Modern Vespa

Consulting the group on this.

Anyways to do a leak down test without removing the carb?

Something that can be made or fabricated you think?

Just for speed purposes because im meeting a lot of folks that seem to have leaks and run an SI carb.

Trying to shorten the time from exhaust off and carb off.

It's just the banjo bolt and 2 hold downs.

Make a plate to use in the carb box if you want to test the gasket between the box and the engine.

Not a super fan of torquing down those 2 bolts. Especially if no other work is done (bearing changes etc)

If they were my own motor, I wouldn't mind at all… it's just a lot of random friends of friends appearing with PX-es.

I'm a little confused. are you talking about a cylinder leak down test or are you talking about a pressure test?

or something else entirely different?

Bung a rag (paper towel) way down in the intake hole... check in morning to see if it has fuel stains or stinks of fuel.

Remember to remove before starting, as Molotov Cocktails are not so popular this year

Ok my bad, not a "leak down test". A pressure test. To check for air leaks.

The usual one to check for seal and cylinder air tightness.

A lot of bogging and leaks around the cylinder base.

And of course, disappearing gearbox oil (no oil at the bottom of the engine).

No way around that, 108.

Even if you were able to plug the two intakes, air will still leak at the throttle hook grommet.

Torque those nuts in unison and you might become a super fan.
Fully torque the nut next to the slide first and you may not

It really doesn't take much torque. Put some rubber over the inlet and use a blanking plate.

I leave the spark plug in and clamp a section of inner tube with the valve removed over the exhaust stub, and pump air in through the stub using a blood pressure tester. Works great.

how do you close off the other end of the inner tube, since you mention it is a section only....

Oh sorry, other end is clamped to a little piece of pvc with a cap. I'll see if I can dig it out and post a pic.

Cool. And does your blood pressure cuff come with a schrader valve or did you have to doctor it?

Yeah that's the way I would normally do it on an engine build. Before the carb goes on.

But I'm thinking about more diagnostics/troubleshooting, where the scoot is running already.

I got it on Amazon, similar to this but different brand:

https://www.amazon.com/MABIS-CALIBER-Sphygmomanometer-Pressure-Calibrated/dp/B00264GT5S/ref=mp_s_a_1_73?crid=1T29K3CVGB4FX&keywords=spygmomanometer-blood+pressure+manual&qid=1675570444&sprefix=spygm%2Caps%2C196&sr=8-73&ufe=app_do%3Aamzn1.fos.006c50ae-5d4c-4777-9bc0-4513d670b6bc

I cut the tube off where it connects to the cuff and stretched it over the valve on the inner tube. Squeezing the bulb pressurizes the crankcase.

Blood pressure display gauge for nibp cuff Aneroid Sphygmomanometer Blood Pressure Monitor Accessories.
https://a.aliexpress.com/_mOXFaOq

I have just the pump, AliExpress to the rescue.

Need to buy the 6mm hose separately though.

A little anti seize on the nuts so that the studs don't want to turn - might help alleviate some of the case stripping potential.

Agreed. Shame to test with cab on - since carb to case is a legitimate contender for leaking.

I always red loctite the nuts, don't want em rattling loose. I just figure all that hardware is a consumable now.

I'm kidding... I'm kidding...

I *do* loctite the studs into the case though. but a bit of slippery stuff is nice on the attachment hardware. however, the newer PX's used Allen head bolts instead of studs, so I can see how this can be a bit of a PITA for a quick diagnostic.

carb=>carb box=>case are all suspects when chasing an air leak (or, even a bad injection leak).

I think there's gotta be a way to fabricate a plug that would fit into the carb. I have some thoughts, but I've been kindve busy with carb tuning.

I'm trying to visualise this too. Any chance you could post a pic of how you sealed the other end of the inner tube section? I was thinking of super glueing it closed but not sure if that would hold.

Still no solution to this yet…

The biggest issue I see is the brass fuel tube in the mouth of the carb.

If that wasn't there, it'd just be a case of using a rubber bung pressed against the rotary valve.

Yeah, I tried briefly to assemble a bung but the spray bar got in the way and then I dropped the project.

Yeah, the bung has to be able to go past the fuel mix screw hole at the bottom…

Still thinking of a solution… might have to be so big it'd be the lid and closing off the air hose hole…?

But what about the oil pump? Lol

Edit:
30mins deliberation, it's not possible without taking the carb off… because you would need to mask off the fuel mix screw hole too… with potentially the screw sticking out.

Looks like it's not worth it for a health check unless you're more invested in the bike.