Revs go up when I shut the throttle!

[ArmandTanzarian]

Your carb setup works? The pilot jet is the right size and it only happens after fast riding?

We've got often the same symptom here, one theory is that through the spitback on fastriding with open throttle, there's a lot of air/fuel mixture left under the panel, which keeps the revs high after closing the throttle till it's gone.

That thought did occur to me and the scoot that does it more has a remote filter and hose that could accumulate fuel vapour but the other scoot has an open carb and I cant see that there would be much left floating about at speed.

[Dazza]

Could it be all that fuel/ air rushing in at speed and at a prolonged rate cause a temperature reduction that might cause the slide to stick open momentarily?

[Doom Patrol]

Without wishing to derail the thread I have experienced something similar on a standard 22mm carb. You knock the throttle back and it continues to accelerate. Would this be a similar issue?

[ArmandTanzarian]

The story so far.

One scoot is a 240 mugello with a reed block that has recently been built up professionally and is suffering really badly with this problem. This scoot is now back with the dealer and I have every confidence that he'll sort it. He does after all have a dyno, a vast stock of carb spares and many years of experience.

The other is a bog standard Monza with a tmx35. That was suffering from exactly the same problem but nowhere near as badly. I upped the main jet to a 330 and then went flat out up the motorway for three miles. No nasty symptoms at all. I went round the junction and back the way I came but this time at a fast cruise of around 60 to 65mph. Pinkety pink, reving like a banshee when I shut the throttle and pull in the clutch. A quick roadside pitstop to raise the needle one notch and repeat. No problem flat out or cruising. It evens seems ok around town with no machine-gunning.

So, in conclusion, it would seem that the problem is indeed caused by the scoot running too lean and overheating when cruising at speed. You don't notice it on short trips or in urban areas because you don't hold the throttle at a set position for a long time and you aren't relentlessly building up more and more heat.

I am completely confident that it will now find some new and interesting way to break down. Lambrettas, why do we bother?

[soosh]

I had this problem and it was down to the fuel rate coming into the carb.Also had to raise the float height do to fuel frothing in the float bowl.This was on a dellorto which you can change the float needle valve.Also had it on a pwk which you have to drill it out bigger as you cant chage them.

[Timbo]

Without wishing to derail the thread I have experienced something similar on a standard 22mm carb. You knock the throttle back and it continues to accelerate. Would this be a similar issue?

I had this and it turned out to be a worn slide..........