Modern Vespa

My motor is a 208cc SS180. My seller recommended a 4% mixture.

I would say it's running 80-90% great, plenty of power while not being lean. However, everything is kind of oily and black, with a lot of black, unburnt residue around the exhaust port and a black spark plug (B8es). So I don't know if it's:

Too rich pre-mix
Too rich jetting
Too cold a plug

Thanks

208cc? How many oversize on the piston?!?

All 2-strokes are sloppy, messy, particulate makers. In my experience piston ported bikes even more so. I run about 3.5% on my SS as modern fuels (and I run ethanol free) are so much better than what they used 50 years ago, but 2% is definitely not enough IMHO. Get a CHT gauge and watch your temps closely.

the GS/SS called for 5% when new with old 30Wt oil

in my GS I run 4% Motul 710 or 800RR

Not sure. From the dude's sales pitch: "Piston is a Mark Broadhurst 208 (Lambretta piston)."

It might be worth contacting MB Developments, Mark Broadhurst's business and asking them what they recommend.

I think the biggest issue is the piston ported design. It is harder to lubricate the big end bearing because the charge does not enter directly on it. That is why Piaggio recommended 5% for those models IMO. The cylinder is not a problem if sized and jetted correctly. I have seen many GS160 and SS180 big end bearings wear out from to little oil mix. Stock rotary valve engine cylinders have no problem running 2%. The only difference is the induction method.

I would use a happy medium with modern oils - 3 or 4 %.

I've run 3% on both my GS and SS for years without any problems.

I ran my Lambretta TV175 at 2% with no issues for many years. when I started taking it on very long trips I switched to 3% but likely didn't need to.

I'm not aware of any data that proves lower ratios of modern oil are sufficient in the old engines, but personal observations have shown me that the lower ratios of modern oils don't seem to do any harm to the old engines.

This alone is probably reason enough to just stick with 4%. I'm tired of working on my bikes.

This reminds me of an magazine article by Gordon Jennings detailing the (somewhat) controlled dyno runs on a Suzuki 250. The gist of the article was that a 15:1 ratio produced the highest BHP with no appreciable carbon build up compared to 20:1 and 30:1

http://www.bridgestonemotorcycle.com/documents/oilpremix6.pdf

I think the salient point that will escape many is the attention to tuning the carb with each iteration of the experiment. It's a sore point of contention with many in the chainsaw world regarding the need to re-tune changes to oil/gas ratios.

This is an excellent article. Thanks