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1981 Honda CB125TB

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Deleted1212
L Plate Warrior



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PostPosted: 11:31 - 18 Nov 2019    Post subject: 1981 Honda CB125TB Reply with quote

Hi All,

Here is my honda cb125tb with a t2 engine. i have just finished rebuilding the engine and carbs, but cannot get it running well. It is running too rich, and i am also not really used to these carbs being primarily a car person! (I have a pair of old BL triumphs that have a single SU, so piss easy to tune)

https://live.staticflickr.com/65535/49084037162_3b96b10c0f_c.jpg

I am learning to ride but so far haven't had much seat time, before the rebuild it was smoking like a 2 stroke from one cylinder so did stem seals and rings and it has mostly sorted it. It still smokes but it's fuel rather than oil. I have played around with the float height to try and get it to ~18mm from the bottom of the float, but it is still running rich and not doing the plugs any good. Any tips on tuning these bikes? Just want to be able to ride the damn thing! Cheers.

More pics of the rebuild to follow
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1981 Honda CB125TB (With a T2 engine)
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Deleted1212
L Plate Warrior



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PostPosted: 11:38 - 18 Nov 2019    Post subject: Reply with quote

the floats definitely "float", so convinced they are fine:

https://live.staticflickr.com/65535/49084037142_0f402db3ba_c.jpg

gear lever had been bent so ordered a NOS one and replaced it -

https://live.staticflickr.com/65535/49083833551_5f576eb950_c.jpg

looked a bit different yesterday morning before the engine went in!

https://live.staticflickr.com/65535/49084037152_b5882e9633_c.jpg

these are the carbs that are on the bike. I have rebuilt both but still not sure they're performing correctly.

https://live.staticflickr.com/65535/49083357023_9e1bd2a6c9_c.jpg
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Easy-X
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PostPosted: 12:08 - 18 Nov 2019    Post subject: Reply with quote

Am I reading this right:

https://www.howmanyleft.co.uk/vehicle/honda_cb125tb

Apparently you have the only one on the road Shocked The DVLA database is a bit wonky for anything before SORN was introduced but probably you have one of the few left on the road Very Happy

Let's assume the carbs are working for the moment. If it's running too rich then either it's not getting enough air or there's some exhaust restriction. Test it with the airbox off and check the exhausts for blockages.

After that are both cylinders firing - both downpipes getting hot - or just one?
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Deleted1212
L Plate Warrior



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PostPosted: 12:35 - 18 Nov 2019    Post subject: Reply with quote

Easy-X wrote:
Am I reading this right:

https://www.howmanyleft.co.uk/vehicle/honda_cb125tb

Apparently you have the only one on the road Shocked The DVLA database is a bit wonky for anything before SORN was introduced but probably you have one of the few left on the road Very Happy

Let's assume the carbs are working for the moment. If it's running too rich then either it's not getting enough air or there's some exhaust restriction. Test it with the airbox off and check the exhausts for blockages.

After that are both cylinders firing - both downpipes getting hot - or just one?


I wouldn't be too surprised if it was the only one! I think i still have the TB engine if i wanted to restore it to completely factory but i'd rather have the t2 engine and goodies like rev counter etc.

will test with airbox off later, haven't tried that yet with the new engine in. Both downpipes get hot, but it seems like when it warms up it starts firing on one cylinder. I wonder if the coils are no good or something and don;t like heat? they are right above the cam cover so would surely warm up a little bit.
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Easy-X
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PostPosted: 12:47 - 18 Nov 2019    Post subject: Reply with quote

Oh yes, the engine swap. How much did you swap over? Fuel system, carbs, electrics, etc.

For example: if you swapped all you could you might end up reusing the original airbox and exhaust system which, unless identical between models, would probably mean rejetting the carbs.

<addendum> well they do look identical to my untrained eye but unless the part numbers [of the airbox/exhaust] match there may be differences you need to account for.
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Deleted1212
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PostPosted: 12:58 - 18 Nov 2019    Post subject: Reply with quote

Easy-X wrote:
Oh yes, the engine swap. How much did you swap over? Fuel system, carbs, electrics, etc.

For example: if you swapped all you could you might end up reusing the original airbox and exhaust system which, unless identical between models, would probably mean rejetting the carbs.

<addendum> well they do look identical to my untrained eye but unless the part numbers [of the airbox/exhaust] match there may be differences you need to account for.


I didn't do the swap myself but bought the bike with the engine and a load of spares. The carb rebuild kit i bought was for a t2, and everything seemed to go on properly - the haynes manual suggests they are the same carb unit so think the jets should be about right now. I think the t2 loom was used for the rest so it should all be t2-spec rather than TB. Think the tank is still original but wouldn't have thought that would make a difference?
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Teflon-Mike
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PostPosted: 15:47 - 18 Nov 2019    Post subject: Re: 1981 Honda CB125TB Reply with quote

rerpha wrote:
I am learning to ride
You do Know that this isn't UK Learner-Legal A1 complient don't you? It may be a 125, but when they changed the laws to up the Learner-Legal power limit to the EU harmonised 15bhp (Manufacturers claim) the CB125T was already outside of the limit, at 17bhp? Wow! To all practical purposes it makes no odds, especially as your example isn't making any bhp at the moment, b-u-t, it may if/when some-one comes to check the paperwork, and it dont match up.... you have been warned!

Elcewise... this is a forty year old motorcycle, that has probably spent a large chunk of its life laid up derelict.... they don't like that much... anything and everything is likely worn out and or gummed up by now.

Personally on all my 126 twin projects my FIRST recourse these days is to do a full and propper Top End Rebuild, on the thing... see handy how2's in profile. These little motors do NOT like worn bores or less than eager piston rings, whilst the valve stem seals have a tendancy to turn to talcum-powder. A Top end rebuild nails just 'soooo many; possible problems at source, and cuts to the chase as far as later fault finding, cos you KNOW that the top end should be good.

I would NOT be muggeing about with the carbs at this stage. Motor is only half the word Motorbike, and even less of one.

Before you 'Go' make sure you can 'stop'.. and preferably go in the right direction.

These have a 'horrible' cable operated disc brake on the front, that was never particularly wonderful even when new, and will NOT have gotten better with age!

That would probably be my first candidate/tack on this thing, and for originality, it would have tgo be properly overhauled, and set up. For the fess, I would probably be thinking about front end upgrades, and either the entire fork-assembly off a TD... 'cos that got a disc brake, and I have more than a couple lying about. though would beg the TD comstar wheel to match, cos the discs have different offsets... Or I'd be pondering bits from later disc braked CG 'cos I 'think' they have the same diameter fork stanchions....

B-u-t the CB100N shared the same cable operated disc brake, and I have NO fond memories of that what-so-ever. It was at best 'wooden' lacking feel, at worst, it just didn't slow the wheel, but worse than worse, it could do any or either any time it liked, and using the front brake lever was like playing Russian Roulette, especially in Brummie Rush-Hour"

Anyway; that niggle targetted, forks rebuilt, I'd turn attension to the back end, where the rear 'suspension' units would almost certainly find thier way into the scrap heat in a nice parabolic trajectory. They lacked damping at the best of times, odds they have any now is near nill.. is they do it'l be rust and crud impinging damper rod progress, I suspect. They would be swapped out. Whilst down there the swing arm bushes would come in for replacement,m and good chance to see how much metal is left in the swing arm.

THEN, onto such luxuries as... mudguards and body-work, and the little niggle of the 'electrics' which are never much fun, but hampered on this particular route to the asylum by being 6v.... a 12v conversion with Benly bits and possibly even an e-start, would be strongly considered.

BUT when I had sorted or sorted a plan for the cycle parts, like that front brake, I MIGHT get around to doing something with the engine, which as said would start with chinky barel kit and a top end rebuild....

THEN carburettors may be a bit of an imperative.... and a bit liker the engine, I would plan a complete overhaul.. after sourcing some nice new hole free exhausts.

I would be wary of the e-bay carbkits... in fact I had an argument with WeMoto over some they listed for the 125Twin, that were actually Chinky bits for the CG, and contained not only the wrong float needles, but wrong size jets.... you have been warned... a-g-a-i-n. DSS I believe ran out of 'official' carb spares for these about a decade ago, so its down to getting a generic CG kit to salvage what bits you can from it, like the float bowl seals and the idle-air screw.... on which note almost certainly worth a look, they have a very fin needle on the eld, wont to corrode into the carb-body... and snap.... oft replaced in ignorance by folk that then say that it makes no difference, more often with two or no rubber O-Rings! More warn!!

B-u-t long and the short of it is that basically on the list of things to worry about.... setting the float height IS a long long way down... and even a well fettled carb will pee fuel from the over flow, cos the needles stick in their guides....

Advice is start from the top, and work methosically down the list, slowly and carefully, with LOTS of known an trusted new bits.. dont 'buy' problems you dont need.

Best of British, with it, anyway.

Add on ED: you say PO did the engine swap.... can you be sure its a T2 motor? Almost all the Benly twins are interchangeable in the frame mounts. The Benly and CM motors oft also have the round rocker covers tat 'look' T" ish, but even they are a quick swap. B-U-T the T's have a 180 timed crank, the CDs & most CM's a 360 timed cran. When you pulled the top off to look at valve stem seals... were both pistons at TDC/BDC together? Can you be sure it's a 180 timed T motor? If not, it would run like a bag off, on only one pot, if it's trying to spark the pots at the wrong time. Whichh dont help much with the carbs... but as said, they would not be very high up the priority list.

On The Topic, worth mentioning the carb-rubbers, which have a tendency to perish, and on this should be flanged for the 26mm carbs, but also tend to get torn with folk ripping the carbs out brutally. Also rather unobtanium... so good luck there and be careful with what you got, if messin in that region. Lots of botches with sily sealant, insulation tape and super-glue are known!
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Robby
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PostPosted: 22:50 - 19 Nov 2019    Post subject: Reply with quote

You'll learn to ignore Tef in time.

Are the carbs all correctly according to the Haynes manual? Bits to check:
- Float height
- Main jet size
- Needle position
- Mixture screw position
- Is the choke working

Bear in mind that it's an old 125, and has probably been owned by 20+ teenagers. They love fiddling with the mixture screw, having no idea what it does.
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Deleted1212
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PostPosted: 23:03 - 19 Nov 2019    Post subject: Re: 1981 Honda CB125TB Reply with quote

Teflon-Mike wrote:
rerpha wrote:
I am learning to ride
You do Know that this isn't UK Learner-Legal A1 complient don't you? It may be a 125, but when they changed the laws to up the Learner-Legal power limit to the EU harmonised 15bhp (Manufacturers claim) the CB125T was already outside of the limit, at 17bhp? Wow! To all practical purposes it makes no odds, especially as your example isn't making any bhp at the moment, b-u-t, it may if/when some-one comes to check the paperwork, and it dont match up.... you have been warned!

Elcewise... this is a forty year old motorcycle, that has probably spent a large chunk of its life laid up derelict.... they don't like that much... anything and everything is likely worn out and or gummed up by now.

Personally on all my 126 twin projects my FIRST recourse these days is to do a full and propper Top End Rebuild, on the thing... see handy how2's in profile. These little motors do NOT like worn bores or less than eager piston rings, whilst the valve stem seals have a tendancy to turn to talcum-powder. A Top end rebuild nails just 'soooo many; possible problems at source, and cuts to the chase as far as later fault finding, cos you KNOW that the top end should be good.

I would NOT be muggeing about with the carbs at this stage. Motor is only half the word Motorbike, and even less of one.

Before you 'Go' make sure you can 'stop'.. and preferably go in the right direction.

These have a 'horrible' cable operated disc brake on the front, that was never particularly wonderful even when new, and will NOT have gotten better with age!

That would probably be my first candidate/tack on this thing, and for originality, it would have tgo be properly overhauled, and set up. For the fess, I would probably be thinking about front end upgrades, and either the entire fork-assembly off a TD... 'cos that got a disc brake, and I have more than a couple lying about. though would beg the TD comstar wheel to match, cos the discs have different offsets... Or I'd be pondering bits from later disc braked CG 'cos I 'think' they have the same diameter fork stanchions....

B-u-t the CB100N shared the same cable operated disc brake, and I have NO fond memories of that what-so-ever. It was at best 'wooden' lacking feel, at worst, it just didn't slow the wheel, but worse than worse, it could do any or either any time it liked, and using the front brake lever was like playing Russian Roulette, especially in Brummie Rush-Hour"

Anyway; that niggle targetted, forks rebuilt, I'd turn attension to the back end, where the rear 'suspension' units would almost certainly find thier way into the scrap heat in a nice parabolic trajectory. They lacked damping at the best of times, odds they have any now is near nill.. is they do it'l be rust and crud impinging damper rod progress, I suspect. They would be swapped out. Whilst down there the swing arm bushes would come in for replacement,m and good chance to see how much metal is left in the swing arm.

THEN, onto such luxuries as... mudguards and body-work, and the little niggle of the 'electrics' which are never much fun, but hampered on this particular route to the asylum by being 6v.... a 12v conversion with Benly bits and possibly even an e-start, would be strongly considered.

BUT when I had sorted or sorted a plan for the cycle parts, like that front brake, I MIGHT get around to doing something with the engine, which as said would start with chinky barel kit and a top end rebuild....

THEN carburettors may be a bit of an imperative.... and a bit liker the engine, I would plan a complete overhaul.. after sourcing some nice new hole free exhausts.

I would be wary of the e-bay carbkits... in fact I had an argument with WeMoto over some they listed for the 125Twin, that were actually Chinky bits for the CG, and contained not only the wrong float needles, but wrong size jets.... you have been warned... a-g-a-i-n. DSS I believe ran out of 'official' carb spares for these about a decade ago, so its down to getting a generic CG kit to salvage what bits you can from it, like the float bowl seals and the idle-air screw.... on which note almost certainly worth a look, they have a very fin needle on the eld, wont to corrode into the carb-body... and snap.... oft replaced in ignorance by folk that then say that it makes no difference, more often with two or no rubber O-Rings! More warn!!

B-u-t long and the short of it is that basically on the list of things to worry about.... setting the float height IS a long long way down... and even a well fettled carb will pee fuel from the over flow, cos the needles stick in their guides....

Advice is start from the top, and work methosically down the list, slowly and carefully, with LOTS of known an trusted new bits.. dont 'buy' problems you dont need.

Best of British, with it, anyway.

Add on ED: you say PO did the engine swap.... can you be sure its a T2 motor? Almost all the Benly twins are interchangeable in the frame mounts. The Benly and CM motors oft also have the round rocker covers tat 'look' T" ish, but even they are a quick swap. B-U-T the T's have a 180 timed crank, the CDs & most CM's a 360 timed cran. When you pulled the top off to look at valve stem seals... were both pistons at TDC/BDC together? Can you be sure it's a 180 timed T motor? If not, it would run like a bag off, on only one pot, if it's trying to spark the pots at the wrong time. Whichh dont help much with the carbs... but as said, they would not be very high up the priority list.

On The Topic, worth mentioning the carb-rubbers, which have a tendency to perish, and on this should be flanged for the 26mm carbs, but also tend to get torn with folk ripping the carbs out brutally. Also rather unobtanium... so good luck there and be careful with what you got, if messin in that region. Lots of botches with sily sealant, insulation tape and super-glue are known!


Ok, so here is the new stuff i have replaced:
- New chinese spec pistons and rings
- New chinese barrel on there which i honed when doing so
- New valve stem seals
- Keyster carb rebuild kit - is this what you meant by ebay carb kits? it said t2 on the box but not entirely sure if it's right or not.
- brake rotor arm - tried a few but the bearing races were very worn and were causing the brake to bind - i know what you mean about them being a bit shite. It is at least now all working properly as we found a NOS rotor arm and replaced it and the cable

I am almost certain it is a t2 engine, the previous owner said he checked the engine number and it was indeed a t2 motor. I think it is timed 180 degrees as there are two marks on the alternator stator/flywheel each side - that being said i think a spare TD engine i have may have the same on its 12v alternator stator, does that sound correct?

Previous owner had replaced most of the rear end from the looks of it, new shocks and springs and the swingarm looks to be in good nick.

The carb rubbers WERE split, but i have replaced them for good ones. O rings and washers were replaced when i did the rebuild and made sure to remove old o-rings.

I just want to run the engine in properly, but if i can't get fueling working properly i'm not sure what to do. I have turned the pilot mixture jets out 1 1/4 turns like the haynes manual says, but im not sure what would be ideal really as im guessing it is basing that off of old leaded/4/5 star fuel?

Either way it is fouling both plugs, not wet just black, so convinced its the carbs.
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Deleted1212
L Plate Warrior



Joined: 18 Nov 2019
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PostPosted: 23:05 - 19 Nov 2019    Post subject: Reply with quote

Robby wrote:
You'll learn to ignore Tef in time.

Are the carbs all correctly according to the Haynes manual? Bits to check:
- Float height
- Main jet size
- Needle position
- Mixture screw position
- Is the choke working

Bear in mind that it's an old 125, and has probably been owned by 20+ teenagers. They love fiddling with the mixture screw, having no idea what it does.


float height was set initially to 14mm iirc, but this made fuel piss out of the float chamber so set to 18mm.

This is the carb rebuild kit i used: https://www.ebay.co.uk/itm/Honda-CB-125-T-Twin-1979-Keyster-Carb-Carburettor-Rebuild-Kit/113851520395?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649

choke has some play but all works (or at least the linkage does)

why can't it just work like my dolomites.... im considering putting a bloody single SU on the damn thing cos these keihin carbs are utter rubbish in comparison!
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Easy-X
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PostPosted: 00:09 - 21 Nov 2019    Post subject: Reply with quote

Just a thought: what were the numbers on the old jets compared to the ones in the kit?
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Shaft
World Chat Champion



Joined: 27 Dec 2010
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PostPosted: 00:26 - 21 Nov 2019    Post subject: Reply with quote

rerpha wrote:

float height was set initially to 14mm iirc, but this made fuel piss out of the float chamber so set to 18mm.

This is the carb rebuild kit i used: https://www.ebay.co.uk/itm/Honda-CB-125-T-Twin-1979-Keyster-Carb-Carburettor-Rebuild-Kit/113851520395?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649

choke has some play but all works (or at least the linkage does)

why can't it just work like my dolomites.... im considering putting a bloody single SU on the damn thing cos these keihin carbs are utter rubbish in comparison!


You could always make up a manifold and fit a DCOE, at least you won't have balance issues Laughing

More seriously, I'm not a fan of pattern carb rebuild kits, I've used a few and they always seem to be inferior to OE kits in some way.

Have a look and see if David Silver do a genuine part.
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pepperami
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PostPosted: 20:34 - 24 Nov 2019    Post subject: Reply with quote

Shaft wrote:


Have a look and see if David Silver do a genuine part.


This ^+1.

They always had the parts I needed for both my 250 Superdream of lovelyness and for my little CG 125.
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Deleted1212
L Plate Warrior



Joined: 18 Nov 2019
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PostPosted: 17:59 - 25 Nov 2019    Post subject: Reply with quote

to be honest i have given up. I have too many projects and just want to be able to ride my motorbike to work etc and get some practice in.

I have put it up on ebay if anyone is interested:

https://ebay.us/iilU7R?cmpnId=5338273189
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Easy-X
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PostPosted: 20:28 - 25 Nov 2019    Post subject: Reply with quote

That's a shame but sometimes you just have to call it Sad
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