touchy 1971 throttle

angus67

angus67

Welder's penetrate deeper!!
Messages
5,288
Reaction score
417
Points
133
Location
Shelton, Wa
My throttle on my 71 is very touchy. It isn't advancing early, it just doesn't have enuf 'slop' in the throttle. My carbs are closing all the way, and some slop at the carbs, but it isn't enuf to be comfortable.
I adjusted the uppers by the throttle, adjusted the idle screws, but when I have enuf slop, it is unbalanced. Its a dual cable on unlinked early bs38's.
Last time I tested the ride, I almost hit tree. I was on gravel, and with the touchy throttle, when I grabbed the front brake, the throttle rolled a little as I puled the lever, and the bike's front almost washed out on me twice.
The cables I have are mikes replacement. Don't know if that helps, just putting it out there. there just doesn't seem to be enuf adjustment at the top. The cables are routed correctly, no weird bends or kinks.
What am I missing?
BTW, these replacement have the cover springs at the carb end, but looking at a set of originals, the original springs are a tighter coil than mikes. they look nice, but are not correct.
I have a set of original cables, last effort will be to put those back on and see if those will give me what I need.
--signed 'touchy bike on gravel':laugh:
 
Angus, I hear ya. I've always considered touchy throttles as a shrewd marketing scheme to make the machine seem more powerful than it really is. In the case of our carb linkage, there's a non-linear relationship between the twist grip and air/fuel delivery. When you first start to twist the grip, a little cable movement produces a lot of throttle shaft rotation. And, air/fuel delivery thru disc-type throttle bodies skyrockets upon initial opening. Some of the early XS ride evaluations note this touchy/jumpy throttle in low speed "about town" riding. Later bikes (and cars) mitigated this by replacing the straight lever arm with a circular pully-type actuator. Long ago, I cooked-up a "soft throttle" mod to reduce this effect:

full


View attachment 35328

This also reduces the initial side loading of the throttle shaft, a good source of throttle bore wear.

Speaking of which, if your throttle shaft and bore clearances are loose, it aggravates this "touchy" throttle. I touched on this subject awhile back, in Carbon's build thread, and gggGary's 71 carb thread:

http://www.xs650.com/forum/showthread.php?p=312847#post312847
http://www.xs650.com/forum/showthread.php?t=33692

One trick you can try, which I've done on mine, is to set the idle timing at the most retarded mark. It'll require that the throttle be opened just a tad more to maintain idle rpm, but it will reduce that explosive-initial-opening "throttle plate to air/fuel flow ratio".
 
please elaborate on the pulley. is it off another mikuni for a different bike? seems that would take up any slack I did have.
 
Last edited:
I would like to know more on this as well. My 73 has the dual throttle cables with the unlinked carbs. Its pretty jerky on the throttle to say the least.
 
angus67 said:
please elaborate on the pulley. is it off another mikuni for a different bike? seems that would take up any slack I did have.
Click to expand...

Yeah, it takes up at least 1/8" of cable slack.

Those pulley sections came from some old radio parts. Remember the old mechanical tuning radio mechanisms with the string and pulleys? The pulleys are thin stamped steel, so mount up easily like a thin washer.

It would be great if some resourceful fellow could come up with a suitable mod for this...
 
hmmm. I have a few transisitor radios from the 60's. another good source-second hand stores. My xl250's kiehin has a pulley system like that, so I will be taking a closer look.
 
springs are pretty tight to keep advance at bay. maybe i should zip tie the weights and see if that helps?
 
angus67 said:
springs are pretty tight to keep advance at bay. maybe i should zip tie the weights and see if that helps?
Click to expand...

Yes, if the springs are too strong, they prevent spark advance at first, but as the rpm's climb, the weights will fly out in one large movement rather than gradually, with the resulting surge in power.

Its another reason I went over to the E-advance, the springs and fly-weights just become an aggrevation after awhile.
 
When I replaced the springs, it was recomended to cut off a complete coil, but I figured Ide just cut off a half coil, because you cant put it back on. It goes full advance about 2500rpm's. But goes past full advance mark by almost 1/4 inch. Think I should take that extra half coil out of the springs? timing is dead on at idle, so my pamco plate is clocked correctly.
 
angus67 said:
When I replaced the springs, it was recomended to cut off a complete coil, but I figured Ide just cut off a half coil, because you cant put it back on. It goes full advance about 2500rpm's. But goes past full advance mark by almost 1/4 inch. Think I should take that extra half coil out of the springs? timing is dead on at idle, so my pamco plate is clocked correctly.
Click to expand...

What you're seeing is the main problem with the mechanical ATU. The fly-weights have 2 tiny fingers that fit into the slots in the "slotted disc". They are a tight fit when the parts are new, but after all these years the fit becomes sloppy as the metal wears away. This results in an expanded amount of advance. In other words, the advance is no longer 25 degrees, but some larger amount.

Spring tension has no effect on the amount of advance.

Its possible to peen the fingers on the fly-weights to get a tighter fit, but its also possible to damage the fingers as you beat on them with a hammer. Ask me how I know:)

The solution is to go over to the Pamco E-advance. It always gives a consistant advance curve. No more aggrevation from mechanical parts.
 
upon further investigation, I re checked my timing. I guesse I had it off before, because now its fireing on the f mark, and is staying within a normal location at full advance. There is still a lot of flutter, but I think my advance rod just needs shimming. 5t has pointed me in this direction. Just eye-balling the slop, looks like a 1/32 of an inch. What other bonuses are to have by putting on the e-advancer?The fingers on my fly weights are sloppy.
If I get the e-advancer, do I have to get the 60* wheel for my basic pamco I have in it now?
 
Sloppy flyweight fingers can give you spark scatter (wandering timing).
Especially if there's no points to drag and hold the advancer rod retarded.

Just for fun, try setting the timing a bit retarded. Idle at the right of the "F", full advance a couple degrees retarded. Adjust idle stop and mix accordingly, sync carbs. See if the explosive acceleration calms down...
 
will do. I have not ridden it since redoing the timing. I plan to tomorrow. If it still runs crazy, I'll retard it just short of the lfl mark.
 
You can get a idea of the amount of spark scatter caused by sloppy advancer parts by gently dragging a finger on the trigger rotor (just a light drag) while shooting the timing light during idling. It will simulate the light drag produced by the points, and force everything back to the stops.

Hope that made sense. '70s think/speak going on here...
 
I am not sticking my finger anywhere near that thing! I stil have 8 fingers and 2 thumbs, and plan to keep it that way. With that thing turning, looks like a bunch of cats fighting in a burlap bag.
 

Similar threads

padge
2-1 intake setup - what throttle + cable?
Replies
4
Views
724
Rasputin
Rasputin
H
91 octane or 87 octane for 1972 XS2 with Mikuni carb conversion??
Replies
5
Views
272
Signal
Signal
julienm94
Dual Pull Throttle Body Adjustments
Replies
16
Views
1K
julienm94
julienm94
B
Hand throttle control
Replies
8
Views
1K
wesleyonoel
wesleyonoel
H
Carb slide springs
2
Replies
21
Views
2K
heepey
H
Back
Top