Honda 1979 125 Twin

[nawrp]

Hello,I recently changed the barrels and pistons on my twin and she won't go above 6000 rpm. Before the motor was running reasonably well topping off at around 100 km/h and 9000 revs but consuming oil. I changed the barrels quite quickly and probably made some mistakes. Any advice would be welcome
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Honda 125 Twin Fan

[Nobby the Bastard]

Time on a tiddler is well spent etc.
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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

[WD Forte]

so you changed the barrels
for what? new ones with new pistons and/or at least new rings?
or a used barrel etc off another bike?

Posters offering minimal info usually means we have to go into a dreary back and forth when supplying a decent amount of into to begin with saves lots of time.
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bikers smell of wee

[nawrp]

Fair enough.New barrels,pistons and rings. On hindsight I did not adjust the rings exactly as recommended nor lap in the valves.The old barrels had significant wear to the left sleeve hence the oil consumption. Also in the old configuration the camshaft was one chainlink off at TDC. Possibly one of the previous owner/s confused TDC for the ignition timing mark advancing the opening of valves?
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Honda 125 Twin Fan

[stinkwheel]

[Nobby the Bastard]

[nawrp]

Thanks for the feedback.The bike was not running perfectly. The motor redlines at 12000 rpm which will take it upto around 130 km/h.It would not go beyond 9/10000 revs with the old barrels hence the decision to change barrels and pistons Nobby. The carbs and ignition timing are all upto scratch including air intake seals. The plugs are sooty but the central electrodes are about the right colour. The camshaft is now correctly set not one tooth off. The moteur seems to have reasonable power upto 6000 rpm but simply will not accelerate beyond this. The motor will go to around 10000 rpm in neutral however but will start to misfire beyond this point
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Honda 125 Twin Fan

[Nobby the Bastard]

There's no way on earth that engine would have spun up to 10k if it was 1 tooth off.

You seriously expect to get a '79 4 stroke twin to hit 12k revs?

My fucking FZR didn't do that.....

I suspect your cam chain is so stretched it looks like it's one tooth out but there's a fuck load of slack being taken up by the chain adjuster.
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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

[stinkwheel]

[Nobby the Bastard]

Well, you live and learn. Those sort of revs are what I'd expect of the pocket rockets of the 90s like the ZXR400 to do.

A auto advance of that era is likely to be a sprung loaded on, relatively easy to check. They tend to be seized in one position if borked.
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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

[nawrp]

Thanks for both of your inputs. I will do further checks and tests and if unsuccessfull will redo barrel and pistons meticuousily. I will keep you updated. Yes indeed 16,5 bhp et 11500 rpm and around 80 mph top speed. I just however want to get it running well and get back out on it again
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Honda 125 Twin Fan

[Nobby the Bastard]

My RG125 was faster...
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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

[lingeringstin...]

That's virtually the same engine as I've got only mine's the 250 version but that means nothing there's only like two horsepower difference between them. Wish I'd known that when I started faffing around with these engines. I'd have concentrated on perking up a 125 instead of the 250's I went with.

The only real difference is the 250 has a 360 degree crank and all the 125's like this I've had my hands on have 180 cranks.

My 250 engines have a longer stroke as well and came out as standard with pitiful concave pistons whereas some of the 125's (probably like yours) had high top pistons so presumably more compression and dual carbs. Altogether it makes them rev much better than bog standard 250's ever did.

I'd say that if your cam chain is old and stretched out it might be putting the valve timing out of whack, which MIGHT account for why it was off by a tooth in the first place. Maybe somebody's been in there before you and done unspeakable things.

Also the standard Honda style cam bearings are miserable in these engines. If they wear out the cam sinks closer to the head and that also throws everything out. Sometimes people just keep adjusting the valves to make up for the cam bearing wear but there's a limit to how far that malarky can go. I replace mine with needle roller bearings.

The valves on these engines can operate OK with a bit of tolerance askew but they do run noticeably better when the valves are spot on perfectly adjusted. These engines WILL run with badly adjusted valves but not optimally. They'll sound and act OK at lower revs but they will not be able to go higher revs, and it can be baffling if you've not come across it before. The valves do need to be spot on to get high revs.

Other things that bugger up your revs could be clogging exhaust (if it was burning oil for a while) and air restriction. If nobody before you has buggered around with the carbs then they're probably jetted about right but some people (like me) do all manner of faffing about and end up with carbs that could never be run with a stock airbox again, so anybody trying to go back and put a stock airbox on my carbs would greatly hinder their performance unless they went back to normal jets and needles again. Was yours buggered with before you got it?

I do know these smaller engines can be revvy. I made a short stroke 202 out of a 125 crank and a couple of YX140 pistons in a cut down 250 barrel bored out to buggery and it easily way outperformed my long stroke 250's.

[A100man]

[A100man]

[lingeringstin...]

Yeah it's known in medical circles as "The Benlys" and it afflicts anybody that's ever delved deep into these Honda twins and their various dubious clones. Very sad. All sense flies out the window and your time gets taken up with memorising and regurgitating pointless minutia about the seedier aspects of CBT and CMX quirks and foibles. Once somebody gets the affliction they stop making any more useful contributions to society. It's a bit like "MZ Nerdery" only with Honda twins. What a waste.