3 1/2 that always stops at the lights!

[Alan D]

My 3 1/2 sport always stalls whenever I come to a stop and the revs run down towards tick-over. It will then always immediately restart on the electric start ( that has been retro-fitted by a previous owner) and tick over. Other than this it seems to run perfectly.
I was thinking it was either fuel starvation or some ignition fault. I've been right through the fuel system checking filters, gauzes, float heights, tank breather etc. and all looks OK so I'm thinking it's the ignition.
Has anyone comer across this problem before and can give some advice on what I should be checking?
Cheers,
Alan

[hendre]

I would suspect the idle mixture or level. Maybe the idle is to lean or to low in revs
Play around with the idle mixture screw (the retracted small one, not the other with spring in sight) with the engine warmed up, if idle speed goes up when opening/closing your mixture is off. To set it correctly play around with both scres on both sides: first retracted untill highest revs, then other to lower revs to usable range and repeat untill rev's are good and retracted screw only lowers revs when turning either way

[EVguru]

A common problem running modern fuel with its high proportion of easily vorporised 'aromatic' compounds is running rich at idle. This is the usual cause of the stumble or fluffing when pulling away, but can also cause a stall after deceleration.

The idle jet in combination with the mixture screw sets the idle mixture and the idle jet in combination with the slide cutaway sets the progression mixture. It's usually progression that's too rich. A smaller idle jet is the easiest thing to try.

[Alan D]

Thanks for the advice. I have been playing around with things again today. I had some smaller idle jets, 45 as opposed to the 50 ones fitted, so they have gone in. (Incidentally the main jets are 115.) In working on the carbs I noticed that one of the carb to head rubber manifolds was in a poor state. I have changed these in the past and having kept the old ones refitted an acceptable original. (Interestingly the older one was a much snugger fit on the spigots than those supplied by NLM a couple of years back. I measured them both and the bores in the older ones were 2mm smaller on diameter. Do they come in different sizes for other engine/carb variants? Maybe I got the wrong ones?)

Anyhow, the bike was less prone to stalling when I took it out for a test ride, a big improvement. It still didn't pull too cleanly though from idle, and has a bit of a flat spot around 4000rpm, so I checked the plugs when I got back and they were a bit sooty indicating a rich mixture. Next job then will be playing around with the mixture screws. I've also got some 110 main jets so might try them too.
Cheers
Alan

[EVguru]

It's been quite some time since NLM stepped up and had a batch of manifolds made when they became unavailable from Ducati (who held the remaining factory Morini stock).

Fuel has changed and I think the NLM manifolds might react to it and swell sometimes.

Small volume production methods have improved and Startgomma in Italy make a range of Morini rubber items that are very nice quality.

Were the plugs just black in colour, or actually 'sooty'? Unleaded fuel doesn't always give the classic plug colours and in any case you need to do a 'plug cut' to assess mixture.

Mid range flatspots can be down to ignition, or you might need a change of atomiser.