just couldn't leave it well enough alone .

[justme1100]

If I am reading this correctly the shorter springs will help compensate for the lack of vaccum. I am running pods and a less restrictive exhaust on my XJ. I do seem to be running a little lean and am not sure yet what the PO has done to the carb rail I am using, it is not stock. I will be doing a lot of reading the next few days as I am working on correcting a few issues I am having with the Ol' XJ. Thanks for posting this info.

[Dylano*]

I might have to give this a try at some point. Seems logical and easy! I'm running open headers and pods. It seems to haul butt no matter what, but I haven't gotten to ride it much lately, so I may notice what you were dealing with.