Engine quits on top end

I just recently built a Schwinn Jaguar with a Skyhawk 66cc, slant head. It runs great from the bottom end to the mid/top end and just when its about to peak either the ignition or the carb is quiting, its just like turning a switch to the off position. I can give it gas but it runs slower. I will coast for about 30 feet and slow down a bit and give it gas again and it pulls hard and then peaks and no power. Feels like the ignition is cutting out or the carb is running out of gas. The engine mods are ported intake and exhaust manifolds and a couple of extra holes in the air cleaner housing. The plug is a nice tan color, the clip is second from the bottom and the stock spark plug is being used, tried a colder plug but it ran worst through out the range. Im stumped! Any help will be greatly appreciated...Thanks Tom

SlowBalt said:

I think your four stroking bad. Try leaning out your air fule mix.

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No its not four stroking at all, actually I think its running lean. The plug is tan and very clean. I did check my motor mounts and notice that my rear mount had gotten loose. I will tighten it up tomorrow, but since I installed this engine, I always had the problem of it cutting out on the top end. The engine only has about 100 miles on it. It is a lean condition for sure, I will make sure the carb is fully seated. I ordered a CNS carb this morning from Spookytooth and might get it by next week, and hopefully it will take care of the problem. If not I will install a carb from my CR250...

in my original post I mentioned that I tried a different plug, it was an Autolight, equivalent to a NGK BP 7HS and it wound up being to cold of a plug and might be better after break-in. Especially here in Florida where it gets really hot. I'm going to to check the float level and try to raise it a little and I will also check for vacuum leaks around the intake flange and carb mounting area. I will keep you posted once I resolve this pain in the ... I have a Flying Horse 66cc. and that thing will rev as long as you hold the throttle open, but it is not ported. I think by porting an engine you will use gas at a faster rate than an unported engine.

Well guys I fixed the problem!!! I pulled the carb off and took it apart, cleaned it out with some carb clean. Then I raised the float some, but not much, well about as high as it could go! Then I took off the intake tube and used a high temp silicon around the mounting flange and making sure none got into the inside of the tube and mounted with the gasket and bolted up nice and tight. Next I reinstalled the carb and dropped the old mounting screw with the phillips head bolt and installed a real bolt with hex head so I could put a 10mm socket on it to tighten it. I tightened the rear engine mount and locktite with the red stuff extra strength! Got it all back together and it ran like a bat outta ****! Found a speed trap sign and went through it as fast as possible all tucked in and clicked off 37MPH! and thats with a stock sprocket! This thing never ran so good, im totally blown away with the power! And to think I ordered a new carb from Spookytooth, the CNS carb, really dont need it now but might install it anyway just for he fun of it to see what it will do. Im so happy now im drinkin some extra potent Margaritas, triple shot!

Here are some pictures of the fixes
P.S. I also changed the fuel line to real motorcycle hose, used for dirt bikes, worked great!

At 37 MPH, my feet were dancin on the pedals, vibration was intense!