Tuning a 125 - UPDATED

[Knowledge]

Here is a tuning question for a bit of fun.

A mate has acquired his first Lambretta since 1970. It is a 125cc (apparently). I fear he will find the a 125 is very slow machine when compared with modern traffic, and probably pretty dangerous. He is not keen to take his full test yet.

Would anyone like to offer their thoughts on what could be done to improve performance?

(I think it wise to keep the discussion to 125cc on a 52 x 58 set-up, rather than go oversize or change engine dimensions)

[dickie]

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.

[MickYork]

It would be interesting to strip it down (possible re-bore if worn) and build it back with correct transfer heights, squish, compression, etc...….run it for a bit then up-grade the exhaust and carb'......and see what difference it makes.

[Knowledge]

Dickie,

I was not aware that the 125 Special was noticeably different in performance than a standard 125. Thanks for the pointer. How do I spot the difference?

The carb is already 20mm.

Mick,

So you are suggesting that I blueprint the engine? Now that is a pretty sensible idea.

Perhaps I should combine these ideas from Mick and Dickie.

[Fast n Furious]

It really all depends upon how much time and money you are prepared to waste on it!
It would have to be very revvy to make any worthwhile power difference, and rev's cost brass.
For shits and giggles though, it would be a lark to see who can make best power from a 125 iron barrel on the same dyno.
How about a power jet carb,+ nitrous oxide injection, a very light flywheel and a decent Jap piston?
Sorry... I've got lockdown fever.

[dickie]

Dickie,

I was not aware that the 125 Special was noticeably different in performance than a standard 125. Thanks for the pointer. How do I spot the difference?

The carb is already 20mm.

Mick,

So you are suggesting that I blueprint the engine? Now that is a pretty sensible idea.

Perhaps I should combine these ideas from Mick and Dickie.

Sorry, I don't know how to tell the difference, but I just looked in Pete Davies's book and the GP125 also makes more than a regular LI125; both made 7+bhp versus 5.5bhp. I have a 64 LI125 barrel on the shelf. I could measure the port heights for you as a reference if you like; I presume that these higher power and also higher rpm motors have higher exhaust ports if nothing else.

Regarding Mick's idea of blueprinting, I think that goes without saying. I'd say that's basically following DT's 'bang for buck' philosophy and should be applied every time we build a motor.

[MickYork]

The 125 has big jump between 2nd and 3rd and the final drive is usually too short, giving a high revving machine with little poke. The Italian Li 150 g/box is the better all round option.

Yes it was DT's article that showed you can get extra performance without major costs or part changes.

[lofty]

If i remember rightly LI 125 had tiny transfers anyway..
centre squish head (gp125?) no head gasket, open ports to match manifolds..
and enjoy the scenery..

otherwise, get an autisa barrel & 54 crank

[jonw]

My son has an original paint low mileage 125LiS3 on which we fitted a GP 150 top end, Scootopia 22 carb and a clubman exhaust. It's an absolute joy to ride, very torquey, pulls well two up even but does run out of legs due to gearing. Great for around town but no rally machine. However he did ride it to Seahouses and back from Morecambe without complaint.

[Warkton Tornado No.1]

If i remember rightly LI 125 had tiny transfers anyway..
centre squish head (gp125?) no head gasket, open ports to match manifolds..
and enjoy the scenery..

otherwise, get an autisa barrel & 54 crank

Did Autisa make a small bore barrel with the stud fixings for a small block? I am aware of the '125 conversion' alloy, large fin spacings for large block cases as I have one that somebody had altered for a reed valve inlet & intended fitting the liners that came with it. I rather wish it had not been altered, as the quality looks to have been spot on......

Just to add my two penny worth, if the scooter in question was owned by somebody I knew, I would see how they fare, & then, when they can 'cope' with all of that power they had(!) & gnashing @ the teeth for more, embark on an engine build with another case with components determined by budget, scope of 'project' & rider ability.

We are all guilty of having pre-conceived ideas when it comes to Lambretta modifications, but the truth is, I have seen many riders with a riding ability that borders on incompetent. It's always a good test to look @ the tyres of anybody that claims to be a 'hot' rider.

Just sayin'.....

[ULC Soulagent]

If i remember rightly LI 125 had tiny transfers anyway..
centre squish head (gp125?) no head gasket, open ports to match manifolds..
and enjoy the scenery..

otherwise, get an autisa barrel & 54 crank

Did Autisa make a small bore barrel with the stud fixings for a small block? I am aware of the '125 conversion' alloy, large fin spacings for large block cases as I have one that somebody had altered for a reed valve inlet & intended fitting the liners that came with it. I rather wish it had not been altered, as the quality looks to have been spot on......

Just to add my two penny worth, if the scooter in question was owned by somebody I knew, I would see how they fare, & then, when they can 'cope' with all of that power they had(!) & gnashing @ the teeth for more, embark on an engine build with another case with components determined by budget, scope of 'project' & rider ability.

We are all guilty of having pre-conceived ideas when it comes to Lambretta modifications, but the truth is, I have seen many riders with a riding ability that borders on incompetent. It's always a good test to look @ the tyres of anybody that claims to be a 'hot' rider.

Just sayin'.....

Chicken Strips

[jonw]

Nobody likes unwanted attention brought to their chicken strips!

[Knowledge]

.... otherwise, get an autisa barrel & 54 crank

Did Autisa make a small bore barrel with the stud fixings for a small block?

Yes, Autisa did make an alloy barrel that worked with a 200 casing and a 54mm crank, but due to their rarity , I thought it best to stick with the standard 52x58 configuration. If someone wants to start a separate thread about that kit, please do and I will contribute.

WT1,your other comments are valid, but I was assuming that the standard kit made 5.5bhp, so tuning it to hit a mighty 7.5bhp should not scare the new owner too much. However, that was before I learnt that the later 125 iron barrels made around 7bhp already.

Anyway, I have learnt that the engine concerned may have a GP125 barrel already. I can work the squish on my new lathe and improve the flow of the ports following DT’s principles and see where I get to. The crank is GP and the carb is SH20. I would like to keep the original exhaust, though I am not averse to altering it. I have no idea about the gearbox, but I hope to get my hands on it by the weekend.

Any more thoughts guys?

[coaster]

[I can work the squish on my new lathe and improve the flow of the ports following DT’s principles and see where I get to.

Before doing that it might be worth concidering Darrell Taylors latest 'findings' following testing of engines pre and post port work. He seems to be implying that enlarging and or matching transfers make no measurable differnce to power. Theres a bit of debate on Scooterotica forum (Wasted Tuning )and also Dan Clare's 'Scooter Workshop' FB Channel.

I have a 125 Special top end if you need anything measuring

Colin

[Knowledge]

Thanks Colin.

I did watch Dan’s “wasted tuning” video on YouTube and I did bear that in mind when starting my thoughts about the 125. Thank-you for highlighting it here.

It is well worth the watch (if a tad too long) if anyone hasn’t seen it.

I might pop up to see you with the engine and try your barrel on and measure it with the Buzzwangle.

[coaster]

Thanks Colin.

I did watch Dan’s “wasted tuning” video on YouTube and I did bear that in mind when starting my thoughts about the 125. Thank-you for highlighting it here.

It is well worth the watch (if a tad too long) if anyone hasn’t seen it.

I might pop up to see you with the engine and try your barrel on and measure it with the Buzzwangle.

More than welcome Martin, Ill be in tomorrow until Friday pm and same next week probably, let me know and Ill send you my post code etc

[Warkton Tornado No.1]

.... otherwise, get an autisa barrel & 54 crank

Did Autisa make a small bore barrel with the stud fixings for a small block?

Yes, Autisa did make an alloy barrel that worked with a 200 casing and a 54mm crank, but due to their rarity , I thought it best to stick with the standard 52x58 configuration. If someone wants to start a separate thread about that kit, please do and I will contribute.

WT1,your other comments are valid, but I was assuming that the standard kit made 5.5bhp, so tuning it to hit a mighty 7.5bhp should not scare the new owner too much. However, that was before I learnt that the later 125 iron barrels made around 7bhp already.

Anyway, I have learnt that the engine concerned may have a GP125 barrel already. I can work the squish on my new lathe and improve the flow of the ports following DT’s principles and see where I get to. The crank is GP and the carb is SH20. I would like to keep the original exhaust, though I am not averse to altering it. I have no idea about the gearbox, but I hope to get my hands on it by the weekend.

Any more thoughts guys?

I'd think about getting a small block OEM type cylinder to use, preferably one that needs boring, & browse catalogues for a suitable replacement motorcycle piston, especially if I had a new lathe with sufficient swing to fit a barrel. A head could be reprofiled & spigotted @ the same time. Just be careful because making a cutter with the correct profile to suit is all very well, but the resultant chatter whilst cutting can cause issues

Although I am not an advocate of raising primary compression, I acknowledge that the shortened barrels (popular years ago) to suit Kawasaki, Suzuki & Yamaha Pistons were very tough, punchy engines & could work very well with, say, a 38 mm bore ''U' pipe exhaust.

Repeating myself here, but dyno tests of two stroke engines before & after with a longer con rod fitted (NO OTHER CHANGES APART FROM PACKING) have proven that lowering primary compression yields more power. So porting of the bottom end may not enhance flow (or appear to do very little) in some tests performed by well proven tuners, but where there is the opportunity to lower an already HPC, this particular 'seat-of-the-pants-non-dyno-non-tuner' will take it. If it were to harm the engine, then that would be a different kettle of fish

[Knowledge]

Sadly, there is a shortage of 52mm piston options if I am to stick with the 52x58 configuration.

Most modern 125’s are either 56 x 50 or 54 x 54. I don’t want to go down that route, as both would make the engine more revvy and that probably won’t suit the 4 speed ‘box.

Oh well, I have got to keep looking. Something with some better rings

[Warkton Tornado No.1]

My reasoning takes into account that an OEM small block cylinder is likely to be cheap & easily available.

It wouldn't affect the original cylinder, just replace it.

Anyway, suggestion suggested, I'd still advocate looking @ something like a KE 175 piston (61.5) & be prepared to overlook the wrist pin diameter of 14 mm, which could be modified to 16 mm easily.

[Knowledge]

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?