Modern Vespa

I am looking for help, hopefully the 10-20% of you current or past Lambretta owners can help me tshoot the issue. My 1963 Lambretta LI125 has a 175 cc top end, 25pbhl carb with a 110 main jet and a knock off clubman box and a UNI foam open filter and it has a 12v stator conversion.

My current problem is that it wouldn't pass 50 mph no matter what, I owned this scooter for close to 9 years now. You can go downhill, you can tuck down but you can distinctively hear the engine just doesn't want to rev up on 3rd or 4th gear pass half throttle.

I can rev the hell out of it on first and second, you can hear the high pitch of the engine and I also hit the 25 mph(1st gear) and 37 mph(second gear) GPS verified so that makes my lambretta highly likely to have the late transmission version(two transmissions gearings were offers in 1963).

What would you do? start with bigger jets? it seems like is bogging down on the top. I have a new BGM fuel tap. I read on a forum that sometimes the flywheel needs to be remagnetized and that sometimes the spark is weak at higher engine demand.

I have zero Lambretta tools. What would you recommend? invest on a flywheel puller and a timing light? change the spark plug and wire? Thanks a lot

Did you change the drive or clutch sprocket? Stock 150s were only geared for 52-53mph. Hard to get too much past that with a 175cc kit.

I wouldn't go with bigger jets. Usually too rich jetting won't allow the engine to rev out fully. What color is your spark plug?

Usually weak flywheel magnets cause problems starting and idling but once running, are fine at higher revs. Have you verified that your timing is correct?

Get a full rebuild done recently? Clutch rebuilt recently?

^^ To be honest, I dislike working on the Lambretta. I was going to try a different plug first. I only have one spare main jet available right now and is a 105. I tried them both in the past.

Last night I googled the same issue and found a person that replaced a bunch of parts, tried them out and it ended up being the stator being too old and too weak.

As far is the speed, I wholeheartedly & strongly believe that there are 8-10 more miles on the top end, The reason why I am saying this is because I can get to 50 mph at 1/3 or 1/2 throttle in 4th gear, it just bogs down in 3rd and 4th but it gets to 50 mph effortlessly.

What I really intended to do is to purchased a Tachometer/rev counter so monitor what is going on on 3rd and 4th gear. I also need to order a timing light.

Can I ask is it gagging on wide open throttle - full throttle?

Before you order a tach, buy/ steal/ borrow some Lambretta specific tools. Flywheel holder, flywheel puller, timing wheel and strobe, spark plug wrench, spare CDI and some different main jets and needles. That way you can inspect the stator, see what color your spark plug is and play with jetting for your setup. The spare CDI is just nice to have since you cant see electricity!

You keep bringing up the stator. Just so you know, 12V stators are comprised of the coils, wiring, flywheel magnets and then CDI. Usually the first thing to go is the wiring. Insulation gets crumbly and it doesnt let electricity flow well. After that coils go. They dont work 3/4 of the time and then crap out. They just die. Period. Flywheel magnets de magnetizing/ trauma is super rare. In 12 years and maybe 200 scooters, Ive seen 2 flywheel magnets so weak they needed to get re-magnetized and one which had a magnet separate from the flywheel. Rest have been perfect. CDI's like coils I've seen fail altogether/ have issues at low revs, but not fail at higher revs.

That said, here's how I'd approach it:
1. Pull the carb apart and tell me what jet, needle and clip position you have your PHBL set at.
2. Pull the spark plug and tell what color it is. Tan? Black? Oily black? Chocolate brown?

Report back and we'll dive further down the rabbit hole.

As someone mentioned before, are you on stock li125 gearing?

I have nearly the same setup as you (but with a 22mm jetex).
My scooter will hit 55mph on a straight and 60 on a downhill. - I don't think you can expect much more from a standard cast iron 175cc without tuning.

I have a 17 front sprocket with a 46 rear on stock 125 gearing, which I feel is geared relatively well for my setup. I do, however, feel like it struggles to pull 4th at times (you need to rev hard on 3rd)

Edit -reread your post. it sounds like you are also having trouble pulling 4th gear - which seems to indicate that your front/ rear sprocket setup is not stock.

Perhaps you are over geared or running rich?

Standard gearing on the Lambretta LI series 3 was either/or 15/46 or 15/47 depending if it was the early or late gearbox. Having done my own speed test(letting it rev to red line on 1st and 2 nd gear) I obtained GPS verified 25 mph(1st) and 37-38 mph(2nd) which automatically puts me as a later transmission but again everything could be far off because I have a few other mods on the scooter that could affect the rpms(different top end, different exhaust, carb, air filter).

It looks like it is inevitable to have to pull the engine casing and buy all new tools just to Shoot but I don't have much choice. Because of the very low speed(50 mph gps) I am very limited on my scooter route. I can't even jump on a hwy or interstate and I am slowing down traffic when I go from a small countryside road. I have been limited to city driving since I got the lambretta. On another hand, I haven't got underneath the engine just to bother to see what cylinder kit I have, I know it has it because people from a local scooter group and the owner of a scooter shop told me who the lambretta belonged before but if I have to guess I would say it probably has a cheap knock off 175 cc kit from either a 175 Lambretta or an aftermarket kit from decades ago.

I am going to pull the scooter out of the shed , post some pictures and start digging. Thanks for the help.

Thanks for the help. It is very likely that I will order some basic tools today. I can use the strobe light for my other scooters and I can also use the timing degree disc as well. fortunately I have a working Ducati CDI laying around, test light and a DMM.

I will need a flywheel puller. I don't know what the clip position means. Do you mean the mechanism inside the bowl with the needle/pin? Next thing is to pull the plug and try a new spark plug wire and cap. For sure I will be posting some pictures this afternoon.

At the top of the PHBL where the throttle cable comes out, are two 8mm bolts that hold the spring, round slide and the needle poking out of it. If you remove the 2 bolts, spring and slide, you can see the needle and the C shaped clip at the top of it. Tell me which of the 3 notches it's set at.

In the bowl, there should just be the main jet in the brass bottom plug (14mm I believe).

Here is the first batch of pictures. I must admit that I hardly looked at this Lambretta, I didn't know on what side the flywheel side was because I never gave it a good look.

The first 2 pictures is what the Dell'orto 25 PHBL currently comes with.
I tested the compression and it reaches 145psi but after 30 seconds it drops to 100 psi. Spark plug looks great to me, it is a BR7ES.

The yellow numbers are written on the cylinder. I was told it had a 175 cc, I believe it. I don't know why the pictures are so degraded in quality.

there is a wire retaining ring that holds the dished cover inside the flywheel fins. There are 2 ends that stick up that you can grab with pliers or something similar. Once the clip is out you can rotate the piece, which has tabs, so that the tabs can come free and the piece come off.

Underneath there is a flywheel nut, like normal, and you need a puller to get the flywheel off the crank.

Plug looks lean. Brown would be better (for this). Timing could be low. Flywheel nut is a left hand thread.
In stock cylinder setup they're not as quick as equivalent Vespas. If it gets to a genuine 60mph you'll be doing well.

Thanks folks.... Any comments about the compression hitting 145 psi but dropping down to 100 after about 30 seconds? small seal leak?

Also, how does the jetting looks? I asked on a lambretta forum but very low replies and contradicting responses. One person said 110 main jet is too rich and the other said it seems too low.

I watched a video of piston-ported on YouTube and they pulled the lambretta flywheel with the center tool and a rubber mallet.

Tbh you haven't answered my first question, so it's pretty pointless trying to move forward 😀

Sorry, I thought I answered. It bogs down in third and fourth at half throttle, we can call it gagging if you want. It sounds similar to when you first try to run a scooter that had a flooded engine with gasoline, it wants to rev freely but it wouldn't clear the revs.

Ok fair enough, I think you've also been asked about the needle and clip position which also has a factor in how the carb operates?

The needle and clip are inside the slide, they need to come out to get at the number which may well be a D26?

Good luck.

Sometimes free revving but bogging under load is ignition. Winding out in first or second is not as much load as fourth or even third where hp comes into play. I'm going to say a weak link in the ignition somewhere.

Hi again, so it looks like the clip is on the top position. A small shipment of parts arrived today from scooter-speed but I still don't have a flywheel puller. I got a new BR8S plug as recommended by scooter-speed and I am about to switch for a different blue Ducati CDI.

On what position should I have the clip ? top(like it is now) middle or lower?

I just came from a 4 mile ride around a main road nearby my neighborhood. I had a few nice flat stretch of road. Here are absolutely the 3 main things that I am seeing:

1) the engine smells hot!!! it irradiates a lot of heat and the idle is much much higher than before. Before this ride the idle use to be very low.
2) The engine bogs down badly on take off, it stumbles bad, so much that you are incapable of rolling with the engine power unless you push it a bit with your feet and pop the clutch at higher rpms to get the engine rolling
3) once it rolls you get a stronger midrange acceleration compared to before but it completely dies on the top end as before.

The only change that I made compare to before is to lower the pin circle clip from the top position to the third position. The engine right now doesn't want to rev freely, it reminds me when you flush your mouth with a mouthwash and you hear that gargling noise, something is making it not rev.

I am 200% convinced that this scooter has another 10 mph hidden on the top end but right now I am back to square 1 , I can't get her to pass 50 mph GPS. The fact that on the midrange now pulls harder it makes me thing that it is not a power issue, unfortunately it bogs down now on the beginning and on the end but it shines in the midrange.

I am changing the plug and putting the clip on the second position. I am also swapping the blue Ducati CDI. Wish me luck and as a reference look at the pictures from the post above. Thanks!

Ok so I definitely think you have a fuel issue that you must address first. Like Jack said, the plug is lean. Do you have anything larger than what is installed currently? You mentioned a 110 MJ. If you dont have anything larger, you'll need to buy a variety pack of larger jets.

Then you sadly need to buy a needle pack. If you look closely, you can see whats stamped into the current needle. It'll be a letter and number ie. D25.

As a general rule of thumb, when you raise the clip (lower the needle), you clean up initial acceleration. Thats why acceleration was better a week ago with the clip at the top position, and why it ran like shit today (when you lowered the clip to the third position).

So, find the main jet that feeds the engine enough fuel so that it doesnt run hot, and then play with the needle and clip to clean up initial acceleration.

Let us know if changing the CDI does anything. If not, got any plans to come to San Diego anytime soon? Let's meet up.

^^ Thanks for following up. I changed the clip to the third position from the top on my first run. I barely made it home. Then I swapped the CDI and put a new spark plug(BP8) but I could barely keep the engine running, it was choking bad.

I then put the C clip on the top position again(original position), the engine fiddles very smooth, accelerates very well from iddle to mid range but still chokes up on the top. I did however reach 52 mph tucked down, that is a 2mph improvement over any previous runs on the last 9 years.


I have to mention that when I moved the C clip to the second and third position from the top the carburator was throwing fuel like the guys in the circus that throws flames from their mouth or when you are drinking and someone tells you something funny that you spill the drink from your mouth. If you put the palm 4-5 inches away from the intake of the carburetor you would get the hand instantly wet.

What I would like to do is to get a tach/rev meter and see what is going on with the rpms. I will post more pictures tomorrow. Thanks for all the help.

I have an LI125 with a 150cc kit on soon to be a 175cc. I currently get 50 mph full licks down hill and I don't expect to go any faster when I put my new kit on. I do however expect to have a bit more grunt up hill. The gearing is what makes top end speed and the power to turn those gears.
I am considering going from a 15 front sprocket to a 16, to grab maybe 4mph more, but then this could be a detriment to the reason for my increase to 175cc which is more grunt up hill.
I would be looking at your gears as sometimes you have to go smaller to allow the engine to get into the rev range you are hoping for.
With Lammy's there are literally hundreds of ratios you can aim for with the amount of things you can change.
My guess is your ratio is two high for your top end.

Well... I think I reach the end of the road and I don't want to deal with this Lambretta anymore. I placed a small order of some bit and pieces but I am not satisfied with the performance. For all I know the 50-51 mph might as well be the performance limit on this lambretta but to me it is utterly insufficient, I need to be in the 60-65 mph (GPS) for me to be happy.

I did order a full set of jets(10 sets), I tried a 120 main jet and it didn't change things. I am honestly tired of being slow and I don't want to dump money on a cylinder kit and other things, that is why I have the 3 vespas with cylinder kits.

It you are interested on my 63 LI 125 Series 3 I will be posting it for sale within the next couple of days. Here is a preview of the scooter:

Original engine case and frame, 175cc top end, 25 del'orto PBHL, Uni air filter, clubman knock-off replica exhaust, 12v dc conversion with Ducati CDI, new BGM fuel tap, new speedometer and a set of jets. The scooter starts on the first or second kick no matter if it has been been started for months, it riddles perfect.

It should need some TLC on the front fork and it needs tires. The scooter has been registered the entire 9 years that I had it insured too. I have the original speedometer that I replaced it on 31 Oct of this year for a new one, the odometer shows 26000 original miles and I have the title for it. The paint is very, very old.

I put about 1000 miles on 9 years, main reason so low is because with a top speed of 50 mph(GPS) I never felt comfortable taking it on the road. I am looking to get $2500 (location Tucson, AZ) and I will consider a partial trade for a Honda Monkey(My scooter plus money)

Hundred bucks, a 12 pack and I'll bring you a burrito. I'll come pick it up.

I mean, you've got a shitty running scooter. Sounds square to me.

(Also, burrito will be top notch)

A last throw of the dice

Try using a 100 main jet.

Good luck

My cast iron 230 uses a 108 MJ with a bgm clubman.

Oh and second clip down on the needle + reduce your pilot jet to a 45

Good luck and keep hold of the old girl.

^^ thanks a lot for the encouragement. And thanks a lot to user MJ rally for his support. Guess what? I put a 100 main jet and problem solved. I gained 8 mph right of way with both cowls out and slight uphill. My third and forth gear keep reving, no more hesitation.

So I received a Tachometer, I plug it in and I was seeing 7600 rpm in first, 6200 rpms in second, 5900 rpms in third and only 5500 in forth gear, that was with a 122 main jet. Prior to that(all the previous years) I ran a 105 and a 110 because that is what it came with the scooter.

I knew the engine had more on it, if the engine really had the 175 cc kit that I was told it had I was expecting at least 55-60 mph. Never in a million years would I thought that I would have to run a 100 main jet considering that I have a Uni foam filter, a 175cc top end, a 25 Dellorto PBHL and a clubman knock off. The gamble paid off!!! the main jet solved it all.

I expect the scooter to reach 60 mph tucked down. I saw 6400 rpm in 4th today (5550 before I change the main jet) and I know the engine could rev up to 7600 rpms (it wouldn't rev that high in 4th though).

Anyways, I am very happy, I had two options in mind: 1) putting a cylinder kit 2) selling it. For the right money I am still open for a sale unless you want to trade your Honda monkey plus I give you cash.

^ ^ ^ ^

That's great news, really glad you got to the bottom of the issue.

Happy scootering

It's all two wheels even if it is different marques.

Best regards

Cgt75b