Careful with 139QMB/ 72cc BBKs

[tpryde1]

Stock 50cc Roketa Maui, with the exception of a performance intake filter in lieu of box. I added only the cylinder kit. Cheap mainland China kit for $29.95. I opted to stay away from a new head as it has been really hit and miss over the decades, with much of the Chinese QC. The scooter has only 1200 miles on it. The only other mod is 7gram rollers down to 5.5. I live in Arizona at 2100 feet above sea level and average summer temp is running about 90 these days. So I changed the pilot jet to a #38 and jumped in with my best calculation to an #85 main. Was dead nuts on the pilot jet and only the most minor tweaking for a beautiful Idle. Breaking in cylinder towards the 3/4 throttle mark, things started to go wrong. Throttle started to bog and it was apparent I was over temping. Shut the thing down in a hurry. Plug chop way too lean heading towards a seize. Temperature and elevation warranted a 84/85 Main jet. Scratching my head I instinctively jumped to a 90 Main Jet. And by pure luck I nailed it. Plug chop looks good, nice tan color. Just finished a 55 mile run this a.m, it purrs like a kitten. Most importantly my speed went from a GPS of 34 mph to 44 mph. A most perfect Apex at only 4.5 h.p wouldn't you say? I know it's been said before, but the lesson here is go overboard on the main jet and then start backing down with plug chops. It may just save you about 4 hours of Labor (that is if you're quick). I am new to this site but I've been playing with mopeds for 40years. I'm a retired Boeing 757/767 Check Pilot. So I'm back in the game, glad I found this site! I must say though I've read a lot of posts, many of them are good, but I find that too many of them are Ludacris. Especially some of the speed claims which often just flat opposed the laws of physics. Thanks for being here, its appreciated.

[Roscoe]

I find it curiously satisfying thinking about an engine so small propelling me down to the store and back. I also enjoy riding the bike which at my age is probably only 1/4 hp at full effort. The batting order at my place, cane, bike, lawnmower, scooters, car, pickup. Right now due to a sore back the cane is used the most.

[sc00ter]

Welcome to the forum! Its nice that someone has some GY6 knowledge to pass along. I only mess with 2 strokes but have done bbk's on a Zuma 125 and a Buddy 125.