Help! Yamaha Zuma 2-stroke idles but dies with throttle

[yamahahaha]

2005 Yamaha Zuma 49cc 2T

Symptom: Starts and idles OK cold and drives just fine when giving it throttle for 45 seconds, then dies. Re-starts and idles fine, but opening the throttle at all kills it right away. If I let it sit for a few hours, it will run normally for another 45 seconds but giving it throttle kills it after that.

Checked: spark, fuel delivery to carb, fuel mixture, idle speed, vacuum line seal, oil level, fuel level, air filter, air box, intake manifold

Replaced: entire carbeurator, air intake box and filter

Now I don't know what to do. Ideas? Thanks.

[sc00ter]

Oh boy, some things are hard to diagnose without seeing/hearing it! Whats the miles? Stock? Crashed/beat-up panels? Has it been sitting for a long time before you got it? Did it just start doing this suddenly or slowly get worse? Did you get it with the problem already there? Not trying to be a hardazz, just trying to find a starting point. I LOVE 2 stroke Zuma's. Currently own 2. There is also Zuma Forums.Net, but the activity has been sloooow. The worse thing ever is when someone starts turning screws on the carb and didnt do a base setting first. I hate tuning carbs!

[yamahahaha]

Hey thanks for your reply. I have several bugeye 2t Zumas & a GY6 150 Zuma. I also spent a summer as a mechanic at a moped shop, and am an ASE certified auto mechanic. Never encountered this problem.

To your clarifying questions:

6k mi

Pretty rough condition, did outside campus duty all its life. Blows a low of smoke when it runs, however it's the strongest Zuma I've ever had in terms of acceleration. The newer ones are terrible. Anyway.

It had not been sitting before the problem developed. It was my daily commuter. The issue developed 2Y after purchasing it. Ran fine before that.

I should emphasize how well it runs during the short time it runs well. It will fire up, respond to throttle, and zip up to 35 right away before bogging down and stopping, unless you close the throttle. The problem began intermittently but quickly became consistent and worse over a few days. The first time it happened, I started it up in the morning, drove out of the parking garage and to a stop sign, then when I opened the throttle again, it died.

Kind of stumped and tired of taking it apart. Thanks.

[sc00ter]

The inside of the spark plug boot/cap likes to break and not hold tight on the spark plug. I carry a spare OEM spark plug cap in the trunk. Mostly happens to modded 50's but its happened on my stock Zuma also. The other thing thats popping in my head, but never saw it on a Zuma yet, is vapor lock. My 64 Impala loved doing it! Last thing I can think of. Is something wrong with the transmission? My 98 Zuma had a damaged torque driver and would just choke out the motor when it got up to speed. Luckily I had a spare torque driver. We have the luxury of swapping out parts between Zuma's if needed. I have horrible trouble shooting skills that come from working in a factory. Its cheaper and faster to just swap out a part then it is to diagnose the problem. Will list anything else I can think of later. You keep us posted to.

[sc00ter]

Oh, wait. We have been punched in the nutz from the carb float "warping" from the ethanol in the fuel. Rare on daily use scooters but have seen it happen. It makes the float sit at a weird height causing interesting problems. The float measured different on each side is how we found it. Like I said, its rare. Another thing is the auto-by-starter thing. We go with a manual choke so never see it happen.

[yamahahaha]

Thanks, I'll keep this thread updated if I figure it out.

[Fahqu2]

Air leak???

[sc00ter]

I always forget about Team Calamari and there tech section. Good site, just dont waste your money on that Fly racing exhaust. Scooter SWap Shop did dyno runs with diffent pipes. Your 05 Zuma should not have a welch plug over the air mixture screw. I forgot to ask my boss about the service manual as he has one for a bug-eye. The older blue/white cover service should have the base settings. If you really need me to look, PM me and I'll swing by his place and look. And Fahqu2 mentioned air-leak. Sometimes Zumas like to develop a air-leak on the reed cage assembly along the motor case seams. We use a bit of Dirko or other like sealant and run a bead along with the gasket before re-installing the reed cage.