24mm phbl too big for a relatively standard Li150 ?

[duncan1958]

In an effort to squeeze more power from my Li 150 I have matched the inlet manifold to the cylinder and likewise for the BGM V4 exhaust.
24mm phbl
100 main
40 slide
D22 needle 2nd clip down from the top
50 idle
60 choke.
Modified air filter assembly and BGM bellows and metal gauze air filter.
Leak test done after replacing bearings and oil seals with the inlet manifold and exhaust stub on,all good.
Original points 6v ,although I have a 12v electronic system for the Li crankshaft if needed.
After a good search back in time through the forum and found some jetting guidelines I went with the above and the scooter would fire up and ran well although slightly on the rich side according to the AFR gauge, I have the lambda sensor on a sniffer type tube that fits over the end of the tailpipe.
Anyways this frigging thing is giving me a nosebleed/headache.
If it starts up it runs fine and pulls nicely through the gears,however sometimes it just refuses to start in spite of not touching it from having ridden it the previous day.
Fuel switched off ,fast flow tap that flows fuel although not checked flow rate.
I haven't messed about with the port timings aside from opening up sideways .
Head and base gaskets used.
Any suggestions from the collective will be appreciated.
Aside from the obvious of chucking the Li crankshaft and cylinder on the shelf and buying a RT 195 kit for it .
I already have the whole monty on my winter model.
Thanks for reading.

[Warkton Tornado No.1]

By now, most people will realise that I am not a fan of the PHB range of carburettors & one of the reasons is in my experience they are not filter friendly on a Lambretta. They do not incorporate a bowl overflow as other carburettors do. Even the VHB that the PHB 'replaced' has provision within the bowl to fit an overflow. Now you might call me prejudiced but the PHB was OEM equipment on the Foxeye Honda 125 that my son owned & in derestricted form was about 33 BHP with no starting or running issues. However, it being a Honda, the air filter housing was a huge plenum box, something not easy to arrange on a S3 Lambretta. The likelihood is that somebody will suggest drilling a hole in the hose, but why would you invite an unfiltered source into a device attempting to keep your engine running on clean air?

Perhaps other Forum users might contradict my findings - that would make sense as the PHB has a massive fan base - but I reckon there are much better, cheaper alternatives, one in particular being the PWK's that are easy to set up & do not have issues fitted with air filters.

[Fast n Furious]

Assuming that all is well with the engine setup I would remove the air hose and leave it overnight.
Next day switch on the choke and it should start fine with no throttle. If it does, then it's likely that there's a restiction issue in the filter tract making the choke over rich.( A 60 Choke jet seems a bit rich to me?)
If it doesn't, then it's most likely a carb, ignition or engine set up issue.
( Make sure that you only have ignition suppression in one place. Either the plug or the plug cap but not both! If your condensor is aged then its wise to replace it, for peace of mind if now't else)

[duncan1958]

Carb is running open mouth at the moment and this morning I removed the points stator and installed the electronic ignition system after checking the values with the ohms tester, all within spec.
Piston stop method to get tdc then timing wheel for 20 degrees btdc.
Checked for spark at plug all good.
Tried kicking with fuel and choke on few kicks and it fired but briefly.
Hand over carb and kicking again and got it running long enough to strobe check the timing and looks to be on target.
Engine off and it won't restart.
I am wondering if worn rings might be a possibility, getting replacements is a bit problematic.
Going to do a compression test next .

[duncan1958]

For those who have been reading/following this thread.
I did two compression tests, 2nd one with some 2 stroke oil down the plug hole.
Noticeable difference.
Pulled the top end off and something had got into the combustion chamber and damaged the edge of the piston at the edge.
Oil had got past the rings .
That's going to be the cause of my problem.

[MickYork]

Nice to get the update for future ref'. Good luck with the new piston

[duncan1958]

Latest situation is the new piston and rebore has sorted out the erratic starting.
2nd prod of the kickstart and the engine fired up.
Been for a few short road tests and all appears good.
No full throttle thrashing just yet

[martyn dwane]

I ran a PHBL 25mm ,open mouth on a 200cc with a RS clubman , main jet was 100,D22 needle,2nd clip, AV266 ,52pilot, 60 choke .ran slightly rich at 3/4 to full throttle, it`s surprising how small a main jet these carbs run.