Ok, my first post, but I've been lurking around for a good while. I am a honda technician and have alot of knowledge when it comes to cars, but, I dont know everything, so I'm gonna post my problem here and hopefully someone can help. I have a 125cc Atv made by zongshen peace. It has a 125cc Loncin engine that ran like a champ until....the screw that holds the choke plate in the carb fell off and into the intake. It lodged between the piston and the cylinder head at idle (thank god). I got it home, took it apart in about 15 mins and removed the screw lodged in the dome. did not touch the valves and the valves will hold water so theyre not bent. set TDC on the piston and set cam timing. Put everything back together and it will crank, has great compression 100lbs and has spark. it is getting gas and air and cranks great with a battery charger, but still will not start!! Im thinking its a carb problem, but when I put my hand up to the carb I get suction and it sprays gas. Ive been working on it for a couple of hours now, and Im just tired, but stumped. any comments would be greatly appreciated. keep up the great work guys!

 

Spray a small amount of either (starting fluid) while cranking - into carb air inlet. No more than 1 second of it !!!!!! (more can blow the engine). If it fires and revs up and dies - then everything mechanical and electrical is OK - the problem is then isolated to fuel. If it does NOT fire up - then you have either a mechanical or electrical problem.

Remove spark plug, reattach plug wire, attach short length of scrap wire to plug metal outer hex casing with vice grips (lightly !!), attach other end of scrap wire to good ground with second set of vise grips, crank it over and look for spark across plug gap. If no spark - then problem is electrical. If you have spark then problem is mechanical.

If problem is isolated down to mechanical - then (probably), the cam timing is 180 degrees out.

 

did you adjust the valves when you reassembled the engine? tight valves will keep the engine from starting.

 

Ok, got home, set TDC again and set cam timing 180 deg opposite. Still no start. I have great compression, great spark, suction at carb and wets my hand when i cover the intake. Im just gonna take it to work with with me and let my 40 fellow techs play with it and see what we come up with. I'll let you guys know what it turns out to be. Thanks for the replies.

 

am I right in setting TDC on the crank by aligning the flywheel mark with the case mark and the cam gear "O" at 9:00????

 

and the manual says

align the "T" mark with on the flywheel with the index mark
align the the camshaft spocket index mark with the cut out in the cylinder head.

the cutout is under the cover where you installed the cam

 

Ok, update...I tore down the head and removed the rocker arms and the valves. Spun both valves on the drill and found 1 valve with a slight wobble! I replaced both valves and set timing and put it all together. It fired! Only problem is it runs like crap! it will idle well and accelerated well in idle, but as soon as you put it in gear it runs really bad and misses badly. Im thinking its off timing a tooth or 2. any thoughts?? carb has been cleaned, and spark is excellent with new plug installed with correct gap.

 

Could be 'off' one tooth OR 'tight' valves - recheck valve last first. GETTING CLOSER !!!