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Hi there.  I'm rebuilding a Cn250cc clone thats been sitting for the last year.  New valves, lapped and compressing well, new gas, head gasket, and more.  Things seem to be working again for the most part, except I'm so far I can't get the bike to idle below 2000 rpm.  It has also been taking a bit to start up the bike cold.  I'll need to try several times to start it, then hold the throttle open until the engine is warm to keep the bike from dying.  Easy start with the bike hot, a little easier warm but still may take a try or two.

  I've replaced the rubber intake manifold thinking a vacuum leak was leading to more air, higher RPM to compensate and a hot engine, and got a new carburetor today that I'm trying out to see if that solves any of the issues.  So far neither of these have worked.  I'm in california so there is an extra vacuum line going to the EVAP from the intake manifold.  The old manifold had 2 ports, and the new only has one so I tee'd off the vacuum lines going into it.  The bike started up fine with this so it looks like there's enough vacuum through the tee.

  I haven't tried opening the EVAP things up; could they be causing a vacuum leak I haven't tracked down yet?  I'm running the engine with the air box and snorkel on, and have tried adjusting the valves with the engine running to be sure they're not too quiet.  I remember that its better to have them a little tappy rather than too tight, but so far this hasn't affected the high idle RPM.  
 
  The new carb has an extra brass port running diagonally from the lower left, I've attached a picture.  The new one's on the right.  Does anyone know what this port would go to?  Can I just cap it off?  It did run without anything attached to it.  I'm assuming this could be at least part of why the new carb seems to be running at higher RPM, with the extra air coming in.  
 
  The new carb also has a mixture screw that can be adjusted, while the older one is factory set, so its another thing to mess with but I'd need advice on what to try. It can only be turned around 300 degrees until it hits the edge of the carb.

   I've thought about gutting the new carb and putting fresh parts into the older one to see if that helps.  It did seem to be running smoother with the new one in, but too low on the throttle idle screw and the engine dies with both carbs.  The old carb is japanese, and the new one doesn't say made in japan anywhere.  Only around 2000 miles on the bike.

    Anyone have any ideas or thoughts?  

Looks like the compression was lost from the gasket ring on the cam//valve cover not sealing properly. I was able to get compression with that off and the valves closed which was a bit of a relieve to not have to take more apart. It looks like the copper spray on stuff actually got me another use out of the metal head gasket. If there are any issues I have the replacement and I'll know what's the first thing to check out.

The timing is now either very very close or completely spot on. Did a wire test through the spark plug and matched it to a mark several times. The camshaft is perhaps a chain link off of top dead center. I'm not sure if I could have done better even by taking everything apart, so I didn't. Hoping that theres a tiny bit of wiggle room on that.

Got the gasket ring where it needs to be and used some assembly lube all around. Things sealed up nicely and the engine ran very well tonight. Warmed everything up, then got things up into the 5-6 thousand rpms and held it there like I was going for a ride. Theres a slight waver in the rpm meter at idle but it could be that I haven't lapped the valves or a slight compression issue still. If it doesn't sort itself out I'll dig into it. But after warming up did some rev and break tests; it was the first time I was able to do those cleanly without the engine stalling, not once. Turned everything off after adjusting the valves. Came back an hour later and everything started up nice on the first hit of the ignition.

Next test will be to see what it does tomorrow cold and untouched. I'm hopeful though. Thanks so much for all the advice and help!

Ok engine no longer starts without sitting on the ignition button for a long period of time.. I have a feeling I'm not getting compression with the flywheel turning so easily. When things cool down enough to take out the spark I can test that.

The shop manual I have says not to use the 1mm metal head gasket, that it can damage the cam. I can't find the .25mm head gasket advertised as such and the kit I purchased only had the 1mm. It looks like a sandwich of 2 .25 mm gaskets with a .5mm steel one in between. Is there a chance I could dremel off one of the .25 mm gaskets and use that? I reused the old one with several coats of Permatex Copper SPRAY-A-GASKET... and have a feeling it is the culprit with a lack of compression. Help!

Ok, I found a mark that is probably a faded T before the F. I was able to rotate the flywheel by hand. I got everything to the point of starting up the bike and it does start and runs on the center stand. I haven't taken it for a ride yet.. the rpm meter jumps more than it used too, and when the engine wasn't very warm it did die going from idle to higher rpm (3-4000). It feels groggy at low rpm still, and there was also a clinch point in the ignition where something was binding, took my finger off ignition and tapped it again, things went back to normal. And the flywheel feels like it's turning too easily; there is no difficult point where the compression stroke should make it hard to turn.

Could this still be the timing not being right?

I can't find any T mark with a mirror, flashlight, and turning the flywheel by hand. I'll see if I can get a friend to help me look tomorrow. But if this T isn't in the right place, could everything bind up like what's happening to me?

Is there another way to get the timing right if there is no T ?

It happens even without pressure, darn it. I'll check out the timing to see if this could be the issue.

Okay I'm an idiot lol... if I put my finger over the spark plug hole I can get the exact same effect with everything locking up right when the intake valve is at its peak. The compression and expansion seem to be the culprit.. while only one of them actually locks everything up.

Should I be concerned about this as I rebuild everything? Am I doing something wrong with the rebuild or will it just work itself out as I put the air box back on etc? I'm still not sure why this is happening, but it doesn't actually seem to be the valve. Is this vapor lock?