1971 CL450K4 runaway throttle/idle

DeadAgeFlute77

DeadAgeFlute77

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Title. When idling bike rpm barely responds to idle adjustment screw on both carbeurators. When using throttle it's good until around 2500-300rpm then screams up to 6000 rpm. Ive had this issue before and as per discussions regarding this site among others I replaced the advance springs with yamaha XS650 springs and those worked for a time but all of a sudden the issue has returned. I inspected the advance mechanism and cut one spring a coil shorter in hopes of tightening the weights. Didn't change how the engine responds and functioned the same with runaway throttle/idle. I have checked for air leaks around both carbeurators using starting fluid and found no change in idle on boots, vaccum seal, throttle seal, etc. Additionally, I had ran the engine previously on each cylinder separately with their respective carbeurator and found the runaway throttle/idle to persist. Thus i believe the issue to lie in the mechanical advance as timing had not changed when the issue arose again. What could be possible solutions to this issue? New springs? NOS mechanical advance unit? Ive convinced myself it's the mechanical advance and want a second opinion before I shell out $300 on a NOS CB450 mechanical advance unit. In addition to that, would a cb500t mechanical advance unit work? Or a cb500 or cb550, etc. I see reproduction units available and am wondering if anyone knows if a different model would fit on a CB/CL450. Thanks in advance.
 
The 500T advance mechanism is compatible, see this thread. I've experimented with some replacement springs and you can read about that here. Near the end there is discussion of a decent choice, better than those in my first round of study, but it requires a slight stretching of the springs.

I've purchased a used advancer from a 500T that was tested in my experiments for comparison and it worked well.
 
Throttle cable routing, throttle tube, throttle cable tension. Rule those out before continuing to fight with the voodoo tubes.

Also, check the vacuum caps for leaks - those little plastic "windows" at the top can crack and then you get all kinds of funkiness.
 
EzPete said:
Throttle cable routing, throttle tube, throttle cable tension. Rule those out before continuing to fight with the voodoo tubes.

Also, check the vacuum caps for leaks - those little plastic "windows" at the top can crack and then you get all kinds of funkiness.
Click to expand...
I replaced the throttle tube, new throttle cable, and have adjusted the tension multiple times. I cannot find any spot where the throttle cable binds at all. Throttle grip snaps back as intended.
 
Have you checked if the clutch cable is sitting/rubbing against the end of the left carburetor throttle shaft and causing it to bind or stick open? I had to use a small clamp on the cable attached to a rear valve cover bolt to hold the cable away from the end of the shaft.
 
DeadAgeFlute77 said:
I replaced the throttle tube, new throttle cable, and have adjusted the tension multiple times. I cannot find any spot where the throttle cable binds at all. Throttle grip snaps back as intended.
Click to expand...

New throttle cable can be a problem - the free play is often different from factory. The biggest pain point is at the splitter - one (or both) sides can pop out of the holder, thereby shortening the cable and holding it to like 3/4 throttle.

Pull the air cleaners off, open the choke all the way and verify that the slides are dropping all the way down when the throttle snaps back
 
ancientdad said:
But again, what were the gaps when you were finished? And did you check the full advance (dynamic timing) with a strobe timing light?
Click to expand...
I set the points gaps to 0.013-0.015 in. using feeler gagues. I did not do dynamic timing. I had not considered dynamic timing. Does dynamic timing indicate if the timing is advancing improperly/properly as the mechanical advance engages at higher rpms?
 
RobMan said:
Have you checked if the clutch cable is sitting/rubbing against the end of the left carburetor throttle shaft and causing it to bind or stick open? I had to use a small clamp on the cable attached to a rear valve cover bolt to hold the cable away from the end of the shaft.
Click to expand...
I had wondered if that was the issue as well but the throttle shaft returns to idle, with the idle screw meeting the carbeurator.
 
EzPete said:
New throttle cable can be a problem - the free play is often different from factory. The biggest pain point is at the splitter - one (or both) sides can pop out of the holder, thereby shortening the cable and holding it to like 3/4 throttle.

Pull the air cleaners off, open the choke all the way and verify that the slides are dropping all the way down when the throttle snaps back
Click to expand...
Yeah i had an issue with the previous throttle cable with the splitter bring worn and the left cable continued to fall out thus I replaced it. I have checked the slides before and they did close fully and responded to the throttle cable appropriately.
 
DeadAgeFlute77 said:
I replaced the throttle tube, new throttle cable, and have adjusted the tension multiple times. I cannot find any spot where the throttle cable binds at all. Throttle grip snaps back as intended.
Click to expand...
I also checked the plastic tops of the carbeurators and they did not leak air and held a vacuum appropriately.
 
stl360+450 said:
The 500T advance mechanism is compatible, see this thread. I've experimented with some replacement springs and you can read about that here. Near the end there is discussion of a decent choice, better than those in my first round of study, but it requires a slight stretching of the springs.

I've purchased a used advancer from a 500T that was tested in my experiments for comparison and it worked well.
Click to expand...
Ahh I see, your thread there was excellent as i have seen that before. I presume the springs that need stretching you speak of are the LEM080C 01 M from Lee's springs. If so I will order those and try those.
 
DeadAgeFlute77 said:
I set the points gaps to 0.013-0.015 in. using feeler gagues. I did not do dynamic timing. I had not considered dynamic timing. Does dynamic timing indicate if the timing is advancing improperly/properly as the mechanical advance engages at higher rpms?
Click to expand...
Yep, that's the only way you can be sure of how far it advances compared to the factory marks.

full advance marks.png
 
DeadAgeFlute77 said:
I used static timing with a test light on the points with the stator on LT and T marks as per the factory manual.
Click to expand...
I just want to remark that it should be LF and F, rather than LT and T. Probably that's what you meant, but it makes a difference as the T marks would mean firing too late. Valve clearances are set at the T marks.
 
stl360+450 said:
I just want to remark that it should be LF and F, rather than LT and T. Probably that's what you meant, but it makes a difference as the T marks would mean firing too late. Valve clearances are set at the T marks.
Click to expand...
Great point, I took it for granted he meant LF and F. Good catch.
 
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