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Old 06-26-2015, 11:27 AM   #1
KirkN   KirkN is offline
 
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Oops. Now I need a manual for pushrod 250 head removal

Sigh. Like the title says....

Started my Baja X250 and while it was warming up, the throttle stuck open. Took me 4 or 5 seconds trying to get the throttle to release before I killed the motor. Tapped on the carb in the area of the throttle slide and it released. (!??) Started bike again, and carb again stuck open. Wasted 4 or 5 seconds dicking around before shutting it down.

I didn't think it was really revving up that bad, but now it won't start. Did some rudimentary troubleshooting and could hear the motor 'chuffing' back thru the carb and have discovered that motor is only making 30psi compression.

Sigh. Must've bent a valve.

I haven't yet done any disassembly, but first up will be pulling the valve cover and having a look. See if I see huge excess valve clearance on the intake valve.

So, anyone got any pointers on head removal? This is a pushrod 250 motor.

Anyone got a line on a new intake valve and a head gasket?

Sigh.


 
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Old 06-27-2015, 05:11 AM   #2
humanbeing   humanbeing is offline
 
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http://josemaco.files.wordpress.com/...h-62ktjam5.pdf
---
As i said b4 in http://chinariders.net/showthread.php?p=181217. Some maker DON'T use mainstream parts. Measure parts is a MUST after disassembled.
Sometimes "chopsticks" is f&%@#d (NOT valve)
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Old 06-28-2015, 01:01 AM   #3
Weldangrind   Weldangrind is offline
 
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Excellent point, HB.
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Old 08-19-2015, 11:27 AM   #4
KirkN   KirkN is offline
 
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Thanks for the link and input.

Finally got to working on this. Pulled the engine out Sunday afternoon. Only a handful of bolts and it's right out on the bench. Had to do a bit of cleaning though. Still covered in Georgia clay from the last outing.








Pulled the valve cover off - nothing unusual at all. Valve clearances all look good. Valves look to open and close properly. Looking in thru the intake and exhaust ports doesn't show anything unusual at all. No obviously bent valve or anything. Hmm....




So, off comes the head. These pushrod motors are a lot easier than OHC motors in that regard. Unfortunately, no smoking gun there, either. No evident 'ding' in the top of the piston, nor anything unusual looking at the valves in the combustion chamber.











So, to check the valve seats, with the valves closed under spring force, I pour acetone in the ports and look for leaks into the combustion chamber. Good sealing valves should not show any seepage for minutes on end.





For the exhaust, all looked good. Dry for 3 ~ 4 minutes until I dumped the acetone out. For the intake, not so good. Drips appeared immediately, even though to the naked eye all looked good. I only had to watch for 30 seconds. I think I've got my smoking gun as to why I could only develop 30psi compression.

Actually, thinking about it, 30psi is a weird value - one would usually expect good compression or zero compression, not something in between. What I don't know is WHY the valve stopped sealing. A bent valve (my original guess) would give zero, not 30psi. And the valve doesn't look burned although I haven't inspected it out of the head yet.

So, that's where I am now. Next steps will be to see if I can lap the intake valve and get good seating that way, or else I'll have to buy a new intake valve. I probably should just buy the new valve anyway and lap it once it gets here. Haven't even tried to source one yet.

I checked the pushrods and they DO have just the tiniest bit of bow to them. I rolled them across a sheet of mirror glass I have, and you can just barely see a tiny bit of bow. I don't imagine that it's significant though.

So, thoughts or comments?


 
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Old 08-19-2015, 12:43 PM   #5
bogieboy   bogieboy is offline
 
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clean everything up real good and see if it was just carbon on the seats but my guess is actually fried rings... or at least a stuck ring.... thats what was wrong with my 200cc pushrod motor when I got the bike... dropped a new piston and rings in and fired right up... mine only made 50psi fwiw...


 
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Old 08-19-2015, 01:13 PM   #6
KirkN   KirkN is offline
 
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Could be, could be.

The thing is, it ran fine one moment, and not at all the next. Would stuck, broken or worn-out rings act like that? I can see it if I got the bike and it wasn't making compression when I got it. But not with it running as good as it had been for me up to that point.

I'm faced with a minor dilemma, though. I wasn't planning to pull the cylinder off at this point (no base gasket, etc). Looking at the cylinder walls shows virtually no wear. The motor has less than 25 hours on it, total. If I don't pull the cylinder off, and the valve lapping doesn't cure the issue, then I'm out a head gasket and the time to go back into it. Sigh.

I'll see what happens after cleaning and lapping the valves. Maybe I'll put it back together temporarily with the old head gasket just to check the compression after lapping.


 
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Old 08-19-2015, 02:45 PM   #7
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I'm gonna ask a stupid question... did you check the compression with the carb off/carb slide all the way open? that can greatly skew the compression numbers.... as for gaskets.... I paper gaskets get replaced with red rtv sealant and metal gaskets get a shot of copper sealant spray and I havent had any issues whatsoever running them...


 
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Old 08-19-2015, 03:21 PM   #8
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Quote:
Originally Posted by KirkN View Post
Haven't even tried to source one yet
For mainstream air cooled CG250:
http://item.taobao.com/item.htm?id=14936130630
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Old 08-19-2015, 03:41 PM   #9
KirkN   KirkN is offline
 
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Excellent - thanks for the link to the parts. Guess I'll have to read up on that sticky about ordering parts from there. It's amazing - the little currency conversion thingy says the pair of valves costs $4.68. I waste more money than that in vending machines for cryin' out loud.

Re: the compression testing - when the issue first occurred, that is, when it wouldn't start after the throttle sticking, I could hear 'chuffing' back thru the carburetor while trying to e-start it. At that point, the carb was on, but the airbox connector tube was disconnected and pulled back so I could see into the carb. That sound is what first made me go get the compression tester, because the sound indicated to me a leaky intake valve during compression stroke. I was originally thinking I'd fouled a plug or wasn't getting gas or something simple.

So, first test was with carb on - one time with throttle wide open (and no choke) and repeat check with throttle slide completely removed!

And, 'cause I'm anal that way and this IS a cheapie Harbor Freight compression test kit, I went and tested my Yamaha XT225, just to be sure the gauge & tubing was working right and got 135psi or something high up like that.

Once I got the engine out on the bench, I used a big battery charger and ran the e-start and rechecked the compression while watching right in the intake port - no carb or even intake manifold. Same thing at ~30psi.


 
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Old 08-19-2015, 09:43 PM   #10
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http://www.aliexpress.com/item/Chine...030049425.html
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Old 08-20-2015, 10:45 AM   #11
Weldangrind   Weldangrind is offline
 
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Quote:
Originally Posted by KirkN View Post
And the valve doesn't look burned although I haven't inspected it out of the head yet.
I'll be most interested to see what the valve looks like, once you remove it.

How much of a bow did you see in the pushrods? Can you measure it?
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Old 08-20-2015, 12:07 PM   #12
KirkN   KirkN is offline
 
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Took the intake valve out last night, but didn't get any pictures of it nor the seat. It looked good, though. If you'd handed me the valve alone and said 'have a look at this valve', I'd have said it was fine. Virtually no deposits built up on the valve. Valve seat looked the same - no evidence of damage or nicks, etc.

I did a very brief lapping job and got what looked like good contact all around. The only thing unusual was the lap band was pretty wide - basically the full width of the valve contact edge. I'm used to seeing the contact band a little narrower.

I put it back together and unfortunately, acetone still leaked thru immediately. So, I'll do more lapping and try again. I'll get some pictures this evening.

The head of the valve was perfectly flat, and there didn't appear to be any runout - ie, stem not bent as far as I could tell. This goes along with no evidence of impact on the piston.


As for the pushrods, I don't have V-blocks nor a dial indicator to check properly, but my pushrods have a different look than the picture Humanbeing showed. My pushrods have larger ends and a smaller, 'necked-down' diameter over their length. When I roll the pushrods on the glass, you can just barely see that there's a bow. I suppose I could use feeler gauges to sort of measure the gap, but I'd say for sure less than 1/32". Maybe about 0.010"?


Hopefully this evening, I'll get it lapped properly. If not, I'll have to order a new valve I guess.



Last edited by KirkN; 08-20-2015 at 12:43 PM.
 
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Old 08-20-2015, 04:14 PM   #13
bogieboy   bogieboy is offline
 
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hmm if lapping the first time didnt get it I wonder about small hard to see cracks in the seat?


 
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Old 08-21-2015, 08:58 AM   #14
KirkN   KirkN is offline
 
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Didn't get to do any lapping last evening, but here are pictures from the 1st session. This is only about 3 ~ 4 minutes lapping, using the suction cup tool thingy and lapping compound. As I say, it still weeped acetone.

I think with a bit more, the valve will seal.

Sure is weird, though, how it went from running like a champ to not running at all. I brought it home from a great dual-sporting trip to Suches, GA area, it sat in the garage for a couple weeks, then I started it up and it was warming up fine 'till I blipped the throttle and it stuck.








 
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Old 08-21-2015, 11:15 AM   #15
Weldangrind   Weldangrind is offline
 
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That is a nice, wide margin. Is it possible that one of the valves stuck open? How are the guides? I'm just not seeing any evidence that points to such a massive drop in compression.
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