Modern Vespa

I cannot loosen the four screws that hold black cap/cover of the Keihen CVK KT carb. I'm putting a lot of weight into the phillips screwdriver and turning, but it's not budging. The screw head is starting to strip out. I've tapped the screw a bit with a drift, and put Liquid Wrench on it. Anyone else had this problem?

I might get a JIS screwdriver, some people seem to have had better grips of screw heads with that.

It's part of a 2006 LX150.

My first post here!

Put penetrating/releasing oil on leave to soak, the next bit might sound strange but used a lot in engineering, Try tightening, then un tightening if it doesn't work do it again a number of times, also you can try tapping the screwdriver handle with a small hammer while doing this.
keep us updated, if this doesn't work there is other ways, also for the small cost I up grade all serviceable nuts bolts etc. were prone to water ingress or heat to stainless steel with the correct grease and torque setting.

looks like you've already moused one of 'em so that's gonna be a toughie to get.

an impact driver would be ideal in this situation.

usually if they strip I get on them with a pair of small cobra grip pliers. some needle nose vice grips work well too.

good luck!

A shot of Sea Foam Deep Creep on each of them, let it soak overnight. If they won't budge, cut a small slot with a dremel tool and cutoff wheel. Use a big straight slot screwdriver. Go to ACE hardware and buy 4 small allen head screws to replace them.

nope

Maybe put a piece of angle in a vise to give you something solid to whack against with a hand impact
If it all fails,
Take a dremel cut-off blade/wheel & slot the heads of the screws
Depending on how thick the wheels are you may have to double up

If you don't have an impact driver, try a Vise Grip (locking pliers). I've used this technique many time. These screws love to get stuck. Replace the screws with an Allen head or Torx screw. Much easier to loosen when the screws stick a bit.

Shot of PB blaster and small vise grips.

Certainly wouldn't use an impactor of any type on an oxidized/ceased screw/bolt 6mm or bellow 15Nm, sheer the head and you're in a different world, there's a reason every nut/bolt and screw has a torque setting, when oxidization sets in and creates a temporary weld, using releasing agent and the correct hand tools, screwdriver/ spanner, you also have one other excellent thing that an impactor hasn't and that is feel, and when you are in the trade over the years it becomes invaluable.

have you ever used a impact driver? that's literally what they're designed for.

and if you bork it up with one of those, you were gonna bork it up regardless.

I've seen more way more busted off fuel inlets and float pin legs than snapped screws on these things over the years.

Yes several, it took me 4 years to learn my trade and use them nearly daily for the last 27 years, Quite agree with your last sentence, this is why I exchange Italian monkey metal fixings for stainless with the correct lubricant saves problems further down the line

Applying a thin coat of antiseize to the new Allen head fasteners will prevent this from ever happening again.

Use a cutting wheel on a Dremel to carefully cut a slot in the top of the screw you have torn up so you can use a flat blade screwdriver when you figure out your next plan of attack. Don't go too deep so you do not weaken the top of the screw and move the cutting wheel back and forth so you cut a flat slot.

4 and 5 inch needle nose vice grips are the best way I've found to loosen carburetor screws once I've destroyed them with a screwdriver.

the small knipex cobra grips are the cats ass for this job. but not everybody wants to spend $40 on some little pliers, which, fair. but the vice grip needle nose are cheaper, easily available and everybody should own a pair.

Finally got those little beasts removed! I put Sea Foam Deep Creep on them for 24 hrs as Abner suggested. I received the #2 JIS driver in the mail today ($10) as Skids said, and that grips so much better in these bolts than phillips. I got three out that way. The third got too gummed up, so I cut a slot for a flathead with a dremel tool. The flathead could grip better, but not enough. Finally what worked was to put the blade of the flathead against one of the roughed up edges of slot I'd cut and tap it with a hammer. That freed it. Same principle as an impact driver, which I don't have. I'm looking forward to replacing those with Allen bolts. The size is M4-0.7. Thanks team for the help and encouragement!

They are kind of weird bolts, with a very long shoulder, and only a few threads.

nope

You'll find an amazing array of nice replacement metric shoulder screws (or "shoulder bolts") at McMaster.com - with Allen heads. Again, I recommend stainless steel with some anti-seize on the threads.