My son and daughter each have Coolster 125 atvs. they're identical. My sons runs excellent. My daughters however, is very hard to turn over. This issue started last year. Earlier this year, her engine seized. So, I bought a new engine from Coolster, and installed it. The only way I can get this engine to turn over, is if I use the battery out of my lawnmower, which is 330 CCA. but even then, that battery will drain quickly. Ive tried 4 different starters, and 3 different solenoids. Ive isolated the starting circuit as well. Still the same issue. Remove the spark plug, and it spins over with no issues. Ive googled the problem, and have found several people with the same issues, but most haven't been solved, and those whose fixes solved their issues, didn't solve mine. I attached a multi meter to the battery, and a clamp meter to the starter wire between the starter, and the solenoid. when cranking, the battery voltage gets pulled down to about 8 volts or so, and the starter draws as much as 112 amps. Did the same to my sons, and his starter only draws 25 amps at most. Ive also attached multiple grounds from the frame and engine back to the battery. All connections are tight and clean. The harness is brand new as well. Ive pulled the side cover off the motor to watch the starter chain, and it seems normal as it spins. All of the rubber guides are in place as well, as is the metal tab that stops the starter gear from sliding along the shaft. Ive also checked the valves for adjustment, and they're fine. The engine is also in time, as far as the valves go. Ive spent almost two months trying to figure this out. I'm about to just junk the entire thing, but don't want to spend more money then I already have. But, I may have no choice. All help is appreciated. Thanks. John.

 

Try spinning it over with the plug in but the plug lead off. It is just possible the ignition timing is out and firing too early, this puts a hell of a strain on the starter, and it would then spin easier without the plug lead on. Causes can be a slipping starter clutch, or timing trigger coil or flywheel not set right (they are fixed, but the factory may have messed up). Main starter leads and earths can be too thin but you seem to have addressed that. I guess compression could be way too high, not a lot you can do about that though. I would be tempted to do up the old engine, they usually seize on piston and cylinder, both are dirt cheap to replace, if it is a horizontal cylinder Honda clone.

 

Thank you merryman. I should have mentioned that the lead is off the plug. The coil has also been unplugged, along with the other set of wires that go into the side cover.

 

I had same thing going on and I went through it all and found out that when I tighten the head nuts down it would make it slow crank as too much compression but if I backed off them a little it would crank like normal…new head gasket worth a shot…

 

I only joined the forum today and just searched this problem after months of like the OP did not find any useful answers.
My issue is exactly the same, i just posted about it in ATV section, all new parts etc and i am a mechanic of 30 years and there isnt much i cant get working or running again but i am stumped.

So are you saying you got this engine new, a bit like me, got the quad and engine which always had this problem, it wasnt new when i got it, so this is interesting and as Merryman says, unless the factory messed up, which now after seeing this thread i am thinking something along those lines is at play here.
Only difference is mine is a CG250 engine but sounds like identical syptoms

If you know how, take off the side cover, you will need a tool to remove the flywheel but then you can also check the woodruff key has not broken and then remove the oil seal and see if you have the problem i have discovered in my thread.
It wont effect the ignition timing which i would bet money on is our problem, i replaced my what seemed like a working starter clutch and still the same as this can sometimes cause similar starting issues.

Merryman, can i ask, what do you think the factory might have got wrong, after everything i have done i cant help but think my flywheel is either wrong or gone bad, new pickup coil etc.

Lets hope we can both our quads running

 

Not sure why you replace an oil seal but my guess is the flywheel keyway or position of the trigger coil must be slightly out, advancing the ignition enough to make it spark too early, could also be a CDI fault making it do the same. As I wrote, if the fault goes away when you remove the plug lead, the problem is spark timing. Also had worn starter gear bearings cause it, again slows the starter due to compression putting a strain on the worn mechanism, so the engine virtually stops just as the spark occurs, sending crankshaft backwards. Also note the post before yours, poster thinks compression is too high, bad machining could cause that too.

 

I didnt replace the oil seal i only removed it so i could see both timing gear marks after i found the crank gear not aligned correctly to the keyway, with the seal in i could only see the crank timing gear mark and it was off. i have posted a pic on my first thread but dont think it has been approved yet.
I replied here as the OP has replaced a lot of parts like me and still have same symptoms...... waiting on a new gaskets in the mail.
Trouble is i dont know enough about these engines and the CDI's etc., hopefully my thread will be up soon.....
I appreciate the reply!