CB125TDC SUPERDREAM (update)

[mrlee1974]

Hi all,i'm a newbie here.I recently aquired a cb125tdc,i was wondering if anyone knew anything about these engine's.it has a motad exhaust fitted (2 into 1) and i was wandering if i would need to up the jet sizes in the carb's.As far as i know the carb's have not been touched and the jet size's are either 68 or 89? i cant tell which way round i should be reading them.The top speed is only 65mph and i know my nextdoor neighbour used to get over 80 on his standard engine!

if anyone as any information as to which jet size's i should try or any other tuning info,it would be greatly received.

Thank's in advance!

[Kickstart]

Hi

CB125TDC won't manage a true 80 (older CB125T might).

Those Motad exhausts are meant to be a direct replacement with no rejetting needed. Possible you might get marginally better results playing with the jetting but don't expect any major improvement.

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[hmmmnz]

65 isnt too bad at all out of a engine of that age, i dont get much more out of my 250.
you don't need to change your jets for a new exhaust on a 4 stroke,
keep the jets the same,
if i was to do any thing, i would check the tappet clearances, balance the carbs, give the carbs a good clean out, and make sure the diaphragms in the top of the carbs are still in good condition and arn't ripped,
change the plugs, and do a engine flush and oil change, the filter is a little peice of mesh behind the right hand case (as you are sitting on the bike) give that a clean out as no one ever does and are normally clogged.
and she should be as good as new again,
remember to do the tappet clearances quite regularly as they tend to become out of spec quite quickly
good luck
____________________
the humans are dead
I kick arse for the lord
Wiring Diagrams BIDNIP it bitches

[Kickstart]

[biker_king]

what is the milage? take a look at your spark plugs to check the fuel mixture , the model after 1982 are restricted due to laws ect so 65mph would bo ok fore this model ,
keep the oil toped up and keep regular checks on the level.
good luck, james.

[mrlee1974]

The bike is in great condition,just want's the carb's balancing up.right side spark plug is clean as a whistle,but left is all sooty.if i was to turn the screw for mixture setting,which way should i try turning it till it's about right,in or out?

And thank's for replies so far guy's

[biker_king]

is the plug black and wet? this would be oil burning in the cylinder.
if its mat black this would be a rich fuel mixture, turn the air scrow out to lean the mixture.

[mrlee1974]

[mrlee1974]

[hmmmnz]

[Kickstart]

[stinkwheel]

Good luck balancing it, I never sucessfully managed, they are an utter bastard to balance.

In the end I used to just settle for doing it by 'eyeball' so the slides hit bottom at the same time. The adjuster screws are so tricky to reach, you can't really get at them properly with the carb in place.

Oh, and before you go looking for diaphragms, there aren't any. It has slide carbs.

65mph is about right. The earlier models would do 80mph but they had bigger valves and a steeper cam profile.

As hmmmnz says, keep an eye on the tappets, they have a stupidly small clearance as standard and can close up leading to scorching of the exhaust valve seat.

Oh, the camchain tensioner. Supposedly 'self adjusting', don't believe it for a second. To adjust the CCT, take the rocker cover off, loosen the CCT nut (back of the engine between the carbs), press down hard on the top of the tensioner blade with an old screwdriver and re-tighten the nut as you press down. It's usually a loose camchain that kills those engines, you should be able to hear the camchain whirring as the engine ticks over, not rattling. Keep an ear out for a "death rattle", if it starts, readjust it again (five minute job).
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[bladerunner]

If it's been used by an old fella that was mechanicly sympathetic maybe a damd good thrashing for a while may help as well. You might find if the oil level is closer to the lower/mid range then you get a bit more rev than if it's filled to the brim as well.

Oh and if it has comstar wheels..check the rivets as these wheels are prone to poping them!!!

[mrlee1974]

Had a go at the mixture settings myself and i dont think i done a bad job,i turned the screw out a fraction till the plug matched the plug in the other cylinder.i think ive upset things a little taking the carb slides out to re-route the throttle cable.I got a huge cloud of blue smoke upon start up and it ran as rough as ya like with rev's stuck at 4k and didnt have the power to pull off.after running it for a while the smoke got less and less.i thought i might have put the slides back in the wrong carb's so i changed them over and there was not really much differance.although it's smoking more than normal,the plugs are dry? could this be a knackerd spark plug? it ran fine after i balanced carb's but everything changed when i removed the slides out the carb's to route the cable!

what do you think?

[Pete.]

First thing to do when buying one of those engines is to check the top-end. They are prone to wearing away the cams & followers. If they are in good fettle then put new good oil in and don't neglect the oil changes.
____________________
a.k.a 'Geri'

132.9mph off and walked away. Gear is good, gear is good, gear is very very good

[Kickstart]

[stinkwheel]

Uncontrolled revving up is a symptom of an inlet manifold air leak. A distinct possability if you've taken the carbs off.

The throttle needles sometimes come adrift inside the throttle bodies too. You've got to drop them in absolutely straight up and down, if they catch, they can be pushed back up into the throttle body.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.