Poor running & loss of power, help needed (Sorted, ta all)

[Nigel. S]

Hi,
Since rebuilding my standard li150 it has served me well, reliable starter, good runner. It was a complete engine strip and rebuild, new seals, bearings etc.
Standard li crank, points ignition, li top end oversized at 57.4. 18mm carb airbox etc.
On the way back from Awfully Pleasant last weekend I had problems. Stopped to fill up then took ages to start it, and when it did start I noticed it had a different tone to the engine, all I can describe it as is a bit blowy, deeper sound, almost a bit like running with an open carb. I took it gently from there and after stopping for a coffee a while later couldn't start it again, faffed about and eventually started. Carried on and a while later just going up a slight incline heard a brief squeal/screech and engine cut out. Not really knowing what to do I left it a few minutes then gave it a prod and it started, so carried on. A bit later I stalled at some lights . would not restart, dragged it to side of road, changed plug and started first time. (yeah, I know... why didn't I do that in first place) Carried on home but still had the different noise, parked in the garage and forgot about it. Long ride and I was knackered.

Took it out an a short local ride out the other night to see how it was, still the different noise and I could swear it's vibrating more than usual through the seat. On way home I gave it a blap, usually I can get the speedo needle tickling 50 if I'm feeling adventurous, but now 45 is tops.

So, I have taken the top end off to have a look.... and need some opinions. No ring pegs missing and to me it looks ok but would like opinions on build up on piston crown and head. I'm thinking drive side oil seal? It's not noticably more smokey, wondering if the squeal/screech was the retaining plate screws.
Don't know if that, and the pics gives enough detail but what do you think? Now thinking just bite the bullet and get the engine off the scoot and on the workbench. This being my first year taking my scooter out and about on a small number of rallies I am still hoping to make Shipston but I am up against it now. I will be Mr Pissed Off from Piss Street, Pisstown, Pissedoffshire if I miss it!!!
The engine was ticking over for 10 mins or so before I stripped it to get some heat in the exhaust to make it easier to get off.
Thanks for looking.



Top plug is the one I took out on way home, bottom is new one but as I mentioned engine was ticking over for a bit before I took it to bits.

[Nigel. S]

By the way, run it on 3%.

[MK Monty]

Nigel. 3% of what fully or semi. The carbon build up would be ok in my opinion. Hard to tell from the pics but have you got brown staining behind the rings or any sighns of a nip. Sounds from your story like a nip. Check the carb rubber for splits. Carb and exhaust studs are tight. It could have run lean and nipped up. When you left it and it fired up again. The innertube leak test would have helped. Im still looking for the 5mph I lost 2 years ago. In my case the extra bag of spuds im carrying may be the fault but not with you...

[coaster]

Like Monty, I also though 'nip up' but the engine internals look healthy in my opinion and the plug colour (first plug) looks good. The 'squeal' that you mentioned is intriguing though and makes me wonder if the problem could be flywheel/ignition related? Points do need regular maintenance and adjustment to counter the wear on the cam follower, worth looking at if they haven't been looked at since the rebuild. I've had the center dust cover come loose as well as the fan cowling and both resulted in squealing noises

[Nigel. S]

Thanks guys,

At the time I did think the noise was from flywheel side but hard to be 100% with full face on and not expecting it. Dust cover was on ok, as was cowling. If I grab hold of flywheel I can move it very, very slighty up down but when engine is running it looks like it is spinning nice and true.
Carb rubber is starting to develop splits but I need to check if they've gone right through. The exhaust stud that's hardest to reach wasn't that tight but it did need spanner to start it off.
I was planning on checking/changing points & timing while its in bits.
Will ponder my next move over brekky.

Much appreciated.

[MK Monty]

If the bore and piston look ok. Check your rings and go down the flywheel route. The circlip on the pin looks like a wire one? I would only expect a modern piston to use those. Most standard pistons have flat bottom circlip slots and the old type ears with holes clips. They can jump out. Worth a look while its in bits.

[Nigel. S]

Just had a look, on piston there is an area that catches your fingernail as you feel around it (visible too), also corresponding on cylinder. Looking up the cylinder from bottom it's around 4 o'clock. Also corresponding on piston ring. Would running the engine with this cause the extra vibes I swear I can feel in my backside? And power loss. I thought the learning curve was beginning to level out!
Carb rubber fine. Yes wire clips. I think it is an ASSO piston, I'm sure it's written on it if I remember correctly, it would've been on old rebuild thread.
So guess looking at next oversize.
Monty, the 5mph you've lost is 5mph you added? Mine should be there anyway!
That might be it today... domestic duties.

[HxPaul]

A friend of mine has a Spanish silver special with a motaplat flywheel and points system.His scooter began screeching coming back from a run.When we got home we found that the flywheel was scraping on the inside of the shroud,on closer inspection we found that the aluminium finned area of the flywheel was slowly coming away from the rest of the flywheel and moving outwards and catching on the shroud.

[MK Monty]

One of the beauties of a cast barrel is they take abuse. Dont be to eager to rebore. The buggers take too long to run in. Basically your piston has transfered some aluminium onto the steel wall. Clean it off with some wet and dry. Do the same with the piston. Check the rings are free in the slots cleaning out any carbon. Check the ring gap in the bore and if ok go again. A new set of rings at worst. Dont be afraid of having a good go on the piston as long as the nip area is away from the rings and use some corser wet and dry to cross hatch it all the textra space and scratches will hold oil.

[Nigel. S]

^ Flippin' brilliant!! I had just logged on to post that very question. I will have a go at it, will get new rings as top one also has damage where its been rubbing against the transferred material.
Thanks Ali.

Looking back at the piston pic the damage is obscured by the barrel stud.

[Nigel. S]

HXpaul... the noise I got only happened the once when the engine cut out. But thanks for the reply I will check that anyway and store that info away for future.
Appreciated

[Wack]

Hard to tell by the photos but the blackening inside your casing could be gear oil , also the piston and head look to be pitted and coated as well. Check your conrod for any notchiness and that there is no up and down play. There is also something on the piston crown, I can't quite make out?

[Nigel. S]

Thanks for reply, Wack.

Piston crown and head have cleaned up ok, the mark on piston is where I'd scraped some of the build up off, if we're looking at the same thing.
On looking at everything again I reckon poor cylinder to case seal, poor u-bend to cylinder seal. New piston rings have a gap of .26mm, old ones had .65 and counting
Had a go at cylinder and piston to remove nip up damage and will reassemble as soon as. (sealing everything properly!)
Had always planned on full strip/check in winter so will wait until then for the rest.

Thanks All.

[nickw]

Not wanting to add more problems but I agree with Wack. There looks to be a lot of gear oil in the different pictures. Have you checked how much oil is left in the gearbox?

[Nigel. S]

Thanks, nickw.

Drained it with scoot level on the stand, only had a tiny jug to hand to measure out the resultant bowlfull but I reckon around 600mls. (+ residue in bowl and still in gearbox.)
I do remember when refilling after last having the side off that oil was appearing at the level plug at about this ammount, even though I thought should be 700mls up to level plug.

Anyway, piston, new rings and cylinder, head all back on. Now gearbox is empty I may as well whip the side off and take a peek.

[coaster]

I can't say that I can see any obvious signs of gear oil and if the base gasket has been leaking then it could only have been slight otherwise you would see oil staining around the joint. I reckon the new rings will make a huge difference.

[Nigel. S]

New piston rings... running in??

Having had the top end in bits I would take it easy anyway, I guess a leisurely day's travelling to Shipston taking my time with a few stops along the way, thick end of 200 miles each way should do it?

[CHRIS in MARGATE]

Not the best way to run in. Make sure you vary the speed and don't labour the engine. Best to change down a gear when engine is under load and let it rev freely.
A good tip to check oil seal integrity is to run the engine on the stand and then block the exhaust tailpipe with a wad of rag. If it stalls then you are probably ok. If it carries on running then you probably have oil seal leak. Might give you peace of mind.

[Nigel. S]

Thanks Chris,
Most of my journey would be A roads through towns with plenty of lights, roundabouts and traffic. Especially the first half. So should be ok with varying speeds.
I do the blocked exhaust thing now and again and it has always stopped almost immediately.
Cheers.

[Nigel. S]

I use a smear of firegum on the ubend before fitting silencer.is it advisable, beneficial or a no-no to use some on the ubend to cylinder join along with the copper gasket. Or flange sealant or copper grease.