Modern Vespa

Hi all, I am very hopeful that someone in this forum might be able to give me a pointer as to whether I'm on the right track in diagnosing and fixing an issue with my LML Star 200 4t!

My LML Star is the 2015 model and has done about 6500km. Regular servicing and maintenance etc. to electrics and fluids. Oil levels always bang-on.

A couple of weeks back, I pulled the bike up (after riding well with no obvious issues for about 30 minutes), and shut off the engine to pop into a store. When I returned about 15 minutes later the bike wouldn't start. I tried the electric start and kick, to no avail, and did some usual checks -- obvious fuel, electrical problems etc. at the side of the road. In the end I had to call the wife and I got the bike home on the back of a trailer.

Back home I first tested the usuals -- electrics, fuel supply, cleaned and checked air supply, but I couldn't find anything obvious. I replaced the coil, HT and spark plug with new items. I drained, disassembled and cleaned the carb and lines. Putting it back together, this time the bike tried to fire very pathetically after the starter turned it over a good 5-10 times. With the exhaust off I could occasionally feel some hot exhaust come out the exhaust side of the head in occasional small puffs. With the exhaust on, the firing was so pathetic you'd not know it was happening.

Interestingly the starter seemed to be turning it over very quickly with little resistance, which lead me to think "perhaps compression?", so I bought a compression tester and discovered that the compression was basically zero (5 psi). This I thought is the issue. I am amazed that I got any heat at all in the exhaust with 5 psi, but I guess this might be simply luck -- enough petrol/air with a spark might occasionally go up perhaps? Either that or maybe my new compression tester isn't working.

Anyway, so this morning I lowered the air box and removed the head. First thing I noticed is significant carbon deposits on the piston face, head and values. No obvious signs of damage e.g. no cracks in cylinder, head or clear signs of massive wear in the piston rings. Gasket looked healthy.

In cleaning the piston face up I notice that the face is slightly burred around one of the valve notches (intake valve notch). On the head, the intake valve has signs of tapping the piston I think -- see photos attached. In addition, the intake valve looks slightly off-centre with its seat, with the valve sitting slightly askew (only very slightly to the eye).

This does look to my untrained eye like valve damage, enough to make me want to sort it out, but I'm not sure if it would be enough to cause the engine not to start through compression being too low. If there's air mixture escaping past the intake valve then I assume that would do it. In your experience, does this constitute a 'bent valve' type scenario?

Edit: Just looking at new cylinder heads on Sip-Scootershop, and by the looks of it I wonder if the whole intake valve seat has been edged forward into the cylinder. It has a pronounced shoulder that is making it stand clear of the cylinder head face (see pictures below), which the exhaust valve does not have, and neither does the intake valve on pictures from sip-scootershop (see: https://www.sip-scootershop.com/en/product/cylinder-head-lml_21301070?q=head).

Edit: Well, I'm pretty satisfied that the issue is that the valve seat has been pushed forward, causing the above mentioned piston/valve contact, as well as the starting issues once the valve seat was far enough in, owing to the fact that the valve is not opening enough to let enough fuel/air mixture in. I suspect this has been coming on for a while, in fact it may never have been right as I always felt a 200 should be able to do more than 80-90kph flat out with a tail wind! If the valve is not opening enough then it won't be getting enough fuel to burn fully. . So -- new head ordered from SIP which comes with valves already in place.[/u]


Piston

Head 1

Head 2

Head 3


Last edited by muszeo on Thu Nov 04, 2021 3:20 pm; edited 1 time in total

Welcome to MV! Someone will be along shortly to help out. Nice pics, by the way.

Thank-you for your reply!

Yes, I think actually I've probably diagnosed it -- I've put a couple of edits into the post above.

I think it's the valve seat on the intake side which has been pushed into the cylinder. Probably gradually over time, with the eventual result being the valve and piston coming into contact. This probably damaged the valve, which was the final straw for compression and thus starting, but the fact that the seat was moving will also have impacted how much the valve was opening and thus how much gas/air was getting into the chamber. It's amazing it ran so well at all, but I'm anticipating that replacing the head (complete with new valves) will sort it out and in addition the bike should develop more power as well (fingers crossed!)

You forgot why this happened ...

TBH I have no idea why it happened. It could be a manufacturing fault, or possibly a design flaw, I doubt it but possibly the tappet set incorrectly? I'm not a mechanic and am feeling my way on it. I'll repost once I've managed to get the new head installed and see if that fixes it.

Might want to pull the cylinder. If the piston casting failed because of a flaw, you'll destroy another head. Has to be a reason the relief notch came in contact with the valve. My SH piston has a larger intake relief notch than the exhaust. The intake valve has a bigger diameter than the exhaust.

By the way. Kudos on the DIY

Anyone aware of any trustworthy sources of manuals online for the LML star range? I could do with something to let me know how to assemble it and get the timing right.

Thanks @breaknwind. That's sage advise, just a new piston as well you think and not the cylinder itself?

The least you want to do is check that there's no slop in the wrist pin. Looks like the contact is offset in the notch. It also looks like the intake valve seat is pushed out compared to the exhaust valve seat but I don't know that engine.

Does the cam sprocket have a 1 and 2 on it? If the crank is on the T mark and the rocker is on the back side(counter clockwise) of the intake lobe, the 1 and 2(1 up 2 down) will be parallel to the head and valve cover contact surface. The cam sprocket bolts are 9Nm 6.6 ft lb on my SH. The tricky part of my SH is the cam chain tensioner. Make sure you know how yours works.


Example of my SH150i


Last edited by breaknwind on Fri Nov 05, 2021 3:46 pm; edited 1 time in total

Im going to go out on a limb and say it dropped a valve seat. The pic showing the gap shouldn't be possible if the seat was properly nestled in the head.

There is a manual for the LML 150 on modernbuddy.com, but the 200 is different enough that it's not quite applicable. Look up the LML Owners of GB forum and someone might have the proper manual.

I'd second replacing the piston, if only because it has kissed the valve a time or two.

Good luck!

If one is found, please email it to me and I'll add it to the 'Knowledge'.

Who made the maintenance of the 6500 miles?

Previous owner, who I know, went through local dealership.

Thanks all, I'll let you know how it goes!

Found a free manual here:

https://www.yumpu.com/en/document/read/27266020/lml-200-4-stroke-service-station-manualpdf

Only online viewing.

Grrr...

The new head and piston have arrived here in NZ from good old Sip-Scootershop in Germany. I've refitted them and can report that she fired up straight away and seems to be running smoothly thus far! Fingers crossed I've not done anything silly.

I've attached pictures of the old head and new side by side -- you can see the damage on the old head where the valve seat has dropped out.

There was quite a bit of faffing around with the piston rings, ensuring they were exactly in the right position, according to the manual, as I fed the piston into the cylinder. The first and second ring seem to go on easily, but the oil ring was a bit of a faff until I managed to get the technique right.

I couldn't find anything specifically written on the proper crank positioning in the manual -- no notch markers on the crank as per @breaknwind's reply post. This is the official LML workshop manual and it doesn't mention getting the positioning right even though it goes into great detail on pretty much anything else. So I had to follow the pictures in the manual as they were shown, taking note in them of the positioning the crank and piston face, and following these to what I believe to be correct (it needs to be assembled with the piston essentially at the top of its stroke). The cam in the head has a natural resting point which lines up with some horizontal marks on the cam sprocket, so this was more obviously right I think and lined up with the manual's pictures as well. When the cam is in this position the marks line up and the cam won't turn by hand/fingers in either direction easily, as turning in one direction will begin to open the inlet valve and the other the exhaust valve, which is logically what should be happening when the piston is at the top of its stroke. So I assembled it as per this orientation, torqued everything up to spec, then manually wound the piston through its cycle and carefully watched the valves and piston/crank positioning to check that they seemed to be operating to my expectation at the right time in the cycle. Just past TDC on down stroke #1 the inlet valve opened, then closed toward the bottom, then on the up stroke #2 neither valve opened, nor for the #3 down stroke, and then finally the exhaust valve opened on the #4 up stroke. That cycle repeated. Seemed logical.

I haven't changed the spark timing or anything that would effect it, so I assembled it all and with bated breath started it. It fired up straight away.

I've adjusted the tappets twice to the spec 0.15 mm clearance -- once when I originally assembled the engine, and once after checking again when the engine had run for a few minutes and then cooled off.

Anyway -- I will take it for a longer ride carefully and see how it goes. Fingers crossed it will continue to run. Given that it has a new piston and rings I will take it easy for a while to make sure it runs in again.

Thanks all for your help!