pit bike will not run

[overbore]

my son bought a china 50cc bike from some kid
and inone day the throttle cable broke which I fixed and
the next day he rode it it quit and will not run.The
strange thing is it run untill he shut it off and then wouldnt
restart.It blows back thru the carb somewhat so I am
assuming its valve adjustment or spark timing althou I have
no info on the mitisubusi china 49cc bike.
It is a dirtbike that supposed to be able to jump even thou
it doesnt even have a real suspension on front .
Anyone know any info on these or where to get valve
clearances,spark timing,valve timing,anything would help.
overbore

 

[Patman]

Try .002 - .003 clearance on both valves. There SHOULD be a timing mark on the flywheel that matches with a mark on the case. Check to make sure you are getting spark also, if not report back.

 

[Patman]

Might also check the air gap on the CDI trigger as this can be an issue, especially on the lower quality knock-off engines. It should be no more than .010", but not much less as the looser manufacturing of these engines might allow some wobble.

 

[overbore]

Thanks patman for the responses.I will try the valve settings.
the only way i have found to find top dead center on compression is watch piston and cam shaft
and mark it best I can.Cant get side cover off where I think the marks should be.
And I dont know where the cdi pickup is to check it.
Dont know how to get cover off side?unscrew or just pop off?
and do you have to take engine side cover off to check cdi as the coil is under the seat and I do have spark but dont know that its at the right time.
Thanks for any and all help.
overbore.

 

[Patman]

The timing marks and the pickup should be under the left side cover (chain side). There should be a few bolts holding it on and then it will just drop off unless it's stuck to the gasket which usually isn't an issue. Chances ar the timing mark is a little point cast in the top edge of the case that points down and then there will be tick marks and possible letters on the flywheel. The pickup should be near where the wires run in to the case in the same general area. There will be a little plastic piece screwed/bolted to the case that is the pickup and a little "bump" on the flywheel. Line them up and set the gap between them.

 

[overbore]

Thanks again.
Like I said it sounds like its trying to fire about every 10 kicks with
popping back thru the carb like timing is off or valves.
do have fire but probably not at the right time.
This should give me something to try.
Thanks overbore.

 

[overbore]

found the pickup under the cover and it looks like 10 just like you said
but when I line up the F on the only mark I see it isnt even close to be
inline with the pickup module like it sheared a keyway or something.
I am assumming two things here,
The marks F and T are for Fire and Top dead center.
And the only Mark that I find is just a little grove cut into the top of the case and its the
only one that I find.
With all that said does it make sense that it would be that far off on lining up or is this
normal?
Thanks overbore.

 

[Patman]

I'd start with how does the valve adjustment look when you line up the T with the groove in the top of the case?

 

[overbore]

Patman that was the right mark to lign up for top dead center as I verified thru the spark
plug hole.Set valves at .003 with same results.
Took impact and got flywheel nut off,but cant get flywheel off yet, but so far like I said before the
pickup doesnt line up when inline with the F mark and the electrical thing that looks to be
coming out of the transmission has been cut completely off and been rewired before my son
bought this thing but I still wonder what its purpose is even thou we know it run when we got it
like this.
I got to get the flywheel off and wondered also if you know if a suzuki,kaw,puller would fit it if I
borrow one?
for a little motor what a pain.
overbore.

 

[Patman]

I have no idea if that puller will fit. The scary thing is that even between the same "design" knock-off engines it might be different just because of the manufacturer on the less expensive engines. Keep in mind these engines are a secondary use when installed in pit bikes, the primary use is in scooters that virtually everybody across Asia rides. I know of at least 2 dozen engine suppliers and am certian there are maybe 4 times that out there and each of them offers between 4 and 10 different versions! :yikes:

 

[overbore]

still havent got the flywheel off so I cant look under and see what is broke besides the plate
one the pickup coil.
Patman have you or someone ever seen also the bike has the cover for the intake plug
drilled and a grease plug looking fitting in it where when it did run it blew oil out all over the
engine which didnt make sense.
I wished that my son wouldnt have bought this thing.
just want to get it fixed and out of my garage.
overbore.

 

[Patman]

Not sure what you are describing with the cover for the intake plug...

 

[overbore]

The engine has screw covers that you take off to get to the valves and the one
for the intake valve has been drilled and a plug like a grease zirk put in it for some
reason which I dont know.
When my son got the bike and it ran for the 2 days that it did run it blew oil out the grease
zirk looking fitting all over the bike.
Not sure why someone would have done this.
Kids buy stuff without even looking to see if its been rigged or jumped very much by the foot
pegs being bent down,ect.
I just need to get it going and then gone gone gone.
Thanks overbore.

 

[Patman]

AH! That was somebodies attempt at a vent. Usually not really needed unless the bike is really run hard such as pit racing. A lot of kids do this "modification" because they believe there is some performance benifit and it's cheap & easy to do. Done correctly it doesn't cause an oily mess. Anyway that shouldn't be causing anything more than a mess at this point.

I hope he got a really good deal because it sounds like it's giving dad a really big headache. We just finished uncrating all of our first run of spare parts so if there is something that is not bike specific that I can help with let me know. CDI, carb, basically anything not internal to the engine our stuff should work. I'd almost suggest the smack with a deadblow hammer to pop the flywheel but honestly I'm afraid the engine couldn't handle it. Many of the low end knock-off engines are basically disposible if you can believe it!

 

[overbore]

got the flywheel off and checked everything out there and everything is alright.
timing might be off by a couple of degrees from the mark but I think that is normal and I have
a good fat blue spark.I set valves at .003 earlier so there has to be something to do with gas or
compression and I am getting plenty of gas.
You dont know what compression this thing should have do you?
I have hesistated on taking it completely apart due to not finding parts and china is a long way to go.
overbore.

 

[Patman]

Yes be VERY careful if you pull it apart because the parts issue is a big problem. You'd think they would be interchangable but there are many pieces that are unique to each engine supplier.

I'd think somewhere over 90psi would be a good number for it. Might want to double check the valve adjustment, I did the valve adjustment on the neighbor kid's XR80 a month ago and it went from 75psi to 100psi just because they were a little off. Made it A LOT easier to start LOL!

 

[overbore]

Patman you there man I need a little more info as I took the time to tear this thing apart
finally and found out whats wrong.
First need to know if when putting the rings on if there is no markings if it matters which side
goes up and do you stagger 120 degrees with compression rings facing exhaust?
And make sure valve clearance is .003 on my feeler gauges which would be american or
someone said .005mm?
want to put it back together tonight if I get this info soon enough.
Thanks overbore.

 

[Patman]

Well it would be nice to know which side of the ring was up before but if there isn't any marking it's sort of hard to tell so flip the coin, you should be OK. The ring gaps should be 60 degrees rotated from each other. .003" would be the clearance you are looking for, .002" or .05mm would work also but might be a little tight on a knockoff engine since the machining and tolorances are a little less precise than on a Honda piece.

 

[overbore]

Thanks again for the speedy response.
I was thinking a few years ago helping a guy do a kxf250 that we had to put the rings
staggered and facing toward the exhaust,but wasnt sure if I remembered right.
And the .003 is in inches correct.
Thanks and I will let you know if I get this thing running cause it needs to leave my garage.
overbore.

 

[Patman]

I really don't think it will matter much on the stagger with this engine. If the rings are not marked there must not be an issue when it is originally assembled in China :)

Yes .003 is in inches and that is the setting I would suggest for both intake & exhaust just because it's giving you a tiny bit of fletibility on a loose engine with questionable build quality so ultimate valve opening should not a big issue, getting it running seems to be the goal. Or should I say getting it running and out of the garage ;)

 

[overbore]

10/4 and Thank you again Sir.
overbore.

 

[Patman]

Anytime.

 

[overbore]

special Thanks Patman for all your help.
This one should be out the door soon as its running fine now.
I believe after going thru everything that I did that it was the valves and timing and I
thank god and you for your help so my son can get something back out of this thing
and move it to someone else.
After going thru it , it seems to be a pretty good bike althou I am scared to keep it for the
parts issue that has been there all along.
The fork boots kept coming apart and found that there was a snapring that came apart also.
woh what a bike.
Thanks again this one is closed.
overbore.