1982 CB125T Project

[mattday]

A couple of weeks ago, I bought my first bike: a 1982 CB125TD-C. I have read a lot of Teflon-Mikes hugely informative articles (on his site) and his posts on here. I am still optimistic this bike needs relatively little to get it on the road. However, I could really do with a little help from anyone who owns one.

1) The back end is in pieces at the moment. As far as I can see, the grab handle should bolt on to the indicator brackets via some kind of spacer, although I don't have any of these parts. I'm referring to part 19 in this diagram. I'd rather not spend £30 on four bolts at the moment, so I was thinking it should be possible to knock something up (M6 threaded rod into tapped tube perhaps). If someone would be kind enough to measure this part, I would be very grateful.

2) The carbs on my bike had the bottom nipples linked by a single tube. I have not been able to find a picture that clearly shows them, but am I correct in thinking they should instead have separate drain tubes running down the back of the engine?

3) Thoughts on my rear shock?...

[Teflon-Mike]

[mattday]

Fantastic!

Thanks Mike, that is incredibly helpful (much more so than the NHS ).

What you say about the shock is along the lines I had been thinking. I asked only because I was concerned it might be a bad idea to leave something that could fail a MOT if it were visible. But assuming it damps OK, I'll focus on getting the bike on the road first. It will probably take me a while to get enough confidence to benefit from new suspension anyway.

I did see a new shock on eBay which seemed quite cheap for a genuine Honda part. Perhaps some kind of licensed copy?

I'll no-doubt be adding to this thread in the coming weeks

[mattday]

I've been slowly getting there with this bike... hopefully not far off getting it on the road now. Had it running on both cylinders for the first time today (video clip attached).

I didn't touch the throttle, and whilst the tach was working, it was showing the idle to be about 5500rpm! Not sure how the tach cable is driven, but it seems to keep coming dislocated at the engine end.

Anyhow, it looks to me like maybe the carb slides are too high with the throttle closed (see pic). I haven't had to detach them from the throttle cable, but I understand there are several positions at which they will attach. Am I on the right track here, or does this look OK? Any other ideas on the racing idle?

[Teflon-Mike]

1/ The pipe poking out the middle, that has a cut-out in the choke flap; that's the pilot air port.
May be my eyes, may be the photo... but LOOKS like it's full of crap.
Air sucked through that hole, aerates the fuel in the emulsion tube as it comes out the jet. If it's blocked, it'll be screwing the mixture right up.

2/ Again, could be photo, but choke butterfly on that one carb doesn't look totally horezontal. The flex link between the two choke butterfys is also a bit flakey, and before you try and get these carbs set up, you have to make sure that, first, both flaps do FULLY open, and close, and also that they are perfectly syncronised, opening and closing together.

3/ No, the carb slides dont attach at different heights on the throttle cable. The idle speed is set, on the screw on teh side of teh carb body that sets how low the slide falls in the venturi.

Tip: when setting idle speed, lift slides by twisting throttle twist grip to lift them off the stop. Make adjustment on the idle screw, then lower slide by closing throttle at twist grip, to let idle 'settle' and avoid merely screwing adjustment screw into soft aluminium of slide!

Note: the thing in the throttle slide that has variouse 'positions' is the throttle needle that changes the area of the main jet when you raise the slide; it has, from memory, five notches in it, and the height is set with a circlip that sits in the top of the throttle body and is sandwhiched by a plate that also retains the throttle cable, pressed down by the slide spring. On most engines, the middle notch, third from top is usually the 'default' needle setting; on the 125 Super-Dream, however, I think its the second notch from top, or one up from middle.

Often found needles sitting high making engines ritch, and not carburate cleanly, where people have presumed usual default on the middle groove.

TIP: book carb settings for needle and for mixture screw just 'work'. I think its 5x 1/4 turns on the mixture screw from fully in OTMH but CHECK THE BOOK.

After making sure idle air port is clear; set the mixture screw; check needle height; make sure chokes syncronised; THEN start worrying about the idle speed, and if you want to get a bit keen about it, 'balence' the carbs so that the idle settings are identicle on both carbs with vacuum guages.

Dont think I How2'd it on here, but is on my webby Balance or Synchronise Carburettors

BUT ought to be last thing on list of service adjustments, after you have made sure that engine's good vis tappet clerances cam-chain adjustment, spark-plugs, air filters etc.

Here and now you need to get it down from half revs, but looking at the slide height in pic; dont think that's cause; pilot air, needle hight and mixture screw could be.
____________________
My Webby'Tef's-tQ, loads of stuff about my bikes, my Land-Rovers, and the stuff I do with them!
Current Bikes:'Honda VF1000F' ;'CB750F2N' ;'CB125TD ( 6 3 of em!)'; 'Montesa Cota 248'. Learner FAQ's:= 'U want to Ride a Motorbike! Where Do U start?'

[mattday]

Those pilot air ports looked clean but both have fuel in them, so I guess that is bad? Not had any previous experience with carbs, so learning as I go along. Thanks for the explanation.

Also correct about the choke butterfly. It is not horizontal because the stop allows it go further. However, I spent quite a while messing about with these previously, added a spacer to the stop and tweaked the linkage etc, trying to get them better synchronised. I gave up in the end as IIRC one moves through a bigger angle than the other and there was always slop in the linkage. Obviously they need to both open fully, but is the synchronisation really that crucial? The design doesn't really seem to permit very accurate synchronisation.

Plugs were new yesterday, air filters are not new, but are clean. Oil looked like it was changed not long before I got the bike. Adjusted the cam chain tension this morning, but had to postpone valve clearances until I have more suitable tools. They are currently all too tight though as I couldn't get a 0.04mm gauge under any of them.

[Teflon-Mike]

[mattday]

[mattday]

In addition to the float needle valves, I removed everything from the carb that would unscrew and made sure it was all clean. Blew everything out with carb cleaner and compressed air. Tried to figure out how the thing is supposed to work and drew a little diagram that is attached. Let me know if I have got anything fundamentally wrong in it. I set the idle mixture screws to the book spec - 1.25 turns out.

The very high idle was due to the slides resting too high. This was mainly because the throttle outer cable had got caught on the adjuster at the twist end. Whilst the carbs were apart, I replaced the throttle stop screws losing whatever setting they were on, but after adjusting them it now idles between 1k and 1.5k.

This would all be very good - it starts easily and the engine sounds OK to my ear. However, the main issue now is that it seems to be idling very rich. The plugs are getting black, the exhaust stinks of petrol and it seems to be drinking the stuff. I have tried backing the mixture screws out a bit. The engine will happily tolerate a bit, but started to sound unhappy if I went another full turn out. There still seemed to be a lot of unburned fuel in the exhaust.

A second potential problem is the main air jets have fuel in them again. I don't see that this would have any effect on the idling. They should only come into play when the throttle is opened up. It is also possible that this is due to me bouncing the bike to manoeuvre it out from storage to where I can run it. The first time after I put the carbs back on, I am fairly sure there was no fuel in these ports.

So I am a bit stumped now. What could be the cause it running rich? Or is this kind-of normal for old carbs?