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Old 10-13-2019, 09:13 AM
vybrano vybrano is offline
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Default Rocker arms (valve lash) won't stay the same as set up, keeps changing

I will set a proper valve lash (eg. 0.08mm on both), but it will hold for just a few seconds of riding.... .

Intake isn't bad, I will usually find it settled at 0.07 - 0.09 (acceptable). But exhaust side isn't holding at all. Either valve lash is 4x bigger than I set up; or it keeps the spring under pressure even in resting position.

It happens almost immediately. I will set up both lashes nicely (using pliers+wrench). Start up engine, ride it for 10 seconds (sounds and rides nicely) and then sound completely changes, performance drops and I instantly know that exhaust valve lash is again at some crazy value.

Any tips&tricks? I have welding machine..., could I? Or something else?
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Old 10-13-2019, 09:22 AM
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You are tightening the rockers adequately, aren't you?
Maybe a stripped out fastener somewhere sliding around?
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Old 10-13-2019, 09:51 AM
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I re-done it about 5 times already, so used force is adequate to my current mood, but at the same time I'm careful not to strip threads.

Intake (good) bolt is spining nicely and easily. Exhaust (bad) bolt is hard(er) to spin, but I don't see any damaged threads.
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Old 10-13-2019, 11:33 AM
wenee9w wenee9w is offline
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Push rod bent? Rocker arm stud stretching?
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Old 10-13-2019, 12:09 PM
Mrmonk7663 Mrmonk7663 is offline
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Is the rocker stud fully seated? I know my exhaust side stud thread was not nearly as nice as my intake thread. I used a thread cleaner to make sure I could torque it down all the way. Take the rocker off and see if you can move the stud side to side with your fingers.
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Old 10-13-2019, 01:15 PM
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Well, first off, if you check it and set it then run the engine up to operating temp it's gonna check different every time due to thermal expansion. Check it and adjust it cold and leave it alone. Set it at .002 cold and stop screwing with it. If you think the nuts are backing off then put a thin line of paint across the stud and nut assembly as a witness mark. Other than that if you suspect bad parts put a compression gage on it. Check it cold the hot. Never adjust it cold then adjust it hot... you'll go in circles for ever unless you really know your stuff...and don't forget to make sure not to adjust against the compression release

---------- Post added at 01:15 PM ---------- Previous post was at 12:30 PM ----------

To add to my above post...since I'm home now...

To check for bent push rods just lay it on a clean flat surface and roll it. You should notice any bends right away...if so replace it. If the ends seem loose or worn out do the same, replace...

You could always remove the springs and check for worn valve guides. But once you get to this point you may as well replace the head...or let's be for real...if it's a predator just buy a new one for a hundred bucks at the next sale...Gotta go again... dang it...
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Old 10-13-2019, 01:29 PM
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Waiiit, is this your GY6 ATV?
In which case, I wouldn't be surprised as all if it's going whack, since it's MADE IN CHINA!

---------- Post added at 01:29 PM ---------- Previous post was at 01:24 PM ----------

Lookie here: https://www.bmikarts.com/Cylinder-He...ine_p_389.html
Brand new GY6 head, $20. Used is $10.
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Old 10-13-2019, 03:34 PM
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If that photo is what you have, its a overhead cam, no push rods. JS speed demon is right, that looks like a GY6 with the anti-backfire mechanism.

If you have been doing any mods, you have to be carefull about valve length. Different setups are too long /too short and will be beyond the range of your adjuster.

Are you prepping properly? Cold engine and the cam follower fingers are OFF the cam lobes (TDC of compression stroke).

Bent valves and sticky guides can cause lash to be intermittent.


Also those adjusters are kind of funky. They are often not linearly perfect, as in, you can turn in the same direction but the gap may get more, more, less, more. And they are notorious for changing when you tighten the locking nut.
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Old 10-13-2019, 07:12 PM
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Gy6? I wasn't even thinking about those...they annoy me actually...
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Old 10-13-2019, 07:42 PM
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I figured it was a GY6, because the engine he was having mystery problems with was a GY6.
And yes, they annoy me too.
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Old 10-13-2019, 11:09 PM
Karttekk Karttekk is offline
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Make sure both valves are closed COLD engine. Loosen the rocker arm jam nut, put the feeler gauge in between the rocker arm and the cam lobe to whatever spec you think is correct. I use a thousands feeler gauge and set them both to .005. Once you have slight drag on the gauge, tighten the jam nut securely then double check your clearance again. If your jam nuts are tight enough it shouldn't need any further attention. Sounds like you aren't securely tightening the rocker arm jam nuts. What does this mean? "so used force is adequate to my current mood." I don't see anything like that in the shop manual. Loosen the jam nut, adjust your clearance then tighten the rocker nut pretty tight. It won't strip or break anything. That's all there is to it. Once and done.
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Old 10-13-2019, 11:38 PM
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Yes, it's GY6. No push rods here.

Yes, I'm adjusting it cold and checking it cold. I will adjust it and ride it in the afternoon. And I will check it&adjust it&ride it next afternoon. So it sits a whole night&day in between.

Yes, I'm setting it up in TDC position.

Karttekk Karttekk, I'm doing it exactly how you described, but it just doesn't hold set value... .

Do you think, that I need a new cylinder head?! I thought if anything is really bad, than I need new rocker arms with adjustment screws + whole mechanism that is holding it in place. Not the head.
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Old 10-14-2019, 05:50 AM
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I mean, it IS made in China. So if you can't figure it out, I'd just get a new head and keep the old one for parts.
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Old 10-14-2019, 07:26 AM
Karttekk Karttekk is offline
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Just for the heck of it, try replacing the rocker arm nuts with Nyloc nuts or the ones with nylon in the threads to keep tension on the nuts. Maybe yours are worn and lost their clamping power. Unusual they won't hold their positions.

https://www.aftfasteners.com/nyloc-n...ert-lock-nuts/
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Old 10-15-2019, 02:34 PM
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These GY6 150 engines work pretty good actually. The China factor would be more in line with it cracking or breaking into 3 pieces. That because they probably have some of my old Chevy Nova mixed in their aluminum.

If your adjustment is truly changing then its because something is broke, threads are stripped or your doing it wrong.

You have to verify that you have the crankshaft in a proper position. You have two TDC. every other one is right, the other every other one is wrong. Verify by looking at the camshaft. The cam follower MUST be off the cam lobe. Look at it.

When adjusting, put your feeler between the adjuster and the end of the valve, NOT the cam.

When tightening the adjuster locking nut its almost guarantied to move the adjustment. Always, always recheck the gap after every adjustment. And DO NOT try to get around that inadvertent movement by not tightening the locking nut completely or you will the nut and maybe the adjuster in your crankcase. Adjust-tighten-recheck, adjust-tighten-recheck.
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Old 10-15-2019, 05:42 PM
Mrmonk7663 Mrmonk7663 is offline
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Good information. If you need additional info download the Yerfdog Spiderbox manual. It details the valve adjustment procedure on a gy6. You have to pop out a little plastic plug by the stator to see the flywheel marks.
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Old 10-15-2019, 07:37 PM
Karttekk Karttekk is offline
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Quote:
Originally Posted by Snaker View Post
These GY6 150 engines work pretty good actually. The China factor would be more in line with it cracking or breaking into 3 pieces. That because they probably have some of my old Chevy Nova mixed in their aluminum.

If your adjustment is truly changing then its because something is broke, threads are stripped or your doing it wrong.

You have to verify that you have the crankshaft in a proper position. You have two TDC. every other one is right, the other every other one is wrong. Verify by looking at the camshaft. The cam follower MUST be off the cam lobe. Look at it.

When adjusting, put your feeler between the adjuster and the end of the valve, NOT the cam.

When tightening the adjuster locking nut its almost guarantied to move the adjustment. Always, always recheck the gap after every adjustment. And DO NOT try to get around that inadvertent movement by not tightening the locking nut completely or you will the nut and maybe the adjuster in your crankcase. Adjust-tighten-recheck, adjust-tighten-recheck.
Whoops, thanks for catching that, I meant the valve. Something odd going on with his setup somehow. Shouldn't take long to adjust the valves, not a whole lot there to deal with.
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Old 10-15-2019, 10:42 PM
vybrano vybrano is offline
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Yep, that's exactly how I'm doing it.

I came up with idea to try to swap parts between intake/exhaust side. Like nuts/studs...etc. what is universal. Of course for example I can't swap valves itselfs. Intake side is holding OK and only exhaust is bad, so maybe with some parts it will shifts.
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