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I thought I had this running good but now this popping is getting worse. I have a 2013 Jonway YY250TT scooter with the CF250 engine. The scooter is popping through the carb when it is running at 4000 rpms to 5000 rpms. It also won't hold steady at thoughs rpms and will actually surge with bigger than a 138 main jet is installed. I have been trying different jet combinations to see if I can clear it up but it will only get worse but not go away with the different jets. I don't find any air leaks at the intake or carb area but I do have a small leak at the exhaust flange I can't stop. I even tried a different cab but it still does it. I also noticed that I have very little response at my air fuel mix screw. I can turn it all the way in and it will still idle great no matter what size jet I put into it. It has always done this with the air mix screw ever since I found a tool that could get to it and that was about a 1000 miles ago or so. I've have 112 to 144 mains in it to test different combinations. Do I have a valve problems or what is going on? I have 3300 miles on it. All highway miles.

Read more: itistheride.boards.net/thread/5754/popping-carb#ixzz2zqaMKZDv

I thought I might post the results of my progress with this scooter incase anyone that has one of these with the CN250 or CF250 vertical engines might be reading this post. I ended up with a 138 main jet and a 40 pilot jet and adjusted the carb after all the other mods I did. These mods are removing the air box, charcoal canister, and installing a foam UNI style pod filter directly on the front of the carb. I routed the crankcase vent hose with a pcv valve and a fuel filter, to keep dirt out, high on the back of the frame. Blocked off the gas tank vent hose and drilled a 1/8" hole in my gas cap because I suspect my tank vent is plugged. I fixed an exhaust leak and in the process of doing that discovered that my muffler had come apart inside. So I cut the end cap off with a hack saw so see if any parts fell out. No chance, everything is rattling on the other end. So I left the cap off and just pushed on the end cap that bolts on with two screws and used that for the end cap. You'll have to use a couple of screws to secure it. The muffler sounds great and I'm sure that frees up some restriction. On my road test drive this thing runs great. I used to cruise 60mph at 5000rpm. Now it does 60 at 4200rpm. When I'm doing 5000rpm, I'm doing 70mph. Big power difference. No flat spots like before and better take off speed. I still think I could use a bigger main jet but 138 is the biggest I have. I am having problems finding bigger ones on the net. Any input or suggestions are welcome. I'm still fairly green at working on these 250s.

OK Thanks. I will take care of both things right away. How do I get the attachments button turned on for me so I can post pics?

I have a Jonway YY250T, 2013, that I think has a carb problem. I noticed from day one that the scooter has a flat spot in the throttle response between 4k and 5k than again at 6k and over. I have rejetted this carb anywhere from a 110 main jet to a 138 main jet and tried different air intake variations. I also tried different pilot jet combos from a 38 to a 42. I always have the same flat spots. These flat spots really didn't bother me until the other day when I was up against really strong head winds. The winds where restricting the scooter where it couldn't get up to the 5k range where it likes to cruise at 60mph. It was making me hang around in that 4 to 5k flat spot area and wouldn't unleash any more power to get out of it. It was bogging, spitting, and back firing if I would open the throttle then would recover when I backed off a little and run ok but was on the low end of the 4k range and only achieving about 52mph. I now want to try and resolve this problem and get this thing running great. I did disconnect the charcoal canister and vented my gas cap. I also added the pcv valve and filter on crank case vent hose and mounted higher than the cylinder head incase you wanted to know about any mods. Also I am running a Uni style foam filter straight off the front of the carb. There is a hose that ran from some sort of gizmo that is attached to the header pipe on the exhaust and vents to the air box. What is that gizmo and what is the correct thing to do to the hose? I just have it plugged off.

Alleyoop, thanks. And everyone else. It was the tank vent plugged. I cracked the gas cap and it is happy days again. I will try and find a vented cap and solve this problem once and forever.

I did notice something about the gas tank pressure I forgot about until you just mentioned it. It builds up a lot of pressure. The hose that comes off the tank and goes to the canister is a vent isn't it? When I was putting things together tonight and draining the tank, I tried to blow and suck on that line and nothing happens. I couldn't blow through it and I couldn't draw through it. I did have the cap off when I did this. Is this normal or is it plugged?

Funny you mentioned that. When I went through the carb I also rejetted it because I felt it was lean. I went from a 38 slow to a 40 and a 120 main to a 130. It ran stronger but still acted up but didn't stall. It should be plenty rich by now you would think. The fuel air adjustment screw was non responsive before I rejetted and now I turn it about 1/4 turn out from flat to reach max rpm. After that, it will slow down. If I completely turn in flat, it will not stall but just slow down a little. This is my set up other than the rejetting. I removed my air box and installed a foam free flow air filter onto the oem air box air snorkel. Removed the carbon canister and vented my crankcase with a pvc valve and filter. Any thoughts?