Vintage Dirt Bike Q & A

I've actually been curious about how the PWK28 or current PE28 would work on the MR. I would've gone that route if there was a PWK30, but as the old carb was working quite well I had it bored out to 29mm instead. The stock carb dimensions are 35mm spigot end, 44mm mouth, 98mm overall length.

Be sure to add some steel plate to the airbox-side area of the MT pipe once it's welded back together. The OEM pipe walls are super thin, and without support from the perforated metal or baffles it'll vibrate itself to bits.

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

That's enough for last night!



Yep I added some straps across the back of the section I welded back on. Still plenty of noise so far, when I put the supertrapp on I guess it'll tone down a little. I had a friend who is a dealer look up what sudco offers for the bike, he said the're is a kit specifically for this bike. But he didn't say what it entails. I'll see what info he's got.

The pipe looks like it fits quite well! It can be hard to get the header bend right. I ended up adding brackets to the frame and pipe top with a rubber isolation mount between.

Re the Supertrapp - the smaller the core, the quieter it will be. I'm using a 1" core / 6 discs / Quiet Core insert / resonator, and it's really quiet (at medium throttle on the street I mainly hear chain noise) but most any ST should be pretty good. 7 discs were too many and 4 weren't enough, for my modified motor at least. The older 2t stuff shows up on eBay fairly often, and I think the 2t core kit they sell now is a 1.25".

This is the type of clamp I'm using now - worm-gear style clamps didn't supply enough 'crush' to keep it from twisting:

https://m.mscdirect.com/mobileweb/produ ... arts=false

I added a bracket on the bottom of the silencer running up to the fender bolt, but my ST is pretty long. The MT125 tailpipe O.D. is 23.5mm.

It would be interesting to hear what Sudco has for the MR175. I wonder if it's pre-jetted and ready to go?

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

According to my guy it's ready to bolt on a stock MR175. It's a Mikuni too, so I'm guessing it's an update to what the Dec '75 Dirt Bike magazine said Sudco offered for them then. I'll know soon enough, got some $ coming in soon, so I'm gonna spring for it and see. After that I'll focus on a fresh jug/piston, seals, etc. And then go back to getting the right hand controls back to stock, and other electrical stuff sorted.

I managed to find a correct headlight at Mid O last Saturday! Metal bucket, in great shape, right lens and wire cover. I'll be putting it on soon. Never did get that air fuel screw worked out, but now that the show is over I will have some time to devote to the bike again. Probably order that kit you mentioned, Ray. I did mess around with lighting on this bike a couple months back, and noticed something seems to be very wrong. Lights blow instantly front and rear, and my voltage is all over the place. Oh well, always something :-/

You found a headlight - great! I think they're just too cool-looking to throw away for most people, so they just lie around waiting for mid-Ohio.

It sounds like the wiring harness tweaks the previous owner did may have included connecting things together that shouldn't have been, or maybe the lighting coil or its connections are funky. It seems like Honda just used the light bulbs as 'regulators' to keep the voltage sort-of in range, but if they're blowing out instantly it's possible the lighting and ignition coils may have been connected together, connect themselves together when the headlight's turned on, etc. I'd unplug the yellow wire from the stator cable and read the AC voltage to ground with the engine running; as a guess you might see 6-10VAC no-load at idle and maybe as much as 20VAC at high rpm, but I've never measured it myself. If the voltage is way higher than that, there's probably something wrong in the harness or stator coil(s).

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 for the info Ray. I've got another update that I hope will help with all the current issues too: a friend in Arizona found a 77 Mr175 that's only missing the lights. Everything else looks to be there! It was cheap too. Just got to figure out how to get it to North Carolina now. I understand Fastenal is a great way to ship large parcels like that for cheap, so I'll be looking into it soon - hopefully before the Barber festival in October. When I do get it. I'm gonna compare bit by bit to mine and connect a few dots. May even swap motors over if the jug is any better than mine.

So another revelation... The manual recommends 20:1 fuel mix. I've been running 32:1. A couple months ago at a meet up, I was offered some 109 octane at 32:1 mix. Wondering if I may as well have dumped lava in the motor now. I guess this also explains why the previous owner had it bored out 3 times over years using it as a "farm bike" and giving his kids a ride once in a while. Learn something every day I suppose :-/

A friend of mine has used UShip for two bike purchases, with very good results. There are a few others that will pick up uncrated bikes as well.

You probably already know that the '77 MR has a different gearbox from the first two years. The '75/'76 box has the ultra-low 1st and is better for tight woods riding, but the '77 box accelerates faster (top speed is the same with all years).

I only use Sunoco race fuel, and in the beginning I was using the 112 octane leaded stuff. A brand-new NGK BP7EV platinum plug would last two rides before it started sputtering under heavy load (hillclimbing etc.). Now I run GT+ 104 octane unleaded, the bike runs just as well and the plugs last forever. I also use Bel-Ray H1R at 36:1, I'm very happy with it.

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

Well next up on my list of woes is the air filter I believe. The original is disintegrating and going in the carb. I see there's a uni one in Australia, just was hoping to find something stateside. Head gaskets too. Csml has a couple, it would just cost about 70 bucks to get them here. been messing with the bike a lot the last couple days (hoping for a trail ride Saturday) but can't get it to run for more than a minute. Then nothing for about 10 minutes, run another minute, repeat.

If your air filter is disintegrating, I'd remove what's left of it ASAP and vacuum out the airbox / air boot / carb, if you haven't already done so. The rotting foam won't damage the motor, but it can find its way into the carb passages and liquify in gasoline, then harden when dry and be extremely difficult to remove.

Have you tried contacting this guy?

http://www.ebay.com/itm/Honda-MR175-197 ... 7675.l2557

The UNI NU-4026 filter is really the only game in town. I've only seen one NOS Honda filter on eBay in the last 8 years or so, and who knows how long something like that would last when put into use anyway.

If your head gasket looks OK, I'd just reuse it - I do it all the time. I use Permatex 2B sealant on it and have never had a leak (the base gasket is another story).

Re the running issues, try pulling your fuel line off the carb and checking the flow. If it's OK, your float valve may be plugged or sticking.

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

Yep, pulled the fuel line and verified flow. Vacuumed out the air box too. "made" a filter out of a 4 wheeler filter and zipties, but yeah I am gonna order the correct one even if it is the guy in Australia. Uni America doesn't seem to stock them. The frustrating thing is only getting a few minutes at a time to diagnose the issues before having to do other stuff. Gonna pull the carb tonight and go thru it thouroughly on the coffee table during Bob the builder and Thomas the train

So over the weekend I did some more work to the old bike. I put the original snail pipe back on after modifying some brackets and seals to sure it up a bit. got the correct air fuel screw for the carb, and made an air filter out of some oiled foam element material I had. It doesn't look oem, but it should be functional and not disintegrate into the carb. I was a tad surprised when I opened up the stator cover and was checking for problems inside to see one of the wire coils was loose on its mount. As in, I could move it with my finger. Looks like maybe the wire is broken, but I had poor light and short time to work on it. Points were also badly hazed and a bit out of spec. Didn't have an ohm meter handy but wondering if Ive located the bulk of my recent problems here. Still managed to do the trail ride on a 97 xr200. It felt like steering a tractor in comparison - but at least it ran.

Whatever the case, I'm out of commission for a while until I get the spare and some ebay parts for this one. I cannot find any fuel issues whatsoever, it's got to be electrical.

So... think you might be ready to throw in a CR ignition? It's rare I say something like this, but here goes... "You won't believe the difference". IMO it would be a mistake to keep the antique low-performance ignition just for the lighting coil, in this era of super-bright LED lighting that runs for many hours on a single battery charge.

I don't know if you've had the flywheel off already, but if not I'd assume it's at least lightly rust-welded to the the crank taper. I use a puller (K&N 82-0150, might be a bit hard to find these days), plenty of heat on the crank end, and sometimes a few 'taps' from a 5 lb. sledge.

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

Absolutely! and yes, I am seeing the light "as it where" on the lighting issue. No reason I couldnt slip a small battery bank into the tool box, and run lights on LED battery only. For that matter, if I get real hard up to charge them somewhere out in the sticks, a small solar panel and a nap under a tree would suffice.

What would I be looking at for this complete cr125 conversion? I do have the aforementioned puller (fits an xr80 as well, and Ive got a small herd of those on the property.) Popped the flywheel off and that's how I found the messed up coil and cruddy points. Now that all this has come up, I'm wondering if an XL100 ignition would also fit in there with its lighting coil.

And as far as believing the difference, the biggest reason I decided to put more time into the snail pipe was that I was overall very disappointed with the difference between it and the mt125 pipe I fit to it. Took all its bottom end away. For good trail riding I think the snail pipe is best for it. I might even take it to someone that does exhaust at some point and see if it could be refined a little. I do love how compact it is.