Modern Vespa

Good afternoon.

I have a Vespa PX 150 fitted with a Pinasco 60mm crank, Pinasco Vespone 190cc kit and 20/20 carb. Jets are 140/BE5/112 with 52-140 idle. I am just running the engine in. I have a Polini Road exhaust fitted.

When I rev the engine, the revs take longer to settle back to idle than normal. I have been told this is an idle jet/mixture screw setting. The mixture screw is 3.5 turns out.

I changed idle jet to 50-120 to richen it out a bit. However, this has made the issue worse. The revs tend to hang for a good 5 - 10 seconds before settling. Sometimes longer. All carb gaskets are new so I don't think it's an air leak.

I am confused how a richer idle jet made the issue worse.

Any ideas?

Thanks.

Air leak?

Seals are all good. Everything seems fine. Although I have not pressure tested it as I have no means on doing so. How would making it richer increase the revs? Surely this would counteract the air leak and balance it out?

Next time you try, rev up and before the revs totally settle back to idle, pull the choke. The choke is an enricher. If the revs immediately come down, the motor wants more fuel. If it bogs and dies, it's getting too much fuel and your idle jet is probably good. Make sense?

Hi.

At idle when I apply the choke, the engine stalls after a few seconds which leads me to believe its not running lean. Thank you for the advice.

Did you rule out a binding throttle cable? Does it do it with the throttle cable disconnected? I would start there and then move on to the carb itself if throttle cable and linkage check out.

Hec

Hi.

I have checked the throttle cable and it appears fine. I will try without the cable connected though just to rule it out. Thank you for your advice.

The engines slows right down and stalls.

Also, when I increase the revs, then back off to zero throttle, the engine pops a lot on the overrun. Is this normal for a two stroke, or is this the sign of lean running? If I run with slight choke, the low revs are boggy but as the revs come down the engine does not make the popping noise.

I have just screwed in the mixture screw all the way and the engine kept running. I then screwed it almost fully out and the engine kept on running with very little difference in revs between. WTFF?

did you do the test yet with the throttle cable disconnected?

Also, just throwing stuff at the wall here, but you're testing the mixture screw on a good hot engine and you're waiting a while after each bit of turn before determining if it did anything, right? Because it's a a slow effect

Yes I did the check with the cable disconnected. I am waiting for the change. The engine was nice and warm after a good run. Normally takes a few seconds for the mixture screw to take effect but nothing. Very odd.

I think you have a clogged idle circuit. I'd pull the carb and completely disassemble it and soak all metal parts in a boiling pot of 25% simple green and 75% water. Boil it for 30 mins and then rinse the carb thoroughly. Blow out all passages and orifices with compressed air and reassemble.

Good luck

I cleaned out the card as best I could with carb cleaner and compressed air. No difference. I have gear oil leaking from the carb area? Weird. Also, when I remove the idle jet when running, the engine dies which is good.

are you sure it is gear oil from the carb area? My guess is it's the typical skanky 2t oil-fuel stuff dribbling out the back (typically around the adjustment screws)? Pics?

Do you notice puffs of vapor blowing out the top of the carb when the engine speed is winding down after revving? Try this, hold a clean sheet of white paper over the top of the carb with the air cleaner removed. Give it a rev and see if there is any evidence of wetness on the paper. If so the rotary valve clearance is too big. This condition would allow unburned fuel/oil mix to escape through the carb. The fuel will then evaporate in the carb box and leave the two stroke oil behind. Maybe this explains the oil you are seeing? I am new to the Vespa Scene so I may be out to lunch on this. Just thinking out loud.

Hec

With the air filter off, fuel does blow out of the carb but I was told this is normal to some extent. If this is an issue, how do you reduce the gap and bring within tolerance? Not too sure about the oil. It seems to gather near the mixture screw and dribble down.

Do you have a YouTube account? Film it and post the YouTube link on here. Otherwise we're gonna keep guessing.

I replaced the idle jet with a 50/120 and turn out the mixture screw 3.5 turns. It seems to run a lot better now. Still not perfect but better. I am sending the bike to a Vespa specialist to get it checked out. I don't want to just mask the issue by making it richer. I will update you when I find out what the issues are. Thank you once again for all your help.

Here is a good video of Rotary Valve Repair.

Save your money. 50/120 is functionally leaner at idle than a 52/140. Put in a 55/160 and then see what happens. All will be near perfect I'm sure.
If anything is actually wrong with it it will be very obvious.

I was told that the new idle jet is richer if you divide the top number by the bottom. The lower the number, the richer it is. This is getting very confusing. Is that jet stack incorrect or pretty safe for hammering it? Why is is the mixture screw does not really do much? Fully closed engine runs. Fully open engine runs.

The pilot jet dividing thing is true but not even half the story. Not surprised you're having issues.
That vespone kit is probably the RX version, which is the best of the range and has a lot of potential if set up well.
To make it go fast and not damage it, the jetting is going to need to be much richer. The 20/20 is small for it but sure you know that already. The current jetting will wear it out before it's run in.
The stock filter can't cope and has to go, run with either a vortex or nothing. AC120/BE4 is not for discussion. 55/160 pilot and main jet at 130 to start with. Hopefully you have the squish at 1mm. Timing set precisely. And a SIP 2.0 tap.
Polini exhaust is restrictive for this kit but it will help it wheelie well.

I bought the bike with the same kitted fitted but it seized on me. It had been built by reedspeed and put on the dyno with the jetting as: 140/BE5/105 with 52/140 idle. It has a upgraded chamber cover and hole in carb for main jet drilled to 2.5mm. It seized on me at WOT after a good 5 or 10 mins. Massive soft seize on the intake side of piston. It was well and truly melted. I have upgraded the main jet to 112 and the plug colour is a medium brown. It feels rich when pulling away as it splutters a bit put then picks up nice. I cant see how reedpseed would set it up incorrect and a main of 130 seems huge. I have fitted a sip 2 tap. Running autolube with 1% extra in tank. Squish is quite high at 1.4mm. Cant get it any lower unless I remove the base gasket. Port height is good for all ports. Timing on stator is retarded as far as it will go to the IT setting. Compression is 170 PSI cold and 155 PSI after a run and warmed up. Its a 2016 PX150E.

Something must be getting lost in translation. Nobody would put a 105 main jet on a "190" kit.

Well that's how it came to me. I put around 600 miles on it before it seized up. If it was that lean, it would have seized a lot sooner than that. I am very confused on the jetting setup. Considering it was put on a dyno, kicking out 17HP with that jetting, it must have been ok.

A Dyno is a tool. They only seize and melt when seriously lean. Not just a bit lean, like 20% lean.
Not saying 130 is the jet, just that with the AC120/BE4 it will be where to start from.

The 2016 model has a rev limiter on the CDI. I take it this has been changed? 1.4 squish big but ok for now. Rest sounds good. Carb drilled especially. Timing needs to be set precisely with your own timing marks.

Will never go properly with the filter in the carb box. Needs to go in the bin.
Get it rejetted it will be fine.

So I have been told that the dyno was complete with a normal air filter on the bike. It now has an air filter with the holes above the jets. So the guy has told me to upjet it to at least 120 and go from there. I find out new things about this bike every day. Yes the ignition has been changed to Zeus unit. I have ordered some new jets so they should be here soon. No wonder it's been a bit off.

If you keep any original style air filter it will end in tears. This kit is way beyond stock filter setups.

Is it ok to run with the filter completely removed or what filter would you recommend fitting? Thanks.

As long as you put top on and snorkel to frame no filter is fine. Keeps big chunks out of system. I installed this bell mouth which helps correct airflow. I was skeptical, however when I had to go up several jet sizes I was convinced, something like 10 sizes.

https://www.shapeways.com/product/ZFBJ7HCAE/20mm-largeframe-vespa?optionId=451888&li=marketplace


After you get this working, pay attention to when and how often you find yourself at wide open throttle. You should be able to run a 24mm carburetor no problem, will bolt on. Intake area is 25% bigger allows more air flow to utilize that kit you have better.

Side note.. have you noticed any difference between running no filter and with this bell mouth? My understanding is there is no real difference. I run with no filter on my 177 tuned Stella.

I have just drilled out a main jet to 1.3mm which I believe equates to a 130 jet? It has made a big difference. The bike will only rev to 6500 rpm and is very smokey. It also ticks over a lot better but the bike still runs with the mixture screw right in. The revs go up but it still runs. Where is it getting the fuel from if the mixture valve is closed. On the main jet side of things, 130 is too big so I will work down from that. Maybe a 120 or 122.

I am going to invest in a 24/24 carb. I will have to then start again with the jetting. I have no real idea where to start. All jets will need carefully selecting. I may be back on here asking for help. Lol.

If your main jet is sooo big, it will bleed into the idle at idle and prevent it from stalling if the air mix screw is all the way in. But you need to find your correct main jet before you start worrying about fixing the idle that won't die with af screw all the way in. Just set idle to a safe idle jet size with the af screw set correctly for your carb.. 1.5 turns out on newer carbs and 2.5 turns out on old carbs.

Both circuits are connected so you need a generic baseline idle that you leave as constant until you get the main closer. Once you get the main closer you can go back and fine tune the idle.

It still runs with the screw closed with an 112 main so I don't know what is going on. Maybe an air leak somewhere. I thought it was 2.5 turns out for newer carbs with the finer thread on the 7mm screw?

Can never keep it straight in my mind.

It's getting picked up next week for a guy to sort it out. I have reached the limit to what I can do. The issues just don't make sense anymore. Pissing me off. Lol.

What size air corrector with that drilled jet? Filter off?
Idle won't work until you fit a 55/160

Keep at it plenty of us here

It's a 140 air corrector and BE5 tube. Filter off with just the cover on.