Jetting problems following clutch change ??? im stumped :-(

[Donnie]

This is really stumping me, having some issues jetting my scooter following changing the clutch.

SPEC first.
Series 2 with GT 186 (extra boost port behind the reeds), TSR EVO Exhaust, Li150 gearbox, 16 / 47 sprockets, 30mmPHBH with the following jetting.

Main Jet 130
Atomiser AV266 or 268 i dont remember but better check i guess.
Slide 40
Needle X7 (bottom clip i.e. richest)
Pilot / idle jet 50
Choke jet 70
Float 8g
B8ES
Float valve is a 300.

So, having renewed my front sprocket and decided to change the rear one to a 47 from a 46 to see if i preferred the ratio, now ive been out jetting before heading to france for a run with Luton Lambretta Club next week.

I find that with the x7 needle on 3rd clip down with both a B9ES and B8ES it's lean, almost grey, but with the x7 at bottom clip ie the richest its very rich at lower revs but seems ok in the rev band, but its making for a terribly rough ride, Im actually tempted to put the bike back on the bench and put the 46 tooth clutch in again! but cannot see how this would be affecting the jetting if im honest. No sounds of head or base gasket leaking, everything looks ok, nothings loose etc, Im just stumped.

[Donnie]

OH, and when I try to go full throttle too quick the engine tries to stall and die so im sure its too rich surely?

[CHRIS in MARGATE]

Try plug chop after a sustained sprint in 3rd gear at fully wide open throttle. That will tell you if the main jet is correct or not. Work back from there and consider richer or leaner needle.

[Donnie]

been doing plug chops at half throttle to nail down the atomiser / needle, thats the thing, the jetting was spot on before the clutch sprocket change?

I think ill be putting it on the bench at the weekend as im working the next 2 days and checking other things eg timing etc etc and going over teh gaskets to ensure theres not a new air leak im missing

[foremanbob]

main looks a tad large....

[Warkton Tornado No.1]

You know that changes to the transmission won’t have caused your fresh issues, but we all know how these coincidences have us all clutching for straws...

If you’ve assembled your carb correctly (plate on top of the needle circlip...) then could it be a worn X7 needle?

[burnside]

main looks a tad large....

I'm running a 135 main on my GT186 with PHBH30, which was how Rich T jetted it. Mine is running on the rich side so will get it on the Dyno soon to fine tune, rides well but can be a little boggy in mid range just before the power band kicks in. This is my set up;

GT186 - 60/110 crank
Franspeed Race
Li150 16/46
PHBH30

Slide - 40
Needle - X13 1 clip + washer from the top
Atomiser - AV268
Main - 135
Idle - 55
Choke - 70
Float needle- 300

[jonno]

Im having similar issues after replacing piston.Could changing the front sprocket caused/revealed drive side seal air leak?

[Adam_Winstone]

If the jetting was OK before then I'd not think that the clutch/gearing change would be the cause of jetting issues now. I can only think that something else has been disturbed during the work.

Good luck sorting it.

Adam

[Scooterlam]

Going by my egt, my jetting behaves differently in 3rd and 4th.


i also see lower temps when the engine is loaded rather than not.
by dropping the gearing you have lessened the load.

just a thought.

[Donnie]

Thanks all, not had a chance to reply as just finished two mental 12.5 hour shifts so im up and raring to go today, going to get it on the bench first this morning and inspect everything too.

jonno, dont think just changing a front sprocket will damage the oil seal, shouldnt' be able to get at it to be honest.

I did very quickly do a little looking and thought about the fact that it was so burbly low down but not rich enough in the mid range so put in an X2 (richer than an x7 or x13) to cope with the mid range but put in an AS266 to lean off the very bottom and actually a very quick run shows it has cured the issue.
However ive not actually done a plug chop etc so could be stupidly lean.

Ill do that this morning and see, but whatever the result going to put it on the bench today, go over everything and even double check the timing too, might even retorque the head etc.

[Warkton Tornado No.1]

Donnie. Good Luck with it & thanks for the feedback.

Unfortunately, by the nature of the ‘beast’ in that Lambrettas are somewhere between a motorcycle & a road compactor, they shake everything, including delicate carburettor components...

Inevitably, the needle & atomiser tend to get overlooked, but I think you are on the right track now

[Donnie]

Well had a quick run on the x2 / as266 and actually still too rich As it was starting to rain decided id get it on the bench with a view to looking over everything, soon as i got the stator side panel off to check with multimeter I notice that the white from the stator to cdi was not connected as it's come loose!

Put that back on after testing the ohms and numbers are within normal, decided to check the timing with the strobe, yep, thats fine too!

So, have popped off the exhaust, ensured that the base, head and exhaust gasket are all ok, no evidence of blow on any and have retorqued the head which was fine. I moved onto the inlet and 2 of the 4 allen head bolts could be tightened up a bit more so could have been some air sucking in there round the gasket instead of just through the carb?

So, now, im going to put it back together and put the jetting back to original and start again once it stops piddling down, so likely to be tomorrow by the look of things :-/

[coaster]

Did you get this resolved Donnie?

[Donnie]

I did and managed to get to france no problem until i buggered the cylinder exhaust stub on a speed bump Was running great up to then. Thing is it was 6 months ago so cant remember what it was that I did to fix it