Series 3 SH carb on Series 2 engine?

[rogscoot]

Hi, wasn't sure whether to post this in the carb section or here, so if it's in the wrong place let me know and I'll move it.

Went out on a long-ish run on my standard Li 150 Series 2 today to the Abingdon Oddballs rally, the scooter was running lovely for about the first 10 miles but then power loss and back fire at closed throttle.
Spark seemed fine and scooter would start up again when cool, but the whole day was a stop..start affair and had to nurse it home!

I think the electrics are fine and my guess is that the slow running circuit in the carb isn't letting enough fuel through when the engine is warm - almost like it's evaporating before it gets into the cylinder.

I'm currently running a SPACO MA19 copy which I'm guessing is the root of the problem, at the rally I picked up a SH1/18 carb for Li150 S3, if I change the manifold to an 18mm type is the carb a straight swap and a worthwhile upgrade? I've heard the MA19 type are often troublesome!

The other option I have is upgrading the original Italian BS5 carb (leaks like a sieve) to BS7 spec with the conversion kit, but that's £45 + extras.
Anyone have any other advice? Or already running a S3 carb on a S2?

Thanks,
Rog

[MickYork]

From the symptoms you've described i'd be more inclined to think it's electrical. maybe just check/change plug, cdi, stator first (i'm presuming it's electronic ignition).

[Tony Hart]

Hi Rog. Your issue certainly sounds electrical. I had something similar on a S2 which cleared when I upgraded the old and worn out points ignition. With regards your original question though I have always ran SH carbs with a suitable inlet manifold on my S2 without issue. Tony

[rogscoot]

It does sound like an electrical problem but I'm fairly convinced it's the carb because the plug looks dry when I pull it out and I've never not had a spark, I'm running points and condenser has been changed and Scootopia remagnetised the flywheel last year, I also had similar symptoms before where it would die pulling up to a junction and this was cured to a certain degree by adjusting the mixture screw, but I don't think any more mixture adjusting on this particular carb is going to help!

[Tony Hart]

Try it with the SH carb but when I had similar symptoms a few years ago I cleaned carb, changed fuel tap, fitted inline filter and cleaned tank because I was convinced it was a fuel issue. When I broke down I would fiddle with the carb and it would restart which convinced me even more it was fuel. I changed the condenser just to keep the people telling me it was the ignition quiet. Didn't help in this case because my points were old and worn. However when I swapped to an electronic ignition due to people's persistent recommendations the problem was solved. Try a proven stator and flywheel if you can. (I also had a spark every time I checked - something must have been going wrong when heated up and under compression).

Tony

[rogscoot]

Thanks for the advice Tony, yeah I think I'll go down the same electronic route as you if the SH carb doesn't sort it out, really not looking forward to running a new wiring loom in if I have to go that way

The tank is spotless and fuel tap is a decent fast flow one so not worried about fuel getting through to the carb only what's happening once it gets inside also just remembered that Scootopia rewired the stator for me too last year (to try and cure the cutting out at idle and replace a wire that I'd crushed) but that didn't make any difference (except bright lights after the flywheel was remagnetized!)

I'm crossing my fingers that it is the carb because it'll be a much easier and cheaper fix then.

I'll keep you updated!

Cheers,
Rog

[HxPaul]

You dont need to change the 6v loom if its in good condition,just swap the 6v regulator for a good junction box and fit the 12v electronic system.