Modern Vespa

I have just rebuild a P200 engine and started it yesterday. The engine picks up revs nicely, but then the revs stay high, even with the throttle fully closed (slide 1mm open).

Is the problem with the carb, or with the starting jet? My logic says if it keeps on revving with the throttle slide fully closed, it must get fuel from somewhere else. And the only place it could get fuel from, is through the starting jet. Or do I have it completely wrong?

(I tested the assembly for air leaks by closing the air screw, and the engine died as it should do. Thus no air leaks between the carb and engine).

The choke return spring is stiff enough to prevent the start jet from being open, but you can easily check it visually as you operate the knob.
It sounds more like the throttle slide is not closing all the way; might want to check that it is properly installed and the cable is not binding.
At worst, it may be that the carb base is slightly warped. It happened on mine after many carb removals and installation without using a torque wrench.
In this case, only replacing the carb will fix it.

Did you meticulously pressure test the engine for air leaks (all seals, gaskets, head, etc., etc.) after your rebuild?

I actually meant the idle jet. (I see some guys refer to it as the starting jet). And no, I have not pressure tested it. What pressure should I expect?

Revs stay high, even with the throttle closed- how long are we talking? 5 seconds? Or an indefinite? If it's a few seconds, play with the air/fuel screw at the back of the carb. If it's the screaming indefinite option, it's an air leak!!

Worst case is air leak and it's running lean with enough gas to support combustion. Pull the choke to bring the fuel mix back to normal ratios/idle. Spray carb cleaner in the usual leak spots or pressure test. Flyside seal, cylinder base and carb base are common culprits

You have an air leak, a pretty serious one by the sounds of it.

To pressure test the motor, you need to block off the carb intake, the exhaust spigot, and the spark plug, then pressurize the crank case to 7 psi. It should be airtight at that pressure. If it's not, you're never going to get it dialed in right.

Any ideas on how to block the exhaust spigot?

Either of these.

Thanks, will test engine this week.

The record of my trials and tribulations with a small frame. Hope this helps.

Smallstate slow to return to idle

I much prefer the radiator plugs (the lower image that fits inside the barrel) myself.

I use a compression tester hose that screws into the spark plug as my pressure source. It also ensures that the piston is pushed down to open the transfer ports when I pump it up.

As others have said, air leak. Happened to me with my Yamaha and it scared the crap outta me. Kicked it over and it was rocking at 10k rpm right off the bat.

Pressure test is your friend. Pump the cases up to 5-6 psi once it's sealed it'll hold that for hours without dropping any.

Hi everyone. I have pressure tested the engine, and no leaks anywhere! However, the slide in the carb was a rattle fit AND the carb was warped AND the casing was buckled

I replaced the carb casing and the carb, and now it runs perfectly. I took the Vespa for a test ride, and it runs like a dream.

Thanks for all the advice.