S2 struggling at higher revs

[onefortheroad]

Hi

I have just finished my Li125 Series 2 and got it MOT'd and on the road. It's had a full resto and engine rebuild and it kicks off and ticks over well. It seems to struggle however when at full throttle. It sets off fine, accelerates and then loses power and begins chugging as the throttle is opened fully. I change gear and acceleration is again fine and same thing happens. It seems fine lower down the revs.

It is s2 125 engine with 150 top end (as far as I know) electronic ignition, standard 19mm Spaco carb jetted for 150 breathing through the airbox as standard. It has a standard s2 casa lambretta exhaust (although it was marked tv175)

Is it
a) a carb/ jetting/ fuel/ airflow issue- ie too much fuel or not enough getting through
b) something to do with the exhaust (are s2 tv175 exhausts different to 125/150 ones)
c) something else- timing, electrical, mechanical
d) the engine just naturally topping out and my expectations are too high for a standard engine/ carb/ exhaust etc.

All suggestions greatly received

Dan

[coaster]

Well the 150 top end won't give you road rocket performance but it shouldn't bog down at full throttle. A tv exhaust wouldn't cause your problem although a blocked one might (unlikely?). My money would be on carb/breathing issues, what size main jet is fitted?

[onefortheroad]

I'm not sure exactly- it's a standard 19mm Spaco copy of a dellorto jetted for a 150. I basically opened the box, made sure it was all tight and put it on!

Maybe I should take a look?

[coaster]

IMaybe I should take a look?

Definitely

[desi]

Having similar issues with S2 Li150 with 175 conversion. Jets are 45, 50, 118 any not sure whether its too much or too little fuel (rich) at full throttle. Putting on choke doesn't help and just adds to prob, not sure whether that indicates I need to go up or down on the main jet size. So got a 115 and a 123 and going to see where that takes me. Anyone know what it is that the choke pull is indicating I would be grateful. Would help to know in this case too.

[onefortheroad]

Main jet is 78 and plug is black but not oily.

Gonna see if I can find the next jet down and try that

[Morgan]

What about petrol tap ? maybe blocked and not giving enough flow ? Sticky's book gives an idea of what the flow rate should be.

Or air restricted, give it a try with bellows of (open mouthed) and see what happens.

When it starts struggling stick the choke on and see if that makes any difference.

report back !!

[onefortheroad]

Just changed the main jet from a 78 to a 73. Scooter now doesn't bog down when the throttle is opened fully but is now ridiculously slow not exceeding 25-30. I've packed her away for the afternoon cos I'm off to the footy but I will try again tomorrow!

I adjusted the air/fuel mix screw with slight improvements before changing the jet- to what extent does this screw affect running with the slide fully open?

[coaster]

I adjusted the air/fuel mix screw with slight improvements before changing the jet- to what extent does this screw affect running with the slide fully open?

The air screw only affects up to around 1/4 throttle, does your carb have an adjustable needle? (I'm not clued up on S2 carbs) if so, try moving the clip up or down.

[Morgan]

Quote: [i"] when the throttle is opened fully but is now ridiculously slow not exceeding 25-30."[/i] are you sure it's not a Vespa ??