Throttle cable adustment

I have just reassembled my 1970 roadster after a complete rebuild, including a professional rebuild of the engine. My current issue is with the throttle cable; which I obviously got wrong. When I first started the bike it was overreving; 4000 rpm. I adjusted the idle screws, with not much effect. Then I noticed that as I turned the handle bars, it accelerated and decelerated; the handlebar turning was operating in the same way as twisting the throttle.

The throttle cable is routed along the bottom of the top frame tube, and tied to it with plastic ties. I reconfigured the plastic ties to try to ensure that there was no movement in the cable. After that exercise the bike revs to 5000 rpm on start up; so I've managed to make the problem worse not better. Any thoughts?

Sounds as if there is insfficient slack in the throttle cables, so the throttle slides are hanging on the cables instead of resting on the throttle stop screws at idle?

Or the top cable is still routed so that the outers are being pulled when the bars are turned?

Or both?

my thought is that there is not enough slack in the cable. I guess one way to check that would be to take the cable out of the throttle control - which will give it more slack, and see if it is still overreving when I start it up. If that is the issue, then I will somehow have to shorten the cable outer - but given that it is a new correct cable that seems odd - unless it can be adjusted further in a way that I have not found; or unless the way that I have routed and fastened it has had the effect of shortening the outer cable relative to the inner cable. I will double check the adjustments on the top of the carbs, but I think that I have them screwed down as far as possible, and the adjuster near the top of the cable is also in.

I have been concerned that the angle of the cable to the top of the carbs is very steep ie a bend from vertical to nearly horizontal - which is necessary to route the cable to the underside of the frame tube; is that how it should be routed?

I had same problem when I installed a new carb the angle is normal the problem I found was with routing and adjustment start over with tank off slacken adjustment re-route cable it may take you a few attempts turn handle bars and watch your slide etc you'll get it.... Doxford

Chris T said:

my thought is that there is not enough slack in the cable.

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One thing you could check is that all cable nipples inside the cable junction are all fully seated in the junction slider?

Also, are you certain you have the correct cables?

The choke lower cable inners and outers are very slightly different in length to the throttle lowers, and I'm not sure which pair has the longest inner, throttle or choke?

Comparing two used pairs of throttle and choke cables, I can't identify which pair are the throttle cables and which ones are the choke cables, but the outers and more importantly the inners of one pair are certainly slightly shorter than the other pair.

If you place a finger on the carburetor throttle stop screw, and snap the throttle shut, you should be able to feel the throttle slide "tapping" or coming to rest on the screw. If you can't, the throttle slides are hanging above the stops, which will help confirm as you suspect that there is something amiss in the cabling.

Hi
Undo the cable from the throttle. start the bike. If you have the same problem the slides are hung up.
I think LAB is correct. I fitted cables that not only didnt seat the slides but also did not have enough range for the slides to fully open. I was told I had used the choke cables.
Chris

Thanks for all the good input. It's going to be a couple of weeks before I get to test the suggestions put forward; I'll let you know how I get on.

Are you running a single carb ? If so then I think that it would be best with an angled connection on the carb.

Don't fix the cable too rigidly. Just enough to make sure it doesn't get caught up when you put the tank on. Nice loose loops are all you need.

I'm running twin carbs. Thinking about the suggestions made I'm thinking that maybe the choke and throttle cables have been connected the wrong way round (that was one mistake that I didn't make - they were put on by the engine rebuilder). I will have to check it all properly; but as a quick test, how much movement should there be in the choke lever - a lot or a little? Mine turns the whole radius of the lever - not 180 degrees, but not that far off it, and I get the impression that it stops because of a stop in the lever, rather than a stop in the carb; I'm wondering if that is because of additional length in the cable inner if the throttle cable has been connected up to the choke connection.

This weekend I got to check the cables, and use some of the advice gratefully received. When I disconnected the cable from the throttle control, I could hear the throttle valve tapping the bottom of the carb - that was not happening when the cable was connected to the throttle control, and by marking the internal cable and connecting it to the throttle control, I concluded that the external cable was at least 1/4 of an inch too long. I looked hard at the choke cable to see if it was possible that the two cables had been interchanged; it is possible, but the connectors of the choke cable where it meets the carb are different; no rubber covers. So it looked like the right cables had been used for choke and throttle. Rather than to attempt to dismantle the cables from the carbs and swap them; and then reassemble everything - which seemed like it could be a bit of a major to see if what I suspected was correct, I cut down the outer throttle cable at the switch where it converts to two cables - one for each carb. I used a mini dremel - and the process was pretty straight forward. It clearly solved the problem; I cut it down about half an inch, and then adjusted it back with the adjustment on the cable below the hand control.

When I started up the engine the problem was clearly solved; the engine was able to start up and remain at low revs.

I spent the next hour or so unsuccessfully trying to adjust the idle so that it would idle without the throttle on. I still have to have the timing checked with a strobe light, so I guess that I might have to get that 100% before I can finalise the idle.

Chris T said:

I looked hard at the choke cable to see if it was possible that the two cables had been interchanged; it is possible, but the connectors of the choke cable where it meets the carb are different; no rubber covers. So it looked like the right cables had been used for choke and throttle.

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I've never known genuine Norton throttle or choke cables to come supplied with "rubber covers"? And I don't think the factory ever fitted them, either?

It is kind of hard to say how he ended up with cables that didn't fit. Everything is pattern parts these day.
Where did you get the motor done in Auckland? I suspect some day I may need to know a good bike shop there.