50cc Chinese engines By triplej56 (Read 2,476 times)

[triplej56]

Help!! We just got back in the scooter business for repairs on Chinese scooters(after a year of shuting business down),and now we seem to be running into a big time problem.Wish my mechanic would send this e-mail(because he could verify what is going on),but unfortunately he does not get involved with computers.Guess I will g ive it a shot(please be gentle to me guys if I screw things up)(LOL!) It seems our problem is two different engines on the 50cc chinese scooters.One engine we were trying to replace because of a bad crankshaft with another engine we had(both 50cc,s).New engine we put in had a good crank so we put it in with a new cyclinder(piston and rings),new cyclinder head(all from Parts for scooters).Now bear with me guys,because I could screw this up(because I am certainly not much in the mechanical aspect of what is going on).Also not the best typer,but again bear with me and I will try my best. We had the timing set on the magnetic rotor(or some people call the ( as far as I know) flywheel). We got the cam gear all lined up and ready to put on the camchain.Camchain installed and ready to tighten down the camseat.Unfortunatly we are about a 1/4 inch away.We gave it a shot to tighten,but when we did seat everything,it was one heck of a tight cam chain(could not even put the chain tensioner in it).We went back to the drawing board to try and figure out what is going on.In our previous experience we had never run into this before.Anyway no luck so far to solve our problem.So we decided to do a little bit of measuring.We measured from where the crankshaft,say goes into the variator and to any point on the engine block.Well though and behold we found out the engine we took out was a 1/4 inch shorter than the one we put in.Now we knew why the cam chain was so tight.We have tried all our scooter parts places.No luck on different cam,cam gear,crankshaft etc.Cam chain maybe?Longer link length(guess you have to take out the crankshaft if it is possible?Sorry if I am getting long on this e-mail but we sure need help!I know some of you guys are pretty dang smart,so we sure appreciate any help you can give us.

[kz1000st]

I would check the engine designation on the side and see if the two engines are both 52QMB or whatever. This is the first time I've heard of this.

[rob63]

Have to agree with KZ, check the engine numbers first to see if you`ve got the same types. The standard 50cc gy6 engine should be 139qmb marked on the crankcase.

[triplej56]

We have checked the engine designations and both read QMB139.

[rob63]

Hmmm very strange, I haven`t come across a 1/4" difference in crankcase measurements before.
Just to confirm, looking at the simplistic pic below, the top diagram is how things should be when viewed from the flywheel side.
The bottom diagram is how the camchain is in your engine ?

photo hosting

[Alleyoop]

Measure the head on the old and the new. The new head may be a little taller, or you have not seating the head on good enough. Also make sure the chain is ON the sprocket on the Crank it may off to one side.
Alleyoop

[tvnacman]

mic out the height of the old jug compaired to the new one .
I know on the 150cc the crank has two rows of teeth . could it be the same for the 50cc . or could you not be seated on the teeth of the crank . Sprocket will be able to explain what I'm refering to better .

looks like the chain could not seat properly very easy . or could you have a kink in the chain ?
www.chinesepartswholesale.com/product_info.php?cPath=1_5_19_204&products_id=62830

John

[triplej56]

The old head and new head both measure the same.We even measured two old head and they are the same.The chain is on the crank properly with no kinks in it.I know because I even double my mechanic on them.Keep plugging guys.

[tvnacman]

you measered the height of the new jug ? tell me what you got , I can measure the new rebuild kits I have here . Did you get all the old gasket off .

John

[triplej56]

Will do tomorrow John when I go to the shop.We did make sure the gaskets were all removed before we put new ones on(base gasket and head gasket).

[triplej56]

John it looks like 1 of our new heads measures 2-3/8ths inches or very close.

[tvnacman]

Post in mm use a caliper . It will be more presice.

John

[triplej56]

John I am out of town but will do it later when I return to shop.

[tvnacman]

Great

[sprocket]

I agree with Alley, check the chain seating on the crank sprocket, they can get twisted to one side..also go by TOP DEAD CENTER on the piston...not the flywheel marks , it is easy to get the chain off by a link... and it runs like snitt...should be able to set it through the plug hole...