CB125TD running issues...

[wreckerman]

Not been on here in years, so long I couldn't even remember my username of email address I'd used, so this is a new profile..

This ones for Teflon Mike as it's about a 1983CB125T that refuses to run properly under 7/8 thou RPM, and the 1 name that pops up about these bikes is Teflon Mike.
So the bike came as a non runner in pretty rough shape, it's got new rings, new pistons, new coils and everything seems to okay apart from the low end problem, I've been through the carbs and everything seems okay, electrics wise everything seems good as well, but it refuses to fire properly under7/8 thou when it picks up nicely, but by this time it fouled up one side so badly to the point the bike has to be stopped, cooled down, and the plugs cleaned, this isn't good living in London as most riding is done below 7/8 thou rendering the bike totally useless..

So Mike if you read this, any thoughts would be greatly appreciated..

[Teflon-Mike]

Proved by Snowie on her Mod 1 test through the radar trap.. a CB125TD-C will pull over 30mph in first!!!! It MIGHT chuck a tappet if you try it... but it WILL do it!!!

So why you grumble about living/riding in urban environ, and slow speed running? If you use the thing as designed, and let it rev, and ride for response, you shouldn't need use a gear over 3rd about town in 30/40/50 limits.. let the little fecker-rev, that's what they is built for; and once moving, in 20/30 limits, you shouldn't really need anything but 2nd.

But Black Plugs. You say you have dismissed anything electrical; not that there's a lot to dismiss; ignition is self exited Magneto, with fixed timing and CDi; tends to either work, or it just doesn't. Its pretty reliable. You say its had new rings and pistons... hmmm... what about the bores? What about the valves and valve seats? What about the tappet clearances? These little engines tend to be pretty sensitive to rings and bore wear; are you SURE they are good?

BUT, leaves most likely culpret IF all else good, is the carbs, and if one plug fouling, the most likely suspect is the wampy flex link between the choke link, and the choke cable clamp on the Left-Hand carb, and the other side choke flap either not fully opening and or flapping around doing whatever it feels like.

Next most likely suspect is the float-needles in the carb-bowls, that are prone to stick in the rebates, and the rubber tips and seats that help them not seal and peee petrol out the breather hole whilst the carb floods engine over-flowing the primary yet.

Next most likely after that.. and related... is the primary air-screw. Has very fine needle on the end, prone to corrode in its drilling, and shear off when any-one tries to twist the mixture screw....

Next and more and/or, is the motor sucking air through cracked, perished, or other wise mangled inlet rubbers.

These are often split by numpties triying to remove the carbs without first removing the air-boxes to give clerance; and or reffiiting carbs without first making room removing air-box and air-box rubbers; and they do tend to go hard with age anyway; aided and abbeted by old hose clamps, and or alternative, and often thicker, jubilee clips that often dont grip, or ruck the rubber, or dont sit in the rebate for them, etc etc etc

If motor sucks air through perished rubber or missing gaskets, then people oft blame the mixture and try richen up to compensate, on the mixture screw, and you get into a cycle of compounding problems.

Main needles are oft fitted on the wrong, middle, groove too, OTMH they should be one down from top.

Go look, see whats what; replace gaskets; clean needle valve and rebate.. and dont assume that is fix and forget, the carb bowls often need a tap with the handle of a screw-driver to make the needles move, and they oft stick if not used or motor run hard (rev the fecker!) very often; check that choke link is actually linked, and gripping; and look carefuly at the mixture screw.. and you do have a whole one!

Chinky carb overhaul kits are often not for the CB's PD carb, but the CG's; they jets are the wrong size, the float needle the wrong type and the float-bowl gasket completely useless... but the mixture screw is good, as are the mixture screw seals; and the kits are oft as cheaper and easier to get than them bits alone.

BUT... if an unknown motor, I tend to rebarel them as a matter of course these days, to KNOW that the bores and rings are good, and whilst in there, clean up the valves, the valve seats and fit new valve-stem seals; and pay attension to the tappet clerances, asjusting with the propper tappet key and lock-nut spanner, that make it oh-so much easier to do, and saves the fine thread on the tappet locknut or chewing the square on the tappet, and risking loosing a lock-nut if 'some-one' revs the dang thing to 15K in first trying to beat 32mph, in first, on the Mod1 radar trap... or anything daft like that Lol!

Sets base line of certainty for further diagnostics, and saves silly questions, before you start; rest is just attension to detail, and not rushing and setting them up EXACTLY to the book, like looking at the grooves on the carb needle properly and counting grooves, getting the air-boxes out the way and looking at that choke link, and bending the tang on the shaft so it actually grops the shaft like it should, etc etc etc.

Have fun... and good luck.
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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?'

[Nobby the Bastard]

TLDR: Check the pilot jet.
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trevor saxe-coburg-gotha:"Remember this simple rule - scooters are for men who like to feel the breeze on their huge, flapping cunt lips."
Sprint ST 1050

[Fizzer Thou]

Mrs.Fizzer had one of these bikes and after two years of ownership she had similar problems which were traced to the inlet manifold 'O' rings (part #20 on the fiche)

https://www.fowlersparts.co.uk/parts/4754885/cb125tdc-1982-e/cylinder-head

She used the bike for commuting up to central London and this effectively cooked the 'O' rings.

With the engine running,spray some WD40 around each manifold.If the revs increase,then you know you have an air leak
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Just talk bikes.What else is there?

Always have a 'Plan B'

[wreckerman]

Took it for another spin and everything seemed a lot better.
I forgot what revvy little engines these things had, I think it just needs a new camchain and a tune up...
It was actually the guy at the MOT centre that said he thought it was something electrical, I just thought that as he should know what he's talking about I'd go with it and the resistance reading from the coils was low and a lot higher on the new ones..

The inlet rubbers aren't too bad considering, slight tear at the very front but not bad, and I used heat wrap around the rubber...

Carbs seem very good, they just need setting up properly.
The needles are on the correct slot.

I'd like to get new barrels, but that can wait until Winter, when I do a Full engine rebuild..

I'm having fun.

Cheers

[T3chN3rd]

Hi, I'm fairly new here, however I just thought I'd post to maybe help a little. Back in April 2016, I bought the CB125TDC for £450 with a couple of months MOT left on it. It came with a Motad 2:1 Exhaust, however the down pipes were rotted a fair amount, so I purchased the two single exhausts. Within 2 months, the bottom end bearing went on the Conrod. I replaced the engine with a 'fully refurbished' engine. Same thing happened. This was due to having a major oil leak, and struggling to keep up with the oil consumption.

After fitting my 3rd engine, I had a problem where it would stop running on one cylinder after the engine heated up. I went through process of elimination. Stripped and cleaned the carbs. Replaced CDI's, replaced HT leads, replaced spark plugs, even putting hotter ones in. Nothing worked. I had the carbs professionally balanced some time ago.

Everytime the engine heated up, the right hand cylinder would stop firing, I take the plug out, it was thick in black soot.

I later found out, that after putting the choke on one morning, when releasing it, the silly ribbon in-between the carbs for the choke had slipped off, leaving the right hand carbs' choke ON.

I linked them both up, everything worked perfectly. I then sold the bike this April for £535.

They are great bikes, comfy to ride, they certainly pull nicely from 50mph. Sound great too. Seems as though the choke thing is a common problem. Another common problem is the rubber bush on the bottom of the rear shock (on the mono shock models), they have a tendency to be worn, due to age, and are difficult to get hold of.

Other than that, great bike, but had too many problems with it as it was my daily commuter.
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Honda SGX50 Sky (non runner) - Honda CB125TDC (sold) - Zontes Scorpion 125 (daily commuter).

[wreckerman]

Crikey two n a half years ago..

[pepperami]

And? Since then.?
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I am the sum total of my own existence, what went before makes me who I am now!

[oilyrag]

If it's magneto and points ignition, I'd guess it was a problem with the points

[Evil Hans]

[Nobby the Bastard]

The best sprints are in British racing green, not red. Red ones go slower.
____________________
trevor saxe-coburg-gotha:"Remember this simple rule - scooters are for men who like to feel the breeze on their huge, flapping cunt lips."
Sprint ST 1050

[Hong Kong Phooey]

[Fisty]

[Hong Kong Phooey]