Throttle stiction

[nickst4]

Among my bikes, the Dart is the smallest and yet it had the heaviest throttle action! Often I'd find the bike starting to slow and realised I just wasn't gripping the twistgrip tight enough, and I started to get cramps: something that's never happened on any other bike. I suspect that the Dart cable rig with it's sharp turns is part of the problem, but I was amazed at how strong the throttle slide springs were. While I was getting new jets, I ordered the lightest springs (.85Kg I think they called them), but it turned out that that was what the carbs had in them already, by the gauge of the wire.

At the time, I assumed the carbs needed heavy return springs to counteract the 'throttle hang' that Darts are known for. However, the real culprit is the ignition timing, and that was dealt with very effectively thanks to info here.

So back to the heavy sticky throttle. I reckon stiction in a clean cable is probably relative to the square of the load, ie the strength of the springs, so I've just reduced the preload on the original springs, the rate staying the same. Free length is normally 66mm, and careful crimping of the coils has brought it down to 52mm. Note that the end-coils are crucial in their fit to the slide and the carb-top, so it would not be a good idea to shorten the springs by cutting coils off, quite apart from the theoretical effects of a reduction in the number of coils operative. 52mm of spring length still gives a decent amount of preload, and my rough-and-ready measure of the pull needed has gone down from 10lbs to 6lbs. Much better is that there is very much less stiction, as was the main object of the exercise.

Test-riding the bike shows that throttle return has not been adversely affected, but the bike now feels even more lively and it's much easier to maintain a throttle setting without gripping the twistgrip so tightly.

Job done!

Nick

[mantaray]

well done

[EVguru]

I don't recall the Dart throttle action being much heavier than the Sport.

The 'square' slide Dellortos rely on the spring pressure to bias the slide to the flat side and seal it (hence the angled spring seat).

Wash out the cable assembly with brake cleaner, particularly the splitter. Then oil the cables, but leave the splitter dry.

[nickst4]

The 'square' slide Dellortos rely on the spring pressure to bias the slide to the flat side and seal it (hence the angled spring seat).
.

Yes, I realised that, but feel that they overdid the preload. Once the slide is raised and air/mixture is flowing beneath it, surely slide sealing is much less critical anyway? My mod has not affected tickover speed or engine-braking, so I think that's the crucial test.

Presumably the extra space around the earlier Morini motor installation allows sweeping curves to the cables, rather than the right-angle bends in the Dart's space-restricted setup which inevitably cause greater frictional losses, however much lubricant is in there. Thus the pull on the earlier Morinis (if they use the same carbs) would be a deal easier, heavy return springs notwithstanding.

If there any carburation problems arising from the mod, I'll fabricate my own streamlined cable rig using stainless pedal-cycle gear-change inners in lined outers with tighter guides into the carb tops and the very minimum of adjusters. Doing that on my Cagiva Alazzurra (Ducati Pantah) made a big difference to smoothness but also improved synchronisation stability a hundredfold.

Nick