All the 2-stroke bikes I've seen, and some 4-strokes only have a single throttle cable, and use the carb spring for the return.  
Whereas most 4strokes I see have 2 cables, for the open and close.

What are the technical reasons 4-strokes need the 2 cables?

cant help you but wonder that too

one pulls it in and one pulls it out. and also if it gets stuck then you can still move it. hope that helps  

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The reason I hear the most is that air traveling below the slide can be enough to overcome the throttle return spring and hold the slide open.  I'm not exactly sure why this is an issue on four strokes but not two strokes.

I don't know how true it is but I was working in a Yamaha shop when they started doing the Double cable deal..
If my memory serves me right it was a Ralph Nader type thing...you know the
Safty Period when everything was looked at real hard to see if it was safe..

                            BC

In general, four stroke carburetors have more internal friction due to accelerator pumps, economizer valves, etc. (many different designs out there)  - the two way cable set-up is to assure the slide (or butterfly valve) closes with all that taken into account.

The constant velocity carbs with a butterfly and coil spring don't need the 2nd cable. The newer Keihin carb only has the flat slide, it may need the insurance of a 2nd cable. I took the 2nd cable off the CVK carb years ago and added a quick throttle, there has never been a problem.

economics 101

2 cost more = more proft to OEM

Burnrider,
I'd be willing to bet that all of these carbs when new or in good shape would not need the second cable - I think the engineers are concerned about things binding due to wear - I would. Could you imagine the litigation from a major injury/death from a sticking factory-setup throttle, old bike or not?
I could see it now, motorcycle and tobacco companies sued together by class action, after all, anyone can be hit by a bike "second-hand" after the throttle sticks. Body-bag ads, the whole works.
I laugh but I cry on the inside.

You guys need to take you throttle cables off and look at the wear. I'm willing to bet most of you will go buy a new one.What you will see is some really good damage on them.The bottem side (close to the carb will have some major wear)Go look trust me.

Ok, since nobody got the right answer, I'll jump in.

4 strokes, especially big ones, have a tendency to ice the throttle valve or slide, like airplanes but not as bad. The second throttle cable is to make sure you can break loose a stuck or frozen slide.
It used to be a pretty big problem when guys would put flat slide (2 stroke) carbs on XR400's and XR600's. I had a 34mm Mikuni on my XR400, and on a cold morning in the Sierras, the carb slide would stick for the first half hour of riding. I used to fly with my brother and I remember him using the carb "deicer" in his plane, so I pulled the boot off the carb one morning and sure enough, it was white with ice. The explanation for the ice is the same as an air conditioning system, moist air (or freon) forced through a small hole at high speeds.

Hope that clears things up.

BTW, for the local Bay Area guys, I rode practice at the GP Track yesterday, there were over 100 bikes on the track at any one time! Mostly Experts and Pros, it was like being on a roller coaster, you had to haul ass for 20 minutes or get run over! It was a blast!

John

I trust ya Macho

I already looked at mine good as new

Well of coarse they would be seeing is I only can handle a 1/4 throttle open  

Not to be an argumentative dork, but if icing is the "right" answer, why don't all four strokes use the double-cable? After all, the vaporization of any liquid, regardless of application, will lower the ambient temperature, this isn't exclusive to any style carburetor. In aircraft at higher altitudes that is a real concern (ex-aircraft mechanic here). I wonder if the XR designs are more prone to such issues than others?  Perhaps a two-stroke would tend to lessen that effect due to it's fuel-oil mix, but I doubt it (just thinking out loud) I'd bet that all the observations here add up to a simple truth, and that is that under certain conditions, these carbs will tend to bind more than a simple single-moving-part slide as found on most two-strokes.    

PS. notice that the mechanical input movement for both 4 stk slide and butterfly carbs is rotational vs linear for 2 stk stuff, i.e. less efficient friction-wise.

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Does anyone here run an FCR carb with one cable? I don't know the slide spring is strong enough to pull the cable friction. Any time I can get to one cable with an accessory throttle, I'll do it.  

Sorry, just read the de-icing theory. That could be a factor for my winter riding, depending on strength of the return spring.

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Does anyone here run an FCR carb with one cable? I don't know the slide spring is strong enough to pull the cable friction.


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I think you bring up a really good point. That is, in order to overcome the internal and cable friction in current 4 stk carbs using a single cable, the return spring would have to be fairly stiff - probably too much so for rider control and comfort - a second cable would help to allow a reduced spring rate don't you think?

Hey guys, can you tell I'm bored this morning?

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That is the reason I have always heard.  Something related to a vacuum above the slide or incoming air below can actually hold the slide up in some instances causing a stuck throttle.  Having the 2nd cable allows you to bring the slide down in a precise manner and/or close the throttle in a run away instance.

Like the backfire screen, it might help in a rare instance, but everyone gets one.  The choice was probably stiffer throttle springs (arm pump) or two cables.

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But that's axe-backwards!!! Moving air has less pressure compared to relatively stable air. That's how airplanes fly and propellers work! If anything there would be a tendency for the slide to close from greater outside pressure. The whole idea of a venturi is to create a low pressure area that draws fuel in.  

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A stiffer spring like a 2 stroke might be the solution if you were hell bent on cable removal. Kev's vacuum theory is what I remember being the most common explanation.  Some riders have reservations about tinkering with an existing design. If that were a problem we wouldn't have an accessory market setup around the dirt riding community. I took 4 coils off the CVK to increase sensitivity. On the CVK this is a separate internal compression spring. The coil spring on the butterfly is the return spring for the throttle cable.

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Most of the differences in the newer "4 stroke" carbs are to reduce slide friction, not increase it. A "2 stroke" flat slide carb has much higher slide friction (in use, not static), that's why they bind or stick on 4 strokes. 2 strokes don't have as high or as consistant of a vacuum signal at the carb as a 4 stroke. In a 2 stroke there's always a little back pressure or pulse that helps keep the slide moving back and forth, so it can easily move up and down. The constant high vac of a 4 stroke tends to suck the slide against the front of the carb. That's why FCR carbs have rollers on the slides. Put your hand over the carb of a running 2 stroke and then a 4 stroke and you'll see what I mean, the 4 stroke will want to suck your hand into the carb!

The reason why the XR's had the problem was simply because of the era, they were some of the first big 4 strokes that people tried to put flat slide carbs on, which were also just starting to be used on the 2 strokes. Before then, most carbs had very heavy chromed steel slides, which had enough weight to close easily and were harder to freeze up.

John

Im working on a NEW TYPE slide that has
teflon,WD-40, and Snail GOO that is dipensed through small nozzles and keep the slide from sticking when it ices up.
Its still kinda large, but as soon as I get it small enough......HEY bye bye
TWO CABLE SET-UPS....

WHO SAID DRUGS ARE BAD?

                          BC