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Re: Tuning a 125

PostPosted: Wed Jun 24, 2020 11:44 am
by Storkfoot
dickie wrote:What about aiming for 125 special port timings and 20mm carb?

I'd imagine there are plenty of 125 special barrels available to buy for not much and just bolt on.


Martin, if he is planning on spending time on dual carriageways, the 125 Special/ GP125 engine will be found out very quickly. But, if he lives in a town or city, that engine with a standard exhaust is quite nippy. I ran one for a few months several years back.

Re: Tuning a 125

PostPosted: Wed Jun 24, 2020 11:54 am
by Warkton Tornado No.1
Knowledge wrote:Hi WT1,

I had a look for 52mm pistons last night and the RS100, RS200 and Aerox100 all have 52mm pistons of 65mm or 60.5mm in length. They all have 14mm gudgeon pins. All have Reed holes in the skirt.

I have found a 20x14x17 needle roller, so it could be adapted to fit such a piston, but just how easy is it to modify a 14mm pin hole to 16mm?


I'd use an adjustable reamer in a Freddie Mercury style. Keep swapping sides.

If the resultant hole was desired to be offset (fiddling compression height) I'd consider cross-slide mounting the piston with a cutter in the headstock.

Increasing the wrist pin size has it's advantages for the selection of the bearing. If I were to go the KE175 piston route it would be my choice. Just sayin' :D

Re: Tuning a 125

PostPosted: Wed Jun 24, 2020 7:40 pm
by Knowledge
Thanks Paul. He isn’t going to do any motorway miles on this engine, so a standard LiS125 will probably suffice, but I am just having a bit of fun thinking around the box and getting ideas from you guys.

WT1, for this exercise I want to keep at 125cc or else I would just fit a Casa 185 kit. That means the KE125 piston is not on my radar.

I still think there is a market for a bolt-on 125 kit that would make 10bhp, perhaps with a reed valve and a better exhaust flange. There are plenty of people who are coming to Lambrettas in their second-age who don’t want to take their full tests, don’t want to be stuck with 5bhp but want to be totally legal.

Re: Tuning a 125

PostPosted: Thu Jun 25, 2020 9:00 am
by dscscotty
Whilst slight power gains maybe achievable, 125's are really hard work to ride as standard, raising the power may considerably affect the drivability further due to the raised rpm required to hit any power band,
As you mentioned the reed IMHO would be a very good conversion, as long as boost ports were added to the cylinder wall, you could certainly achieve a good power spread, along with a higher exhaust port duration, as long as the piston and rings were able to cope with reed valve mods and higher rpm. I managed to get a 125/130ish vega to almost 12bhp using additional ports, reedvalve etc, whilst maintaining a good power spread and drivability, so be interesting to see what was achievable.

Re: Tuning a 125

PostPosted: Thu Jun 25, 2020 1:26 pm
by lofty
if you are looking to get the best out of a standard barrel/carb/exhaust/crank then surely its just a case of finding a suitable piston and working out some useable port timings

next step, maybe reedvalve and boost ports, maybe conrod length would help/hinder power delivery..

if a 'bolt on solution' for purchase and general use, then perhaps a sleeved down GT/mugello/casa/BGM would be an option..

or off the top of my head.. the new J & vega kits from rimini, would need work and/or some kind of adaptor but could they be grafted on?

food for thought or fantasy.. sometimes i wonder myself :D :D

Re: Tuning a 125

PostPosted: Thu Jun 25, 2020 3:12 pm
by Scooterslag
funny I was looking at something similar a little while ago as I had a old 125 barrel and head and thought it would be cool to make it practical for modern road use. I was looking at various pistons and was going to research using a Piaggio hexagon 125 (2-stroke) piston for a suitable conversion but never go much further with the investigation. If memory serves me - I'm sure it used a similar sized small end bearing and the piston height wasn't far off. Paul

Re: Tuning a 125

PostPosted: Thu Jun 25, 2020 6:54 pm
by bike grim
I found a standard DL125 was fine for general use even up to 50mph on dual carriageways. Nippy off the line as well.

Re: Tuning a 125

PostPosted: Thu Jun 25, 2020 10:10 pm
by Andy B.L.C.
+ 1 The DL 125 that I had in Cologne that was scrapped by the German powers that be, years before they took the van... A standard, never taken apart & bunged together with scrap by eejits 125 really impressed...

Re: Tuning a 125

PostPosted: Sun Jun 28, 2020 6:34 pm
by Adam_Winstone
125 Special and DL125 are basically the same motor (gearing, carb, barrel by part number, etc.), although there are 2 different jetting configurations for the 20mm carb, and for their size they are excellent motors. The short gearbox makes them pretty quick off the mark and their ability to rev-on, compared to standard LI, give them a reasonable top speed too. I'd rather ride one than a standard LI150. However, they're still a 125 and top speed will be limited.

All depends what you're expecting from a motor. I loved the standard 175 III that I put on the road a couple of years ago... loving it for about a week and then getting bored of it again ;)

Re: Tuning a 125 - UPDATED

PostPosted: Sat Jul 25, 2020 8:52 pm
by Knowledge
So I have finally found time to measure the ports on the 125 barrel on my mate’s scooter. This has bigger ports than the 125 barrel I had hidden at the back of the workshop, so I suspect it is the DL spec. Nice.

Inlet. 140
Exhaust. 147.5
Transfers. 117

The exhaust port and transfers are very low figures. This also makes the blow down a very, very low 15.5 degrees.

Darrell Taylor documented in Scootering the development a commercially available 200 Indian steel barrel using his “bang for buck” tuning techniques and achieved excellent results. I am applying the same to the 125 barrel, but I thought I would share this with you before firing-up the grinder.

So my first action was to slip a 1mm spacer under the barrel to bring the bottom of the transfers up to the top of the piston. The results were:

Inlet. 136.5
Exhaust. 149
Transfer. 122.5

The inlet can be easily adjusted back to around 140-145 and the transfer figure looks good. The exhaust needs work, but it is accessible. However, it would be a big change to get blow-down up to DT’s suggested figure of 25-27 degrees.

The exhaust would need to be pushed up to 172.5 degrees to achieve this. That means increasing the port height from 16.5mm to 23mm (6.5mm). The port could also be widened from 29.4mm to 32.2mm. That would mean increasing the exhaust port area by 50%. Wow.

All these figures need double checking, but I am throwing this out there for some feedback.

(I am planning on using the standard exhaust and 20mm carb, at this stage)

Re: Tuning a 125 - UPDATED

PostPosted: Sat Jul 25, 2020 10:55 pm
by dickie
These are similar figures to the ones I ended up with when I used bang for buck as the basis for my attempt on a 175.

I really like mine, it's a nice ride, but like all 2 strokes, it's dominated by exhaust. Bgm clubman was the best so far, but that ended up in the scrap. I suspect a standard pipe may be best for a 125.

Initially mine ran with 25mm dellorto but I'm trying a 28mm when I get to spend some time at home. I'll let you know if it's worth the bother.

Re: Tuning a 125 - UPDATED

PostPosted: Thu Oct 20, 2022 10:31 pm
by Scooterslag
Any more updates on this cool little tune Martin?

Re: Tuning a 125

PostPosted: Wed Nov 16, 2022 12:35 am
by Bilko
Storkfoot wrote:
…..But, if he lives in a town or city, that engine with a standard exhaust is quite nippy. I ran one for a few months several years back.


Oddly enough, a mate was raving about his all original factory-spec DL150. He wanted to share his joy in standard Lambretta’s (he’s got a cracking collection). So I took it for a few days and riding it round Belfast was really enjoyable and more than capable, even on a couple of nippy ring roads. It’s been many years since I’ve rode standard and it’s inspired me to write a piece about it in my blog.

I’m considering doing it to my S1. Or maybe a slight increase in carb size and CC’s.

Re: Tuning a 125 - UPDATED

PostPosted: Sat Nov 26, 2022 9:30 pm
by Knowledge
Scooterslag wrote:Any more updates on this cool little tune Martin?


Actually I gave up. So much so, I fitted a 185 kit to the engine that I intended to use for the 125 experiment and weighed-in the 125 barrels I had accumulated.

You can only polish a turd so much.

Re: Tuning a 125 - UPDATED

PostPosted: Sun Nov 27, 2022 12:57 am
by Fast n Furious
50 years I've been polishing that turd...... :lol: :lol: :lol:

Re: Tuning a 125 - UPDATED

PostPosted: Tue Nov 29, 2022 4:43 pm
by jonw
Knowledge wrote:
Scooterslag wrote:Any more updates on this cool little tune Martin?

Actually I gave up. So much so, I fitted a 185 kit to the engine that I intended to use for the 125 experiment and weighed-in the 125 barrels I had accumulated.


I thought the motivation to keep to the 125 was that your mate hadn't passed his test?

Re: Tuning a 125 - UPDATED

PostPosted: Wed Nov 30, 2022 8:02 pm
by dickie
Knowledge wrote:
Scooterslag wrote:Any more updates on this cool little tune Martin?


Actually I gave up. So much so, I fitted a 185 kit to the engine that I intended to use for the 125 experiment and weighed-in the 125 barrels I had accumulated.

You can only polish a turd so much.

I was determined to build a totally original 125 for my s2 but was fortunately persuaded otherwise when I was at AF. Sadly, it was just after I'd paid for a rebore on my 125 barrel.

210cc now :D

Re: Tuning a 125 - UPDATED

PostPosted: Thu Dec 08, 2022 10:28 pm
by Knowledge
jonw wrote:I thought the motivation to keep to the 125 was that your mate hadn't passed his test?


I fitted a Casa 186 kit for him. He wasn’t bothered about the 125cc limit.

Re: Tuning a 125 - UPDATED

PostPosted: Fri Dec 09, 2022 10:12 am
by Rich Oswald
Knowledge wrote:
jonw wrote:I thought the motivation to keep to the 125 was that your mate hadn't passed his test?


I fitted a Casa 186 kit for him. He wasn’t bothered about the 125cc limit.


Tut Tut!!!