Modern Vespa

Serious About Air Leaks

I've had a lot of frustrations with my PX engine over the past several years, and I believe I have finally found and eliminated the root causes. I did this by comprehensively checking for leaks in the cases using equipment I assembled myself and I found not one, but three sources of leaks. I know others said "it sounds like you have a leak somewhere", but there isn't a lot of information how to go about finding and isolating them, so in the hope that someone else can benefit from my experiences, I'd like to share what I came up and how I determined where the leaks were.

First, once you understand that a 2-stroke engine works very much like an air pump and that the vacuum induced by the piston moving upwards is what the carburetor responds to at any given throttle position, it becomes obvious that any unwanted air leaks into the crankcase makes trying to dial in a carb a pointless exercise and increases the likelihood of other more serious issues popping up sooner or later. I cannot emphasize strongly enough how important I now believe leak testing is if you want to save time and frustration when dialing in or problem-solving your engine.

To my knowledge, there is no such kit currently available for the Vespa on the market, so here is what I came up with...

To get started, you have to devise a way to seal up the crankcase and introduce about 5-6psi into it - no more than that or you risk damaging a seal - and there's only 3 places to do this, the inlet port, the exhaust stub, or the spark plug hole. The inlet port won't work because on a rotary valve engine the inlet port will never be open at the same time as the transfer port, and since I couldn't find a very practical way to do this through the exhaust stub, I opted for the spark plug hole, which seems to be what is usual, and the easiest way I found to do that was to use a compression tester (which I already had).



Since my compression tester was a one-piece type, I had to reconfigure it with quick-connects so it would work for both leak testing and compression testing.



Note: The compression tester hose will have a Schrader valve in the spark plug end that only allows air into the hose towards the gauge. This needs to be removed for leak testing or else you won't be able to pump air into the crankcase... but don't lose it because you'll have to put it back in for the compression tester to work again. (Compression testers use a special low spring-tension Schrader valve marked with a white band - supposedly auto parts stores carry these).

Next, the inlet port in the case half under the carb box needs to be closed off. Using a hacksaw, file, and drill, I cut and fitted a cover plate out of a 2x24" piece of 1/8" flat aluminum stock obtained from the local hardware store, then cut a matching gasket from a piece of plumber's gasket rubber - get the black neoprene stuff, not the red rubber variety, it will hold up better to the light coat of grease that's used to seal the gasket. I deliberately made mine big enough to cover the autolube holes as well - I'll explain why later. A couple of appropriate-length 7x1.0mm bolts and washers and this part is ready. (Note that I was originally going to connect the tester to the inlet port until I realized the port timing issue and had to plug up the hole.)



Next, the exhaust stub needs closing. Again, the plumbing section of the local hardware store had the answer - a flexible "coupler" with a plastic pipe cap in one end.



Lastly, comes the main section... a 'T' fitting that will accept the compression tester hose, a low pressure gauge of some kind, a tire pump, and be closable with a valve to seal off the system. I wound up going with mini brass fittings because I was making a lot of this up as I went along and wanted to be able to move sections around easily until I settled on a final configuration - plastic pipe sections would probably work too and cost less. Parts used: 1/4NPT ('T', quick-couplers, hose barb fitting), 1/4 to 1/8 NPT adapter, 1/8NPT (mini ball-valve, 'tank valve').



I already had an old combo vacuum/fuel-pressure gauge from my car tune-up days past, so I used that, but 0-15psi low-pressure gauges from Winters can be had from Amazon for under $10. All the brass bits I either got from Amazon for whatever bargain price I could find, or picked up as needed at the local hardware store.

Here are all the parts put together for positive pressure testing.



Note: If you're putting a tester together for the first time, don't remove the Schrader valve just yet so you can first test your assembly by pumping air into it and dunking it into a bucket of water to make sure it doesn't have any leaks. A leak tester is no good if it has its own leaks. Once you've determined it's airtight, the valve can come out for testing.

I found Windex worked quite well to check for leaks: it could be sprayed where needed, bubbled up visibly around a leak, and didn't leave much if any residue.

Basic benchtop testing procedure...

Connect the hose to the cylinder head, smear a light coat of grease under the inlet port cover and hand-tighten it into place, apply a light coat of grease on the exhaust stub and clamp the exhaust stopper into place. Next turn the crank to BDC to open the transfer ports (it may spin there under pressure if you don't), connect the tire pump and gently pump about 5-6psi into the crankcase, close off the mini valve and watch what happens. An airtight crankcase should not leak. With my engine, after chasing down and repairing the 3 different leak issues I had, it held 6psi positive pressure and 6psi negative pressure for 45 minutes without dropping at all. I didn't wait any longer because I didn't see any point. (In hindsight, I'm not sure there's any value to vacuum testing with negative pressure. Positive pressure is all you need to find the leaks. If anyone knows of a reason otherwise, please let me know).

At this point, I've positively determined that my crankcase is now an airtight system and the carburetor should respond 100% as expected (that is, unless there are leaks around the carb base or airbox).

Next, I fashioned another cover plate, one that goes inside the airbox itself. This way I can make sure there aren't any leaks under the airbox at the gasket. Also, this plate will come in handy should any suspicions about leaks come up long after the engine has been installed and operational... I only need to pull the carb and exhaust and use this plate with the exhaust stopper to re-test the integrity of the system without having to remove the engine from the bike.



Some troubleshooting thoughts and tips...

If there is a leak, first look for it on the outside by spraying Windex on the seams of the case halves in the crankcase area, the cylinder base, head, flywheel side seal, around the 4 crankcase stud nuts, etc.

If it's not leaking on the outside, it must be leaking on the inside and most likely is the clutch-side main seal. If you have a bike with the seal on the inside of the bearing, you can test the seal without splitting the cases by removing the clutch cover and clutch, turning the engine over flywheel-side down supporting it as level as possible, filling the main bearing races with gear oil, and with crankcase under pressure watching for bubbles escaping through the oil in the races. (I own a PX, but if you have an older Vespa with the seal on the outside of the bearing, checking it for leaks should be even easier).

If there aren't any bubbles, it can't be leaking there so no need to pull the crank to inspect/replace the seal, just turn the engine back over onto its clutch side and let the excess oil drain from the bearing. But if there are bubbles there is definitely a leak there, and the crank will have to come out and the seal inspected. If the seal appears 100% intact, it may be leaking around the edges of the seal due to the bearing housing being deformed from sloppy bearing replacement procedures. This is an issue I discovered I had and was able to fix the leak around the oil seal with Loctite 641 Retaining Compound, a more viscous version of "green" Loctite 609.

If you can't find the leak on the outside, and it's not the clutch seal, it has to be leaking into the gear box some weird way. This is probably pretty unusual, but happens to be another issue I discovered as a result of two small fractures caused by a small-end bearing catastrophe a few years ago that resulted in needle bearings flying around inside the crankcase. To confirm this, closing up the gearbox is required. This is done by fashioning a 3rd cover plate for the selector box and in combination with the inlet cover plate which covers the autolube holes, pumping up the crankcase and letting it leak down while keeping the clutch breather hole stoppered up or covered with your finger. If when removing your finger air is found to escape from the breather hole, then it's definitely leaking from the crankcase into the gear box, and since you've already ruled out the clutch seal, the next step is to figure out how it's getting there. This will probably require intense visual inspection of the interior of the cases.





As an added bonus, even though the gear box isn't required to be airtight, by pressure testing it you can determine if there are likely spots it will leak oil and address those while it's up on the bench. I found 2 issues this way... the o-ring on the kick starter was leaking slightly, and the other was odd... the thick square washer underneath the nut that fastens the lay shaft was slightly rotated despite the square shape and a corner was pressing on the clutch cover just enough for it to slightly fizzle there. When I realigned the square washer so that it didn't touch the clutch cover and tightened everything back down, the leak went away and the gear box now holds air. In theory, this engine shouldn't leak much of anything anymore and should run pretty clean.

Lastly, if you know anyone who's tossing out a damaged clutch breather, ask for it and see if you can salvage it by cutting off the bell on top and/or cleaning up the threads. Adding an o-ring makes it a handy test hose fitting for the clutch breather.



In conclusion, an airtight crankcase is essential to proper induction and carburation.

Great post!

My leak test setup is a bit different, I use an expansion/freeze plug for the exhaust port, spark plug at the cylinder head and the tester at the intake side. I also set the piston at BTDC and pump to 6 psi, this was done on a smallframe engine.

After an engine rebuild, I found out that one of the case bolts under the cylinder had a massive leak. Instead of opening up the engine to reseal the gasket, I gooped up the cylinder stud bolt with hondabond and let it sit overnite. Re-leak tested in the morning and it held pressure without any bubbles forming from a spray on soapy mixture.

An ideal test is zero loss of psi in any amount of time.

The results of my re-test was a 0.3-0.4 psi loss in 25 minutes.

Do you know the minimum tolerance that is accepted?

I looked high and low and an expansion plug that would fit the 125/150 exhaust stub just didn't seem to exist, which is why I went with the flexible coupler instead (I got the idea reading motorcycle forums discussing 2T bikes).

I'm surprised you're using the inlet port for air. I originally was going to do that, but realized the transfer ports woudn't be open and air would have to leak around the rings to get to the head, so switched to the spark plug. Are you saying your inlet port is open at BDC?

I don't recall the tolerance because in my case it didn't appear I needed to worry about it, but it was mentioned on various motorcycle forums and it was somewhat variable.

According to this document, more than 1lb/min is a problem.
http://www.dansmc.com/vacuum_instructions.JPG

Other sites say any leak is a problem and is not acceptable. Personally, I'd prefer to have none. If you're going to bother chasing them down, why not fix them all and have a perfect induction system?

If you add a regulator to you gauge tool you can use an air compressor.You'll need a good air gauge to test that everything shows low psi before testing on motor.On dirt bikes 5-6 psi max but if leak is found at 2-3 psi that's good.

The engine is reed valve to case induction, the intake port has been matched to the manifold and the rotary pad was bluntly chiseled away. The crankwebs no longer seals at the intake port and transfers are open at BTDC.

I have read no more than 0.5 psi loss under a minute which is borderline tolerance. The re-test showed zero loss after 5 minutes but soon after 0.1 psi leak every 5 minutes. Its not air tight but an acceptable pass.

That's when a regulator is handy ,timing test along with finding the leak.Man you guys have done some home work.

I didn't even think of a reed valve, that makes sense. That would certainly simplify the tester when used with converted Vespas or Stellas.

Ive also heard of filling the crankcase with K1 to find air leaks. Although that is not pressurized and would probable only be good for the rotating seals.

I'm in the same boat as you with tuning issues that never seem to end. I would need to do this leak down test very soon.

Side note: Did finding these leaks help with your off-idle choppiness?

So, where were your leaks?

I use a Mityvac hand pump and some various bits I've made to plug things up.

keaton85
I don't know just yet, but I think that's just a jetting problem that I haven't quite sorted out yet. What I will say it has completely changed how the engine responds to any adjustments I do try to make. Everything seems to respond now as expected.

Where were the leaks? I've already covered that in a drawn-out post here Dies going down steep hills [SOLVED] and mention it in passing in my OP above. Quick recap: around the edges of the oil seal, and in the crankcase wall between it and the gear box, one being on the side towards the rear and the other on the bottom towards the kick starter.

I set up my rig for vacuum testing too, but wasn't sure it was going to tell me anything different than pressure testing, which reveals leaks easier using soapy solution, Windex, etc.

My Mity vac does pressure and vacuum.
Also be aware that the hoses that come with the compression tester kit and thread into the spark plug hole, they have a shraeder valve incorporated. You need to remove it if you are checking how long it holds pressure

I did mention that in my OP above, but thanks for pointing it out again.

P.S.
One thing that frustrated me in writing a long post like this was that there appeared to be no way to imbed the images in the text as on other boards, but instead they all have to be piled up at the bottom, which I think probably makes it harder to follow.

if you insert pics as your writing they stay where you want

I believe

I didn't see any way to do that with uploaded pics. I think I'd have to host them elsewhere, which I don't have set up.

Yeah... easiest way is to use a image hosting service like photobucket or flicker.

Upload you pic, get the image address, and insert it ...

... with "img" tags. Like this.

Regardless, really informative post. Thanks!

Yeah, Photobucket is pretty good for hosting images, Jim.

Just upload your pics to it, go into your library and hover over the image you want to embed and click the little gear icon.

Click 'get links' and then just click on the IMG code (for forums & boards) and it will automatically copy it...you can then just paste it wherever you want in your post. That's it.

You can resize your pics in Photobucket also, if you want to.

I like imgur.com myself. It's what I use for posting my pictures. My favorite feature is that I can upload at any size and then generate bbcode links for "large thumbnails," which fit into my posts nicely, but allow someone to click through to get ridiculously large images if they want them.

There, did it! Much easier to read now.

Pdxjim, Thank you for the thorough write up, lord knows I may need it someday. Gave you a thumbs up.

Thread resurrection request.

After rebuilding my 150cc engine with all new seals, bearings, gaskets and a Malossi 166 kit I have a nasty air-leak somewhere and cant get it to run right, it revs on its own like crazy!

Can Jim please reattach all the photos from the first post as I've searched for a few days on this issue that I have and this thread seems to be the most detailed on on air leak.


Thanks.

Hmmm... I see that Photobucket now wants ransom money to continue hosting my photos. Good this I still have them all saved in a folder. I'll have a look around for another hosting site and edit the post to fix this.

Imgur or tinypic...

I was just reading about that. I wonder if photobucket is owned by someone whose last name rhymes with Shkreli... but I digress.
It looks like Tinypic is owned by Photobucket now, so thumbs down on that one. Imgur looks good. I'll see what I can do.

Oh, shit. Didn't know that about tinypic. And I have lots of photos uploaded there. Shitstorm started with imageshack, next came photobucket and I believe others will follow...

On another forum I've lost lots of detailed threads to the Photobucket ransom situation, the last few years I've been using Flikr though (owned by Yahoo), which hasn't been affected and is good for 1Tb of free storage (free for now, but for how long?).

Using any third-party hosting site runs the risk of your images being lost in the future, which is one of the reasons I notice it isn't recommended on this forum; if you look here:

FAQ: How do I upload a photo?

You'll see this has been covered in detail, and how and why direct uploading is preferred (see "Attaching an Image to Your Post (recommended)").

I'm going to get out of the habit of using Flikr and direct uploading on this forum instead.
Maybe it's too late for repairing a thread like this one, with Photobucket Holes though - there's plenty of it about; it's done a lot of damage.

The drawback to local hosting is that photos can't be imbedded in the text of a lengthy post such as this but wind up all heaped up at the bottom. In fact, I originally had done it that way until the alternative was pointed out to me. Still, it may be the only good solution anymore.

Jim, its great to hear that you still have all the pics.

I went through this thread few times, and the pics would really help before I take out the engine again.

Please find the time to upload them again. 🙏

Before I started my build thread here I also ran with the same problem with photobucket, so decided to upload pics here.

I think its the easiest and safest way.

Rehosted and reposted.

Ta da!

Wow, that was fast!

Thanks a lot!

Will start assembling a air-leak testing equipment tomorrow.

I'm still hoping that I will manage to find and fix the leak with the engine on the bike.

Yes, I see that you mentioned it before, and just tried it myself and found the same problem. Good of you to relink it all - nice job.

I think I have an air leak too.
1980 Smallframe with a new Pinasco Zuera piston, new clutch, new seals, new crank, new carb. ( Dellorto 24mm. )
I have tried for over 2 weeks to dial the carb in but am having issues just getting it to idle. Tried idle jets, 50, 53, 55, 57, 60. The air mixture screw is out nearly all the way and the only chance to get it stop revving is to use the choke.
I have an idea for an easy way to test for an air leak. Please tell me if you think this will work?
Drop the motor part way via the rear shock.
Connect a bicycle inner tube to the manifold and the exhaust fitting with clamps
Leave the spark plug in
Pressurize to 6 pounds
Wait

Hooked up inner tube and tried to fill with air. Getting a high pitch whistle but can't tell where it's coming from. Air escapes too fast

Use soapy water in a spray bottle. Suspect your block off points first.

Found a pretty massive air leak by the intake manifold. I put a bit heat silicone on both sides of the paper gasket and tightened. One bolt tightened down fine the other seems stripped.
Looking at bolt holes one looks bigger than the other. The manifold came with 2 bolts. Neither works in the larger hole. Can't find the originals in my tub of vespa stuff.
Could they be different sizes?
Could the previous owner retapped it?
Did I strip it?

Is this the first time you've had the manifold off?

I'm not a smallframe guy (yet?), but I'm going to guess that there was a previous air leak there, maybe a warped manifold base, and the previous owner tried to overtighten the bolt to compensate and instead stripped it out.

a Helicoil or Time-Sert is probably your best bet to fix it.

I did use a helicoil on the intake and that's fixed, I found another air leak on a clutch side case bolt. (The hole was oval and air was coming out) fixed that.
The 3rd one is odd, behind the flywheel there is a small hole and the air just hisses out of it. There doesn't seem to be crack in the case, but I'm not sure how it got there. The scooter ran okay before I rebuilt the motor so I'm sure I'm responsible.
Will JB Weld fix this?

This is from porting the cases from the inside. Most times if you go too deep, this happens...

Tes, JBWeld can totally fix that. I have fixes like that on a couple of my motors where I've gotten a little over-excited with the ol' Dremel.

Google or search the site and you'll find lots of examples of how to go about it.