Modern Vespa

Spanish GS 150
Really difficult cold starting, will only start with a hand over the carb, the choke which is part of the air filter housing works but it won't start with just it.
Lumpy tickover, and won't run below 1300.
Won't take any load, stalls if you select a gear.
Compression feels really good, but I'll also test it.
Carb has been ran through an ultrasonic cleaner, it likely has some wear but no blockages.
Motor rebuilt around 10 years ago but not ran until recently.
Vape static ignition timed to 24 BTDC (3 degrees lower than standard, from other posts that seems to be the norm when converting to CDI)

Is it worth doing a leak test?
I've never done one and would need to buy/make a kit but with these symptoms I think it might have an air leak.

Edited to correct title, leak test not leak down test

You've likely diagnosed the issue - now it's time to find the leak.

We don't really do leak down test as much as leak tests.
It's easy to build a test kit - search it here and show us your results.
Lots of helpful hints on how to.

Search leak test rather than leak down test.

I've had a rake around the workshop and found everything pressure or vacuum related along with an old inner tube. There's no obvious way to join any of it together so I'll need to work that out. I don't really want to cut the compression testers hose.

Also checked compression which was 118 kg/cm2 compression from cold.
Spark plug looked like new so very lean.
There's an obvious leak at the head as it's dripping sponge.

You have half what u need there.
No need to trash it.
Get a cheap old fashion blood pressure tester from Amazon or the drug store.

Use the tube that screws into the spark plug hole from your compression test kit.

Get a small t fitting from the hardware store.

Use the tubing, bulb and the pressure gage from the blood pressure test kit.

Remove carb and exhaust.
Use carb box with rubber under it to hold down and block off inlet.
Use rubber stopper from hardware to block off exhaust port.

I'm not sure this bit will work "This tube. U will have to stretch the blood pressure tubing over the other end." but I'll give it a go or It might join and seal with tape.

Edit - glue lined heat shrink would work to join them and the glue melts again when reheated so it could be separated again.

Should I fix the leak at the head first? It looks very minor just oozes a few drops.

Fixing the leak at the head won't take very long and you might be lucky, if not carry on with the leak test.

I just made one yesterday. I used a piece of aluminum and a rubber gasket to seal off the intake with the carb box removed. Rubber stopper in the exhaust stub for the cylinder. Bought the blood pressure kit off Amazon for $16. This way was easy for me cause I have everything laying around the shop at my work.

Cheap exhaust plug for leak testing.

https://www.supplyhouse.com/Wal-rich-1218012-7-8-Crown-Rubber-Trap-Plug-Large?utm_source=google_ad&utm_medium=Shopping_tm&utm_campaign=Shopping_TM_New_users&gad_source=1&gclid=Cj0KCQiA5fetBhC9ARIsAP1UMgEZhVoznpEmOLoxj0drD1aoeZHbKPcCRgcfbuRFsH_BPWbkzFcVC6AaArkZEALw_wcB

I made a pressure tester with stuff I had kicking around the garage. Most of the leaks I found were in the works themselves, the cylinder or the reed block on LML engines. I will probably make something better as the tester itself always has leaks I have to fix each time.

According to a post on another forum that I found via Google the pilot jet should be a 60 not the 52 that is currently fitted. (MotoVespa 04m dellorto ua 19 S1)

Fingers crossed that the leak test goes ok and it's as simple as changing the jet. I'll probably not get to work on it until Thursday.

No pressure test yet, parts haven't arrived.

Took the head off and lightly lapped it to confirm it was already flat. It looks like it may have been skimmed previously, both the lip on the head and the recess in the barrel measure 2mm with calipers so I'm going to need to use some blue to confirm it still touches.

Annoyingly it looks like whoever rebuilt this motor just put a new piston in without even honning it, the last two stroke I bought was the same. It's also a few sizes over already at 58.5mm. I'll still probably run it as is to check what does and doesn't work before rebuilding or replacing.

Confirmed the lip on the head doesn't touch the cylinder for the whole circumference so no wonder it leaked.
Removed the cylinder, found a fin chipped off, a hole in the exhaust stub, and it clearly hasn't had the port work done that was claimed.
I don't feel ripped off, when I agreed to buy the scooter I didn't have any info about the motor and the price was good enough to gamble on it. It's only when I collected it that I was told it had been rebuilt and ported, but it looks like someone along the line has been ripped off.

I'm not sure what to do now, this barrel may be salvageable but as well as the damage it's already 1.5mm over standard, I haven't seen new ones anywhere but they might be out there. Quattrini M1XL obviously appeals but then I'd also need crank, clutch, carb, may need cases welding to be able to match the ports etc. this was meant to be a budget build first scooter just to try it out. Replacing the whole motor with a 3 or 5 port might make more sense, but everything I've seen for sale in the UK lately is either very rough (edited typo rough not right) looking and/or from India.

To clarify, it's a MotoVespa piston ported motor, so a largeframe with a similar top end to the gs150, I don't know if parts are interchangable between the Spanish largeframe motors and the GS but I doubt it.

The only barrel I could find was in Spain and they don't ship to the UK, I'm sure I could get it somehow, it could be an excuse for a holiday. But I decided to see if I could salvage the old one.

Confirmed the base made enough contact to seal despite the missing bit, welded up the hole in the exhaust stub, lapped the top surface until the head fitted ok. Did a little work on the ports copying three threads I found on Spanish sites. Waiting for a hone to come in the post to deglaze it.

Found a new potential problem, both ring ends cross over the inlet port. I haven't changed the width of the port. Every 58.5mm vespa piston that has clear enough pictures online seems to have the gaps in the same place.
I thought this was something you should never do, but I've found very mixed info when I've googled it, apparently some husqvarnas have the gaps over a port from new.

I think I've missed something obvious, I'll double check but I don't think the piston rings pass over that port, it opens to the crank case not to the combustion chamber.

i have no idea what the stud pattern is. . . .but if it's the same as GS150, you have to move the studs into order to run that M1XL.

you *might* want to look at just building or buying a large frame engine to run for now . . and keep your eyes open for a Spanish cylinder or Spanish engine for sale in the future.

What a tricky engine.

I've read that the stud spacing is different to the GS150. I think it may be standard largeframe spacing, everything else looks fairly standard largeframe.

Leak test started, no movement after 10 min.
The red bit on the kit is glue lined heat shrink that I only heated at the ends so it won't mess up my compression tester hose. There's a silicone cap on the exhaust like the one pictured on the inlet.

It looks like the leak at the head wasn't the problem.
I'm going to troubleshoot further this afternoon but I suspect ignition timing, basically because that's all that's left since it has a spark, it has ok compression and the plug is wet. Timing was set according to the vape instructions and double checked, but I'm clutching at straws, I may try setting it 5 degrees either way of is current position to see what effect that has.

With the choke on, how many kicks does it take to start? after it starts, does it run okay? Does it start up just fine after ran and warmed up?

To save time, it's not the ignition timing. This can be a very long way out, still starts, runs and rides.
Here's something to check. That looks like a new piston. With a piston ported cylinder a specific piston is required. One that fits a rotary valve cylinder is not the same one as for piston ported. The inlet timing determined by the piston window, which has critical port timing, when there is no additional Reed block.
Confirm the new piston has exactly the correct inlet window before trying anything else.

20+ kicks didn't start it. That's all I've tried since fixing the leak.

Before fixing the leak it would only start with a hand over the carb or on starter fluid and even then it took a few kicks. Once started it would tickover a bit lumpy at 1300rpm and would respond to the throttle. Once warmed up it started in 1 to 3 kicks but it won't take any load it stalls if you select a gear. I suspect it'll behave the same now, I'm just about to go to the workshop to start troubleshooting.

The choke which is part of the air filter housing moves it's full range. The pilot jet is a 60, this seems to be the biggest available, someone on a Spanish forum was running this size, I haven't been able to find jet sizes in a manual another member here thought 50 pilot and 80 main was standard, I had 52 84 fitted but went up on the pilot since it seemed lean. Adjuster 1.5 turns out, difficult to adjust in situe so hard to guage it's effect, doesn't have an o ring on this carb so probably leaks air in when the lock nut is loosened, also can't be accessed with the air filter fitted and is jammed against the cowl, all round bad design really.

Carb has been through the ultrasonic cleaner twice. I've also verified each passage is clear but spraying carb cleaner and watching which holes it comes out of.

The piston that's fitted doesn't have a window, it just has a small notch out of the bottom edge.

I've got a few pages bookmarked to do with this motor as there isn't much info out there on it. This link shows two pistons used in someone else's MotoVespa 150, mine looks the same as the one on the left, that's what I mean by a small notch rather than a window.

https://www.germanscooterforum.de/topic/247947-motovespa-150s-von-1964/page/4/

Although that only confirms my piston looks right, the inlet port timing could still be wrong.

Just to reconfirm the symptoms are the same as before fixing the head leak and passing a leak test.

Just stared it up on a tiny bit of starter fluid, warmed it up, turned it off and it then started first kick.

Still lumpy/stuttery and still stalls if you select a gear.

Just had the Strobe on it 28 BTDC at idle, 25 with revs. I couldn't read rev counter at same time but once it hit 25 degrees it stayed there, this fits the vape road curve as it's -3 at 1000rpm, although the sip speedo claimed it was idling at 1600rpm.

Standard timing for this motor is 27 BTDC.

I think that confirms that it's not timing. It was easy to do though so worth fully ruling out.

Looks like I probably need to measure the piston, or use the degree wheel with a feeler guage in the inlet, but either way I'm not sure where to get a stock measurement to compare it to.

The one on the left in the picture has been modified to fit, using a piston from a rotary model.
I would definitely be getting a degree wheel on the crank and find out how many degrees BTDC the inlet opens. On these non reed engines, even 5 degrees variation here is massive.
I would also confirm the piston skirt is long enough to not show conrod through the exhaust port at TDC.
Either of these being wrong will make it run like crap.

Before I took the measurements I checked the temp and started it, the head was 12C and it started first kick.

I make it 66/67 degrees for the inlet starting to open, fully open at 33.
No rod visible through exhaust port, the piston skirt covers the port at tdc.

I don't know how reliable these numbers are but I've found a post where that say the inlet was 63.5 standard and they modified it to 66.5.

So not the inlet either. 66.5 degrees is good to go. It's down to blocked carb or blocked exhaust.

For the carb, remove the pilot jet and start up without one. Should be bogging rich if the carb is good.

Similar for the exhaust. Take it off and ride without it. Will be slightly noisy but a valid test to see if it is blocked.

I tried the carb first as the exhaust is new. It started 3rd kick without the pilot jet fitted, very boggy. I couldn't really test for more than just a cold start due to the amount of hailstones blowing into the workshop.

I left it an hour to cool and it started and ran better with the adjuster out 4 turns, but I wasn't able to fully warm it up or test ride it. I'll see if it starts tomorrow from fully cold. The 60 pilot may still be too small, it came with 52, I haven't found any documentation confirming what size was fitted from new.

It started first kick today. 60 pilot adjuster 4 turns out.

Ordered 65 and 68 pilot jets, based on the rule of thumb of going up a size or two if you're over 3 turns.
I can't run it as is anyway as the cowl won't fit if the adjuster is over 2 turns out, it's jammed hard against it on any setting, the air filter doesn't allow the carb to be rotated to gain any clearance.

What concerns me though is I'm now jetting for cold start, it might be too rich when warmed up, but the ability to start from cold is essential. Plan B is a phbg carb to get rid of this weird airbox flap choke.

If the carb is 1 point per 1/4, a 68 jet will put it at 2 turns. Doesn't quite work like that but either the 65 or 68 is going to work on the hot engine.
Before getting too excited, the main jet will affect the hot condition, so that will need to be adjusted before finalising the pilot jet.
At least if it's now ridable, that's solid progress.

just some perspective-

These older machines work with a true "choke" system. Generally, a flap on the airbox closes off the air. Whearase the later SI carbs and pretty much every modern side draft carb work with a fuel enriching circuit; an extra jet is exposed and throws in more fuel. The fuel enriching circuit is a better design, not right or wrong, just easier to start.

from my perspective on owning and running bikes with true "chokes"- widebody bikes (v30-v33, VN1, VN3, VB1, and GS) as well as the VNA platforms, these bikes can take anywhere from 5-12 kicks to start with the choke "on" and the bike completely cold. Once warmed up, bikes start 1-2 kicks.

something to think about.

I've had a few cars and bikes with a true choke, but they've always been a precise fitting part of the carb and have been really effective, this is the first I've had with a crude flap in the airbox. I could live with 5 to 12 kicks if that's what it takes as long as I can be confident that it will actually start, until now no amount of kicks was going to get it started.

I've also ordered main jets, I'll fit one that I expect to be way too big and work down, maybe start at 100, current is 84.

I can't really refine the jetting until the registration is sorted so I can ride it, waiting for NOVA number/waiver at the min then need to fill in DVLA paperwork.

The next bike I need to fix also has a starting/choke problem, a Yamaha R1-z 250 which only starts on one cylinder till you coax the other one into life.

it looks just like a VNA carb, UA/16S1. would be nice to throw on a known good carb and see what happens. That engine is stock so should run on stock jetting.

Same carb range but 19mm UA-19-S1 I couldn't find documented standard jetting although I've been told 50 80 which is probably right since it came with 52 84, it's now 60 84 and needs the pilot adjuster >3 turns to start at all from cold which does seem a long way from standard.
Motor is not quite standard, 58.5 bore, sip road 2 exhaust, head had been skimmed (1.6mm squish) and I couldn't resist getting the Dremel out when the barrel was off but I didn't do much to it and it had the same idle and start issues before and after.

I'm happy that it'll now start, select a gear and pull away, but I don't have enough space to do more than that. I'm hoping that there's nothing else wrong and it just needs fine tuning once it's on the road.

Wow! Albeit that carb is extremely hard to fine. Thanks for the info.

It's just dawned on me that the adjuster on this carb is probably an air screw not a fuel one. It's on the filter side and has a fairly chunky tip these are normally signs of it being an air screw. Therefore more turns out equals leaner, so 4 turns out means I need to go down pilot jet sizes not up.
I'll get to the bottom of it, it'll become obvious when I start messing with the jetting.

It is an air screw not a fuel one so I had been going the wrong direction and I don't need the bigger jets that I bought, just a bad assumption based on cold starts normally being ok if it's rich.

1 turn out with the 52 pilot jet that it came with might not be far off, even though I can't get it to start at 1.5 turns, I'll come back to it when I've got the main jet sorted.

Fitted a 100 main jet, I'm confident it's too rich so it should be a safe starting point to work down from.

Finally got the registration through, and dropped down a few main jet sizes over 3 or 4 short test rides.

No difference in top speed between 95, 92 and 88 mains, all 55mph. No faster down hill so feels like something is holding it back.
Plug fouled with the 92.
Max under plug CHT is 275F with the 88.

Going to need to do some plug chops, I suspect it's still rich with the 88 but with no change of speed or feeling after each change I've lost my nerve and need to take a look.
Stock is believed to be 82, only relevant changes are the sip road 2 and some DIY porting.