Honda cb 125 tdc engine problem

[Briandude]

Hi can anyone help me out, got a honda 125 tdc it's only done 3000 miles from new, it has sat in a garage since 1984, everything is original, only things bad were the exhausts they were rusted away, battery had split. My main problem is that the pistons are seized in the head, and can't get it off, is there anything I can do to free them up with causing any damage, everything else engine wise is immaculate. Thanks for any help, Brian

[Teflon-Mike]

Make life easy for yourself:-
HOW2: Top End Rebuild Honda CB125 (Benley Motor)
You might be able to free the bores off, with two-stroke oil down the spark plug holes and gently working the crank round on the rotor bolt, BUT.
These bikes suffer horribly from long lay-up. They have tiny pistons & they are a very high performance motor for thier size, reving to 12,000rpm, and they start to go 'off' and loose performance just for slightly worn piston rings.
Layed up rings stick in the piston grooves, long before the pistons sieze in the bore. Freed-off, they will run, and often run reasonably well, but within 500 miles they will die... horibly. The gummed rings let the piston run on the bore, you get incredibly accelerated bore wear, burned oil and then the motor just stops running.
Other things is the valve seats rust, and need grinding, and you cant remidy that without opening the engine....
AND you presume the motor is siezed due to pistons stuck in the bores, I would not discount the fact that the engine was pre-siezed, by being run out of oil..... good reason it has sat for two decades unused that.... or two decades has seen oil replaced slowly by condensation & the main crank bearings rust.

I have done a number of these bikes, and have had to rebuild at least three engines from long-lay-up problems, which weren't so bad pistons were glued in bores.

New barels & pistons are about £90, I would PLAN on doing a top end rebuild as 'course', but hold of ordering expensive bits until I had opened the top and seen what I was dealing with and whether it was really worth doing, and had some idea whether the crank was serviceable.
____________________
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?'

[Briandude]

Thanks Mike, I have read a lot of your articles and they have been extremely educating, I've got the head off and can see the pistons there was plenty of rust in there, I've tried oil and at the moment has been left to stand over night with wd 40 over them, from reading your other stuff have already purchased a complete set of engine gaskets and seals and was waiting to see if I needed anything else was the engine was apart. There was plenty of oil in the bike which was pretty good considering, just a mm of congealed oil in base. There was also oil around the crank as well, so I am hoping its just pistons.. Will let you know.... Thanks Brian

[Briandude]

Finally after a lot of heating up, lubrication, and in the end had to resort to a puller. Just hope I haven't damaged bottom end.

[Teflon-Mike]

https://www.bikechatforums.com/download.php?id=86638
EEK
did you do that trying to hammer piston down the bore, or was that pre-fucked, by a dropped valve or summat?!

Way it''s standing, I take it the crank is siezed?
____________________
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?'

[Briandude]

No mate the other piston was quite high up in the cylinder and freed up fairly easy, but I tried everything to get this one free, 3 days of lubricating and heating with blow torch, it just would not move in the end I bought a puller, put the legs of puller in the heat fins and screwed down onto piston. It eventually gave way and came out, there's nothing else seized in the bottom end it's in nice condition, going to get barrel kit off eBay tomorrow, do you think I should strip bottom end as well!

[Teflon-Mike]

[honda cg 1992]

hi dont suppose anyone could help i brought a cb125 superdream which has had the cb250 superdream engine fitted but i am having problems it is only running on one cylinder and on the odd occation the second cylinder will fire up but not for long i changed the coil packs as i though it could be a dodgie spark but it didnt cure the problem it revs up to about 6-7 thousand if you let go of the throttle it will die and when the other cylinder fires it revs up fine its really confused me

[Teflon-Mike]

[nightshaddow]

mike and i may help post the pic of the bike and the engine (close up) also the engine number itself will help
____________________
"WHAT DO YOU MEAN 100 MPH THATS IMPOSSIBLE Its chinese OFFICER."
Current rides CB 125 TDE(the next project) NS125RK ( now on the road ) CB125TDC (soon to be project 17) melody ( the nephews )

[Briandude]

Ok stripped and removed piston fragments from crankcase, and rebuilt. Just put new pistons on and barrels, but I have now noticed there's only two holes that will take dowels and seals, is this correct.. Have looked at your pics mike and it looks to be the same, do I just fit the two or have they sent wrong barrels,,, will post pic tomorrow night, cheers

[Briandude]

Engine is now rebuilt an ready to go back in frame, next job is the carbs. Thanks for all the info on here Tef would have been stuck without it. And your right about having a manual invaluable, anyways thanks again Tef will let you know if I get it road worthy..

[Briandude]

Ok finished the carbs, front and back wheels brakes, mono shock, engine back in, engine turns over but will not fire ;( , getting a good spark at both plugs, had carbs off again made sure they were spotless, that's it so far will keep at it, any advice always welcome...

[Teflon-Mike]

[Briandude]

It's alive woo hoo, I was getting a bit miffed with it all, re read manual, re read your articles, I used 2 stroke to lube everything on rebuild, I thought I may have done timing chain wrong, triple checked that and rechecked tappets they were fine, noticed a bit on oil on front of cylinder head bolt the one on its own at the front so downloaded conversion app and changed the pounds to Newton metric and retorqued head down, I thought sod it so got plugs out again and cleaned with emery paper and refitted, and it started first turn, so don't quite know what the problem was, but it works.

I was lucky enough to have another tde to rob from, it was a bike of eBay that was a bag of crap it had 12 months mot on it, god knows how it was falling to bits but nicked tank of it cos the one on elf y was rusty and the tap was blocked.

Anyways again thanks for your help teff, I will post some pics of it when I get chance,, then when that's finished I got a cb125rs to sort, glutten for punishment..

[Briandude]

Just a quick update, took bike for MOT today and happy to say it passed with couple of advisories which were both to do with the monoshock, i will sort that later, for its first time out it ran pretty good got me there and back with no hassles, still a few niggles to sort out carb wise, need to make a manometer and get them sync-ed, had to take it with a different fuel tank the original one has spent a few nights with hammerite rust remover in it which has worked really well. here are a few pics of it.

https://imageshack.us/a/img844/2219/20121127213515400.th.jpg
https://imageshack.us/a/img4/1646/20121127213426400.th.jpg
https://imageshack.us/a/img31/8881/20121127213416400.th.jpg
https://imageshack.us/a/img442/8026/20121127213402400.th.jpg
https://imageshack.us/a/img545/5389/20121127213357400.th.jpg
https://imageshack.us/a/img96/6566/20121127213350400.th.jpg