Vintage Dirt Bike Q & A

I have just acquired a '75 MR175... new to trail bikes... wondering if there are any aftermarket, race-type heads that were made for these 175cc bikes? It seems the biggest market was the 125cc for the aftermarket, DG etc. race heads.

Thanks...
Nate

The only one I know of is the DG radial head. Mine needed a lot of work (detonation damage, poor quality control, head bolt spacers, etc.). The combustion chamber is similar to the CR125 DG heads. They come up on eBay every now and then, but I don't think a lot of the MR175 ones were made. You can convert a CR250M head, but it's not all that easy. Converting the stock head to high compression w/conventional dome & squish band works well.

Ray

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

Thanks... got to doing some reading and find that what I really want is a custom port-job on the cylinder with matching work done to a factory head. Not cheap... but I would think it is worth it to get the response I want from the bike. Think I am just gonna make her a dedicated trail bike and keep looking for a bargain CB bike for the street. I got this one that has a resto job started with lots of NOS parts... clean lil gal. Got her for trade with a S&W .357 mag plus a small booty.

Thanks... got to doing some reading and find that what I really want is a custom port-job on the cylinder with matching work done to a factory head. Not cheap... but I would think it is worth it to get the response I want from the bike. Think I am just gonna make her a dedicated trail bike and keep looking for a bargain CB bike for the street. I got this one that has a resto job started with lots of NOS parts... clean lil gal. Got her for trade with a S&W .357 mag plus a small booty.

The bike looks nice! After dumping several grand into my MR, the biggest bang for the buck was definitely the CR125M ignition (it totally transformed the motor; a complete set from a '76-'78 would be ideal) set to about 2.0mm BTDC, higher compression (there are gasket sets with copper head gaskets .010" thinner than stock) and an aftermarket pipe. If you're going to stick with the stock pipe and ignition I wouldn't bother with porting it, but I've got excellent specs if you need them. There's an old magazine article floating around with porting specs, definitely do NOT use those (I tried it, what a disappointment). I'm working on a reed-valve conversion now, it's gonna take a while.

Ray

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

I am more a car engine gear-head... the 2-cycle World is new to me. I have started a tear-down of the engine. Trying to get the case apart so I can inspect the crank. Is there anything to be done to the crank that will help it? Have it and the rod and piston balanced?

I would love to "upgrade the ignition" to the 125 ignition, but I have no idea what components are considered part of the ignition? My shop manual mentions testing the battery, but I can't for the life of me see where a battery would sit under the seat? Did the '75 have a battery? I would assume the ignition would go from a points type ignition to a pointless type? Will have to do some searching articles for that process.

Also... Is there a trick to getting the AC Generator Cover pulled off without the special tool puller?

Thanks...
Nate

Nate,

Actually, it's a very good idea to have the crank balanced for the CR ignition, so if you go that route you might want to consider it. Both my motors vibrated quite a bit after the CR ignition swap until the crank was balanced, now it's much better.

The CR ignition consists of a rotor, stator, CDI box, and coil. The rotor & stator fit the MR crank and ignition cavity as is. The CDI box will need to be mounted under the tank, as will the coil (which replaces the existing coil).

You may have a hard time getting the AC generator/flywheel off even WITH the puller. I've used PB Blaster and a heat gun on the flywheel center with the puller torqued down good and some careful hammer blows, and gotten three of them off so far without damage. If there's no rust (none of mine had any), the problem seems to be the Woodruff key, which deforms and jams everything together. You may be able to use a large bearing puller as I've done with the smaller CR rotor, but clearance is much tighter with the big MR flywheel.

The MR175 never came with a battery. Are you sure you have the correct manual?

Ray

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

The shop manual shows some tests using a 6v battery I think... that is what confused me!

I finally got her tore down... had to improvise with a friends tools and used a few diff pullers to get everything apart. The biggest problems were with the bolts! Those dag things had been in there since '75 and even tho the manual tells you to remove them, the case and the 5 bolts that hold 2 thrust plates... those things are barely deep enough to get a philips head down in it and def not deep to get any type of torque on them. Had to use a chisel on most and mess them up. Will just buy new bolts and replace the case bolts with torx head bolts!

I am gonna send the cylinder, head and carb off to a guy that has done 2 cycle work for about 25 years and still rides them. He is gonna add a power-port to it for low-to-mid RPM and torque. Does some tricks to the head and builds the carb with special dividers and a few other mods to really perform. Not cheap work... but it will perform the way I need it to for the type of trail riding I want to do. I will sacrifice top-end RPM for better low-mid performance.

Any suggestions on who could balance the rod/crank/piton as a combo? I will not know if I need a #1 over until I can get the local 2-cycle guy to mic the bore. The entire bike was very clean inside... really hated to crack the case... but ya gotta do what ya gotta do to get to the dang crank. Learned alot along the way... my mechanic friend did also... first motorcycle engine he had tackled also.

Thanks for the ignition upgrade info. Best to find that stuff on Ebay, or buy it new?

Nate

Nate,

Do you have an impact wrench (air or electric)? My electric one's the best tool purchase I've made in decades. All my case screws, bearing-retainer screws, crank bolts/nuts etc. came out in perfect condition.

Here's a link to another thread here with a bunch oif MR175 stuff in it:

http://vintagedirtbikeforums.alp-sys.com/viewtopic.php?f=21&t=2091&hilit=mc175&start=15&sid=cdea8ade2d5f843d1720c92a4cb536ec

... and here's the porting your engine guy should use, the result of 300+ simulations using the engmod2t 2-stroke engine development software:

http://s399.photobucket.com/user/rayivers/media/MC175PortingDiagrams4chordal.jpg.html

While you're there check out my other photos, you might find something of interest.

My buddy went the vintage-tuner route you're planning on, the results were... mixed. He's using the porting above now and loves it. If your guy has actually built MR175 - not just CR125M - motors before and will also design/build you a pipe (a 2-stroke's only as good as its pipe, as I'm sure you know), then you should be OK. By the way, there's absolutely no need to sacrifice top-end for bottom end on this motor, although you can do the reverse if you're looking for a more OMG! type of power rush. I did many boost-port simulations, it added a small amount of power above 8,000 rpm.

I've only used Crank Works for crankshaft stuff, I think they're excellent.

Ray

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

We used my friends impact on several bigger things... did not think to use it on the case bolts and the bearing thrust plates. Oh well... I live and learn!

I will read the posts... like I said... new to all this. The guy I was thinking of using has a site I will post. Not dead set on anything and not sure how many MR175s he has actually done... just that he cut his teeth on Hodaka 2 cycles.

Thanks for the advice and links!

http://richstaylordporting.com/modern_2 ... rting.html

Nate

Man... I really love the black restored MR175. Looks great! I was wondering if altering the stock color was a bad idea or just do what you like cause it's your bike :>) I am not the biggest fan of the Honda green on the '75... it's not horrible... but I would like a color that suits my tastes. Do you have a source for the tank you used? I pulled my tank and as one would think... it is pretty rusty in there! I just looked at the original instrument cluster and she shows 1,435 miles! If those are original... no wonder the tank paint looks almost new, minus the few dents and everything inside the cracked case/tranny looked great. The original seat has one separation in it... I have an idea that the bike was garaged all of its life.

I've never used Rich's Porting, but the web site looks great and his work looks top-notch. That machined piece on his reed-valve page gave me the idea of cutting out a section of cylinder to end up with nearly the same thing, which I did recently with a Kawasaki KDX200 cylinder.

Thanks! The black thing just kind of happened, and since those pics were taken I've added to it with a number of black-plated parts. I fought the rust for a while, then gave up and went with a plastic tank (Clarke 1362X universal tank, P/N 11362-03 in black). Yes, your bike does look well preserved, definitely not a barn bike like mine (I bought it for parts, now it's my main rider).

Ray

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

Rich does awesome work, I had him port my mt250 and add a reed valve. Here are some pics of the unfinished bike. I have other projects so its not finished yet. I have ridden it and the engine is nice. Thats a cr250 cylinder thats got the port work he matched the cases to the cylinder and jetted the carb too. If I needed another bike ported I would definitely call Rich.

http://s201.photobucket.com/user/wooshe ... t=3&page=1

I guess being a 2 cycle newb... no question is too silly. Like I said before... been a car engine guy for decades... but how do you take the connecting rod off the MR's crank? Is the crank 2 pieces welded together after the factory piston is in place with the bearing? Jus curious!

Nate

Nate,

The crank pin is pressed in. Just FYI, Crank Works disassembled my NOS crank, balanced it, then reassembled it looking better than new. They also rebuilt my friend's MR175 crank using Kawasaki KDX200 parts, as many of the Honda parts - especially the rod - are extremely hard to find.

Ray

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric