2002 Zuma Fuel Delivery Problem ???
 

I'm new here and posted about installing a 72cc performance kit in the newbie thread:

http://www.scootdawg.net/showthread.php?t=56185

I had more time to work on it today, the K&N air filter came in, so I slapped that on, HUGH difference over the stock air box. The engine now revved up again and I got somewhere about 45mph out of it, however, not for very long.
I was doing a full throttle speed run for about a mile, then it just lost power, felt like I ran out of gas. I pulled to the side of the road, the engine still running, and idling well. I sat there for a minute to let traffic go by me, then took off, and had no problem getting back up to speed again. I again ran wide open for about a minute when the problem reoccurred. WTF ???

Now here's what I'm thinking, the fuel valve is closing under full throttle due to low vacuum, and the carb is running dry due to the increased fuel demand of the larger cylinder and jets. When I back-off the throttle, sufficient vacuum returns, the valve opens back up, and gas is once again free to flow into the carb.

Does this sound like a reasonable conclusion ???
And if so, is there a fix that doesn't include replacing the vacuum actuated fuel valve with a manual petcock ???

I have never replaced the factory petcock as I have never had a issue using it with a Malossi cast 70cc kit. I also use all the stock vacuum lines and oil pump. As long as you jetting is correct the stock carb will work fine with a 70cc kit, but if you go with a 19mm get a Stage 6 carb as they come pretty close in the pilot/slide/main AND they use the correct atomizer. You may have had a soft seize. Pull the exhaust and look inside the exhaust port for piston scrapping. A cast iron kit can handle a soft seize, you just replace the piston. A aluminum kit will get trashed and need a total replacement. Look in the exhaust hole for piston scoring......

This is the kit I got :

https://www.zumasrusstore.com/collec...erformance-kit

And it was $399 a few weeks ago when I bought it.
I also used a new Malossi 6117010 belt.
The plug I used was an NGK BRP8HS and it was a nice tan color with a #95 jet. I'm still missing something in the mid range powerband, I may go up to a #100 jet and see what happens. Gotta' get this other fuel system problem fixed first. Maybe I'll go with a manual petcock, those were invented for morons anyway, I've had bikes, ATVs, lawnmowers and snowblowers with manual fuel valves, it's no big hassle. But I'm not yet sure what type of bracket I'd have to fabricate to install one.
However, I would like to understand the stock vacuum valve a little better.
Is the fuel fed strictly via gravity, or does the diaphragm pulse with the vacuum signal creating a pumping action, pushing the gas downstream under pressure ???

There is no need or reason to replace the petcock. All the vac line does is open and close it. No "assisted pressure", just gravity fed. Did you pull the exhaust and check for scoring? If you have a soft seize to many times (3 max with a aluminum kit) you will loose all compression and possibly damage the main bearings. Out of curiosity, did you do a leak down test on the motor after installing the kit? Did you do a heat cycle after installing the kit? The smallest air leak, usually around the reed cage, will cause the biggest problems.

Sorry for my late reply, I don't have a garage to work in and its rained here in NH almost every day for the last month. I also work 6 days a week, and with what odd free hours I do have, I haven't been able to squeeze-in any time for my Zuma until today. Although my lawn cries out in desperation for a good mowing, the Zuma project finally won out.
I'm still not sure what a "soft seize" is, I did look in the exhaust port and didn't see any scoring on the piston. The plug was a bit whiter than the optimal tan color, denoting it's too lean, so I pulled the carb and upjetted it from #95 to #110. My kit came with jet sizes up to #120, so I can still go higher if needed, but I don't think it'll be necessary.
Got it all back together and took it for a blast.
HUGE difference.
It has great acceleration right up through the powerband, not sure what the top speed is yet, but it easily buries the stock speedometer.
I can now cruise in traffic 35-40 at less than full throttle, and still have some more punch when I goose it.
I pulled the plug when I got back and it's a bit darker than optimum, but I'm ok with that, I'm not looking for gas mileage, if I only get 50 miles to a tank I'll be happy. Also, it being a two stroke, a bit too rich is always better than too lean.
BTW, I only run 91 octane ethanol-free gas in it, same stuff I also run in the ATV and all my small engines from the snowblower to the weedwhacker. Not easy to get everywhere, but a few jugs lasts a long time.
Anyway, you were right, it wasn't a fuel petcock problem, but it was still a fuel delivery problem. Hopefully solved.

I always start with the largest jet and work down till the bog goes away. Still do a leak down test. Your aluminum kit will not handle many soft seizes, and a leak down test will help locate issues before they do damage. Also note that the mid section and idle section of the carb will need to be addressed. Drop the clip one notch for starters. My Zuma running buddy has not pulled out his Zuma or Vino in 2 years. Mine is still stock as I lost motivation to mess with it, and I understand the time issues you face. Moped Army has the best tech section on the web. Scooter Swap Shop has a good video section on tech stuff.