YPVS Compression/ Probs

[looey]

Can anyone tell me what the lowest & highest compression figures for a YPVS 350 top end would be acceptable ?.

And what would be the symptoms of having very low compression ?.

I've basically purchased and fitted a complete replacement top end for my YPVS engine, which previously had completely knackered valves and a cracked barrel, but I've now gone from it running perfectly but spitting oil out of the inner valve seals, to running like a bag of sh*t, spluttering and miss-firing and won't even pull through the mid-range .

I took my time putting it together, genuine head gasket, re-torqued after first idle heat cycle etc etc. but it's not very well at all. Setup is identical, carbs, reeds etc. just removed from the old and fitted straight on the new.

Did seem ok for maybe the first 1/4 mile being ridden very very gently but then just like a switch was flicked started running horrible

There's no visual signs of any problems, no leaks and it's not pressurising the water system or anything.

I didn't take compression readings before I took it apart (wish I had now), but the other 2-stroke twin I have has 175psi on both cylinders. This top end is giving 110psi on the left cylinder and 115psi on the right when hot, which doesn't sound very good to me

Anyone got any ideas, loosing the fight with this one

Cheers

[steven]

Hi,
CP= 17-20 x CR

CP=Compression Pressure
CR-Compression Radio.

YPVS Compression Ratio = 6.0:1
17 x 6.0=102psi
20 x 6.0=120psi

So for your YPVS the lowest figure you would expect is 102 psi and the highest would be 120psi
You say you have a 5 psi difference between the two cylinders, that sounds minimal and I would say is an acceptable differential.
Least you know your compression is ok.........just need to check everything else now.
HTH.
steven.
ps, whats the "other" 2 stroke twin you have? and what is its compression ratio?

[looey]

Cool thanks for the reply/info, that's what I was hoping to hear . So assuming I don't have head or base gasket issues, then it might not be a strip down again.

Can anyone advise what bad running symptoms could be caused by a base gasket leaking ?. Only reason I ask is that when I was riding it, one moment it was fine, then in an instant the engine note changed at it started running like this

My other 2-stroke twin is a GT250 X7 but we have 2 in the household. Mine measures 175psi on both cylinders and the other bike 170psi on both cylinders.

Cheers again

[cb250g5]

This isn't the answer to your problem, but remember you've gone from a set of Mick Abbey barrels to ?? presumably standard, so when you sort the issue, you may well have to revisit the jetting.

It ran like you describe when I first rode it. I ended up junking the airbox & running foam filters, it was the only way I could get it to run right.

[jessy03]

Just a thought - it's not electrical is it, if you had the engine out is the stator connectors securely fixed.
Jess

[looey]

May 7, 2013 20:24:11 GMT 1 cb250g5 said:

This isn't the answer to your problem, but remember you've gone from a set of Mick Abbey barrels to ?? presumably standard, so when you sort the issue, you may well have to revisit the jetting.

It ran like you describe when I first rode it. I ended up junking the airbox & running foam filters, it was the only way I could get it to run right.

Yeah I was intending to re-visit the jetting once it had settled in and done some miles, as these barrels look like they are a bit more tuned than the ones I took off, certainly not standard. Thing is, this is all just pootling about on low throttle inputs, so the main jet isn't even in the equation then and like I said it ran ok for the first 1/4 mile then just like someone flicked a switch it started running like crap .

The engine has not been out, just coolant drained, rad off, carbs off, pistons removed, new pistons with new small ends and circlips, reed blocks swapped to new barrels, then re-assembled with new genuine gaskets.

[steven]

i recently, pressure tested the crank case and cooling system on my 4L0.
i read up a fair bit about it on the net first, and i have done a bit of pressure testing at work over the years.
most of the leaks were on the test gear, and that was the most time consuming, sorting out the test equipment and getting it air tight.
after that loads was leaking on the engine.
I rebuilt the engine with everything that it needed, to include crank and re bore by Paul at PJME, all seals were genuine Yamaha, it was an Athena gasket set, and all the time in the world needed to prepare gasket surfaces etc, etc.All fixings were torqued using a calibrated torque wrench.
the engine had loads of air leaks, head gasket, water pump gasket, washer on top of the thermostat housing, (that doesn't house a thermostat).....barrel drain screws, ... but the most concerning was that both gaskets on the reed blocks were leaking.
the remedy was, which is on the net.......use some kind of sealer or silicone, i used hylomar, which i also used to seal the crank case half's.
After coating most of the gaskets in hylomar, it passed the pressure and vaccum tests.
the only gaskets i could get to seal without goo were the base gaskets, and yes, i did have to trim them, as they were overlapping, thus causing a double gasket thickness...... i discovered this when checking the squish band, and realized... the cylinder was sticking up in one corner
i put the base gaskets on and then the barrels, and holding one barrel down firmly, i trimmed round the side of the barrels with a brand new craft knife... a blunt one just tears the gasket material.
after buying 3 or 4 gasket sets, i managed to get the engine to pass the relevant tests.
steven.
ps.....with brand new rings and bores at 1.5mm o/s, and everything else standard, the engine compression figure cold, with oiled bores and 0000/0 miles is 8.0 bar (117.6 psi) on both cylinders.