Vintage Dirt Bike Q & A

Thanks to this forum, info on ebay items and its knowledgble memebers, my 1975 MR175 with now 983 miles on it runs like new! Thank-you all very much!

Its 100% stock and with the exception of a UNI Air Cleaner, New Tires and some hex bolts to make taking the alternator cover off a little easier, its 100% original. All it needs for me to mend the rear fender and add a tail light.

For Ray,

After some careful thought about the carb circuits regarding what was going on I decided I had a rich carb. That was even more apparent this morning when I had a couple of oil drips on the floor around the exhaust port.

So just like the igniton except this time with the carb, I started off again by redoing everything except to tighter tolerances to control the fuel flow. That includes the float which I dropped 1mm down to 19mm instead of 20mm.

Noticing my plug was still wet at idle I went up one full notch on the needle (2nd notch) which allowed it to dig a little deeper into the needle jet. My hunch worked and the 1/4 to 3/4 throttle response is now perfect. You crack it in low and the front end rises right up. Additionally my idle cant be better. Air mixture screw mimics factory specs at 1.25 turns. Not to mention my calibrated ear says its right on at an idle of 1400RPM

For high RPM I went all the way down to a 115 main jet. I started at a 120...it sputtered and smoked just like with the 122. The 118 sputtered a little bit less but it sputtered and with high rpms the plug was still a little damp and black colored, not grey. I even noticed a little oil drip from the exhaust meaning its still rich. I went back to the bike store and they had a 115. That was the one I needed...... perfect. High RPMs in 2nd gear left the plug the prettiest grey color you ever seen. Ill ride it hard this week and check the plug some more for signs of heat, but I dont think Ill get to much. If so Ill go back to the 118. No sence of buring a hole in the piston.

As for power, Ist gear the wheel comes up, second gear its comes up a tad and when I hit third its hitting near 40mph in a flash.

I cant wait to get it in the dirt!

Thanks to all...........

Larry

Larry,

Hey, that's great! About the oil... what premix ratio are you using? I looked back through the postings and couldn't find it, but maybe I missed it. I'm running 36:1, Bel-Ray H1R.

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

Ray,

I am @ 20:1 with a synthetic racing oil. (cant remember the Brand name) I know it sounds rich but I am throwing caution to the wind. The bike has not run in over 16 years, I wanted to make sure that the lower end, piston bearings, cylynder wall and rings got plenty of lube just in case any thing happens to break loose (carbon, small metal peices etc.) In all rights since it was running so rich I really didnt need it. But it didnt hurt anything either! I am confident after 4 miles of rich running all has been lubed very well.

Ill move it up to 40:1 for the next tank and if required put a new plug in it. Of course it all depends how the plug looks after I leaned out the carb.

Also I put a 5W-30 Honda oil in the gear case with a high detergent content. Since its been cold and I doubted I would get the gear case up to a good operating temp while I was dialing everything in, I wanted to make sure that again I had no gear case ot clutch issues. Pluss fresh detergents also do wonders for cleaning out old crud and deposits. So it more or less was used a flush oil as I ran this for the last 4 miles and last night I went back to a 10W-40. After the gas tank gets empty I will again change the oil.

Cant be to careful with engines that have been in hybernation. Especially 2 strokes with no oil pumps or oil filtering. How does the saying go "better to waste a $3.00 Quart of oil then spend money and time on $1,000 repairs"

Also, I am still spraying a "little" WD-40 in the plug hole and rotating the piston a couple of times. WD-40 does wonders on removing carbon. But you have to make sure you move the piston a couple of times before installing the plug, if not the WD-40 will ignite and youll get a hot spot on the piston.

I aslo use WD-40 to lube the carb rubbers. It reall helps in getting the carb back in. Only on the filter side rubber though as I learned from you, it easier to disconnect the manifolds bolts then try to slide the carb out of the manifold rubber.

Ray again thanks for all the help......

Larry

Larry,

OK, I remember now about the 20:1 synthetic. I bet the oil drip will vanish at 40:1. Your jetting will run a bit richer with 40:1 and also with warmer weather when it arrives, so you might find yourself tweaking that air screw again and/or throwing in a 48 or 50 pilot after all. A good check for proper pilot jet sizing is to see how far you can turn the air screw out before the idle speed stops increasing; ideally that will happen between 2 and 3 turns out. I really had to work overtime getting a decent idle on my bike with the tiny CR rotor and suoer-high compression, and probably spent more time on the pilot and slide than all the other stuff combined.

I change my gearbox oil every 150 miles. The first two times it was entirely dark gray and sparkly, the next four or so times it was half dark gray and half the original clear yellow color (split right down the middle in the drain pan, gray oil from the clutch side), and the last couple of times it's been all clear yellow like it should be. What a long strange trip it's been.

I'm also a big fan of overlubing rather then the reverse. And always glad to help if I can!


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

Ray,

Thanks for the advise regarding what may happen when I lean out the oil mix.

A little talk about the igniton. As you perhaps are aware I did buy a 125 CDI off Ebay and arrived today. It looks in great shape. however, as mine is stock and I plan to keep it that way, I dont think it will be any better then the points other then the points will wear.

Besides Ill lose my lights and since I plan on keeping it 100% original that just wont work. But we'll see after I ride it awhile.

Okay heres another question..........have you come across any replica rear fenders? The OEM one I want cost alomost $70.00. I am hoping to find one for 1/2 that.

Larry

Larry,

This may be difficult to accept, but the CR ignition completely changes the nature of the MR175 motor. The massive reduction in flywheel mass and diameter, combined with an electronic advance curve and perfectly timed, intense sparks makes for a HUGE improvement over the points ignition - which is perhaps as a good a reason as any not to install it (what you haven't experienced, you won't miss). If I was forced to return to the points ignition now that I've gotten used to the electronic one, I would sell the bike - but then I like a hard-hitting MX type of power.

As far as lights go, there are high-intensity LED flashlights available now that are probably 5X as bright as the stock headlight with a much more focused beam, that will run 8 hours on 4 AA batteries. A small gel-cell battery (as used in alarm systems etc.) would easily fit on top of the toolbox or wherever, and would probably power an LED replacement bulb for 30 or 40 hours before needing a recharge. Just throwing this out to give you some idea of how far lighting has come from the 1930's-era technology of the MR175. My lighting system is unplugged but still 100% intact, and if one day I want lights again I'll definitely go the battery/LED route. I also wouldn't be surprised if Ricky Stator could add an LED-lighting or battery-charging coil to the CR stator plate.

Re the fender - check out eBay item 260389162667:

http://cgi.ebay.com/ebaymotors/1977-OEM ... 0007r14155

These are the nice '77 fenders - the front is especially cool. I was unable to find any aftermarket MR175 rear fenders, and it appears no other Honda model has ones that are really close (an MT125 rear could perhaps be made to fit, with a fair amount of work).

And have I mentioned yet the easiest and most effective MR175 mod I've done yet? A 13T front sprocket! There may be a bunch of reasons not to try it, but performance is not one of them.

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

Ray,

Glad your not my neighbor or my arm would of been twisted off by now!

Thanks, Ill wait until this Sunday on the fenders. Maybe Ill bid on them.

Larry

Larry,

Yeah, sorry about the arm-twisting - I do that sometimes. :>) I really didn't expect much to result from the CR ignition changeover, so I guess it made a big impression on me or something.

I wish those fenders were a little cheaper, but I guess MR rear fenders aren't too plentiful. You also might want to try checking completed auctions of recently parted-out MR175's and ask the sellers about the rear fenders if they weren't listed or didn't sell - they might be happy to get rid of it for cheap.

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'm going to make a post about LED lighting options. Your post about flashlight batteries caught my eye when seeing if anyone else improved this lighting situation. Many of the bulbs are readily available still, but the small instrument bulbs 6volt 1.5 watts are spendy and have to be ordered.

One trip to radio shack for a small LED and anappropriate resistor solves that problem with a more efficient light that most likely will not ever need replacing.

I'll post details with links to a resistance calculator later. This is easy stuff. Most of the headlights are "sealed" and cost a small fortune while sucking tons of current from this already starving system.

I did not want to change the classic nature of my bikes, but light bulbs are not a permanent change and nothing will be visible. Also throwing away bulbs every few years and buying new ones is a waste of money and further adds more garbage to our environment.

If you go the LED route, you might consider connecting several high-intensity white LED's in series, which should result in a pretty bright headlight. My LED conduction-voltage chart doesn't show a number for white LED's, but the voltage drop varies from about 1.2 volts (red) to around 3 volts (blue), with the other colors spread out between these values.

You could just try clip-leading six or eight LED's in series across the headlight bulb terminals for testing, start the bike, and see how bright they were at idle - then add or subtract LED's and/or a current-limiting resistor as needed. Once this had been finalized, the LED's could be soldered together in a cluster to the base of a burnt-out headlight bulb, arranged to point in the same direction. I'm thinking it will surely make a better light than the OEM 6V bulb. It's also quite possible that a high-intensity multi-LED replacement lamp unit with the same basing already exists, possibly as a flashlight upgrade bulb.

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