Why Two Throttle Cables?

[Person8]

On a thumper's carb, why is it neccessary to have two throttle cables?  One pushes, and one pulls.  Why the push cable?  I recently damaged the push cable, which caused the throttle to stick open, so I did a trailside repair by pulling off the push side.  It worked fine and allowed me to finish the day riding.  It made it much easier to twist the trottle with only one cable too.  The second cable forcibly pulls the slide back down when you let off the throttle, but there is a spring on the carb to do that.  What I'm wondering is if it would be OK to leave it the way it is and just run it with one cable?

 

Larry

 

 

[MikeyG]

Hey Larry,
I have a complete guess as to why it may be like that. The newer thumpers have accel pump carbs, and if you take off the cover on the carb where the throttle cables go (well on a 426 anyway) you see pulleys, and the parts used to actuate the accel pump. The second throttle cable may be needed to correctly control the pulleys, thus correctly controlong the accel pump. This is a guess however, maybe the more enlightened and help more. ;)

 

[Rcannon]

I worked a bit with the dual cabl esetup on a DR 350 suzuki. I wonder if it is just not some liability issue. I think the 4 stroke create a huge vacuum on the slide. The extra cable is there to make sure the slide will close in case of a broken main cable. On the suzuki, I could remove the extra cable and it worke djust fine.

 

[Ol'89r]

Originally posted by Rcannon
I wonder if it is just not some liability issue. The extra cable is there to make sure the slide will close

Bingo. :thumb:

 

[Person8]

I was thinking it might be due to a vacuum trying to hold the slide up, caused by the high volumes of air rushing past beneath it. So is this a real big deal? Can't the spring on the carb take care of it?

Larry

 

[dirt bike dave]

Originally posted by Person8
Can't the spring on the carb take care of it?

Usually it can. However, you probably generate the highest vacuum at the time you least want the throttle to stick open - engine braking in a low gear down a steep hill. Combined with overheated brakes, that could be a recipe for disaster. Also, the carb slide and body will wear as the bike gets older, making sticking more likely.

To ensure the throttle will close under extreme conditions, you would need a super stiff spring. The manufacturers must feel a dual cable is a better compromise for safety, ease of operation and reliability. It certainly isn't cheaper than a stiff spring.