Idling issues

[Highpiper]

My 1979 3-1/2 Sport starts easily enough with the kickstart lever (unfortunately never with the electric starter), and use the choke lever on one carb to warm it up. It's usually then, when it warms up, that the idling takes on a life of its own...revving higher, then dropping lower, where it usually cuts out eventually.

This strangeness occurs even when starting a warmed up engine (after riding it for 30 minutes). I'm very familiar with Amal carbs, from years of tuning old British Bikes, so I've spent a lot of time doing very fine adjustments to air mixture screws and the idle speed screws to get them 'just right' and the idle set nicely at 1000-1200 rpm.

Since I put the original airbox back on the bike, I can't seem to eliminate the popping on decel and the cutting-out at idle, particularly at stoplights. I know, this all suggests a too-lean condition, but I have turned the air mixture screw almost all of the way in, and the idle speed screw in a bit more---but this produces the crazy pattern described above---surging and revving.

Do I need to raise the needle one notch? Adjust things differently? Or just live with the popping and set the idle higher (1400-1500 rpm)?

Thoughts welcome. Thanks,

Peter

[EVguru]

I know, this all suggests a too-lean condition, but I have turned the air mixture screw almost all of the way in,

Well for starters, it's not an air screw, it's a fuel screw, so you're making it leaner still.

The Dellorto tuning manual isn't hard to find on-line, here for example;

http://www.thisoldtractor.com/gtbender/dellorto.htm

[Highpiper]

Thanks. Just found that same link online a short while ago. Got some reading to do today. Cheers,

P

[adrian650]

Hi,

I'd also be surprised if you manage to get a sustainable 1200rpm idle with stock ignition. Most of us have given up trying to get that. I suspect that spark and fuel/air are all too 'on the edge' to stay correct over any reasonable range of temperature and time.

Now I'll step back and await all those who have 1000rpm idles...

[3potjohn]

Well my tacho is at 500 ish without the engine running. I guess I have a slow idle speed though,it can read 1000rpm and seldom ever cuts out.If it is set to tickover higher then I am assured of eliminating cut out but then you're heading into tricky neutral selection territory. I've previously rebuilt the carbs and cleaned them ultrasonically. They had mismatched jets in them (this may have been a tuning mod but I have them the same now)
Treble check for air leaks in your system.I had some K75-like pops and bangs last week.Turned out to be a split in the intake rubber hidden by the jubilee clip. I also previously had a carb top retaining screw not all the way in, due to muck or fur on the threads,so the top was not closed down properly,which could not be a good thing.
Talking of carb rubbers,there should be a harder plastic collar under the jubilee clip going onto the manifold side shouldn't there. I found these seemed to want to slide out as I tightened the clip.Eliminating them seems to make it better but I am worried about splitting or cutting the rubber with the jubilee clip.Anyone got a view on this?

[Highpiper]

Thanks for the interesting replies. Last night I spent some more time in the garage working with the idle to see whether unscrewing the 'mixture' screws (versus turning them in) actually improved the bike's ability to idle steadily (and reduce the popping on decel).

The first thing I noticed was that turning out the mixture screws (about 1.5 turns, vs from the 0.5 turns out, where they were) generally raised the speed and intensity of the idle. So far, so good. However, every few seconds as the bike sat there idling, there would be this surge of revving on its own--up to 3000 rpm. Crazy.

My natural inclination at this point would be to "tame the surge" by adjusting the idle speed screw, but then the bike would drop down to 1200 rpm and just cut out. So I started leaving it alone...and the bike did its usual surge upward, then gradually, over about 30 seconds, fade back down to about 1400 rpm and stay there, without cutting out.

A brief ride around town showed less popping on decel, though some remains, especially when closing the throttle quickly to stop for a traffic light. Less 'cutting out' at lights waiting for them to turn green. I guess this is an improvement...more noodling with it tonight.

The other thing I'm noticing is that while holding the clutch lever in, and keeping the bike in first gear at stoplights, there's a little bit of surging going on...may have to adjust the clutch to help 'hold' the idle steady in these situations.

Cheers,

Peter

[dunk 1]

My 79 sport pops on deceleration when the slide stops/idling screws are not exactly alligned. I have accepted a higher idling speed but have a question in that one mixture screw is fairly sensitive whilst the other seems to have no effect at all and can anyone suggest an explanation for this - thanks - Duncan

[Highpiper]

Duncan

I can report the same phenomenon on mine. The front cylinder's carb is far more sensitive to minor adjustment of the mixture and idle speed screws than the rear cylinder.

The same is true for their reaction to using the front cylinder's carb choke lever--it produces a far greater surge in rpms (up to 3000-4000 rpms) compared with using the rear cylinder's choke lever prior to starting the bike. Can't figure out why that seems to be the case...both carbs are at the same angle, with the identical settings and components.

Peter