Modern Vespa

hi,
I am using Vespa PX-150. I am interested to purchase a compression gauge. I don't know the size of the head where the spark plug is inserted. Can someone suggest to me which one may I purchase so I may check the compression of my engine?
One of the posts in which I searched about compression is 1965 sears Allstate trouble starting .

Well If I purchase the same one what is your opinion about it?
Need your kind guidance.

Thanks in advance.

The plug is 14mm. The kit should have that adaptor. You will also need a means of blocking the exhaust port and the intake under the carburetor. I used a patch cut from a bicycle tube for both.

Thanks for guiding me.Please can you show the picture of exhaust port and the intake under the carburetor.
How can i practically use this tool box ?

No need to plug the intake or exhaust. Just make sure the kit comes with that 14mm adapter (and pretty much every single comp tester comes with that size). When it comes time to test, screw the adapter into the head, then give the kickstarter about 5~7 hard swift kicks. Pretty much keep kicking the engine over until the needle quits climbing on the gauge. To test for worn rings, perform a "wet test". A wet test is when you squirt some oil down in the cylinder. The oil in the cylinder will boost compression if the rings are worn. If the rings are not worn, then your compression will be about the same without any oil inside the cylinder.

Or my favorite push it down the drive in gear, to turn the motor over.

Durrp. What he said. COMPRESSION not PRESSURE test. I read and answered too fast. I used the pieces from my compression tester to build my case pressure test rig.

This article saves me writing the same. https://www.klemmvintage.com/squish&comp.htm
non return valve in the plug thread, is a 2 stroke tester.

Thanks for the reply and your valuable guidance .Well it has 4 brass adapters consisting of

10 mm
12 mm
14 mm
18 mm

As Eric mentioned that the plug is 14mm so the adapter 14 mm will be suitable for my need. Rest it will be useful for my other vehicles.

whodatschrome appreciate your guidance. Well, I am is a little bit confused about the "wet test". First I have to warm up the scooter by adding oil then unplug my spark plug from the head, insert the compression gauge and then again kick 5 to 7 times. Is it the right way I am writing? The second thing I want to clear in my mind that may I drop a fresh gear oil inside the carburetor chamber (when the engine is ON) where the fuel drop inside the piston chamber?

Whatever you buy, it must have a schrader valve in the 14mm fitting. Otherwise, our small displacement motors won't show an accurate reading.

By using the compression gauge what is the perfect range (psi) of a healthy engine ?

I'm sure you'll get a lot of opinions, on a new top end with squish set to ~ I.00 mm I usually see 140 plus. My p225 Pinasco was around 160, my DR177 was close to 170.

I think people like 100 psi to even run.

With a squish around 1mm (and no other cylinder head bowl work done to it), the compression will be pretty darn high (140,160,170). On a stock top end the compression will be MUCH lower.

Cylinder psi = your local atmospheric pressure X your scooter's compression ratio

Cylinder psi = 14.7 psi (sea level east coast USA) X 7:1 comp ratio ("64 Allstate)

Cylinder psi = 102.9 psi

If you want to do the calculus based on various specific changes to your engine's scenario, this link is pretty good:

http://www.csgnetwork.com/compcalc.html

yeah, you're looking for 90+ and ideally 100~120ish

I had a P125 with 75, and it would take a million kicks or a looonnnnggg running push start but eventually it would start and run.

but if you're suspecting poor performance or running from compression, this at least is an easy one to rule out.

I'm just adding on to the "local atmospheric pressure"...
Elevation does have a huge impact on the PSI readings! There could be a 20+ PSI difference between a reading in Delaware or a reading way up high in Colorado.

Well currently my scooter is giving 100 psi I am not too much technical for calculation .What is your opinion about the reading i.e 100 psi of Vespa PX-150

100psi is not great but not horrible either. It'll start and run forever but it's not going to fly up hills or win any races.

Yes, you are right, So what is proper psi for this model (Vespa PX-150 ). Yes for hills or racing in my opinion it is also bad.

What is the reason behind it? The ring pistons are weak or due to some other issues?