Modern Vespa

Stock p200.

Can't remember how much the seals can pressurize up to. 5-6 psi? My bicycle inner tube doesn't want to go past 4.3 psi before the tube balloones so much the tester leaks. Pressure is best in the 2.5 psi range. Slight ballooning but pressure holds for minimum 5 minutes.

Everything fly side isn't showing bubbles (flyside seal, cylinder base, exhaust stub)

Motor runs like shit and won't idle. Swapped carbs, rotary two stroke oil test takes about 2 minutes to drain. Stator rewired, consistent spark. Compression @ 120.

Scratching my head. Anything scream out at anyone? Get a better inner tube?

Definitely a leak somewhere if you can't get past 3psi.

Usually aim for 5psi for the test.

Compression test doesn't mean much on a 2 stroke vespa as a ring gap and bore measurement will give a better idea of cylinder health, there's no valve leaks to check on a 2 stroke.

Did you check on the clutch side seal? Pour oil into the bearing and see if there are bubbles…? It's difficult to do with the engine on the bike.

Leak culprits are cylinder base gasket area and crankcase gasket around the kickstart gear.

^^^^ this.

Plus, I spent 2 days chasing phantom leaks before I realized that it was my test rig that was leaking. Fix/build a better rig first. Save the angst.

Even if the pressure test wasn't perfect, it would still idle

Changed the exhaust?

Check the stator ohm readings on the green/white and red/white. Check that the float/needle isn't leaking gas.

Get some drain plugs instead of an inner tube. Something like these, but measure for your particular outlet.

No better options but a blanking plate and silicone mat or inner tube for the carb inlet. Pressurize it via the spark plug. I use the hose of my compression tester. Make sure whatever you use doesn't have a check valve in it, though.

I'm Team Air Leak on this one. Hopefully not the clutch side seal for obvious reasons.

And if it is leaking, build or buy a smoke tester. It'll find leaks that you would NEVER find otherwise. Fill the motor with smoke by removing a plug, filling it with smoky air, then re-pressurize it and you'll see the smoke coming out the leak, including if it's your tester's seals.

If you get a leak be sure to spray some of your soapy water over the ends of these. Ask me how I know.

Haven't pulled the motor. Still in the frame doing what I can till I have to drop it. If I do, I'm changing seals just cause!

Good to know. I'll swap it tonight and see.

Revisit the airbox and carb mounting too.

I get 496 ohms and 108 ohms.

Float looked good but I cleaned the original carb. Swapped it with a spare and same problem.

I'll have to dig out my old Malossi side draft inlet. Use that instead of the blanking plates I built. Cap the exhaust with something.

I definitely will! Airbox was tight without the carb but the case to airbox gasket was definitely compressed.

Yep! Found all the leaks in my pressure tester and none at the cylinder base/flyside seal

You can use a drain plug there, too, if you go that route.

If it's leaking past the inner seal, the gearcase will be pressurized and you should be able to detect it at the vent.

Yep. Pulled the vent and didn't hear any noise from that area. Didn't feel any pressure and no noise change if I pressed down with my finger and tested and then released.

Gear oil didn't rush out at me when I checked the level. Smelled like old oil, not gas. Consistency felt good too.

All good signs

So the winners are Jack and whoever said to change the spark plug. Lapped, cleaned and new gaskets on all and still had rough starting. Swapped exhaust and consistent starting with rough idle. New spark plug and purrs like a kitten.

Thanks all for the reassurances. Was wracking my brain late last night doubting myself.

I had a plug that gave me intermittent stalling and misfires. It wasn't until I tested it that I found carbon tracks on the insulator was causing it to spark from the insulator to the threads. Actually I felt it first.

Good idea ! Do you know the right diameter to fit a stock 200 exhaust manifold ?

Just measured a spare cylinder and the exhaust stub measured outer diameter 44.3 mm and inner 37.3mm.

You want to size it to the inner diameter.

Thanks,
I just ordered an expansion plug made for swimming pool pipes, looks like the one from the link
The minimum size is 38mm, it should fit tight.

Couldn't the air escape in between the cylinder and the manifold ? That is a long thread but could false the measure

Now I have to cut a thick plate of aluminium to shape, drill and tap a hole for the breather, I already have a blood pression tester.

The exhaust stub in the 200 has a ridge inside it that makes it difficult to seal, so the plug seal didn't work for me. I used a rubber pipe sleeve from the plumbing department, hose clamps and a big pill bottle.

I just checked on mine, no ridge
Will see when the plug arrives

I made my pressure test kit





Unfortunately I have a leak... I just finished totally rebuilding this engine, this is very frustrating... Could not find the leak with a spay a soapy water, no leak on the flywheel seal, nothing at the base of cylinder, nothing between the cylinder and the head, clean on the spark plug, the plug on the manifold is ok. I can't believe the oil seal on the clutch side is leaking, it is brand new, the crankshaft is brand new, I have the special tool to pull the crank from the clutch side and I know how to use it, please no, no, don't tell me I got a failed oil seal, please.

What is the last check ? The exhaust manifold, hopefully the engine is on the stand, I rotated it 180° the have the manifold pointed upwards and I was able to spray from the top and see some bubbles, and YES, a lot of bubbles ! it leaks from the manifold where it is screwed to the cylinder, good news, it doesn't matter finally !

So guys when you have a leak, think about the manifold connection to the cylinder before splitting your engine

Now you need to build a smoke tester and use it to pump air into the motor. You'll see the smoke come out wherever the failure is.

It finds leaks that no other method has found for me.

You are absolutely true ! I need a smoke machine, I'm going to try to find on the net how to built one myself

How fast is it leaking?

With the inlet and exhaust suitably blocked it was just a matter to connect the pump and gauge to the bleed nipple on the inlet bung and pump away. If the motor is healthy then you should expect it to hold pressure, or more likely to leak very slowly. It has been suggested that the threshold of acceptability is losing less than 1 psi (or in this case 50mmHg) per minute.

https://archive.scooterlab.uk/engine-pressure-testing-leak-cheap-tech/

Short version since I'm on my phone: baby oil and a rag in a jar with a cheap soldering iron.

Search on here will also turn up examples.

I don't get it (English is not my mother language, french inside ), you mean using a tube like in the video ? I dont have an intake manifold for that kind of carb, SI 24 only...

I could remove the exhaust manifold and try to block the exhaust port but I dont have anything that would fill the hole. Maybe a cone shaped silicone plug would fit ? Don't know

You said you have a leak. I am asking how fast does it leak? 50mmhg/ minute like the article says? Or faster?

Faster, I'd say 100mmHg/min at least, maybe 200 ! But the leak from is manifold is large, as soon as I got soapy water on the threads I could hear the sound of leakage + lots of bubbles.

200cc stock cylinder and exhaust stub? If so, you can unthread and retread the stub on. Make it a two person job so it is extra tight.

Yes 200cc stock cylinder/stub. I took the stub off the cylinder to clean it, maybe I didn't screw it back tight enough (cylinder was in the vice so I put quite a lot of force).

Or use some heat resistant silicone on the threads (for fireplace application, 1600°C grade)

The HVAC industry uses smoke matches.


https://www.amazon.com/Arctic-Hayes-Strikes-Smoke-Matches/dp/B01BRL6A2W/ref=pd_vtp_h_pd_vtp_h_sccl_1/142-3106694-8485819?pd_rd_w=OWTfu&content-id=amzn1.sym.e16c7d1a-0497-4008-b7be-636e59b1dfaf&pf_rd_p=e16c7d1a-0497-4008-b7be-636e59b1dfaf&pf_rd_r=WZEZ7BFN86B34Y5TGQME&pd_rd_wg=lPFLx&pd_rd_r=236382da-9445-45d8-b5e3-cb319e102531&pd_rd_i=B01BRL6A2W&psc=1

Nowhere near enough output, plus you still need to get the smoke inside the motor.

I mean, sure I could just buy one, but where's the fun in that?

Swisher Cigar at the corner bodega.... $1.00

https://www.youtube.com/shorts/0TuVwLUugU4

I used these on my vmc super g exhaust stub
https://www.amazon.co.uk/gp/product/B08JTW5JRV/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1