Lean

[scooter_italiano]

Hello, I've got a long time problem with my GP200 standard engine.
Standard 200cc barrel, 22mm Dellorto carb and varitronic, B9ES spark plug.

At high revs I've got detonations from the exhaust and a grey spark plug.
I've changed the main jet from 118 to 120 and the atomizer from 5899-2 to 5899-4. Now the spark plug is black and oily very difficult to run at low revs even with half a turn mixture screw but still lean at high revs!!

how is it possible?! and what to do?

grazie!

[eden]

check the timing with a timing gun!
check the fuel flow!

[Adam_Winstone]

#4 atomiser is weaker than 2 at most, if not all throttle positions (exception being full throttle BUT only if you had a massive induction pull that could make benefit of the larger central drilling... which it will not do). When going for a weaker low rev atomiser (e.g. 4 or 5) you normally then go considerably bigger than standard. I am one of many people with road and dyno experience of standard 22mm carbs using a 4 atomiser with 123 to 138 main jet (dependent on other elements).

A good pal had his standard SIL GP200 with 42mm clubman and standard GP induction dynoed to use #4 with 123 main, which has not missed a beat since, with better MPG, cleaner running through the range and considerably higher top speed, as it no longer bogs out at 3/4 throttle. I could list out a number of other mates running similar setups too.

Ignition timing may also be a factor but I note you are running a non-standard ignition and the auto advance/retard function means that you should not be judging your jetting according to standard settings, which will be quite different because of the standard static ignition.

Adam

[scooter_italiano]

#4 atomiser is weaker than 2 at most, if not all throttle positions (exception being full throttle BUT only if you had a massive induction pull that could make benefit of the larger central drilling... which it will not do). When going for a weaker low rev atomiser (e.g. 4 or 5) you normally then go considerably bigger than standard. I am one of many people with road and dyno experience of standard 22mm carbs using a 4 atomiser with 123 to 138 main jet (dependent on other elements).

A good pal had his standard SIL GP200 with 42mm clubman and standard GP induction dynoed to use #4 with 123 main, which has not missed a beat since, with better MPG, cleaner running through the range and considerably higher top speed, as it no longer bogs out at 3/4 throttle. I could list out a number of other mates running similar setups too.

Ignition timing may also be a factor but I note you are running a non-standard ignition and the auto advance/retard function means that you should not be judging your jetting according to standard settings, which will be quite different because of the standard static ignition.

Adam

non ho capito....
do you mean that I should run a bigger main jet with #4 atomizer?
I've tested a 122 main jet and it was impossibile to go low revs around town.
With a standard 118 main and #2 atomizer it's lean at high revs and for long distance run too, perfect around town.