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

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.

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!

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.

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.

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.

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

If it was a broken steel reed it would not run at all. Did you check the compression? I dont think thats your problem but you never know. Have never seen a stator fail on a Zuma either. Still giving it some thought for you.

I was wondering about the reeds, I'm not too familiar with 2 stroke engineering so I'm not sure what the symptom is if the reed fails.

I'm going to keep focusing on the fuel/air system. I might try putting some fresh gas in it?

You said you changed out the whole carb, was it new or a proven used one? I take it the symptom stayed the same after the carb swap? Like I said, we have gotten lazy and just swap out parts from each others Zuma's. We have went as far as to swap out a motor (I have a working stock spare) to narrow down a electrical issue in the loom that was a bear to find! Loosen the fuel cap and see if the symptom changes, just to rule out vapor lock. You miles are just high enough were stuff starts to need adjustment or wears out. And again, make sure the plug cap fits on correct and is not loose. Friend had a SYM Jet 50cc that he purchased with almost identical issues. We (friend and I) messed with it and just told him he was stupid for buying it with a problem and gave up. I wonder if he ever got that piece of junk running? Actually, it was a nice scooter, just would stall at random for no reason. But he never should have purchased it with an issue like that. Will ask him if I see him this week.

Not the same in any respect but a friend has a Honda Metropolitan, the liquid cooled version, with crazy high miles. He got gas one day and the scooter started running funny. He thought it was bad gas and flushed it all out. Problem did not go away. He asked me to take a look at it but does not have a service manual! How do you work on a Metro without a service manual? Im not looking at a tiny screen on a phone. Anyways, we could not figure it out so he ask a friend who is a MASTER MECHANIC at a huge lawnmower shop in town. Turns out it was the fuel pump starting to die. I never would have figured that out, even with a service manual! I like simple 2 strokes for my scooter toys but I do love the Honda Ruckus, and it has the same engine as a Metro.

I got it running a bit better by fiddling with the fuel mix screw on the carbeurator. I think it would run for a bit because of the electric choke, but wasn't getting fuel during part throttle only. Turning the mix richer resolved the stall and it's running better but not perfectly.

I forgot the factory settings, but I think they are listed in the pre-emissions service manual. Will look if I get a chance. I also seem to think Team Calamari has/had the adjustments listed in the tech section. Again, I despise carbs and stink at adjusting them.

Agreed!

Here is the kicker toward my dislike of carbs, I work at a factory that makes small 2 stroke motors. I work in the engineering department. If I run into majer carb issues and I cant figure it out I hit up a friend of mine who has a much better understanding of carbs. I've even hit my boss up before. He has a doctorate gedree in mechanical engineering/engine studies. My old boss built FAST 2 strokes but got kidnapped by Google X.

Air leak???