Modern Vespa

Hi Folks!

I just bought this Vespa 100 Sport small frame and it's got the infamous carb to intake manifold leak!!



I removed the tank, removed the carb, and checked out the intake manifold. It's got the sleeve we're all familiar with but there doesn't seem to be anything between the sleeve and the intake pipe. No "felt" or any other kind of washer as I've read about (but never seen a photo of).

How do you get this thing sealed up?? Is there a washer I can use? Some Dellorto SHA carbs have a plastic sleeve piece that goes over the front of the intake and seals to the inside of the carb face, but this is just metal-on-metal so it doesn't seem to seal.

The leak isn't terrible, it still runs decent, but after warm-up when you return to idle it is high and then comes down, so there's definitely a leak, and also when you spray some carb cleaner at the intake/carb joint at idle, the bike shuts off, which is a dead giveaway. The plug is very dark, so I don't think it's dangerous, just not running optimal.

Can anyone advise me? I'm familiar with the thread everyone links to, but honestly that isn't of much help to me. What's the definitive solution to the smallframe intake to carb air leak?

Thank you!!!

Take a peek of the felt ring.

MV user Voodoo brought up a neat trick for minimizing air leaks by filling the slits in the carb and sleeve with silicone RTV. Then install a felt ring with a little grease on the outside of the carb

The felt ring should go into the gap between the intake pipe & the aluminum sleeve just before the carb slips into the same gap. It seals against the end of the slotted carb stub. In a pinch I have used a fat O-ring. I use a length of one by two wood scrap to gently pry the carb back pushing tight against the felt ring(or O-ring) while I tighten the clamp screw. That w/ sealing the slots and I have zero air leaks among several smallframes.

If you get exasperated, grab yourself the clamp on Polini CP21 from Treatland. It has a plastic bushing that fits the stock manifold perfectly. You ditch the stupid sleeve thing and clamp directly to the manifold and will not leak again, guaranteed.

Try the other ways first, though. They're cheap and don't make you muck about with jetting.

I love my Polini CP carb though. It's been a good upgrade.

-John

Where does one get a replacement felt o ring? I did not see one on my intake. Is it common for the felt ring to be squished inside the manifold sleeve? Can a rubber one be used? Which type of RTV is recommended?

Yes it's common for the felt ring to get pushed all the way in. If you have a pick set you can try and see if you can catch it. If not, swing by SF Scooter Centre and ask Barry if he has one.

I have no experience with the rubber ones so maybe someone else will chime in.

I think any kind of RTV will work. It doesn't get hot, it's just needs to be fuel resistant. I had Blue RTV lying around.

Hi Folks,

Visited Diego at SF Scooter Center yesterday who showed me an intake and carb that had been pulled from a bike, so I could see how it needs to seal. He said the felt ring is designed to stop crud from the engine getting into the carb, not to seal the carb from air leaks.

He suggested I seal the slots on the carb mount with hylomar seal. I just cleanly plugged the slots with red RTV, as that's what I had, let it cure for a couple hours, then re-installed the carb (no gas!). I'm giving it an overnight curing and will test today or tomorrow. Hopefully no more air leak!!

OK.

I sealed up the slots on the carb mount with red RTV and gave it a day to dry. Fired it up today, sprayed carb cleaner at the manifold and the bike shut off, so there's still an air leak.

Has anyone ever thought to put a head shrink tape around the intake pipe to give some kind of gasket?

Not sure what else to try honestly. I don't really want to pull the engine off and all that. What else can i do?

Thanks!

If you go for a ride does the idle creep or stay pretty mellow? It's 40 year old technology so I lower my expectations a little when it comes to getting the carb perfectly sealed.

The $99 dollar solution... http://www.treatland.tv/dellorto-PHBG-21mm-p/dellorto-phbg-21-as-2557.htm

This carb clamps on with the use of a shim. I haven't used it but considered it for my build because that stock carb set up is so tricky. These guys are located in the Mission.

You won't be able to use your choke cable with the Treats carb since it's a knob style.. however I do believe SIP sells the kit for that problem.

So no greased felt ring on the manifold before you installed the carb? I think that could be your problem regardless of what it might do, It does seal the end of the carb against the shoulder in the aluminum tube.

You can also pull your shock bolt to swing the motor down to make sure the manifold to case nits are tight. Cou;ld be leaking there.

Treats also sell the cable choke conversion.

Great trick tip: Polini CP21 on a smallie - making a choke cable

Rode the scoot around on Saturday. Ran great, but after fully warm you could definitely hear some racing when returning to idle.

So I had another go at it yesterday. Tried an old moped trick that I used to use on the Polini SHA carbs. I used a rubber o-ring inside the mouth of the carb, which would push against the inside of the intake, and I used another, larger o-ring in place of the felt washer (which I could not find) at the end of the intake. You can see in the photo the inside o-ring, and also see that I attempted to use this stuff called "wrap-fix," non-adhesive tape that adheres to itself when stretched, to attempt to wrap the outside of the carb to better seal it. That would have worked great but unfortunately, it wouldn't fit underneath the sleeve on installation, even a single layer, so instead I smeared some white lithium grease around the outside of the carb in an attempt to further seal it up.

Took it for a run around and it wasn't racing at all. Although I will need to run it harder and longer to tell for sure. A carb cleaner shot at the carb mount was the best it has been so far.

My understanding is unless you want to replace the carb and possibly intake, you're going to have to settle for a tiny bit of air leaking from this stock setup, correct? I figure my gf can ride it around town and if she ever rides it hard enough to seize it up, I'll just build up the engine then