Topic: CB100/CB125 Hybrid aka CB112.5

[Tugboat]

Only a couple of weeks until Barber and I didn't think I'd have time to properly line the tank, so I found this one on ebay for $100 shipped. Clean as a whistle inside and the paint is in pretty good shape. No dents.


So with a clean tank & working petcock I was finally able to fire it up and ride it around. It didn't want to idle unaided, though it did run well enough to get me up to 50mph on flat ground. When I got home I pulled the plug: looks like I'm running rich.


I pulled out the air filter & left the filter box lid off to see if that leaned it out - no effect. I had the air screw set to 1.5 turns out.. turned it out another 1/2 turn and that improved it but not much.. still won't idle on its own. I replaced the plug cap - no effect. I know it has stock jetting (105/38) and am pretty sure the float height is correct. A couple of times it would overflow once I shut it off, but not every time and never while running. Static timing was set when I installed the new points plate, though I haven't checked it while running.

[cbornotcb]

Check out the needle jet Tugboat, they tend to "oval" after constant use, that'll mess up your jetting.
Tiny

[Tugboat]

Messed around with the carb last night. First, the float valve seat was loose. Problem #1.


Next, I checked the float to make sure it floated correctly. Turns out, one side has gas in it. I have a buddy's CB100 carb with a known-good float in it, so I "temporarily borrowed" it. Problem #2.


Checked the slide needle - clip is in the middle position & the needle doesn't look too worn.


Put it back together & ran it about a mile down the road - still running rich, though it does idle a little better (with some extra throttle and the idle screw all the way in). Still won't idle unaided though.


Double-checked the schematic, and stock mains are #105. I'm running a #110 so hoping to pick up a 105 from the shop today if they have one in stock. Regardless - sounds like there are at least two more problem here right? (1) It should idle if the idle jet is correct and otherwise unblocked [jet is not stamped, so not sure if it's #38 like it's supposed to be. Must be aftermarket]; and (2) running rich in the upper throttle range. I can get this thing to do ~50mph in current form, so I think some plug chops are in order to find where I'm too rich. Stay tuned...

[Tugboat]

Pulled the bowl off and looked at the slow jet. I tried to take a pic but my camera wouldn't focus that tight.. suffice it to say that what should have been a #38 looked like a damn main jet! Some PO probably cleaned it out with the smallest drill bit they had and now it's... maybe a #80? HUGE!! The main is a 110 (stock is 105). I have access to a Keyster kit which only had another 110 main, but it also had a #38 slow jet so I replaced that one. I also replaced the float, which turns out to be the same as on a CB350 and my buddies at Nashville Motorcycle Repair actually had one in stock. Put it all back together and took it out for some plug chops. Results:

Idle - at 2 turns out of the air screw


Mid


WOT - in 5th


I should have done WOT wound-out in 4th, since 5th gear is essentially useless. Not sure if it's the gearing or what but I can get up to about 55mph in 4th, but 5th doesn't seem to have any additional power. Also, WOT looks a little lean and it did help to give it a little choke.

It has a rattle too. Not sure if it's the top-end or what. I changed the oil this weekend and there were some non-ferrous fine metal flakes in it. Some small chunks in the oil pump screen too.


I tightened the cam chain while it was idling. Not sure if I tightened it enough, but I backed the tensioner all the way in/loose and then backed it out until I felt some tension. Might look at that again when I get home. Double-checked my timing while I had the oil drained out as well. Here it is on my way to work this morning.

[DustyRags]

Ah, the fine art of mangling jets! Good catch!

Bike looks nice.

[Tugboat]

Here it is with one of it's cousins at the Barber Vintage Fest in Birmingham, AL this past weekend. Ran great. Turns out the forks were twisted, but we loosened the triples and got it all back straight again. Picked up some clubmans that I might use (right-side up, of course), a new headlight bucket, front brake switch.. never did find side covers or a front fender.

[Tugboat]

Now with clubmans...


Finally got the carb tuned. I was running pretty lean at WOT with the #110 main. #105 is stock for this motor/carb setup. I figure the CB125 air box I'm running  is less restrictive than the CB100 one. I've gone up in steps of 2.5 and finally settled on #117.5 - five steps up from stock. I started out getting about 52-53mph at WOT in 4th (5th is pretty much useless) and yesterday I got it up to 60mph with the proper jetting.. a 13% increase.

Next up: re-routing some cables, cutting and painting the fenders, new grips.

Also, I should note that I have a weird rattle. It changes with engine speed, and is the likely culprit for the aluminum sparkles in my oil. Probably a cam bearing failing? I need to open up the valve caps and make sure I'm getting oil up there. I have a whole new top-end I can swap over when/if it fails.

[Tugboat]

AHHHHHHH>>>>

Well, it's been a while since I've had time to mess with this thing. I've used that time to assemble some parts and finally got around to pulling the motor. Might I add that engine tear-down on one of these little things only took me & Spanner1 like an hour or so, and that was including trips to the beer fridge.

The good news: cam & journals actually look great. Normal wear, and no signs of oil starvation. Didn't get a pic of the journals but here's the cam.


I did, however, find the source of the ticking noise.


The cam chain must have gone un-adjusted for some time and got stretched out and started flopping up against the side of the case. The tunnel on the cylinder looks similar. It also damaged the cam sprocket.


I'd already bought a "new" head that's in good shape and its matching cam, and had new valves installed and the seats cut. Also have a new cylinder, piston, rings etc that I'll be installing.

I guess what I'm wondering now is what to do about whatever residual aluminum flakes that may still be down there in the bottom end. After my first oil flush, where I found the aluminum, I'd replaced the oil and ridden it around some. Again, pulling 60+ mph in an effort to get the carb jetting right. Also, this motor had undoubtably been run for some time while the damage was occurring, and by looking at the cam & journals it had never been enough of an issue to block any oil passages or cause any wear. Finally, when I drained the oil the other night prior to pulling the engine, I didn't see any metal in it. So should I just put it all back together & go for it? Flush the bottom end with oil or diesel or something?

[grcamna2]

 Nice CB112 Tug 

[Tugboat]

Oil "filter"


When I drained the oil prior to tearing down the top end, I noticed that there wasn't nearly as much "sparkle" to it as there was the first time I drained the oil. Looks like the centrifugal "filter" is doing its job?

[grcamna2]

Quite a few aluminum chips in that spinner/filter there Tug...    I think a few good changes of oil and clean the spinner out a few times with-in a 250 mile run-in should help.