Round town running problems

[thackers]

Hi, I'm after the wisdom only this site brings on technical issues with a Lambretta.
Recently I've had problems when in slow moving traffic, crawling along a walking pace, 30 mph ish types of speeds. I might be in 2nd or 3rd gear and the bike is spluttering and jumping like a Bronco. It then takes time to clear and when traffic speeds up it's ok then when around 55 mph to 60 it will give another jolt, you can almost feel it coming like it's clearing its throat with a small cough.
This higher speed thing is a recent problem, although the slower speed thing has always been present but no way as severe and totally manageable with throttle management.
I've checked and cleaned the carb and jets, spark plug is the correct colour and gap. It's getting plenty of fuel from the fast tap, starts up ok but I'm stumped as to the cause of the spluttering.
I changed the piston rings last year and ever since I've been getting a thick black sludge type of blow back onto the exhaust. The manifold is tight and I guess the goop is out of the stub/exhaust connection, could this be an indicator?
Set up is
Mugello 225 stage 5
30 mm dellorto open bellmouth
devtour exhaust
Varitronic electrics
Fuel mixed@ 3% with 100% synth oil
Sorry for rambling, and I appreciate your time and help with your thoughts and answers
Cheers, Dave.

[Stampede]

Have you tried another CDI?

[thackers]

No mate I haven't. The Varitronic system has different cdi and regulators and I don't have spares. I hadn't thought of electrics as it starts as usual and runs ok from cold, it's just when I hit slow moving traffic the problem starts and the little cough at higher speeds mostly up an incline which I forgot to add
Cheers for the fast reply.

[paulnobodyimportant]

It might be that the grade of your spark plug for a tuned motor is running too cold in heavy traffic. My tuned scooters do the same if I'm stuck in slow moving traffic and then clear as the revs blow out the unburnt fuel and try to clear the gunge from the spark plug.

In general, I use as B8 plug in my tuned motors and may change to a colder B9 plug for long motorway runs. If yours is this type of plug, try changing to a hotter B7 plug, but only for around town etc.

Until I started looking at spark plugs on Google, I didn't realize how much you can interpretate from them. The link below will give you some food for thought, even if the spark plug isn't your problem.

http://honda-tech.com/showthread.php?t=3063102

[molibdimum]

Is it possible that the Varitronic ignition could be faulty and not giving you the correct timing Dave?
Ive got a very similar problem with my TS1....runs very rough at slow speeds oiling the plug and I'm going through plugs like nobody's.
I have an M-Tech fitted but I really need to check my timing.

Bye the way I'm running a B9 and thought of trying a B8.

[Donnie]

mollliiiiiiiibbbbbyyyyy!!!

"I have an M-Tech fitted but I really need to check my timing"

Ye're asking for trouble surely???

[Donnie]

Im presuming Thackers has set up the timing as per the instructions and strobed it to ensure it is retarding as it should, and if so have you rechecked it to see if it still is to eliminate this?

[molibdimum]

mollliiiiiiiibbbbbyyyyy!!!

"I have an M-Tech fitted but I really need to check my timing"

Ye're asking for trouble surely???

Nope....I need to make sure the stator hasn't slipped from where it was set when I put the M-Tech on.

[Donnie]

ah! Phew!!

What did you set it on originally then?

[Stampede]

I would change the spark plug first (against a new and perhaps hotter one). If that doesn´t help, I would strobe the ingition, if it fires where it should.
Do you have a second ignition in your shed which you could try or borrow one from a mate?