Modern Vespa

Hi, thoughts please on the spark plug after riding 500 miles, mostly urban circuit. Last 4 or 5 miles was riding home at low revs, idled for 1 minute before shutting off. I'm aware it may be inconclusive, this wasn't one of those WOT, clutch on, cut engine for a proper analysis of main jet, even so what do you think? Its black but not wet, cleaned it with a clothe on a few seconds.

Ps: Btw nothing wrong with the engine running, only pulled out the spark to install the new kick starter easier.

Thanks!

Update, forgot the setup:

GTR 125 ported engine, BGM Pro 177 (57mm), Extend intake Polini crankshaft, Polini ignition (variable advance), SI24/24E, AC160/BE3/140 main, 55/160 Idle, SIP Road 3.0 exhaust

Bottom line is: Keep jetting as it is or make it leaner?

All we can really tell is what idle looks like. I like idle or low end a little rich, that way if scooter gets a bit hot and I roll off throttle it gets more cooling oil and gas, instead of getting a leaner mixture.

Thanks for your feedback Christopher. So I guess the black we see in the picture *can* be mostly from the idle jetting? it does get dark fast and that's the reason why the WOT/cut test? Makes sense.

Regards

You are correct about cut test. A cylinder head temperature gauge is nice also.

If you'll continue to ride the same areas, your jetting is probably close enough. 500 miles and no issues means you're good enough for your conditions.

Are you planning on going to the countryside? Out to the desert? Freeways? If not, leave well enough alone and enjoy!

Hmmm.
Well - yes - kind of pointless to look at a plug and try to read specifics of jetting - when left to idle.

That said - the photo appears to show some small balls of aluminum.
Not good.
Detonation.

Given that its quite black - and it idled - I would guess:
Rich down low - lean up top.
You should be careful with that.

Aluminum balls on the plug means you are eating up the piston.
Its not major - yet.

If you want to provide a bit of detail - perhaps people can weigh in and help.
What is the jet stack?
Have you drilled the carb's float bowl hole?
What exhaust and carb are you running?

Thanks for feedback! The spark was black when I took it off. Those are dust I picked over my workbench, the bright dots at right is just clean surface of the spark without oil I incidentally touched and cleaned - but thanks for the warning, I'll have a particular attention on that next time I take it off ! Setup details on my post above, I don't think I'm running lean up top. Mechanic referred something about the float bowl hole not needing intervention as it was already larger? maybe a later model carb? dunno...

I have a question tough ... if I want to make a test and analyze the spark on up top revs and not letting it idle, do I have to clean the spark before the test or if I run WOT it will "clean" by itself due to the higher temperatures? sorry if this a dumb question ...

Thanks. That was my thought too, no issues so ... why mess with it? I just took the opportunity when pulling the spark out for the 1st time, I couldn't avoid looking ... But yeah, I think your logic is correct.

Well I have a DR180 and I cane it most days on WOT and the plug is perfect . However if I leave it running on stand or pootle about local slow it can look rich. As I'm more concerned about seizing at 60 mph plus I haven't attempted to iron out idle running rich. It sounds a bit boggy too in very low revs in third but I don't think I can get it perfect chocolate brown through all running conditions. All I know I'm unlikely to get a poor jetting seizure, so live with the slight gurgle low revs and this black looking plug you show.

Impossible to read a clean plug.
Colored plug will change color in 30 seconds - which is why all over the internet - they show people killing the motor and coasting down.

Best to perform in a place you can have some load on the motor.
Think slightly up hill in 3rd gear for example.

Before you perform - you want to make sure you're close...
Ask me how I know...

The hole in the bottom of the float - is best measured with a drill bit.
If you can't fit the butt end of a 2mm drill bit in it - well - then - its not drilled... for the record - if carb float hole is not drilled - that 140 is doing nothing for you.
and with that 160 AC - you will need about 140 or real life main.
I wouldn't run it hard/wide open without making sure that float hole is drilled.
Safer play on jetting is to use a 120AC - which will take your main down dramatically - ensuring your float hole can feed it properly.

Thanks Charlieman22 for you feedback!

I am relying in mechanic words saying the carb didn't need to be drilled. He was building another Vespa at same time, "same" carb SI24/24E, and that one he drilled. I did run WOT a few times (and for a few miles straight!) and didn't notice anything strange, but you never know when you're walking along the cliff, right?

If the main jet is 140 (1.4mm), does it really need a 2mm hole for fuel to flow freely? if the hole was i.e. 1.5mm 1.6mm ... wouldn't that be enough? I don't know the size of the original hole tough

Regards

There is fluid dynamics at play that I'm not certain of - which cause this apparent discrepancy. It may be that the two orifices combine in some fashion to restrict flow, or relate to their proximity to each other, or the 90 turn, but it's real and it will be apparent if you drill.

The original hole is 1.5mm I believe.
It will feed up to about 120MJ without starting to restrict too badly (but with some restriction), in my experience.
If you drill your carb to ~2.2-2.3 mm, your main jet will fall down to about a 124.
That's how much restriction you have now if your carb is un-drilled.

Wow, couldn't imagine it would have such an impact!
I'll definitively address this subject with the mechanic, thanks for the insight
Regards

Can confirm the phenomenon PSMA -- years back when I first waded into this stuff, I got a Polini 210 touring cylinder and stepped my MJ up all the way to a whopping 142. I was mystified how this gigantic hole wasn't making the engine gurgle and drown. Only after drilling out the SI float hole to 2.5mm (which is overkill according to many) did that 142 have the expected spluttery effect. Turns out that setup was *actually* needing a 132, 130 in the summer.

I've done a fair bit of elevation changes on a PX, like 4000 metres in both directions in a day, and the effect is dramatic. To go from 1000m above sea level to 5000m, the engine becomes essentially undriveable unless you step the MJ down 10-12 sizes, and even then the 'sweet spot' is very narrow. Point is, these things are *sensitive* and you've probably got some artificial jetting going on with that float bowl hole.

Thanks Jim. Very informative

I asked the same question a year or so ago. Doesn't make sense, does it?

Jack221 replied the biggest reason is better fuel economy
My guess:

Main jet is an orifice. The channel is a tube, subject to friction. Fluid running through a 1.5mm tube isn't the same as fluid running through a 1.5mm nozzle.

Using garden-hose fluid dynamics, a larger diameter hose will feed the nozzle more efficiently at a given pressure. That doesn't mean the water will not flow freely(up to a point) with a narrower hose, but will take more water to get the same spray power at the nozzle.

Yes Ray, that makes sense. Anyway, a larger channel won't harm a thing and its better to play in the safe side. Thanks