KR-1S.co.uk Forum

I know there are a few Gurus on here so what do you make of this wear on my Barrels for my YPVS engine
I have always run it on 40-1 Silkolene comp 2 pre mix but they looked o.k. last few years i pulled the barrels off
Put it up on the other forum and had a few suggestions but would like as many opinions as possible as i don't want to get this again if i go for a new rebore.
I do think though i'm going to change to a full synthetic oil and increase the 2 stroke ratio a tad.
Ironically it has ran it's best this year with the best power output on the dyno (61.3 bhp) but i think i have ragged it more in the red line
Dyno showed it ran slightly rich at full throttle



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I'm far from expert, but it just looks like a bit of piston slap, and blow by from worn rings to me, hence the brown stains and wear, where the piston has been rocking in the bore.

Could it have been 'running it's best, with most power this year' because it was running slightly lean?? and not far from a nip-up. How many miles had the pistons/rings/bores etc got on them. and were the pistons matched to the re-bore. Synthetic oil sounds like a good move to me.

The mixture was slightly rich on the dyno and told it was best to leave it at that

Mileage on bore has only been about 2800 miles It has always rattled though and drove me nuts
I changed the rings last winter and the wear the wasn't there
I always suspected the original bore was never to the proper tolerances but the last thing i want is excessive wear on the barrels/pistons again in such a short mileage

My gut feeling is with SP - piston slap, but the hone marks suggest the bore is still good and round, but with that sort of 'good running in mileage', the honing normally isn't so visible - so could be clearance as you say.
If the bore's good, piston clearance is good, then - worn rings or gummed up rings can have the same affect ? Could you have been given rings that are too small ?? Partially seized big end ??

At least you've caught it now !!

Re the mixture, obviously the dyno reading is accurate, but seemingly looking at the colour of the underside of the pistons is a good double check - you'll need to search it, but I think you're looking for the same brown as a healthy spark plug (I think).

Tommy

I used to have a YPVS with a stage 3 tune,ran it on normal two stoke oil while i ran it in then changed to fully synthetic oil and always made sure it was at operating temp before canning it.If you get the barrels rebored check that the bores are round and not oval.Sounds daft i know but it is always assumed they are perfectly round.You will need to find an engineering firm with a CMM(coordinate measuring machine) to check this.Also make sure the ports have fully chamfered edges otherwise the rings will snag and wear out rapidly.Lastly while running a bike rich is better than lean it does have it's problems in that any unburnt petrol will wash the oil off the bores leading to premature wear.Hope this helps,i am by no means an expert only passing on what i have learnt over the years.

What was piston/barrel clearance when you assembled the engine?
What was the ring material were they nikasil rings or iron barrel rings?
are you running an air filter?

Many Thanks for the answers
I don't know the piston clearance,i will have to get info off a local engineer for that.
The pistons are Mitaka and their rings are RIK

I currently use Ramair clamp on filters and always used them since the build
It's caught me unawares really

Local engineer has them now and he reckons the bores go tight at the bottom and loose at the top
He going to measure them properly so will see what the outcome is
He reckoned it wasn't an oil problem though

There are a few sins, the port windows are not correctly chamfered, the oil or base stock has not been sufficently refined leaving the verticle lines.
Bores do go tapered but only when hot, if you want to be 100% you would need the both cylinders bolted to a dummy case with the other cylinder a old head with a big hole enough to bore through and the water inlets plugged up and water at 65deg to fill the jackets.Every bolt torqued to spec, then bore and hone the cylinder.
Look up 'torque plate honing' but that is for cylinders with through studs, there is another process for base studs(I think the above is correct)