Modern Vespa

I'm nearly out of ideas with troubleshooting this issue so I figured I run it past the braintrust here before I spend any more money.

Problem: Rough idle and missing once warm. Starts right up and idles fine when cold. High idle when coming to a stop that can be brought to normal by giving it a fist full of throttle with the rear brake engaged. (all symptoms pointing to lean idle correct?). No issues getting up to speed or missing when on the throttle.

Background: This is a barn find that I'm nearly finished bringing back to life. Everything appears to be working with the exception of this idle issue. This entire scooter was recently torn apart including the entire engine. The bike was fitted with a Malossi 218cc kit. Carburetor was completely gone over with new jets installed but I did not touch the auto choke. Valve clearances have been checked and double checked. I've removed the SAS system with a block-off plate and the evap canister. I've installed a brand new intake manifold and checked everything for vacuum leaks. No amount of adjustment on the fuel mixture screw seems to make a difference.

Next steps: Remove carburetor and remove the auto choke mechanism to clean and check operation. Remove the 38 idle jet and replace with a 40 (anyone know where I can find a 42 or larger idle jet for the Keihin?).

The symptoms in my mind are pointing to a lean idle mixture that is compensated for when the auto choke is open. I just don't understand why giving it a fist full of fuel would bring the idle down and stay there? I'm also at elevation (6500 feet) which is why I'm hesitant to start richening anything because usually I have to go much much leaner on jetting compared to the rest of the world.

Any ideas are greatly appreciated.

Have you checked the float height ? Low level would bring on lean running. Giving it throttle will result in more fuel coming through so would mask the problem.

Only a thought , Im sure better heads than mine will be along with a solution

Interesting thought. I did check it the first time but let me ponder that. You might be onto something.

Keep the ideas coming.

This is a red flag - normally the GT200 is fairly sensitive to the air/fuel idle mixture setting. So double-check everything in the carb.

I should restate that. Adjusting the air/fuel screw certainly does something. I can easily stall the bike out by turning it in too far, but it has no impact on this particular situation.

Just as a sanity check, this is an air screw and not a fuel screw correct? Meaning turning it in is closing off air and making it richer. I'm currently running at about 1 1/4 turns out.

Holy cow I think I've got it backwards. Thinking I've been making it richer and I've been making it leaner. I'm just gonna go make a big adjustment (like 3 turns out) and see what that does.

Stand by.

I think you should be 2 1/4 to 2 1/2 turns out.

I just cleaned the carb on mine. The idle jet can be finicky to get all the varnish off so much so that I bought a $100.00 Ultrasonic cleaner. I use the strand of wire technique to poke through and then it takes a 30 minute bath in the cleaner.

When I put the mixture screw back in I started at 2.5 turns back from completely closed (lean) It ran ok and I tooled around town for about 20 miles and then filled the tank and did 130 miles on a morning trip. I noticed it still wasn't idling really smooth so I adjusted the RPM a little higher with the throttle adjustment until it smoothed out a bit and started leaning in the mixture screw until it nearly died, then I went back to rich, which like you, I noticed doesn't do too much at idle.

Back to lean where it wanted to sputter and then slowly enriching it-about 2 1/4 turns from closed is where I wound up in Las Vegas at about 2200 feet elevation. The RPMs picked up just a hair and I had to decrease the RPM's again with the throttle adjustment screw.

It's running really smooth now. I think I saw a video that said the mixture screw should never be more that 3 turns out from closed, if it is there's a problem with the idle jet. Hope this helps. I would also make sure there's no air leaks from the rubber intake.

6Sigma Jet Kit.Com is where you can get jets for your carb
They also have different stages of tuning kits that can be tailored to your specific riding conditions
These guys know there stuff plus they sell a nos kit for the gt200
I have there jet kit and nos kit on my 05 gt200 with 25k and have had no problems what so ever.

We have jets for the carb. They're pretty cheap. But replacing one piece of brass with a hole in it with another piece of brass with a hole in it isn't going to fix this. And raising or lowering the needle on a CVK carb? Who does that?
Find someone who can show you how to set the idle circuit screw. It's way off.