Modern Vespa

Just rebuilt my P200 motor, and popped a Dell'Orto rebuild kit into my 24/24 carburetor. The bike starts fine and idles a little fast, but turning in the airscrew has no apparent effect on the idle speed.

I realize this means that the motor is drawing air from somewhere, but I am focused for the moment on the carb (which was leaking like a sieve and giving me all kinds of problems before I buttoned it up). Does anybody have any ideas what could be wrong INSIDE the carb that could cause this?

As I mentioned, I realize that the air also could be drawing in through the cases, but the bike's idle is consistent, the power is smooth (and strong) and there are no leaks. That, and the fact that the carb has been apart multiple times, lead me to focus on that first.

Maybe spray some WD40 around the suspect areas (throttle housing) see if the idle speed fluctuates.

Or try richening the low end mixture screw instead or closing the air screw

Lower the idle speed as low as you can and try again.

Shouldn't the air mixture screw ALWAYS kill the motor unless there is an air leak somewhere? So even if the throttle slide were pulled out, the air screw should still kill the motor, right?

Do what Groover says, and if that doesn't change the idle, Turn the Air screw back to default, then work on the idle screw. You might have to work both to get the right settings. Search the NSM forms, this topic has come up alot and there are plenty of good postings.

Just for added info, is your top end stock? and the jets in your carb stock? and is your idle jet free and clear of debris? nothing clogging the holes?

Thanks! Will disassemble and spray some cleaner in there to try and dislodge anything that may be stuck (like some old gasket sealant).

Top end is a 2nd oversize. I think that the jet went up a size or two when I moved to a 24/24 carb, but that was years ago and it has idled perfectly well ever since. It was actually idling pretty well AFTER the rebuild but BEFORE I tried to fix the leaky carb. The only thing that has changed is the use of gasket sealant to make the float chamber gas-tight.

The idle mix screw only does a fine adjustment, with most done by the idle jet. When adjusting the idle mix screw you are only looking for a small change in rpm. That change is unlikely to be large enough to kill the engine. If you have the right idle jet then there will be a (RPM) maximum at some point as you adjust the idle mix screw. If you don't find a maximum then it's not the right idle jet.

You might think about flattening the base of the carb just in case it has warped - use wet or dry type paper (with water), maybe 360 grit on a piece of glass and rub in a circular motion.

I hope I read that correctly.

No, the throttle slide needs to be almost fully closed at idle. That prevents fuel being "sucked" out of the venturi spray nozzle or the off idle transition hole in the bowl side of the slide grooves. If the idle speed screw is turned too far, and the throttle slide is opened too far, fuel will be drawn from other than the idle circuit and it will render the idle mixture screw virtually useless.

With the air filter off, engine running, if you stare (safety glasses) at the little nozzle protruding from the side of the carb's throat, fuel should not be coming out of it at idle. The slide should be closed far enough that air is mainly going through only the little notch (cutaway) at the end of the throttle slide.

Idle mixture fuel comes out of the little hole that is opened/closed by the idle mixture screw as you turn it in and out. Don't turn it in too tightly. You can see the pointy end of it as you tighten it up closing up the idle mix hole. If that hole gets enlarged or buggered from overtightening the mix screw, bummer. It can't reduce the idle mixture flow sufficiently, but will run with the idle speed turned up (slide opened) to provide more air for too much idle mix, followed by high idle speed.

What oops and they said. Lower the idle speed (close the slide some more). Then the right idle jet size, usually 55/160, will make it noticeably too lean or rich.

If it won't do that, make sure the idle jet and idle passageways are clear. Remove the idle mixture screw and jet. Plug one hole with a finger, stick the carb cleaner straw in another hole and seal it off with another finger. Give it the finger. Watch for spray from the remaining hole. Do the various combinations. Stick the straw on the bottom of the idle jet and blow.

See what happens if you loosen or remove the idle jet while idling. It should idle rich and usually die if the passageways in the carb are clear. That also may indicate the idle jet is stopped up.

Most gasket sealers and fuel don't mix. Possibly a glob problem. Too much early morning coffee, too many words.

Absolutely sensational advice. Thank you!

As it turns out, also absolutely correct. As a few of you noticed, I misunderstood the function of the air screw in the fueling circuits. I looked at the carb again last night and, sure enough, I had the throttle adjuster backed out of the carb box, placing tension on the throttle slide. I screwed that far enough into the body to give me some slack at the grip, and voila!: the idle and air screws are functional again, and the bike idles beautifully.

Thanks again to all!! Now I have to resist the temptation to go WFO at every opportunity for the next 300 miles.

Nice work!

Here's a diagram I put together a while back to help illustrate the inner workings of these Weber-style carbs by Dellorto.

BTW, what's indicated as the "transition circuit" is aka the "pilot circuit", and aka the "idle circuit", though "transition" is probably most accurate since it continues to contribute to the fuel/air mix across the entire range.

As blackbart noted, the transition orifice is closed (or nearly) when idle is at a bare minimum due to being covered by the slide, making this the position you'll most notice the effect of the idle mix screw.

If you have an older style carb there will be an additional passageway (not shown) because the air corrector and fuel jet for the transition circuit will be in two different places.

I have ~150 smooth-running miles on the rebuild but have a problem that needs to be addressed.

When I first assembled the motor, and after rebuilding the carb, I had gas streaming out of the carb "tower" where the float assembly is held. I carefully disassembled, reassembled, and torqued down the carb body to ~12 ft/lbs. Problem went away.

Over the last few days, the problem has come back, but the gas appears to be coming from the mating surfaces for the carb / carb basket. Apart from new gaskets, any suggestions for fixes? Would anybody use gasproof gasket sealant here?

Unless the body is cracked or has a leaky lead plug, it's probably not really 'leaking' fuel out the base because at that point it's atomized and being sucked down the induction port. More likely it's still leaking from someplace else and collecting in the back. A leaky base will give you a lean running condition which is not good.

If the carb base is warped from being over-torqued sometime during it's history, you can try lapping it on something flat like a piece of sheet glass, the thicker the better. Search the forum for suggestions on how to do that. Make sure all grit is removed so none can get sucked down into the rotary pad. I know the new PX150 manual says to torque the carb to 12-15 ft-lbs, but some say that's too high because they're easily warped and stick more to 10-12. Maybe someone else can chime in about that.

Most of the time the only sealant needed and recommended is grease. If you really want to try something serious, I would recommend Hylomar. It's gas and oil resistant, never hardens, and removes easily with acetone. It's used in both the racing and aircraft industry and is not difficult to order online. But if all things are right, you shouldn't need it.

that picture is worth two thousand words. Thumbs up!

What exactly is the "lead plug" you refer to here? The carb body does not appear to be cracked anywhere, but it may be warped. I tried to follow the Haynes procedure for installing the motor by myself and at one point I had the entire motor wedged in the frame, hanging on the carb body! I know, I am an idiot.

I will check the level of the bottom of the carb body with a level, as well as that of the carb box. I have ANOTHER rebuild kit that I will throw in there, but I do not hold out much hope for that fixing the problem. Other than this leak, the rebuild of the motor went perfectly.

The carb body has several little lead plugs that close off holes drilled during manufacture in order to create passageways. On second thought, these probably have nothing to do with it and I probably shouldn't have even brought it up.

Have you tried lapping the base yet?

Going to try lapping the base tonight, though I would be surprised if that fixes it, given that I get a lot of gas dribbling out of the bike when it is parked (with the tap OFF). The drip will always subside, but it is clear that it is coming out of the carb somewhere, and it seems unlikely that the base is the culprit.

Can you post a photo of the carb, minus the filter?

Will do. It is a Dell'Orto 24/24.