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Discussion Starter · #1 ·
I got a new GPX motor off of ebay about 5 months ago and ever since the first day i got it running it ran horribly. It will not rev up past +/- 2500rpm and when you give it full throttle it sounds like a Honda "ricer" car bouncing off the rev limiter and backfires, this is while riding it and it is under load. And the motor has a tick coming from the head that sounds like the valves. Then finally it has a very strange squealing noise that sounds like it is coming from the cylinder head. The motor has barely been used and probably has 5 hours on it. I would tear it down but i dont know where to get gaskets or parts so it would be useless. I have tried to return or exchange it but the seller is dead set on ripping me off. So i am stuck with the motor and would like to fix it without tearing it apart completely. I would appreciate any comments or suggestions. Thanks:confused:
 

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Valves should be set at .003"-.004"



Tear into the top end if that isn't helping and let us know what you need, we have parts :)



Also make sure your exhaust gasket is installed before you slip your pipe on (silver ring). That may be some of the cause for the backfiring and such.
 

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Discussion Starter · #5 ·
Ok. I am a mechanic but i have never taken apart one of these 125cc motors. To measure the valve clearance do i just need to take off the 2 covers over the vales and check it with a feeler gauge? If so what should they be set at so i can adjust them?
 

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1998 Honda Z125R said:
Ok. I am a mechanic but i have never taken apart one of these 125cc motors. To measure the valve clearance do i just need to take off the 2 covers over the vales and check it with a feeler gauge? If so what should they be set at so i can adjust them?




Yes just take off the tappet covers and use a feeler gauge. Valves should be set at .003"-.004"
 

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Discussion Starter · #7 ·
I just checked the valves and the intake was out of adjustment. The exhaust was fine. That took care of the ticking sound.



The motor just stops revving at 2500rpm and from then it just boggs but it will continue to go the same same speed. The bogg sound gets deeper as you turn the throttle more and at wide open throttle it just backfires.



I have adjusted the main jet in the carb but it makes absolutely no difference.
 

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Discussion Starter · #11 ·
My carb is a 22mm sheng-wey. I have tried a different carb from a running motor, still has the same problem. I have tried a new spark plug and a CDI box from a running motor and checked all the grounds and wiring. I pulled the exhaust off to check to see if it was clogged but it is perfect. I am positive it is not a fuel problem as i have swapped a running carb on it. And i have run it without an air filter with both carbs. The only thing that i have not checked is the coil and the compression. If i rule those out then it is something internal.
 

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my 05 thumpstar is doing the exact same thing cleaned the carb and filter about 3 times already.... im gonna check all the grounds,and the sparkplug and maybe the vavles tomorrow if i get time...
 

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Make sure you have the motor grounded correctly. Ground the ground wire to the frame and strip the paint where you ground it. A bad ground would make it run like that and sometimes it wouldent even run at all.
 

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Discussion Starter · #16 ·
It could be the timing but i dont know how to check the timing on these 125 motors. Is there a way to do it with a timing light? Is there a way to do it at all?



It might be the timing because it idles cold perfectly, might be advanced alot. And the motor gets worse every day.



How can i adjust the timing?
 

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Discussion Starter · #17 ·
Today 07:18 PM

orraclekid15 Yea it kind of does sound like the cam timing is off, maybe it jumped a tooth? Pull the ignition cover off and see if the timing marks line up.



So check the marks on the flywheel and cam? How does the cam cover come off, just unscrew it?
 

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There is a bolt on the opposite side of the head, directly across. Take that out and the cam cover will fall off. Then just line the t mark on the flywheel with the mark on the case and the mark on the cam sprocket should line up with the mark on the head. If not then you know the problem.
 
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