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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: 15
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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76 r75/6 -- mine since 1980
76 mr250 -- a project, coming along nicely!


Last edited by shoeman1952 on Sat Mar 10, 2018 1:18 pm, edited 1 time in total.

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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: 995
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: 15
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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76 r75/6 -- mine since 1980
76 mr250 -- a project, coming along nicely!


Last edited by shoeman1952 on Thu Mar 08, 2018 10:32 am, edited 3 times in total.
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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: 15
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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76 r75/6 -- mine since 1980
76 mr250 -- a project, coming along nicely!


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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: 995
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: 15
thx very much, pretty sure i'm gonna give it a try.
rlp

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76 r75/6 -- mine since 1980
76 mr250 -- a project, coming along nicely!


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PostPosted: Tue Feb 20, 2018 5:24 pm 
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Joined: Tue Jan 27, 2004 8:51 pm
Posts: 136
Location: Gilbert, AZ
My $0.02 worth.
I always pull the right case off and leave the transmission and crank in the left case.
I install a hose clamp on the shafts to keep all the gears and shims Etc in place.
That way when pushing the crank out from the left side no parts and pieces fall out.
take pictures before during and after As you may need one of those pictures on reassembly.
Have fun and let us know how it goes

Rick


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PostPosted: Wed Feb 21, 2018 7:11 am 
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Joined: Tue Jan 11, 2005 6:10 pm
Posts: 161
It's been alot of years since I had a early CR apart but the bottom ends are very slimier. I had made a tool out of angle iron with a bolt in the center to push on the crank and holes at each end that lined up to bosses cast into the case for this use. You'd bolt it on the case and turn the center bolt and it would push the crank and crankcase half apart. I seem to remember it was the left case that it bolted to leaving everything in the right side case. A car steering wheel puller may work in this instance too.


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PostPosted: Sun Feb 25, 2018 11:36 am 
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Joined: Mon Jan 29, 2018 10:50 am
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hello again, and many thx for the replies. here's a quick up date on two fronts. PO told me he had installed new shock seals but may have overfilled them, as they were locked. possibly hydraulically locked? turns out both rods were badly bent. i fully disassembled the shocks and straightened the rods using a press, v-blocks and a dial indicator. and patience. just a side note, the small diameter threaded end is peened over to secure the nut. this must be ground off so that the threads aren't damaged. with the internals in place and the rod retracted inside the tube, i filled the shock until the oil level was just under the locking nut/seal retainer. the action was nice and smooth thru-out the range, so at that point i installed the springs.
Attachment:
shockdisass.resized.jpg
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i borrowed a honda three-jawed case puller from a friend of mine, a vintage bike mechanic. care must be taken to fully thread the three studs into the case's drilled holes, and that the center rod is perfectly axial w/the crank. i snugged up the center rod, and smacked the upper case all around with a rubber mallet. i tightened a bit more, maybe 1/16 turn, and repeated this step until i heard a small pop as the case was released.
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the left-hand case easily separates from the right, leaving the crank and trans in place.
Attachment:
cases.disass.resized.jpg
cases.disass.resized.jpg [ 280.68 KiB | Viewed 669 times ]

the left hand crank bearing sounds and feels "rumble-y" and i can see some minor damage on the outer race in a few spots. new case gasket, 6308 C2 bearing and seal will be ordered. NTN, FAG, Steyr and SKF all make great bearings, just have to be careful about counterfeits. edit: i had previously removed the right case and clutch pack, then checked for vert movement of the crank end on that side. feels great.

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76 r75/6 -- mine since 1980
76 mr250 -- a project, coming along nicely!


Last edited by shoeman1952 on Thu Mar 08, 2018 10:43 am, edited 2 times in total.

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PostPosted: Thu Mar 01, 2018 4:50 pm 
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Joined: Sun Nov 30, 2008 12:10 pm
Posts: 995
Location: Connecticut, USA
Here's a link to my Showa 360 shocks rebuild thread, which are pretty much identical to yours except for valving and springs. I think your shafts may have bent due to overfilling / hydraulic locking, and I believe yours are overfilled again right now; there should be at least 20cc of air space inside - you can wait to see if your shafts bend again, but... I ended up putting 110cc of 3W Bel-Ray shock oil in mine, and they worked fine for thousands of hard dirt miles before one seal started to weep and I replaced them with YSS RZ362-395TR units.

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: Thu Mar 01, 2018 11:17 pm 
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Joined: Mon Jan 29, 2018 10:50 am
Posts: 15
hi ray,
and thx very much! have read your tutorial and while i wait for the center gasket and crank bearing to arrive, i'll disass one shock and measure the amount of oil i have in there.

with the rod fully compressed, i recall leaving the oil level such that it would just cover in internals and still leave a space for the threaded cap to be installed, with the rod fully compressed. if i left too much of a void, this resulted in a bit of rod movement { 2 - 3 inches maybe } with zero damping. this didn't seem right, but i'll disass one and put in 110cc and check out the oil level just to compare.

i'll report back and let you know what i find, and thx again!

r l p

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76 r75/6 -- mine since 1980
76 mr250 -- a project, coming along nicely!


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PostPosted: Fri Mar 02, 2018 6:16 am 
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Joined: Sun Nov 30, 2008 12:10 pm
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Location: Connecticut, USA
Quote:
with the rod fully compressed

Oh, OK - sorry, I should've read more carefully. :) It's been a while since I had mine apart, but I remember going through pretty much the same procedure regarding full stroke damping and trying to eliminate any damping-free zones. IIRC the goal was to get the oil level just above the top of the piston at full extension, which resulted in consistent damping and still provided enough air space to prevent transient hydraulic lock on sharp hits.

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: Fri Mar 02, 2018 12:37 pm 
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Joined: Mon Jan 29, 2018 10:50 am
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turns out i had 135cc in there, and now have 110cc.

i reckoned that since the valving would be still submerged even with less oil in the system, damping in both directions wouldn't change. and as air is compressible while oil is not, i considered that removing a little bit of oil would only add a bit more air damping. didn't really see a downside, especially considering your experience with that oil level. so i elected to disass the shocks, measure what what in there, and refill with 110cc.

prior to installation of the springs, i checked and found that compression and rebound damping is still consistent throughout the full rod stroke as expected. both shocks now reinstalled, with mounting bolts cleaned up and re-loctite'd. bike off the stand, action feels great at both ends. took about an hour.

thx again.
bp

ps local shop had a Honda OEM crank seal in stock, while crank bearing and center gasket are due this week. stay tuned. :wink:

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76 r75/6 -- mine since 1980
76 mr250 -- a project, coming along nicely!


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PostPosted: Fri Mar 02, 2018 5:19 pm 
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Joined: Sun Nov 30, 2008 12:10 pm
Posts: 995
Location: Connecticut, USA
IMO 110cc is a very good idea, especially as I think Showa may originally have used 100cc or so. I remember one time stroking the shock a bit too fast and ending up with a big ring of oil on the bench, and I realized there are huge instantaneous hydraulic-pressure variations inside the shock during use, which are (mostly) absorbed by the internal air mass.

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: Tue Mar 06, 2018 8:11 pm 
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Joined: Mon Jan 29, 2018 10:50 am
Posts: 15
after cooling the new crank bearing in the freezer overnight then heating up the sleeve, installation of the new crank bearing was a snap. cases are now back together with new center gasket and crank seal, with the original "cheesehead " screws snugged up. i've ordered a complete set of stainless steel hex head cap screws { allen bolts } for the cases and engine covers so i can get a more consistent feel when tightening everything down, and to make it easier on the next guy. there were a few hex head fasteners installed, but had the black oxide finish and were ugly/rusty. i'll use a dab of neversieze to prevent any problems with the aluminum/stainless interface.

i did note that one of the wrist pin clips wasn't seating fully in the piston groove, so i took 'em both out and found one was 1.0mm diameter, while the other was 1.2mm. new clips on the way, and hope they're both 1mm.

side note: have zero radial play at the crank now. went from a rumbly, loose bearing to one that now feels perfect. it's beautiful, too. mission accomplished!
Attachment:
gears.resized.jpg
gears.resized.jpg [ 219.23 KiB | Viewed 670 times ]


gearing/shift drum look very nice, with original hobbing marks seen.

_________________
76 r75/6 -- mine since 1980
76 mr250 -- a project, coming along nicely!


Last edited by shoeman1952 on Thu Mar 08, 2018 10:17 am, edited 1 time in total.

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