Modern Vespa

Hi all,

There has been tons of posts around this subject but I can't find anything about the strange (or maybe not) idling I'm getting.

The engine: PX EFL Kickstart 125 with Malossi 177 Sport Aluminium Kit

Pilot Jet: 52-140 (2.69 ratio)

I've adjusted the mixer screw to the point the revs settle as best as I can get it. Any further in, the revs become more unstable, much further out it starts to get boggy. I've adjusted the idle screw for a reasonably low RPM. When I blip the throttle it picks up fine. It settles to idle quickly after revving.

However I get weak double pops every 4 or 5 strokes and it wont fully settle at idle. Idle revs tend to fluctuate in a cyclical fashion every 15-20 strokes.

I've tried it with the old 48-160 (3.33 ratio) idle and this is way too lean. I adjusted the mixture screw as far out as it goes. It doesn't settle at all and takes a long time for the engine to settle to idle after revving.

I've got a video, if anyone wants to see it, I can send a link to download it.

I've ordered a 52-120 pilot jet to see if that makes any difference. I'm not confident though as the standard procedure of idle adjustment seems to have worked apart from the odd idling.

Is this a sign of something wrong or nothing to worry about? Any thoughts appreciated.

Is your carb warped? Could be that the slide doesn't return all the way back to its set point. Some one will be along with the jetting question I'm sure. I can't help you there as I struggle with that.

Jet is too lean. But on these carbs it could be the main jet or the pilot jet.
If you have one, how many turns out is a 55/160?
What's the rest of the jetting?

FridayMatinee is a brand new carb so would be a shame if it was warped. I've checked the slide and seems to return to closed position ok.

Jack221 about 3 turns out on the 52-140. Went all the way out without settling at all on the 48-160. Haven't got a 55-160 but this is leaner than the current 52-140.

Main stack - Air corrector 160, Atomiser B4, Main jet 115

I should also say it is a SI 20/20 carb.

Try a 140 air corrector and also make sure your ignition timing is correct and not missing.

It sounds like some electrickery may be complicating your idle issues.

And please share the video. It would be a massive help diagnosing.

https://we.tl/t-te3x6ZRFYi

We transfer link for the vid.

Thanks all for responding!

Here it is for those who don't want to download:

Thanks very much for Youtubing it chandlerman! I'm a bit of social media phobe

Also to say, I did set the timing to the recommended 18 degrees BTDC.

Listening to the video it's quite a lot too lean. Sounds like only a jetting issue.

Change the Air corrector to a 120 first. May get away with a 140 but I doubt it.

The dividing thing on the pilot jets is not an exact science. Like with 55/160 and 48/140 being nearly the same number. They don't ride like they're the same number.
When at tickover the 55 bit is more in play. You need a jet with more small number. Get the main jet stack done first. It's all going to change.

Thanks Jack221 I'll give both of those things a try. The permutations are mind boggling!

If you are rotary intake, Malossi suggests cutting the crank and extending the intake opening. Did you do that? According to Malossi, it makes a 10 point difference in the main jet.

Also agree with others suggesting you go back to an AC140. The rest of Malossi's jetting suggestions assume you will retain that.

Cheers for the response Kowalski!

I put in a BGM touring pro crank with a larger opening recommended for tuning kits. Good to know on the main jet. I think the original was a 102, so I'll get some options around that +10 (I've currently got a 105 and 115).

P.s. I read an epically long thread you created about bogging issues for your 177. Lots of good info in there. Did you resolve and what was your solution in the end?

Solution in the end was just using the BE4 mixer supplied with the kit. Once I woke up to the necessity of doing that, the rest fell into place. I have the cast iron kit on LML reed cases, so YMMV.

With a rotary intake, the +10 is supposed to be what gets you from the MJ105 to the MJ115. If you pair it with an AC140, the MJ115 should be pretty close in your case.