Lambretta sx 150 soft seize after 7 miles

[garethwyn66]

Just had a 1968 sx 150 with 125 engine tuned to 175. Plug is burning perfect chocolate brown. The scooter seems to have quite a bit of vibration when I throttle it up. But after about 7 miles I can feel the engine tighten up and if I carry on it soft seizes and I got to leave cool down for 20 mins before she restarts. Have got the timing checked and it was out by 3 degrees. Set to 19 degrees. Whent for a ride not as much vibration seemed ok. But after 2 rides seems to have gone back to vibrating and it overheats again. Im using rock oil premix fully synth as 2 stroke oil now but when I first got it I used the same on I use for my vespa rockoil injector oil but im told it works as a premix to any ideas. Is it possible after a soft seize the barrel is scored and this is why is overheats. Is it passible for the
timing to move on a lambretta. ??????

[Muppet]

125 bored to 175 ???????

[garethwyn66]

I was told it is a 125 tuned to 175. Sorry i m not a mechanic. Dont know much about engines. Its deffo a 125 engine tuned to 175.how they have done it dont know.

[10 inch Terror]

Sounds like the barrel walls are now too thin and are warping when the cylinder gets hot. I remember having 150 cylinders taken out to 175 in the 80's but they have more 'meat' to play with.

[bristolmod]

is it in fact an SX150 with a replacement 125 engine or an LI 125 Special?

Anyway, if the original cylinder was a 125 item (52mm) and its been bored to 175 (62mm), then the walls are way too thin and they warp when hot = seizing.

Replace top end with a stock 150 or a 175 cylinder and a conversion piston.

Chris

[garethwyn66]

is it in fact an SX150 with a replacement 125 engine or an LI 125 Special?

Anyway, if the original cylinder was a 125 item (52mm) and its been bored to 175 (62mm), then the walls are way too thin and they warp when hot = seizing.

Replace top end with a stock 150 or a 175 cylinder and a conversion piston.

Chris

It s an sx 150 with a replacement 125 engine

[rosscla]

The engine blocks are the same for small block, so it may say 125 but no way to tell what cylinder is fitted without dismantling it and measuring.

[bristolmod]

the blocks are the same- the cylinders differ on thickness- although not ideal, you can get away with boring a 150 out to 175.

Boring a 125 cylinder to 175 cc's is pushing it to say the least. You will get thin walls that can distort when hot and catch the piston.

Chris

[rosscla]

Sorry I should have been clearer. What I meant was that the block may be stamped 125 but that the OP has no way of knowing,without at least partially dismantling the engine,how the cc increase alleged was done or if it even was. It could be a kit conversion or bored out.


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[mick1]

the blocks are the same- the cylinders differ on thickness- although not ideal, you can get away with boring a 150 out to 175.

Boring a 125 cylinder to 175 cc's is pushing it to say the least. You will get thin walls that can distort when hot and catch the piston.

Chris

If the blocks are the same then the spigots must be the same. The cylinders differ on thickness but i would have thought that only means the 125 is thicker.That means a 125 and 150 bored to 175 would end up with the same wall thickness ? or am i wrong ?

i agree that 175 on a standard barrel leaves the walls on the thin side.