Advice on minor nip up.

[eadmr04]

I had a couple of nip ups the other day, was riding along and just sensed something was wrong, pulled the clutch in and it died straight away. I coasted to a standstill put it on the stand and it started first kick and tickover was fine. Pulled over a bit further and checked the plug which seemed ok. Carried on and it happened again exactly the same symptoms and result so I just limped home taking it easy. The scooter starts fine and ticksover ok. I thought I better check it out and this is what I found. Any answers to the following questions would be really appreciated.

Any idea what may have caused this?

I think the piston crown and inside of head look fine am I right? If so can I assume that this has not been caused by a timing issue and that my timing is somewhere near correct?

The barrell seems ok with no gouges and the rings are free on the piston. Will I be able to clean the piston up and re-use, if so how do I clean up the piston and rings? (Idiots guide please)

It is a 175 barrell on a 150 casing, any idea what sort of piston this is, as can been seen it is only 2 ring and has 6222 GPM on the inside.

Cheers.

Also,

It's a 150 barrell bored to 175 and I'd been told it had been run in but not 100% sure especially as there is a lack of carbon build up on the crown and cylinder head. Would the timing appear to be ok?
Set up is:
150 bored to 175
42mm Clubman
MA19BS5 with following jetting
Main 108
Atomiser 260B
Pilot 40
Slide 50
Starter 55
6.5 gm float
Airscrew 1/2 turn out

Plug Nippon Denso W24 Es

Speed when nipped up about 40 to 45.

Thanks.
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[Snails]

I'm no expert on this but the carbon deposits look about right. I had several nips on a barrel and piston I ran in when I opened it up a little after 600 miles or so and the deposits look similar as do the marks on the piston. This was with an iron barrell 150

So after thinking about my options I cleaned up the piston with some wet and dry sanding the high spots off and then washing with petrol. Ran in the piston for the last 300 miles and then tried opening up again and it still nipped up at around 45 - 50 so I got the bore tolerance checked and it appeared a little tight so I ended up having it rebored again and changing the piston to something better after all. Maybe I should of done that straight away and saved myself the 300 miles running in as I by then had to do another 900 miles for the new rebore so I ended up doing 1800 miles running in

[bristolmod]

That nip up seems quite severe IMO. I notice that its a "GPM" piston, whilst fairly adequate , isn't one of the best available.

I'd be inclined to cut your losses at this stage and invest in a better piston. The carb could be improved upon, and I would replace this with a S3 type unit correctly jetted.

Mixture wise I'd go at 4% semi for the first 500/750 miles then fully synth thereafter.

Chris

[shocky]

id hone that barrel with a draper honer there abour £18 and clean the piston with a file not emery or anything with grit/sand in it or you will leave sand/grit in the soft alloy and that will "lap everything and poss go on the big end ,them gpm pistons arnt expensive anyway

[joespeed]

that will be a common looking lambretta piston,
file offthe high spots on the piston and polish with a fine wet and dry paper (180 grit) refit piston into cylinder and check how tight the fit is,it will need a hone to take off the scuff in the bore and give more clearance,how much running in have you done? how many miles on this set up?
regards
joe

[shocky]

what oil you using running in iron barrels i use wilkinsons cheap oil for 1000 miles @4% then 3% fully synth

[tavspeed]

why dont you walkerise it,que special x.

[lifes a drag]

i would bin the piston if i were you , get the bore checked like snail seid , if ok hone it like shocky seid but id would never use a piston that had seized , im not saying you cant but it will always be on your mind that that piston has already seized once , no mater how smooth you get the piston the grooves and high spots that are microscopic will cause heat build up you will be back to square one

[Dazza]

Another option- Run a junior hacksaw blade across the piston where it seized forming a cross hatch pattern, that should help it retain a bit of oil in that area.

[corrado]

why dont you walkerise it,que special x.

If you Walkerise them when new you might avoid nip ups altogether as it removes any high spots. It's only a 10 minute job (plus half an hour cleaning it up afterwards.)