Modern Vespa

P200 engine ran when I got it, then stopped. I checked the compression and it was too low, below 100. Ideal compression for a p200 is 130-150. I pulled the cylinder and there were light, nail-catching grooves.

I bought a new oversized piston from Scooter Mercato, it came with rings, a new piston rod and circlips and had it delivered to a motorcycle shop that does overbores. Two weeks, $120 or so in parts and labor later and now my compression is even lower. It maxes at 70 and I've used two separate compression testers.

My best guess is to pull the cylinder again, check and replace the base and head gaskets and to lube + torque the exhaust. The head bolts were all torqued to specs.

Anything else I may be missing?

Off the top of my head - the bore and hone is not right as matched to the piston, the ring gap is off, rings are installed wrong (if possible), or rings need to be run in. Post some pics if you can.

What style is the comp tester? Does it thread into the head?

And to test, you need to put the scooter on the centerstand and kick it over REALLY hard at least 5 times.

You could also lap the head surface ever so slightly. And there should not be a head gasket on a stock machine.

Piston installed right side up?

Are you holding the throttle wide open when cranking? Who did what on the piston? Did you install piston or the shop? I would be checking ring end gap, and piston to cylinder wall gap if you haven't. Did you check the head to make sure it isn't warped? You can lap it using some 600 grit wet and dry sandpaper wet and see what gets polished. You can also do the cylinder top, at the same time. What is the squish, I've read the factory setup has a fairly large squish and lower compression. Maybe the low compression wasn't the grooves in the cylinder wall.

Throttle doesn't have to be wide open.

Stock P200 doesn't have a head gasket.

If the 2 compression testers were not 2 stroke compatible, then the readings mean nothing. Does it run?

this is my first guess. followed by incorrect ring gap or rings, or wrong size piston/bore. under 100 is low, it'll run, if you can get it going, but it ain't right.

check the exhaust too, might be all plugged up? quick and dirty look see on that at least.

-g

Like Jack said your compression numbers are only as good as your guage. that being said ive never heard of a two stroke compatible guage. maybe it is time to learn something. I have two Snap/on guages that i have used for years....mainly on four stroke engines. Recently got into scooters.....hmm. Two stroke compression guage. Help me out here. scott

a compression gauge is a compression gauge. I have the same one I used on old cars that I use on the bike. On healthy bikes, it shows >100 psi, on not healthy ones, nope. plug in, kick until it stops rising, throttle open or not is up to you.

I did the install. I checked the ring gap, I'll pull the head and confirm the head-arrow direction and pull the exhaust to see if there's anything blocking it.

The only consideration I've ever heard of with small engines & compression testing is the hose length. The volume can be enough that it produces inaccurately low readings, so you can't really use them to evaluate the absolute pressure, only the delta on compression.

My concerns are what bore clearance they bored the cylinder to... did they measure the vespa piston at the correct area or rely on the numbers quoted with the oversize piston

what bore clearance dis they set the machine up to?

ie measure piston.....add eg" 4thou" and bore the cylinder to that size??

who ever did it should be well versed in vernier scales and should be able to give you numbers they used..

some ones hand had to set the lathe to the amount bigger than the piston required (I like to use 4 thou)

can you talk to them?

I am worried it was bored too big

You won't see if something is blocking it. Just try testing compression with exhaust off the stub.

A compression tester that works on a 2 stroke has the non return valve in the bit that is screwed into the cylinder. Ones with the non return valve in the gauge could be reading 50psi under depending on the length/bore of hose.

thanks Jack: here in the states we call that the shrader valve. and it is located in the tip of the hose that is screwed into the cyl. Basically it is just a one way check valve. Anyway thanks for the info. scott

What make and model do you use, I've looked and haven't come across one yet?

I have two of the snap/on compression testers, but i think most of the compression testers have a shraeder valve on the end of the hose that screws into the spark plug hole. The shraeder valve is what keeps the compression in the hose so you can read it. without the shraeder valve your compression reading would go to zero as soon as you stopped kicking the engine over. scott

On my tester, the schraeder valve is up by the gauge, not down by the plug, but it's a short hose and the results seem close to right, so I roll with it

Thanks for the help everyone.

I pulled the head and the piston's SC arrow is pointing towards the exhaust port: the correct orientation.

So the idea as I understand it would be to bring the cylinder to a shop to measure the bore size. Correct me if I'm wrong but I can't find those tools commercially at an AutoZone and a pair of calipers wouldn't be accurate enough.

It was a pain in the ass to install the piston in the frame (due to the pesky clutch side circlip) but I'd imagine I'd need to bring the piston in too. I can pull the rings. and measure the gap with a few feeler gauges.

You can leave the clutch side clip in. Once I get the cylinder off by pulling the studs, then removing the clip you can see. I insert a 1/4" drive deep well socket, something small that tapers down like a 5mm in my set, into the clutch side and push the pin out. I did that three or four times at least during my last rebuild.

I have no experience with this make or model but that is the tool that I use.

https://www.amazon.com/Bore-Gauge-0-0005-Increment-Set/dp/B07569BJYG/ref=asc_df_B07569BJYG/?tag=hyprod-20&linkCode=df0&hvadid=331670439271&hvpos=&hvnetw=g&hvrand=4939040586562942048&hvpone=&hvptwo=&hvqmt=&hvdev=t&hvdvcmdl=&hvlocint=&hvlocphy=9019817&hvtargid=pla-644275680660&psc=1
[url][/url]

Its a bit too tight in there to try and knock the wrist pin out from one side only...it works.....proportional to amount of swearing.

I find it way quicker to remove the studs and take the entire top end off...either with vice grip on the NON THREADED part of the stud after head is removed....or with two nuts on the head and a spanner. as one nut hits the other it turns the stud. Nice. Safe. Loctite on assembly

good luck

I'm not concerned about the wrist pin or the piston circlip, so we're kind of getting off topic there.

I called around to some parts spots, none of them have that tool, but my brother in law luckily does so they're going to help me out, so I'll have to pull the cylinder.

They're saying that if the cylinder wasn't properly honed after the bore that, that may create this issue. Can anyone confirm?

Squirt some oil in the cylinder, and perform a "wet test". That will tell you the possible condition of your rings.

Doubt it has anything to with the hone ... more likely the bore is off.

So cal is correct....most likely the cyl. Bore is off. You check the bore with a tool called a dial bore guage. The hone is so you can achieve a good crosshatch pattern in the bore, so you get long life from the piston rings. Usually the person performing the machine work ( boring) will also have the new piston, so they can measure and get the bore correct. Usually they will bore the majority of the cylinder and hone maybe .003." All this being said you need a good machine shop. Not something you can do at home.
On another note: some times it may be cheaper to just buy a new cyl and piston. They are not that much money. The last engine I rebuilt, it was cheaper to buy a new cyl. And piston. As to pay a machine shop to bore and hone.

How certain are you about the health of the engine below deck? If you had low compression before the new cylinder and you still have low compression if the machine shop measured appropriately maybe the problem is not related to the new piston and cylinder?

I'd put the rings in the cylinder and measure gap. If that is OK then probably bore is too. If this is too big maybe bore is too.

+1 crankcase leak could be the real issue

Picture of the rings in the bore would help

Update, with more specs:

This is a second oversize piston for a p200. 66.9 size piston. It's stamped so on the piston's crown. As mentioned, I had it drop shipped directly to the machine shop that did the overbore and hone.

We measured the bore size yesterday and it came up as 66.929mm (2.635 in) The measurements were consistent throughout the barrel: top and bottom of the cylinder, measuring north-south and east-west.

the ring gaps are both at .025 gap. As I understand, there's a difference between a stock piston and the range of oversized pistons. From what I've read .2 is minimum so i took it a bare step back from that when i did the install.

Piston to cylinder wall clearance

Matches this information


Did you measure the piston size to verify its diameter?

something that had just occurred to me... how's the condition of your clutch? and is the cable seated fully and everything is operating correctly?

my thought is, if the clutch is slipping then you're not getting the full juice of the kick so your compression reading may be coming back low.

just a WAG that i came up with while staring out the window and cooking....

-g

that shouldn't move his compression number though - so long as it turns over and gets some revs, he should still get a good gauge number. but slipping will make it harder to get enough RPM that it wants to start. kind of like why it's harder to start a lammy than a vespa - less engine revs per kick due to the gearing reduction happening before the kick start.

Heck, I would never had thought of this. Good man.

UPDATE:

did a fresh hone on the 3rd over re-bore.
bought a new set of rings, gapped them to .025 and

the compression is still at <90psi.

head nuts are torqued to 15psi, done in the X-seqeunce.

fresh base gasket. hi temp rtv silicon on both the base gasket and i applied some directly to the head

I have to get a deep socket 13mm and then I can continue cranking down the pressure on the exhaust manifold nut to reach 55 ft lbs.

i checked the head for warping using the glass-and-sandpaper trick.



"low compression= air leak somewhere between piston/ring/head gasket/head."



what am i missing? for a quick backstory:
this is a new-to-me bike. it started up after a decoke-and-carb-cleaning. then it wouldn't start.

i pulled the cylinder to find gouges, and got it overbored to 3rd over w/ fresh rings, only to get even lower compression than before it was overbored.

Piston upside down?

nope. SC/Arrow are pointed down, at the exhaust port.

What did you find wrong with the second over bore?

How does it feel when you kick it over with a plug? Maybe your gauge is jacked up and lying to you?