Modern Vespa

I've been chasing down an air leak for a week or so now in my P200 that seems tied to my air box...pretty sure the issue is the surface that joins the carb. I've repeated the issue with both a 'known good' old carb and a new one out of the box.

Since I've been unable to track down a replacement airbox, are there any janky solutions out there? I've lapped the airbox surface that contacts the engine case, but solutions for the inner surface are definitely lacking, given the tight access.

have you tried lapping the carb itself? you can do that. but the inside of the carb box is a tough one. there's a fair amount of meat there, it shouldn't warp. a good thicker gasket like the blue ones SIP sells can take up any unevenness.

Have you sprayed around the base of the carb with WD-40 while at idle yet?
Was there an RPM change?

In that tight carb box anything you spray at the base will be ingested at the filter and you won't be able to tell much, been there

Remove filter. Small, toolbox-size WD-40. Worked for me.

Wd40 maybe, carb clean or starter fluid just go up in a cloud and get sucked in. These are the typical suggestions, so that's where my brain was at.

It's a cheap way to verify.
http://scooter-speed.com/dellorto-si-carburetor-casing-carb-gasket-sip-premium-non-oil-v9g-92976030/
Also cheap. Nice guys there too.

One important issue worth mentioning:
When you screw down the carb box it isn't torqued, right? That's just holding it down for the real spec torque, which happens via the two carb bolts.
If you don't finesse those two carb bolts down equally, like if you do a full spec torque on one first, it'll rock over to that side. The other bolt will torque at spec, but some of that resistance is it fighting the other bolt.

Both bolts have to be torqued equally once under tension, so it slides down nice and flat.
A quarter here, a quarter there..

The carb cleaner test created a huge RPM drop when sprayed around the carb to air box connection, hence the issue. Both carbs are confirmed flat.

I was thinking about resorting to some gasket goo of some sort. I improvised a way to lap the inside of the air box using emory paper and a prescription bottle top (seriously) but there's nothing else I can come up with to flatten that spot. I got a little relief but not enough.

The gasket ray8 linked has depth and squishiness, I'd use that with grease or a paper one plus hylomar or aviation form-a-gasket, which are far better for this use than RTV. If you use the sealers and the blue one it's a bitch to clean the next time you want it apart.

I would use Micrometer blue. its a paste you apply very thinly to a surface and it marks the other surface where it touches.

Put some on a glass sheet, press the carb onto it to check it really is flat.
when you're happy it is, apply some to the carb face and bolt it to the airbox without a gasket. get a scraper and carefully remove the blue area and repeat until it's blue all over.
my mate did a car cylinder head this way so yours should be a doddle

The likelihood that your cases are warped around the carb is low. Similarly, the likelihood that both of your carbs are warped is low. Let's not start with filing the surface of the cases to be flat - a gasket will accommodate the common irregularity. What's left? Is the cover of air box tight?

The carbs warp all the time if you over torque them, use a torque wrench for sure. They are quick to check with a sheet of glass and fine wet/dry sandpaper, with water.

The air box lid doesn't matter at all except to keep out dirty / hot air but cannot crest a lean condition.

The cases are surely ok but make sure the mating surface is clean.

The box is the hardest to check. Best to borrow one from a mate.