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Discussion in 'The Perfect Line and Other Riding Myths' started by toates89, Apr 25, 2014.
Sorry I posted my real world experience.
Yes, the vast majority of bikes over the past 20 years have used CV type carbies with twin cable systems. And yes I read your post where you clearly stated the dual cable system was neither designed or would be effective to force the carby shut, but was rather designed to stop "Surging is when the piston's pulling so much vacuum that it causes the slide to 'jump' and the push/pull throttle is something made up to try and sidestep this" You're exact words.
No need to cut & past from a clymers as 40+ years experience as a professional mechanic is enough to know your wrong. Like to share your "real world experience" ....dude?
Sounds like he's still living in the land of dirt bike carburetors, Mikuni VMs and such, that CAN'T use a 2 cable setup.
I've ridden many open class 2 strokes and never experienced this "surge" he's talking about.
BTW "vacuum" no matter what is pulling on it can never get over about 32" mercury (depending on the atmospheric pressure of the day) so I don't think it matters what engine the carb is attached to, once the pressure drops to 0 (actual pressure, not gauge) "vacuum" is done.
If your 500 2 stroke surges when you close the throttle, you probably have a vacuum leak and are jetted a little rich to compensate.
I happen to like that world :) Toss a PWK or a FCR on somethin' and I'm happy :)
Mikuni does make a flat slide pumper that can use a 2 cable setup and works real well on 4 stroke big singles. HSR42 and 45.
Here's my opinion in a little more detail of why the designer's use a twin cable system. Basically it's to achieve a light throttle. This is particularity an issue on an I4 with 5 springs, the main return spring and 4 small coil types at each of the butterfly valve shafts. If they fitted the weight of springs that would essentially guarantee the carbies or throttle bodies would shut all the time you'd end up with an unacceptably heavy throttle. So they fit light springs that will do the job when everything is working perfectly. To cover the event that the throttle may not close due to lack of maintenance etc they fit the second close cable that should add no weigh to the throttle if working properly as a fall back safety solution.
So IMO, the reason for a dual cable throttle system is to safely have as light a throttle as possible. If you remove the second cable you negate this design safety feature, and save what $15? You can then go buy a Big Mac and Coke with the money.
It's there. Anyone with a dr350 who's fitted a TM33 flat side (not CV) will tell you it'll do it. It's not bad, just at high speeds, WOT, then quickly closing, it'll lurch. Using both cables doesn't help. The cables are also not interchangeable unfortunately, so you can't really keep it as a spare. I recommend buying an actual spare, then zip-tieing it along the length of the original, along with a clutch, and decomp if you have it.
I'm running one cable. The TM33 has a heavy spring and large pulley and is well built. In the (ridiculously unlikely) event that the cable snaps, cutting power will be a safer option than using the other cable to bring it back down to a position that will still leave the engine running.
The spring is not going to snap.
I wouldn't describe the likelihood of an accelerator cable breaking as "ridiculously unlikely". It's a bit more common than that. But anyhow if the cable breaks the carby will just close shut.
Ask anyone old enough to have regularly ridden old English bikes with Amal carburetors about sticking throttles if you think this discussion is purely academic or the manufacturer decided to over engineer the bike for no purpose whatsoever. Google "Amal slide sticking" and you'll be able to entertain yourself for the next week reading the hundreds of pages of posts on the subject.
I think this is part of the reason for the development of cv carbs.
sent from a device designed specifically to annoy scottrnelson YES I do know how to change it, just never bothered as I was too busy riding my motorcycle.
I can still clearly remember as a young apprentice in the 1960's and working with a guy who was a very serious and moderately successful racer (both track and speedway). Anyhow this other guy brings his newly purchased second-hand Triumph Tiger 100 into work and basically pressures the racer guy to take it for a ride. He's gone for quite a while on this ride and when he returns he's quite shaken up. The throttle had jammed wide open, and magneto kill button didn't work. He'd had to reach behind the engine to the fitted carby velocity stack and use his hand to choke it to a stop. Image trying to do that with the bike going flat out along suburban roads.
Heck, why WOULD it overpower the pull cable (or open cable as it is aptly called in the Kawasaki parts diagrams)? They are adjusted to work together in a synchronized fashion, not to "override" each other.
Of course if the open cable failed, the light springing of the butterflies could cause the throttle to remain open due to air flow, as pointed out. Likely to happen? Probably not, but a close cable would pull it closed.
One side note, the old flat slides used on superbikes in the 70s/80s were known to stick due to the vacuum pull of the engine, part of the reason some high buck ones have actual ball bearings on the slides to run in the guides - low friction regardless of vacuum pull by the engine.
I will say I do go with the lighter pull on the throttle. With the two cables you could even eliminate the spring entirely, but they don't.
You'd be surprised at the amount of torque you can apply towards closing the throttle when it sticks wide open and your ass is at full pucker.:eek1
On my KLX250S, the double cable throttle set-up isn't really "push-pull" but "pull-pull". When turning the throttle, one cable goes into tension and the other goes slack (at least as slack as you have play in the set-up). Anyway, opening the throttle pulls one cable, and closing the throttle pulls the other cable. So you'll have the same leverage to pull both cables individually and aren't' actually pushing on either.
This happens to those who distort the carb bore by overtightening the flange bolts, especially with an insulating spacer between the carb and intake flange. It's the quickest and easiest way to destroy a perfectly good carburetor. Mikuni fixed this by making the VM series with a spigot mount.
OK I'll get involved in the semantics. The typical free-play adjustment between the two cables is 3-5mm. When you twist the throttle grip open it pulls one cable, and pushes the other one. The only way it wouldn't be pushing the second cable through is if there was inadequate free-play adjustment.
So when your opening the throttle its a push-pull system, and when your're closing the throttle its a pull-pull system. Everybody is a winner.
Actually, it's pull pull both ways.
When you open the throttle, the "open" cable pulls the bellcrank on the carb which in turn pulls the "close" cable, and vice versa when you close the throttle, except there is a spring to help with that.
Bobby Bowden invented throttle cables?
Maybe in Aussieland. Here in the USA its hard to push a rope.
this is why I always cover the clutch and routinely practice stuck throttle drills so that I don't have to think to pull in the clutch