1975 CB125S

Jmunk

Active Member
I saw this on the local Craigslist and it was worth a look. It followed me home and a new project begins. I enjoy the single cylinder Honda’s and couldn’t pass this up for the price.

1975 CB125S, the most interesting part is only 123 miles. I know it’s better to run an engine vs having it sit. Hopefully the fact it was stored indoors it’s life will go in my favor.

I have the carb off soaking and the gas tank off with some E85 gas in it. I have found that E85 will eat up varnish nicely. The tank had a small amount of varnish and appeared to have been stored fairly dry. The tank has a few small areas of what looks like flash rust in a few areas. I’m undecided if I will runas-is with an in-line fuel filter or trying something like Evaporust. If anyone has used something like that on a tank I’m curious of how it does long term.

It didn’t appear that anyone had messed with anything on the bike and the carb was virgin. The interesting part is the main jet is a 98 and idle jet is a 38. The Keyster carb kit I bought has a 38 idle jet but a 105 main jet. I may reuse the original main jet? If anyone knows anything about these bikes/carbs it’s a OEM Keihin stamped 660A.

The compression feels decent but I haven’t tested it yet.

I think some light cleaning, upto date maintenance, new tires, new gray cables and some grey shearing, and it should be good.
 

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69ST

Well-Known Member
Sweet find!(y)

Those were good machines. Didn't see many of them, I imagine the limited power of the 125 motor resulted in more buyers looking toward the 175 & 350 twins.

Cleaning and reviving the fuel/induction system should be straightforward. You'll want to verify that the disc brake assembly functions freely and without leaks. If it's healthy, a system flush, with fresh D.O.T. 3 brake fluid should be all it takes.

The money question is the motor. If it sat, for all those years, with both valves closed the motor could very well be in the same condition it was back in 1975. OTOH, if one valve was open...there may be rust in the top end. A good shot of oil, some cranking, followed by a compression test can tell you a lot. Or...you could just rebuild the carb, fog the cylinder with oil, add some fresh fuel and attempt taking it for a ride.
 

Jmunk

Active Member
Sweet find!(y)

Those were good machines. Didn't see many of them, I imagine the limited power of the 125 motor resulted in more buyers looking toward the 175 & 350 twins.

Cleaning and reviving the fuel/induction system should be straightforward. You'll want to verify that the disc brake assembly functions freely and without leaks. If it's healthy, a system flush, with fresh D.O.T. 3 brake fluid should be all it takes.

The money question is the motor. If it sat, for all those years, with both valves closed the motor could very well be in the same condition it was back in 1975. OTOH, if one valve was open...there may be rust in the top end. A good shot of oil, some cranking, followed by a compression test can tell you a lot. Or...you could just rebuild the carb, fog the cylinder with oil, add some fresh fuel and attempt taking it for a ride.

The front disc brake is actually cable operated. I was surprised but given the age it’s a good thing as the caliper would be toast by now.

I squirted some oil in the cylinder the other day and let it sit. I did a compression test today and it topped out at 90 PSI (carb removed, cold, Harbor Freight tester).

I would have liked to have seen the compression higher but maybe once it runs it will come up a bit. 90 PSI should be enough for it to run.
 

69ST

Well-Known Member
A cable-operated disc brake...wow(!). Hey, if it works it works, that's what matters. As for juice-operated disc brakes on these bikes, I think you'd be surprised just how well they've held up, on bikes of this vintage. And service parts are available.

I agree with you on the compression. If it'll fire and run, you might get lucky. It very well could improve with some miles. What have you got to lose in the attempt? Worst case, you'd have to overbore or replace the cylinder and give it a valve job...probably one valve only. The rest of the motor should be like new. 123 miles isn't even enough to begin the sludge formation process.
 

Gary

Well-Known Member
That might be the very first small Honda's to have a disc. They have some special cam system to operate both pads which was pretty ingenious. I'm really surprised they still had grey cables in 75- as you can see in my picture in '74 it seems some models were black. But now looking at some old pictures of my 73/74 CR it had black cables as well, maybe the off road models had the black ones. I'll bet the carbs are very much the same,they were used on the XR 75 as well. Pilot jets clog real easy,if I don't run the gas out it will plug up in a week of sitting. Northeast Vintage Cycle might be your best choice for parts. You'll have fun with it too I'll bet,back in the day I had an '71 Emerald Green SL 125. Liked it so much I bought the TL because of it. If I had to get rid of any of my bikes my 50 and the TL would stay
 

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Jmunk

Active Member
That might be the very first small Honda's to have a disc. They have some special cam system to operate both pads which was pretty ingenious. I'm really surprised they still had grey cables in 75- as you can see in my picture in '74 it seems some models were black. But now looking at some old pictures of my 73/74 CR it had black cables as well, maybe the off road models had the black ones. I'll bet the carbs are very much the same,they were used on the XR 75 as well. Pilot jets clog real easy,if I don't run the gas out it will plug up in a week of sitting. Northeast Vintage Cycle might be your best choice for parts. You'll have fun with it too I'll bet,back in the day I had an '71 Emerald Green SL 125. Liked it so much I bought the TL because of it. If I had to get rid of any of my bikes my 50 and the TL would stay

From the limited amount of research I’ve done, this year was the first for this model to have the disc. The cables have some fading on them, the front brake cable is the only one that appeared to be hard to find a replacement for. I located a OEM one on eBay in Indonesia for $14. A lot of OEM parts on eBay for these bikes appear to be in Indonesia. Honda must have kept these models alive there much longer as the smaller CC bikes are the norm there.

I’m on the frugal side and found a product call Rust911. Seems to be the same thing as Evaporust but much cheaper. I order a 16oz concentrate which they say you mix 8oz per gallon of water. It cost $23 shipped for the concentrate. If you wanted it in bulk you could mix it for around $4 a gallon if I recall correctly.

I plan to give the Rust911 a try on the gas tank and maybe try the exhaust collar which has some light surface rust to see how it does on chrome.
 

b52bombardier1

Well-Known Member
My CL100 has more problems with clutch plates sticking together than anything. Hopefully, you can avoid this problem and she runs well for you.

Rick
 

Gary

Well-Known Member
When I bought my TL in 95 I just went about things like I knew how so I used regular oil in it. Plates stuck all the time as well. My fix was to use GN4 oil in it,never had a problem since. Seem to remember hearing a fix involves removing the plates and sand blasting the steels something with them being so smooth the oil creates a suction ?
 

69ST

Well-Known Member
When I bought my TL in 95 I just went about things like I knew how so I used regular oil in it. Plates stuck all the time as well. My fix was to use GN4 oil in it,never had a problem since. Seem to remember hearing a fix involves removing the plates and sand blasting the steels something with them being so smooth the oil creates a suction ?
Interesting, as the only stuck plates I've seen with the 49 & 72cc horizontals were due to rust...and those were rare exceptions that had ingested water.

Could very well be that automotive oil formulations reached a tipping point ~25 years ago. If you recall, in the 1970s, the changes were almost yearly but for the better. Zinc & phosphorous levels weren't seriously hammered until 2006 but, friction modifiers have been evolving for decades now. I agree with you, GN4 is cheap insurance; I've also seen tech analysis, it's better quality than most realize. I imagine that any JASO-spec/wet clutch compatible, bike oil, should be a safe bet. As cjpayne has mentioned, 20W50 Valvoline motorcycle oil delivers slightly (but noticeably) better shift feel.

As for bead blasting, or sandblasting clutch plates, that will work. I just don't know for how long. Some clutch plates have dimpled friction surfaces, with a very precise pattern...easy to do, for an OEM. OTOH, I've seen glazed clutch plates that caused no problems. Wet clutch operation is very similar to automatic transmission clutch packs in automotive & truck applications. Clutch friction material composition is the realm of engineers. Two basic principles I do understand are dynamic friction and static friction...a.k.a. as "sticktion" (no joke) to engineers. The former describes parameters involved with engaging & modulating clutch engagement as from a standing start. The latter describes how much force a clutch can withstand, before the onset of slippage, once fully engaged. Most of this is above my pay grade. There aren't many choices in replacement parts anyway...a good thing, imho, as it could be mind-numbing.

When dealing with a slipping clutch, I just replace the discs & plates, springs too if available. Gotta teardown the clutch pack either way. So, it's the same amount of work, oil and gaskets...only real difference, once you've gone this deep into the clutch...is the price of plates + discs.
 

Jmunk

Active Member
Gas tank is filled with the rust remover and water mix. I used distilled water, may or may not help any.

Here is a before photo. I’ll give it 24hrs or so and see how it looks.

If the rust remover works I’ll capture it and put it back into the gallon jugs as it claims to be reusable until it’s black in color.
 

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Gary

Well-Known Member
Hope you can figure a way to keep it from rusting again after you get it cleaned out. Luckily it doesn't look too bad to begin with.
 

Gary

Well-Known Member
Interesting, as the only stuck plates I've seen with the 49 & 72cc horizontals were due to rust

When they stick what a pain in the butt as well. I don't know Rick's fix but I live on a big hill so I'd put it in 5th and down the hill I go. Soon as it starts I'd pull in the clutch and ride around in 1st or 2nd until it broke free. This hill has seen numerous bikes get pushed down it to start,the Honda's 99.9 didn't need to be pushed- the old CR's and MR have a strange kick start lever when it gets to the end of it's travel somehow your foot slides off and your shin hits the peg,so it was more convenient just to hop on. Triumphs on the other hand 99.9 times had to be pushed back up as well. After that bike I never owned any thing but Honda's
 

69ST

Well-Known Member
I agree with Gary, the inside of your tank looks pretty good. I've had mixed results from Evaporust, it's good on lightly-rusted metal but won't touch really heavy rust. Going by what can be seen in your photos, so far, I think that the product you're using (likely similar, or the same as Evaporust) should do the job. Believe it or not, acetic acid a.k.a. white vinegar, works more slowly than Evaporust...but does better with heavy rust. The process can take days, or weeks, but does work. Phosphoric acid would be my preference, it leaves a light phosphate coating behind which adds a little corrosion protection but you've gotta be careful not to get any on painted surfaces. If you can be that careful (hint: mask the tank in layers) you can apply tank sealer. That's the only way I know of to keep rust from returning, with today's oxygenated fuels.
 

Jmunk

Active Member
If the rust is removed, and returns quickly I’ll either try the phosphoric acid or the POR 15 tank sealer. I’d rather not do a sealer if at all possible.
 

Jmunk

Active Member
24hrs in on the gas tank soak, I’m going to let it go another 24hrs. My garage temp is at the lower end range of what they recommend for its working temp range. I see on their website they say the warmer it is, the faster it works. It’s making good progress, another day shouldn’t hurt.

I put a dribble of gas into the intake and gave it a kick, it came to life.
 

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69ST

Well-Known Member
First tank I chemically de-rusted took 5 days of soaking, punctuated with vigorous shaking...a couple handfuls of old nuts & bolts added. That was using phosphoric acid, in 80F weather. It turned out beautifully. The KREEM sealer remains intact, just yellowed from fuel exposure, 19 years later.

Most tanks take less than 72 hours. The longest acid-soak took five weeks(!)...using distilled vinegar. It turned out beautifully but developed flash rust almost immediately after rinsing with hot water. A brief hit of phosphoric acid cleaned the flash rust. As yet, I've not figured out a reliable & repeatable way to prevent flash rust with anything but phosphoric acid, other than a generous shot of WD40.

With chemical rust removal, there's almost no such thing as too much patience. And, usually, that means an extra day or two. It's the difference between a successful result from a sealer kit and a godawful failure. If you do opt to one day seal the tank, the POR-15 is more forgiving. KREEM will fail if the prep is less than perfect; but, a white tank interior lets you see how much garbage has accumulated.

I don't blame you for wanting to avoid using sealer. And, while some light rust is all but unavoidable, its presence isn't necessarily cause for alarm. Keeping the tank either full of fuel, or empty & fogged, should keep the tinworm from invading. Most steel tanks has a little bit of surface rust. Clear, inline, fuel filters are not only good protection, they're a damn good way to tell when there's a problem. If you see an appreciable amount of rust grains, it's time to do something about it. That might never happen. Worst case, it won't take you long to prep & seal. I doubt you'd let it get that far.
 

Gary

Well-Known Member
One of the advantages of a 2 stroke- my MR's tank looks like the day it rolled off the line,the exhaust is in excellent shape as well. My TL's tank was a little corroded maybe the same or a little less than your CB. I ended up cleaning it and coating. I coated my 50 as well since I have seen so many of those tanks rust out on that bottom seam. Unless you can get pure gas I'm not sure leaving it full would work,that --*-- alcohol might make it rust again as it might attract water?
 

Jmunk

Active Member
48hr mark. It warmed up today and that seemed to make a difference.

Might try another 24hrs.
 

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