Modern Vespa

Chuckled.

Last couple days, I noticed the scoot would randomly stall at idle.
Quick chat with SC, advised me that it was likely running too lean.
Too lean? But I have a richer rod in it. Weird.

Then yesterday - my idle started creeping up.
By the time I got home - it was around 2700.
Weird.

Maybe I have an air leak.
Oh man, maybe all this abuse has cracked the welds I made in the case.
Or maybe its a main seal.
Or perhaps its the head.

But the head would cause detonation - not fast idle.
And my seals are Viton.
So perhaps its something simpler.

Rather than tear it apart to leak test - I read my own scouting report and used a trick I'm a fan of.
I'm sure most of you know it - but I'll post a vid of it hear.

Using carb cleaner - with a tube on it so I can be precise with my aim - I run the motor and squirt it at the likely candidates.
When you hit the spot - idle will die down.
Even better if you can pick up the idle higher for the test.
If you watch the video - you can see how one side of the carb had no effect - but the other killed the idle.
Note: I fabricated the intake manifold using some pretty rudimentary tools.
The parts fit up well enough - but when I welded - I got some warp.
I am recalling now that I kinda sanded it with the belt sander - then just cranked the bolts to flatten on install. Not my best moment.
Plan to pull it and either re-make, or see if I can machine down flat.

Hmmmm. Not sure that's enough of a leak for occasional stalls though.

How's the plug look at idle? Dry?

The Smart Carb will "flame out" (stall) if its too lean at idle.
I spun the idle clicker about 5 clicks - and the stalling disappeared.
I thought it was kinda weird at the time - but rationalized it away.

Day later it was back.
By that afternoon - it was idling at 2700RPM.

Today - to warm it up before I ran that test/shot that video - I spun the idle mixture clicker a couple full turns richer - until it knocked the idle down.
I actually turned the screw a bit back leaner before I shot this video - so that I could test it under higher idle.

I think the carb manifold is the culprit.
The spay takes the idle right down.
Will confirm with a little clean up pass to flatten the base - and re-install.

Pretty sure this is my culprit.

Wow the carb is that sensitive? Did not know.

I've tested it with the usual suspects pwk, tm, phbh, vhsh and pulling the carb from the intake rubber to simulate leaks to test idle jetting, just to make sure everything is running properly. Normally still runs with a big ass gap, but is boggy until push everything back together. Thought they'd take a bit more abuse.

Granted that's with a Reed intake… I imagine the vacuums involved are different.

Interesting. Sounds like there is a difference.
One of the things about the Smart Carb - is that it exacerbates any issues.
This according to SC owner - and I have found him to be right.

A touch of additional air in - changed the tuning about 15-20 clicks.
Of course - the scoot doesn't run well anywhere else in the throttle now - because as you open it - the vacuum weakens in the intake manifold - and it stops sucking air through the leak.

But that small leak is picked up by the Smart Carb.
When you ride it - it feels very "precise" for lack of better word.
Very small adjustments to fuel or air have significant effect.

I'll clean up the intake and re-assemble.
Suspect I will be 20 clicks too rich when I do.

The adventure continues…

Always appreciating the pant-browning test runs and experiments in the name of Vespa science…

Weirdly - copy didn't post.
Here it is:

Battling through an air leak.

108 offered some insights/reaction to my vid and comments.
Kinda stuck with me as I worked: is smart carb really that different - or is it likely I have a bigger leak?
The welds?
The main seals?

The carb can give a "fuel injected" feel - but it gets there through tight toleranced, highly atomized, fuel flow.
So tiny air leak = all sorts of 4 stroking and what not.
Great if everything is tight.

My homemade manifold is elongated at the back to allow a bigger sealing surface.
Ironically - this was done to ensure no leaking...
I used a piece of Viton rubber sheet to make a gasket.
I was surprised it was leaking.

First thing I did was pull off the intake and check for flatness.
Looked a little warped - probably from welding.
So I flipped it over and chucked it up on the mill and took a few clean up passes.

Reinstalled - but found that spraying carb cleaner (as before) when running would stall it completely.

Neighbors are a car repair shop. Great guys.
One walked over - inquired as to the issue - and then came back 5 minutes later with a smoke tester.

This thing operates like some kinda power bong!
It's combo pressure tester and smoke maker.
It came with this stethoscope shaped part - with a replaceable sticky face - that I was able to plug right to the front of my carb.
Hala-fu*king luya.
I can pressure test with smoke and include the carb?!
Holy all things holy!
This is like waking up at 30 and realizing broiled broccoli with olive oil and salt and pepper is fantastic!

Still - I couldn't find a leak!
At one point I thought I had - only to realize - smoke was coming out the tail pipe. Hahaha.

So I wrenched down harder on the manifold's nuts (as one does), and tried the carb cleaner trick again.
This time - no change of idle.
Good to go!
(CM suggested it might only leak when vibrating - thus the smoke tester didn't show. Thought that was a pretty good explanation).

Scoot is back to running exceptionally.
Lil vid of me coming into Laguna last night - tick over set to 800-900.
No stall, nice lumpy idle.

https://youtube.com/shorts/lN5L_uGIcWg?feature=share

Oh yeaaaah… idle sounds great.

Apologies for the self induced paranoia.

I've had engines run ok with air leaks. Decent idle, no consistent stalling etc… they just lacked power and the jetting is all over the place…

So was a little surprised about your tiny manifold leak affecting the SC.

Well - it did sound great - until it seems to have stopped wanting to stay idling down low again...
108 - you may have been on to something.
I have some wierdness happening.

Time for proper leak test to sort out what's what.

In the mean time - in a classic Charlieman re-organizing the deck chairs on the Titanic sorta way - I decided what I should really do is build an intake manifold base plate reinforcement.

Have the CNC machine reanimated, and wanted to try my hand at some photo to part CAD that I saw on the internets.
Turned out to be super simple - kinda blown away by the accuracy.

I love it!

Nothing better than the "it was a learning exercise" explanation for the complete overkill solution of the new base plate.

I'm going to learn Fusion360 just so I can start sending you stuff to fab for me

Im just glad I was able to sell the house before you pulled the gates off

Probably just as well. It would have been awkward when I borrowed your truck to take them down to SoCal's place, too.

So nice to be able to mill your own parts!

You can import pictures into Fusion (insert canvas) and then scale them (right click canvas in the model tree and select calibrate). Then you can make your drawing on top of the image.

I use this a ton on existing parts. I also use my scanner instead of a camera for gasket surfaces as it gets rid of any parallax.

Cool!
Will try that.
Scanner makes sense…

Total weirdness continued today.
Going up the hill - I could hear pinging.
Pinging!!
Audibly!!

Editors note: I can never hear it
Looked down to see 403 degrees (head) only half way up.
Sheesh something is terribly wrong.
But what?

Laid off throttle - it felt fine.
Pulled into shop and commenced prepping for leak test.
Think I've found my culprit - or at least one of the issues if there are multiple.

Carb petrol inlet was cracked and loose.
Have been pulling a Chandlerman running without full petrol feed.
See below.

Ran the leak test and it was imperfect - but hard to know if it's real or related to my testing apparatus - given how small. See vid here if u have a minute of your life to waste and wanna weigh in.

Sadness.

Unfortunately, looks like a leak somewhere if it's that speed. Unless it's slows down and stops at around 220mmhg.

It's small, probably gasket related rather than seals. Enough to mess with jetting though.

Exhaust side leak is enough to mess with jetting AND temps too.

Ask me how I know…

Oh, when was the last time you pulled the head and cleaned the top of the piston?

no, don't do that. Nothing but sadness, pushing, and expensiveness awaits you down that road.

You got away lucky looking at all the detonation spatter on the piston. I'm going to guess the head doesn't look much better. Good news is that you can run it like that just fine. Maybe clean it up a bit with some sandpaper.

And I have a very similar mod on my GL's carb. The difference it that I pulled the old brass insert and replaced it with a 90 degree, then slathered the entire thing with JBWeld. A good reminder to me to keep an eye on it, though, just in case.

You need to borrow your neighbor's smoke machine again and keep looking until you find the leak. It's going to be tiny enough that smoke and a bright light are probably your best bet to finding it. And you're certain it's nothing inside the crank case? i.e. Does covering the oil vent slow the rate of loss?

Good thought. I may try the smoke machine with carb and exhaust off to see if I can spot anywhere. I can also pull and plug the gear box breather to see if it slows at all.


Seals are both serviceable from the outside - but the clutch side is likely not manageable without pulling the lump and throwing it on the bench.
I think I made a little tool for sliding it over the cranks edge. Perhaps I could finagle by just lowering.

I lose about 1/2 lb per minute - but it slows.
Perhaps I will also blank off the carb and ensure my leak test equipment is not involved in the seepage- before I go full Monte - by which of course I mean - working on the scoot after stripping down nakey in my shop.

TMI, TMI!

Wait a minute...does this mean you don't all wrench in the buff? I thought "Doc's n' Socks" was de rigueur when working on vintage bikes.

Insert obligatory crank puller joke here: ________

I'm thinking fast orange wouldn't feel too good on sensitive areas…

And Petcock jokes!

I routinely wrench (and weld) in puffy bedroom slippers…but otherwise fully clothed.

I feel like the HS coach who teaches rocks for jocks.
Quiet class.
Quiet!

Turned down a 90 degree fitting from the hardware store and pressed it in to the carb (after drilling a slightly larger hole in the carb...)
Also - replaced the bad O rings on the exhaust.

The pinging is gone (probably because I am not starving the carb for fuel which was just pouring down side of motor...)
Still in search of my mystery leak tho.
Slow but present.

Blocked off the exhaust and ran second round of smoke test.
Removed clutch cover breather.
Turned off lights and used flashlight.
Even filled motor with smoke and pressured.

Zero sign of smoke escape.

Pulled the exhaust flange and shot a side pic of piston.
There is some browning - not sure if this is normal - or if it suggests either oil sucker case issue/clutch side seal/ or nothing.

What do you guys think?

Banjo looks great..!

Brown marks looks strange… never seen that before, especially that uniform.

If the leak is still super slow, I don't think it'll be visible with the smoke.

I'm guessing it a paper gasket related leak.

I'd try a process of elimination, blanking off the exhaust (without the o ring and manifold) testing that. And then blanking off the intake (taking off the manifold) and pressure pumping through the exhaust.

I don't think that staining on the piston is a problem. Looks like a little exhaust is just blowing back into the cylinder a tad early, before heading up the transfers.

I agree, it could be staining, from rich conditions, and a expansion pipe. I've been experimenting running a condition, so rich, that it would not fully reveled, to see what temperatures that produced going up the hill.

My recollection of seeing pictures of pistons that were in motors that suffered from the oil sucker issue, was that they were stained 360°, starting right below the ring. Is that true? Does anybody have any photos of an oil sucker piston?

I have an idea for another test I plan to run… When I get back in my shop, I'll give it a go

I feel oil suckers are a bit of a myth…

I'm definitely glad someone's pointed out the crankcase gasket is a weak link at that small spot where there's not much surface for the gasket to sit. I do think it can leak in that area. But I think the majority of the time its oil sucked through the crankshaft seal and there's usually staining on the crank siting next to the seal and below the seal.

Might be controversial, lol. But willing to stick my neck out for it.

The reason why I believe that is, after opening a dozen crankcases on different mileage bikes, none have had a leak like the FMP videos. There's usually a big pool of oil, mixed with fuel at that area, however I think is due to over rich conditions (probably from damaged rotary pads) and it's just collecting at the bottom of the crank area. Because fuel here is a bright blue/green, it's easy to spot the difference. When you open up the crankcases, that pool of collected fuel drips down towards the gearbox giving the impression it's leaked through.

Good stuff 108.
Tks!

Mine might be particularly prone in that area - I did a lot of "learn to weld" on these poor old cases - so they likely have a bit of warp in them from the heat.

CM has had a lot of luck with the smoke machine.
Less so for me - even with the fancy test machine.
Maybe if the motor were out on the bench.
¯\_(ツ)_/¯

It before any of that: I have an idea for a test that might isolate the issue.
Easy to perform while still on the scoot.
Stay tuned…


🙂

Mad science method ftw!!

Did you take the clutch and flywheel off to check the seals while under pressure? Ruling out everything else would then point to a crankcase leak. Some sensitive microphone on a stick might get behind the primary to listen.

Dark colour on the piston coming from the rings, indicates non sealing rings. Dark colour isolated lower down maybe nothing.......or could be gear oil burnt on by the exhaust gas. Where the leak is will confirm either way.

Haven't pulled cover and clutch yet - waiting u til I have some seals arrive.

I'm using non std seals.
They are viton - 25x32x5.
Found some Honda ones that are same only 4.5mm.
Also found a metal outside version that are 4mm

Not much room on the crank for them to seal to - so thinner would give a little reassurance against any crank movement.

How long do your seals normally last on tined motors ?

Good point, never thought about a number.

Usually the seal is thrown out when bearings are changed… usually with engine rebuilds, or if the crankshaft is wobbly when attached to the bearing.

Not gonna say anything else to jinx it.

This. I've had a couple of fly side seals fail me, but they were usually installer error on my part.

And, yeah, seals get replaced when the cases are opened, and even good cruciforms wear out before the seals, so I don't have a lifespan number on my seals.

You'll eventually find the leak with smoke, I'm sure. You just haven't gotten there yet.

And as to oil suckers, I think that they are real, but agree with 108 that they take up more space in our minds than they really deserve. Of course, if you *do* get one, it's going to be a huge PITA to both find and to fix, so it's one of those "High Impact, Low Likelihood" risks that you really don't want to be chasing down.

I say that even though on my smallie when I had too little surface area on that spot after machining the cases for the long stroke crank, it blew out almost instantly once I ran it.

Not tried smoke, but it must be better than soapy water… that's just messy and it's not that easy to find in reality.

I think CM2's leak is so small smoke is dispersed before you can spot it.

Fingers crossed new method of pressure testing finds the culprit.

I've found leaking pores in the cases which had been there from the factory with smoke. Tiny, tiny little things.

They're almost easier to find than larger holes, because the smoke sprays out like a geyser once you get the case filled with smoke and then add the pressure. It's like an inch or two long pointer right to the leak.

One of my biggest gripes with soapy water is that you spend half your time playing, "Are the bubbles from my spraying the water or from a leak?" Smoke is much more definitive.

This… with you on the "is that spray or the leak"

I ended up pipetting soapy water in the end.

Pores I can see it working, but deformed side of casing or past gasket paper, that's what I'm wondering?