LCGB Forums

li 150 series 1 full restoration 2 years ago

fitted a 186 mugello, 26 phbh delorto, ram air filter, new gp crank, ancillotti exhaust, 12v varitronic ignition, ngk b9eg plug.

the squish was set at 1.2 mm

synthesis 2 oil at 2%

had it set up on the dyno and checked 18 months later

have done 5300 faultless miles mostly 2 up. always starts first kick

i noticed the exhust got louder and so did club members

there were no signs of any blowing so i took the exhaust off, iv ordered a new one to see if it made any difference

looked like there may have been a small leak from the base gasket so we checked it with an air test and it held up

i decided to take the top end off and fit new base gasket and this is what we found.



















the barrel looks worse than it is. there is no score in it its just marked.

it looks like the build up of carbon on the piston has been hitting the head.

i have ordered new rings and piston.

any ideas why carbon has built up so quick?

would this make it louder?

also the plug looks much too light, would carbon build up make it run hot?

any ideas please

At 2% it's not your oil mix that's causing the carbon build up but maybe the barrel wear? With this much carbon has the drive side oil seal been leaking? I had a base gasket leak on a Mugello 186 but this was more down to poor seal on the transfer port surfaces at the crankcase due to porting modifications. I holed a piston. It was cured with an alloy base gasket from MB which gives a much better seal due to larger contact area.
Paul.

It looks like worn rings have caused the problem.Check the gap.I run a Casa 210 top end on my series 2 and the rings needed changing after 1300 miles.

I had a similar problem with a Mugello 198,although not as bad as yours.my solution was to service/clean the carb.After scraping the carbon off the piston & re-assembling it I have had no issues since.I'll just add before fitting the 198 kit I was using a 186 kit for 5years & many miles with no problems.

Would running a B9 plug contribute towards carbon build up as it's a cold plug? I run the same setup but with DC electronic ignition not variatronic and find that a B8 plug is fine but even that can foul up a bit riding around town

It's burning gear oil which has then gummed and worn the rings. Gear oil isn't a lubricant in the top end which explains the running hot and the louder exhaust will be down to the baffles being gummed up.

I'd have thought the leak test would have shown a leaking seal ?

If the seals are all ok it looks as though the rings have stuck (or very badly worn) and it's just a gradual build up.

Another option may be a too retarded timing or not giving it enough rev's often enough.......

burnside wrote:Would running a B9 plug contribute towards carbon build up as it's a cold plug? I run the same setup but with DC electronic ignition but variatronic and find that a B8 plug is fine but even that can foul up a bit riding around town

+1, the Be 9 will cause it to run too cool and that will increase the ware rate.

B9 is too cool a plug, other than for long runs ie doing 100 plus miles, generally Ill move to a B9 on a run to a rally and once im back to more local runs will stick the B8 back in.

Also, 2% is too little, 3% is better, it's not to do with the top end but the fact that the big end will get less lubrication, by the time the 2% goes into the cylinder, gets burnt and then the big end its less than 2%. On a vespa the path of the petroil is the reverse.

And lastly, Ive always treated (and been told to treat) alloy / nicaseal barrels just like a t5, ie ring changes every 4000ish miles and its always stood me in good stead.

Oh dear!

Lots of advice, but which is right?

There’s nothing wrong with your oil, apart from it may be doing too good a job. Synthesis is as good a racing two stroke oil as the likes of us can buy. BTW. The big end is the first moving component to be subjected to the fresh rush of petroil into the crankcase.

What can be seen from the images is classic blow-by which may be bore to piston clearance as well as rings. What are the bore clearance & ring gaps?

As for the crown of the piston, presuming that you don’t suffer from a limp right wrist, there is a lot of carbon there. What is the underside of the piston like? That’s a good place to look as an indicator of correct fuelling.

Other than you are using too cold a plug (as others have said), it would be interesting to know your carburettor & ignition settings.

Running 2% quality oil is fine and will give adequate lubrication to the heart of the engine. Incoming fuel goes down first intk crank area. People run with more oil for peace of mind but in my opinion that's an unnecessary expense and that isn't the reason for this failure or a discussion for debate on this thread.

thanks for all the replies.

had a surprisingly quick delivery of the piston and rings i had ordered.

tried the new piston in the barrel and it wouldn't go in even with out the rings on it!!

cleaned up the old piston and rings which came up like new. the rings were not stuck in the piston.

the under side of the crown of the piston was a very light brown.

i checked some measurements, the piston to bore clearance was a tight 4 thou. ring gap was 24 thou.

the timing is set at 21 degrees with a strobe.

put it all back together and set the squish at 1.2 mm. i put the new exhaust on and put a b8es plug in.

started it up and it sounds like it did when new. rode about 5 miles and it runs beautiful and feels sharper

i think the problem with all the carbon was that the b9 plug was too cool, it was recommended with the varitronic ignition. also i probably dont rev it enough to clear it.

i dont do motorways but varied riding coast to coast, the derby 150, club rides and rallies

i am going to use it as it is for now and probably check it in about 1000 miles.

thanks phil

Big ring gap.

Hi Phil.

Thanks in return for your frank response.

The choice of plug is relevant but I don’t think the root cause of your issue here.

As I suspected, I think your bore to piston clearance is way too much. Please check with an expert on alloy/nicasil barrels, but I would have thought 0.002" was the maximum for a 200 (Ø 66 mm) so with your set up, I'd be aiming for 0.0015" maximum. As CHRIS in MARGATE has pointed out, your ring gap is too much as well.

Sorry if I'm p*ss*ng on your chips!

I must confess to having never used a 'standard' piston in a Mugello though I have used the barrels.

The pistons should be graded to give the correct clearance. I suspect they are & that is something you could do well to find out because it sounds as though you need a grade between what is in your engine now & the one you tried that won’t fit!

Whatever you decide to do, Good Luck!

Piston crown should be stamped with letter A or B or C (not sure how many sizes ) the barrel is usually stamped with same letter. Make sure your new piston has same letter stamped on it as the old worn one.

With nikasil lined cylinders I didn't think the piston or cylinder were graded.I bought a new piston from Rimini Lambretta for my Casa 185 kit and it had no grading marks on it and it went straight in.

when I holed the piston in my imola they asked me what letter was stamped on the old piston when I rang to order a new one , in my case it was A. The TS1 kits/pistons are graded this way if I remember correctly.

Alloy/nicasil barrels for motorcycles & most performance scooter barrels (such as TS1) are graded, typically ‘A’ to ‘D’ but I would advice you to check out the physical differences.

It may be that in returning the ‘oversize’ piston you bought, that you could still end up with a proper bore clearance if you swap for the correct grade.

As for the rings, you definitely have too big a gap as previously mentioned.

This all needs a little more research if you really want to get to the bottom of this problem as I’m sure all Forum readers would like to see & share in the resolution, as they have all offered their well intentioned advice in good will & hope only to help but learn in the process.

Hi

Many thanks for your replies,

Since my last post I contacted the supplier explaining my problems with the new piston which I was returning.

He said ( as in Bazza and Tornado No.1 posts ) Mugello pistons are graded A B and C, with new rings fitted at 7-10k miles and probably two sets before an upgrade to a B.
and that mugello's do get blow by sometimes due to piston rock.

The new piston and my original are both stamped with the letter A.

Because of the variation in the two piston sizes and with no reference point re: original piston to bore size, It is difficult to know how to proceed may be the original piston is undersized.

There is no lip or wear marks to the bore the fine honing is visible. i think its just the rings that have worn.

Unless any one can suggest a better option please?

I will order another A piston and a extra set of rings.

Ride scoot as it is now for about 200 miles to let it colour up, Then remove barrel and check piston for blow by,
if it is clean then the piston rock was probably caused by the piston hitting the head due to the carbon build up which was in excess of 2 mm.

I hope the new A piston fits, If it doesn’t I’ll put the new rings in so I can ride and have a re-think, maybe a new kit.

This is my first nicasil kit I would like to resolve the problems, It has been amazing.

I could have been riding in blissful ignorance but for a noisy exhaust. lol.
I will post when I have some progress.
Thank you for all your help.
Phil

Thanks for the update.

Do try & get the correct bore clearance, if you can. The dealers that supply these kits should state what you need to achieve.

I'm not pushing as I sense you've had enough hassle, but does anybody know the differences in Grades A, B & C so that you can make an educated guess? The diference between two pistons of the same grade should be minimal. It really makes you wonder! Are they the same mnufacture of piston?

The only ways I know of to make your piston 'grow' is to bead blast which upsets the surface or, as Jerome (Readspeed) used to do, get your piston hard anodised.

But really, there ought to be a combination that should work for you.

The rings aren't such a problem as you can get deliberately oversize & gap to suit. That may be the best that you can achieve.

This is bl**dy frustrating as an onlooker! Gawd knows how you must feel.