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TOPIC: 1972 JT2 MX Mini Enduro compression

1972 JT2 MX Mini Enduro compression 06 Dec 2017 12:02 #1

  • eschrader
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Hey guys,

I picked up a mini enduro back in the summer. The bike stalls going up hills (even the slightest grade). I am only about 170lbs but I know these things arent exactly a rocketship now that I am older.

How can I cheaply/easily test compression? Think I can probably just get away with replacing the rings and honing the cylinder on my mill?

Thanks,

Eric Schrader

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1972 JT2 MX Mini Enduro compression 06 Dec 2017 14:29 #2

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I wonder if you should be looking for another cause. You could feel on the kick start by hand if it has reasonable compression & to stall like you say i'd expect it to have almost zero to feel or a very slight feel on the kick start. If that was the case she'd likely need push starting or only just fire up with a real good kick over. Sorry to go on like a broken record--bit more info please. Had two of these for the kids back in the "70's & have one now for our grandson. This one i bought pretty stuffed but despite that still runs reliably. Great little bikes, replaced by the GT50 & GT80 down here in NZ. Just thinking back, two things can kill the performance & 'totaly' blocked exhaust is one, have seen them grow over in the exhaust port or where the pipe screws on plus in the small tail pipe but would be severe to do what you're describing. Other is ignition points wear down on the fiber heal & points gap closes down retarding the timing but even if retarded off to the point of almost no gap it would still go better than you describe. I give up lol--till more info anyway. Thanks. ps, with the pipe you could unscrew it at the cylinder just to try, just undo a few turns, although if grown up in the port then that won't fully prove a point without removing & scraping out. Run it & put your finger over the tail pipe & see if you can stop the motor, providing no leaks anywhere else. But wait--there;'s more lol. On the YG1 similar rotary valve motors back then we had numerous bikes [farm bikes YG1T] that stripped the center out of the rotary valve disc [where carburettor feeds the motor]. The stripped center would revolve & the disc would follow as it desired, totally cocking everything up. Would run but super sluggish. So that's just another thought. You'd have to be careful but put something in past the valve & carefully turn the mag flywheel til it touches the give a little slight force & if solid then it's ok!!. If it's stuffed then running with carb cover off & it'll blow more than suck if you give it a rev most likely & consequently hardly run at all. All pretty unlikely scenarios but anyway--have a good day & hope i haven't ruined it.
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1972 JT2 MX Mini Enduro compression 06 Dec 2017 15:02 #3

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The kick starter feels like throwing a hot dog down a hallway, barley any compression which is why I asked.

Now what you said about the exhaust might totally be it. The exhaust was hitting the back tire, I tried to bend it out and it collapsed the bend to have about 50% blockage of the diameter of the tip of the exhaust. Should I just cut the tip short out of the muffler before it gets to the shock?

The frame is all jacked on this bike but thats how I got it. Gotta make do with what I got.

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1972 JT2 MX Mini Enduro compression 06 Dec 2017 15:07 #4

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Can I take the exhaust pipe off completely and see if it runs, or does it need the expansion chamber to run somewhat decent since its a 2 stroke?

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1972 JT2 MX Mini Enduro compression 06 Dec 2017 15:22 #5

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I was looking at the parts diagram. Can I just take off the Exhaust Outlet (#2) and see if that helps?

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1972 JT2 MX Mini Enduro compression 06 Dec 2017 18:52 #6

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yeah didn't realize the end came out [too many years gone by], so take the end out & anything restricting it should be obvious in that area but don't forget the front end where it buts up to the cylinder. Can run it minus the complete exhaust if your neighbors can stand the noise. If it is blocked enough to cause a power loss like your're experiencing then it's a wonder the exhaust noise isn't very quiet. i had a 100 twin LS2 & both headers were 90% blocked. be lucky to get a pencil through it. All that said--if it's not that then we'll start again.

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1972 JT2 MX Mini Enduro compression 07 Dec 2017 11:20 #7

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The tip was super blocked. Cleaned it and re-bent the pipe tip out a bit.
The compression isnt as bad as I thought.
I fixed the float because it would piss gas sometimes. The gasket was rubbing on the float so I trimmed it back and added a small washer to the jet. Works good.
Same issues when I start it, it goes but craps out at upper rpms and loses power and will stall. Might be related, but the first gear when popping it in "clicks" like its half engaged and I hit it down to second and back up to first and its fine. Maybe its the clutch? or the carb at higher RPMS?
Here was my cleaning process:

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1972 JT2 MX Mini Enduro compression 07 Dec 2017 17:04 #8

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yes idealy it needs burning out which is easier said than done in this world with emission laws etc. We're out in the country a bit so get away with it. put my pipe off [forgotten what it was] in my brothers big rubbish pile of builders wood& other stuff & it came up clean as a whistle, albeit needing painting after. Oh yeah, was a yamy 100 farm bike AG100G, i put the separate tail pipe short muffler in our log fire in the house. Anyway--i don't realy think this is your main problem but you could leave the ring nut at the cylinder loose a few turn for a try to prove a point. Regarding gearbox issues, if it sits there with the clutch disengaged but in gear then it won't be a clutch issue but do wonder if the internal centralising stop pin [bolt] is bent from having been dropped/crashed & forced the lever past the stop point. Number 4 in the link is an off center screw so adjustable for half a turn to equalise the movement from the resting position. All that's just guess work on the problem--let's get it running right first. Air filter not choked up, don't forget the rotary valve. Over & out for now or until other come up with a better idea. I tend to jump in the wrong end when someone says my bike stopped & bugger me it's only out of petrol lol.

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