running rough

[colsx150]

Hi, my sx150 has a mugello 186, 30 dellorto, af clubman and electronic ignition
its been on the dyno and set up and runs fine flat out but at low speeds it runs like a piece of crap.
at low speeds it splutters, sounds rough and feels like it struggling to move but as soon as you give it a bit more throttle its fine and rides ok.
was told that the jl 4 exhaust i had on it before was causing this so changed it for a clubman and its still doing it.
i dont know anything about engines and dont understand how its still doing it after being on the dyno
has anyone any ideas as to why its doing it, its driving me mad
thanks
ps ive read somewhere about 4 stroking, is that what mines doing ??

[dapper]

Did you change the exhaust since its been dynoed? Was it running rough after it was dynoed with the JL4 exhaust on? It sounds like it is 4 stroking at lower revs which points to the carb not being jetted correctly and running rich.

[Minority]

Always had problems with 4 stroking with my 30mm phbh (225 Rapido, JL3) at low throttle openings, swapped to a 28/30 mm Oko (Keihin PWK copy) from ScootRS and it's much smoother at low throttle openings.

[colsx150]

Thanks dapper & minority for your answers.
The scooters only done 350 miles since being rebuilt.
Had it dynoed when the jl4 was fitted, then it was dynoed again with the clubman fitted.
And both times it's had the same problem.
I will nip it back to the tuner to see if he can sort out the carb.
Might try a oko one if it's no better
Is a 30mm carb to big for a 186cc, Would it run better with a smaller carb on it.
Thanks col

[storkfoot]

There is nothing wrong with 30mm PHBHs "per se". Dell'Orto are one of the biggest carb manufacturers in the world.

I would hazard a guess that there are hundreds and hundreds of well running scoots in the UK with these carbs fitted. You may, of course, have a problematic one, in that it's old and worn out, but the point I'm trying to make is that before you just go out and buy another carb, especially one that is a copy with a somewhat chequered history, it is worth trying to alter the jetting.

Another point is that if it was dynoed the dyno operator would, surely, have checked the fuel/air metering to identify where, if at all, the carb was lean or rich

[Minority]

Thanks dapper & minority for your answers.
The scooters only done 350 miles since being rebuilt.
Had it dynoed when the jl4 was fitted, then it was dynoed again with the clubman fitted.
And both times it's had the same problem.
I will nip it back to the tuner to see if he can sort out the carb.
Might try a oko one if it's no better
Is a 30mm carb to big for a 186cc, Would it run better with a smaller carb on it.
Thanks col

Tonydevon has posted on this forum about his experience going from a 28mm dellorto to a 26mm on his 198 Mugello and gaining power & torque.
I went for the OKO because of the superior D slide technology (and it was cheap ) and have been very happy with the quality and the performance. There is a question on it's capability to flow enough fuel through the float bowl at full throttle, but there are some mods to help this and may not be a problem on a 186.
ScootRS also do a 24mm version which may be more suitable for your Mugello 186.