Vintage Dirt Bike Q & A

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PostPosted: Sun Feb 11, 2018 9:38 am 
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Joined: Mon Jan 29, 2018 10:50 am
Posts: 4
hello,
my 76 mr250 project is very nice in all respects and has 3930 miles with 150psi compression and nice spark. off the road since 2012, previous owner said it was hard starting and sold it to me. found the timing nearly at "T" due to worn points follower. set the new points at "F" and started with much white smoke. carb inlet/outlet, top of piston and plug wet with gas. noted maybe 1/2inch fuel in the crankcase. thought i'd deal with that and carb tuning in the spring so i'm in the middle of a complete mechanical refresh at the moment.

just noted vertical clearance of 0.004 at the flywheel, with zero side-to-side and zero endplay. flywheel is not loose, removed the nut and can obtain same movement at the crank. my service manual gives no spec, but this seems excessive. am ok with top-end work, but have no tools for splitting the cases. am wondering if i should continue on this path or part it out? many thx,
bob p


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PostPosted: Sun Feb 11, 2018 2:18 pm 
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Joined: Sun Nov 30, 2008 12:10 pm
Posts: 983
Location: Connecticut, USA
There shouldn't be any radial crank play at all, and the MT250 shop manual has no spec for axial crank end play so I assume that's zero as well. My CR125M crank had about .010" radial play when the LH main bearing failed, dropping its inner ball cage & rivets into the crankcase. :( It's safe to assume your main bearings are shot, and will get worse and worse until one or both fall apart.

A local MC dealer can split your cases easily if you bring them the bottom end, possibly for a very reasonable charge. Changing out all the case bearings is a fair amount of work, but doing just the crank bearings really isn't once the cases are split - again, the dealer could easily do that along with the splitting if you provide him with bearings (I'd use OEM Honda ones).

The MR250 carb was ahead of its time and few people knew how to repair or jet it back in the 70's, so I'm not surprised yours is having problems. As long as it's intact, setting the float height / cleaning all passages & jets / replacing the power jet O-ring etc. should get it working a lot better. Obviously getting that gas out of the crankcase will help a lot too.

Ray

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric


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PostPosted: Sun Feb 11, 2018 7:07 pm 
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Joined: Mon Jan 29, 2018 10:50 am
Posts: 4
i usually have a winter project or two that i may sell or ride a few years, depending on the bike. have refreshed many cb750s, a cb550, and a few two-strokes. liked my 76 cb750 and also my dt250, kept them both for awhile. sometimes i sell them as a nearly done project, if predicted expenses such as bodywork (done by others) or mufflers (omg pricing) won't be recouped. my 64 ca77, 69 cd175 and 69 cl125a all fell into that territory. i left it up to the next owner to decide their own path -- i.e. race bike, full-on resto, daily rider, or chopped into a cafe'. when researching this bike, i noted it was on this site back in 2012 and you gave some well-thought out and patient advice. with 3930 miles, i had assumed the bottom end would be fine and the internals certainly were super-clean when checked. this project was coming along nicely until i started looking for problems as i am want to do, as i never sell a bike with hidden issues. this bearing problem was a complete surprise.

i looked up the nominal radial play on a 6306 bearing (C3 class) and its roughly .0015, so i need to decide my course of action. luckily a vintage honda mechanic is nearby and he's loaned tools as needed in the past, and helped when i've been stuck. when i was overly cautious and asking for help checking the valves (all 16 of them) on a dual overhead cam cbr600, he pushed me into doing it myself. being the usual tappet/screw type, it was easy (no shims!)

i'll visit him this week, and discuss my next step. maybe i'll split the cases and bring him the crank side? i have a service manual. really too bad, as i just polished all engine mounting bolts, painted the brackets, and re-installed them. all torqued and Loctite'd. a shame really, as everything on this bike is real nice with the exception of locked-up shocks. i think they're over-full, the PO mentioned he rebuilt them. piston was in backwards too, but luckily no damage.

well, i have to mull this over. many thx!
rlp


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PostPosted: Mon Feb 12, 2018 7:16 pm 
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Joined: Mon Jan 29, 2018 10:50 am
Posts: 4
am thinking more about splitting the cases, and had a quick questions about the process. my service manual goes straight into removing the trans after the cases are split.

once the left cover is off and primary gear removed, can i pluck out the crank while leaving the trans in place?

thx,
rlp


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PostPosted: Tue Feb 13, 2018 9:55 pm 
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Joined: Sun Nov 30, 2008 12:10 pm
Posts: 983
Location: Connecticut, USA
Yes, I believe so. On the 70's 125 and 175 2t engines, you can definitely remove either case half and leave the transmission as-is in the remaining half, with the aid of gravity of course. I've done it in an engine stand, but a bench should be fine with proper support.

Ray

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric


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PostPosted: Wed Feb 14, 2018 5:39 pm 
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Joined: Mon Jan 29, 2018 10:50 am
Posts: 4
thx very much, pretty sure i'm gonna give it a try.
rlp


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