This ones about beat me !

[cobby56]

Now I don't profess to know everything but I've a fair bit of experience with Lambrettas, built quite a lot of engines etc , got over various problems as you do , but this has about done me. Put together an Indian GP last June for a bit of rally work but also as an everyday ride. 2nd hand engine I rebuilt with new bearings, seals, decent chain etc and brand new Rapido 225 top end. Must have put 6 or 7 motors together of this spec with PHBH30 and JL. Did a fair few rallys including some reasonable distance ones and rode it right through the Winter, everything fine until after the Morecambe pre season in March, must have covered around 5000 miles all in when this issue first arose.
It starts fine, ticks over fine and pulls away fine, but if you give it a decent hand full it splutters, seems to cut out, judders etc, if you ease off the throttle its OK, you can also work through this and its fine -- until you change gear, it does it at the same or similar throttle position in each gear.
Changed the whole ignition system as similar to previous problem on another scoot, where it was breaking down under load (BGM stator, BGM CDI, AF mid weight flywheel) also put on new HT lead, plug cap, plug, still the same.
Tried another carb off another of my scoots in case there was a problem with my carb I couldn't detect, this carb is exactly the same spec, still the same.
Fitted new fast flow fuel tap in case fuel blockage, still the same.
Replaced front sprocket spring and also clutch pressure plate and steels which were showing some wear, still the same.
I've had top end off, nothing obvious, piston to bore clearance and ring gap still within tolerances.
I think I'm going to have to completely strip motor but just wondered if anyone had any ideas or suffered similar symptons, If I hadn't done so many miles I don't think I would be so stumped.
The only thing I haven't mentioned is the crank which is an AF race crank that came with the motor, but I don't know why that should be the problem as its run fine for 5000 miles.
If you've managed to get to the end of this without falling asleep thanks, any pearls of wisdom greatfully received

[Donnie]

Carb mate, I'd put money on it. Yes I know you are going to say you replaced the carb for another one, but unfortunately you also said it was set up exactly the same.

So, I'd say there's an issue with either the atomiser or the needle personally.

[cobby56]

Carb mate, I'd put money on it. Yes I know you are going to say you replaced the carb for another one, but unfortunately you also said it was set up exactly the same.

So, I'd say there's an issue with either the atomiser or the needle personally.

I would as well mate under normal circumstances, but I've tried the same spec carb off one of my other scoots also I fitted brand new needle, atomiser, jets etc in both carbs to hopefully rule that out. The second carb is also running bang on my cutdown and I've done 30000 miles on that baby .

[Wack]

The AF cranks are known to twist which could knock the ignition out at certain revs? Have you checked out the loom and ignition switch and that the throttle pulley is not catching any wiring?

[cobby56]

The AF cranks are known to twist which could knock the ignition out at certain revs? Have you checked out the loom and ignition switch and that the throttle pulley is not catching any wiring?

Hi Wack, thanks for input. Checked loom which was new and happy with it and all earths, brand new Casa ignition switch fitted a couple of weeks back (forgot to mention that in original post) Think you might well be on the money, I don't know the history of this crank as it came with the motor when I bought it, guess it could have twisted as its been ridden quite hard at times. Think I'm going to have whip motor out and try another crank / bearings.
Looks like I'll have completely replaced everything then if that don't work there's a lake over the road from my house

[torquewrench]

Before you head over to the lake….
If everything mechanical and electrical has been checked and is O.K., and the problem seems to be linked to throttle position, have you considered that fuel starvation might be the issue ?
If so, could it be as simple as a blocked breather in the filler cap, which is impacting your fuel flow rate at higher revs ?

[cobby56]

Before you head over to the lake….
If everything mechanical and electrical has been checked and is O.K., and the problem seems to be linked to throttle position, have you considered that fuel starvation might be the issue ?
If so, could it be as simple as a blocked breather in the filler cap, which is impacting your fuel flow rate at higher revs ?

Yes considered it. Both carbs used stripped and blown out with air line, new fast flow tap fitted as mentioned, new fuel line used. inside of tank checked for debris - clear. Also tried a couple of other filler caps off other scoots. Thanks for suggestions. Its hard to cover everything I have actually tried since March as its been ongoing. Dead frustrating as its my daily ride, it is ridable - just. Lakes still tempting

[torquewrench]

Lakes still tempting

In that case, you should be ecologically sensitive, and drain the fluids first.

[Rich Oswald]

Hi Cobby, have you done a leak down pressure test. There just may be a hairline crack in the casings that you can't detect. Just a thought mate,

Rich'

[MickYork]

........new plug ?

I'd maybe consider a Dyno session, might save some hassle

[cobby56]

Rich - could be an option, I did a leakdown test when I built the motor, might be worth doing another.

Mick - not sure what a dyno would achieve, the carb set up is right proved by the fact I've previously done around 5000 trouble free miles (and nearly 30000 on my other identical set up), the timing has been strobed at 18 BTDC. What I seem to have is a failure somewhere that has developed over time, not sure a dyno would identify this ? Oh yeah plug, must have tried a dozen since this all started
Thanks for your input lads, its all appreciated.

Reckon I'm going to try another crank, Wacks post underlined what had been niggling away at the back of my mind.

[Wack]

Just as a thought before you pull it apart why not strobe it at high revs to see if the marks break up ?

[Teglease]

I understand your frustration ! Bloody lammys , but being a fisherman can you be careful if you go down the lake route? maybe just slid it in slowly, also would it be the whole scoot or just the engine? as I have a spare that I could use if you let me know the lake it's in. (sorry light relief)

[Paulnobodyimportant]

Still think it could be carb settings. I've two tuned engines that have done lots of miles with same setup. Then, both a differening times have suddenly run rich and boggy. Posted a few threads on old forum, but in the end had to change, fairly radically the needle settings to get it to run leaner. And as I speak, some 1000 miles later, both starting to run rich again and spluttering at low revs.

So to conclude, i'd just play around with the needle settings before that frustrating strip down.

[MickYork]

The dyno session would conclusively (hopefully) eliminate any niggling doubts. I found it very useful when it helped identify loose magnets in the flywheel.

My gut feeling is that it's electrical. The likelihood of 2 dodgy Lambretta stators ??

[JETEX]

Have you tried running it with the headset top off ?. As said previously, if it's happening at the same throttle position, it could be that the throttle roller is pinching the wiring in a certain position ?

[foremanbob]

Have you tried running it with the headset top off ?. As said previously, if it's happening at the same throttle position, it could be that the throttle roller is pinching the wiring in a certain position ?

How would a pinched wire on the 12v supply side affect a distinctly different ignition circuit. I would have though that that would impact the regulator/lights/horn etc before causing a missfire......

[JETEX]

I was thinking along the lines of the throttle roller hitting the green wire, thus shorting it out perhaps ?. You know the thing, straws, clutching, at ( re-arrange ) !

[foremanbob]

I was thinking along the lines of the throttle roller hitting the green wire, thus shorting it out perhaps ?. You know the thing, straws, clutching, at ( re-arrange ) !

I could see that now..... or ignition switch/kill button being affected by vibrations at a certain rev range....

I guess disconnecting the green from the headset would rule that one out.....

[MickYork]

I'd disconnect it from the cdi to rule out all the cut out wiring.....