Modern Vespa

Hi guys I need to do a pressure test on my Vespa T5. Where can I buy one or how do I make one please.

Following this one. I've wanted / needed one for a while and think this next build I can justify it.

Here's my set up.

1.25" (I think) expansion plug
Blood pressure cuff bobber thingy
Low psi gauge
Couple of hose barbs to attach the hose to that brass manifold
A compression test fitting from a compression test kit I had.
And some thread sealer


All told it was less than $25

There was a whole thread about this, but it wandered way off topic several times, plus most of the pictures were shared via Photobucket and are no longer available.

That being said, most folks here have made their own pressure testers. Here's mine:



Mine used parts off Amazon and the hose from my compression tester. I have radiator plugs to fit the cylinder stub and manifolds for my side drafts. I just put a piece of silicone baking mat under an old carb or blanking plate for si carbs.

How to build a pressure tester is easy. The more interesting question, IMO, is, "how do you find (and fix) leaks when you have them?"

For that, you have a number of options. You can use soapy water and look for bubbles, similar to looking for a hole in a tire. That'll find obvious, big leaks around the cylinder head and base, or the fly side seal.

It won't help you with leaks in places you can't see, e.g. the crankcase or the clutch side of an already-assembled motor.

Personally, I like to use a smoke tester. It'll find all of those, plus even tiny leaks like pinholes in the castings that you would NEVER find otherwise. You pump smoke into the case and then look for where it sprays out.

Also, if you remove one of the plugs so you can quickly fill the motor with smoke, then close it off and pump more smoke in, that'll get you faster results.

This is what I got coming to make one. I will show yous when it's done. I'm using the rubber sheet to cut out a part what goes on the cylinder where the exhaust goes and another part to go where the carb goes

For the pipes

I'm taking the pipes, the bump part and the clock. I'm not using the part what goes around your arm.

Inner tube valve

A few suggestions. None of them mandatory, but maybe worth consideration.

1) Get expansion plugs for the exhaust stub. Something like this: https://www.amazon.com/gp/product/B08B1RP11K (you'll need to measure your stub). It'll work much better than trying to get some sheet rubber to seal.

2) You may find yourself chasing leaks around those barbed fittings, so be ready for that.

3) Your carb cover plate will be much nicer than mine. Usually, I can't even find it, so I put a piece of silicone baking mat under an old carb body instead.

3) Your drawing is waaaaay too nice. NSM design drawings should look like something that a dim four year old would want you to put on the refrigerator door.

. Thanks I could do better with my drawings 😆. If i glue the fittings together with puncher patch glue and clamp them after they could hold. A expansion plug would be better. Just got to find one to fit a 39mm hole. It's 16mm deep will it stay in.

My Vespa T5 cylinder

Here's a radiator plug that should work for your exhaust. I assume you can also get it on your side of the pond : https://www.amazon.com/Dsnaduo-Automotive-Neoprene-Expansion-1-1/dp/B09P12PNW9

Gasket surface looks tiny on the exhaust end…

Wouldn't be surprised if you had an air leak there.

Running a box exhaust? Wonder if it's worth installing the exhaust and blocking that end off.

Out of stuck on the US amazon but I found them on the UK amazing 👍

I not sure if I had a leak it was running ok when it was on. Got the mallossi on now. I can try blacking the exhaust

Here is my setup...

Might be tough if it's a one piece expansion pipe.

You can install the first section and block it there if it's a two piece.

I have a sip box exhaust on it. I have coming an expansion plug coming today so I will remove the exhaust and plug the cylinder now. It's properly best so it doesn't leak

Yeah, never done it before (not a huge fan of box exhausts)… but worth installing the box exhaust and blocking off the tail pipe.

Then you'll definitely know you don't have a leak between the cylinder and exhaust.

Drawing air through that gap will give you lean conditions. Normally there will be oil dribbling out at that area too.

after you pointed out about testing the exhaust for leaks I'm going to test it with the exhaust off and plug in them test it with the exhaust on. If the exhaust is leaking do I get a gasket for it. As far as I know my exhaust doesn't have one.

You should be able to clean it up good and then seal it with hi-temp RTV Silicone. Some exhausts (e.g. the new SIP Road 3.1) have an o-ring there, as do many of the higher end expansion pipes.

Ah right, got you.

Yeah, could do that… might be easier to find leaks that way.

Depends on how flat the exhaust stub, exhaust end, (might be worth carefully sanding it down on a piece of sandpaper on glass to get it flat) you might want to track down a gasket the right size or cut your own. If things fit or don't fit, high temp sealant will be the way to go.

Exhaust paste usually needs a tight fit.

I made one and l did a test. The gage is in mmhg and it only go upto 300 then it goes over so I pump it up till 120. That's 440 mmhg it wouldn't go anymore. I read to pump it up to 10 psi witch is 517 mmhg. It went down slow. I set my timer for one minute and it was on 120. 440. By the end of the minute it was on 117.

I will take photos of it tomorrow

10 psi is too high. 5-6psi is all you need friend. Don't wanna blow out a seal after all.

That's 258 mmhg. That's ok then my gage goes to 300 mmhg. So if it was too high and it the pressure was going down slow that's means my main seal is ok then. I'm going to test it again tomorrow. I fort my main seal needed replacing because of the smoke. It just needs a good run then after I replacing the main engine gasket.

Pump it to around 220-230.

If it drops after 5mins or 6hours, you have a leak.

It might take about a minute for the pressure to stabilise the needle after initial pumping.

I will test it properly in the morning with the right pressure because it was too high the test I did. 5 mins to 6 hours ?

Yeah, I used to do the test and wait 1 day. Zero needle movement is a pass for me. That's safest, but can't wait that long anymore with builds. Especially if they're not my own.

So I usually test, wait 15mins, if the needle moves even a little, I'll let it wait 1hour. If it moves again, I'll find the leak.

If I have the time, if theres no movement after 15mins, I still wait an hour or two.

Usually the leaks I see on new builds are at the intake area or cylinder head gasket and base gasket. Sometimes exhaust manifold.

On old engines I didn't build myself, I usually see leaks on the main clutch seal, main crankcase gasket, intake area or cylinder head gasket and base gasket. Sometimes flywheel seal, but not too often.

This is why I did my test with higher pressure. I got this off another forum.

I did a test pumping it upto 230 and set my timer for 5 minutes. 143 in 5 minutes

143

Leaking somewhere. Need to find it. When the engine is hot it could be way worse.

Er… never ever heard someone say a seal only holds when the shaft is spinning… that's just weird…

So oil leaks out of the drive shaft when it's not moving?? Something tells me I don't think so.

I think someone is trying to rationalise their leaky engine.

Yeah, there's a leak…

Shouldn't move in 5 mins.

But takes forever to find slower leaks and I have only used soapy water and a water dropper which takes forever to find.

Sounds like you'll need to check the clutch side seal which is a pain because the clutch has to come out and you need to tilt the bike.

Sometimes I think it's easier to rework it, just double check all the work done, going backwards.

Biggest issue with slow leaks that I found is that they play havoc with your jetting, especially when the engine is hot.

Rich conditions go lean when you're at traffic lights etc..

Now it's time for you to get or build a smoke tester to go with your pressure tester...

Yes I seen a video of someone had the engine on its side and put oil on the main barring and bubbles come up. My clutch is out so I could put the scooter on its side and test it. I got a feeling it's the main seal anyway because I changed the oil two weeks ago and it was smoking a lot. I fort it wasn't the main seal because I had that go two times in the past and had fuel in the gear oil so I only charged the main engine gasket. It still was smoking after. That's why I wanted to do a leak test to save me opening up the engine again and have to by gaskets again. I was hoping it might just need a run to clear the smoke. I've got a main seal spare. I'm kicking my self now I should have changed it when I had the engine open.

I'm working on it. Just thinking how to pump the smoke in with the gage on and how to make the smoke.

You don't need the gauge when you're smoke testing, just a little bit of pressure.

My smoke generator consists of a jar containing an old rag and baby oil. A cheap soldering iron sticks down into the rag, which acts as a wick. It won't catch fire and burn, but the oil will smolder, producing smoke.

I have a schrader valve (inner tube valve) that I hook my bike pump up to for pressure and a compression test hose that I connect up to the spark plug hole.



I've found not only my self-inflicted leaks, but also pinholes in the castings that I'd never find (or even look for) otherwise with smoke.

I have it all to make one. I thinking using a fish tank air pump. I have one somewhere. I wonder how much psi it is. How much pressure you think I need for the smoke testing