175 four stoking??

[Peterp]

Evening all,
Just finished my refurb and took it for test run, gently as a rebore and piston fitted but nothing else apart from increasing the main jet by one stage.
It may be me but it feels slightly off colour and runs a tad uneven on low revs especially after changing up. The minute the throttle is opened off tick over it seems to smooth out, but when I back off the the slightly it comes back. Lots of excess Lambretta popping. Could it be the larger jet or would it benefit from richening the mixture screw. I’m a little in the dark on this so any help gratefully received.

[hullygully]

you haven't said what carb & exhaust & timing bud?

[Warkton Tornado No.1]

Congratulations on getting this far, especially after you were ‘away’ for some length of time

Inevitably, you are bound to have set the ignition at a retarded amount to which your machine was originally set. Whilst that is necessary, it really is a grey area (IMO) as to the extent required/necessary. The thing is - in the case of retarding the static ignition setting on a two stroke - ‘less’ is sometimes ‘more’ in the instance of a relatively standard Lambretta engine.

Bearing in mind that the principle reason for retardation will be due to the difference in the fuel we use nowadays, I reason that with an optimised engine, very little retardation is required to function well. Habitually, many of us Lambretta enthusiasts have a tendency to follow the herd, so will often apply a principle that will be best practice ‘& then some’….A prime example is squish. An engine rebuilt with first class, new components, specifically bottom end bearings, crankshaft & little end really should be extremely safe to run 1 mm squish with a centralised head to piston with profiled head profile with no ‘squash-able’ gaskets involved or extremely high RPM. The best race engines will run even less than 1 mm as the importance of heat transfer is an important factor in ensuring reliability.

Ignition was 23° BTDC when your engine was built & - much like yourself - a friend, returning to the fold, rebuilt an engine using that setting as he was oblivious to current thinking. In the event, his engine had what can best be described as a ‘sparkling’ performance for which it had no right, though it had a PWK carburettor & a mass produced expansion chamber of unknown origin. If memory serves, I think final setting was 19° BTDC. That’s only 2° allowing for the better fuel atomisation of the PWK & the better exhaust.

BTW I may be a dinosaur in many respects but I really do know the benefits of a good ignition set up & was using the race Motoplat -that advances prior to retarding - decades ago.

I reckon that many of us need to ‘unlearn’ some of the knowledge gained via car/four stroke ownership as different rules apply. Ignition is one example but compression ratio & primary compression are often subjects where old rules are applied….

[Peterp]

Yes, I should have mentioned set up. It’s standard carb and exhaust, I did have rebore up to 63.4mm and obviously a new Gol piston.
Ignition timing 17btdc. As I was getting excitedly close to 185cc I upped the main jet by one thinking it would give me that little bit more. As yet I’ve still to put it through its paces as I’ve heard “four stroking” can be harmful to two stroke engines.
Today I put the original jet back in, I’ll try it tomorrow and see. I may have been premature asking bothering you fellow scooterists, we’ll see eh.
Being honest, I’m worrying and stupidly disappointed with my efforts.
I have to agree WT, as an ex vehicle mechanic/fitter I perhaps do overthink it. I may have mentioned earlier that my previous scooter experience is very dated, back to the 70’s, series 3 machines were quite new and I didn’t require refurbs. And, I had not become part of the tuning circuit.
I’ll let you know tomorrow’s outcome.
Thanks again both

[Peterp]

I meant to add that my reference to “four stroking” is what I think is happening. Open the throttle and it seems ok, back off the throttle and again seems ok. It’s when I accelerate and then back off slightly, what I call “trailing throttle” that it seems to be “poppy” and the scooter appears confused.
Thanks again,
Pete

[hullygully]

have you allways had the timing @ 17btdc?

[Peterp]

Hello Hully, yes I believe so. I bought it last year knowing it needed some tlc. Well, a bit more than tlc to be honest but there we are.
It’s now timed to 17 with a disc.

[Storkfoot]

To me, what you describe here is very like the classic pop pop sound of any single cylinder two stroke engine, especially one in standard form with a box type exhaust. Unburnt fuel and exhaust pulses have a tendency to ignite or echo inside the hot exhaust pipe when you come off the throttle. I do not believe that this happens in four strokes when you are dealing with valves rather than ports. That said, I have no experience with four strokes.

Four stroking in the two stroke world is different, in my experience, and tends to happen when you are on the throttle and either the ignition timing and/or jetting is incorrect at that point in the throttle progression.

From the video you sent me, when you are revving the engine hard, I do think you need to either clean out your box exhaust with a caustic soda solution to ensure it is not partially blocked or treat your scooter to a new mushroomless late SX/GP exhaust.

The air screw setting is a bit of a dark art but it is primarily very useful in identifying what pilot jet you need. From what you have said before, the engine ticks over nicely on no throttle so I suspect the air screw setting is close to what it should be.