honda cb125 twin

[bub2006]

Well been and bought another bike. The vision was just not quick enough to get up a rather large hill near me and due to no cubs for sale round here i have been and bought a 1985 Honda CB 125. Hardley used this year so when I bought it its runninh bit rough at low revs. I never had anything with more than 1 cylinder, does any one with an idea who lives anywhere in detbyshire or Nottinghamshire mind having a like and helping me outu??I can travel. Cheers guys ride Safe

[Jon]

The usual reason they run rough at low revs is sadly, because the cam bearings are fecked. Basically, the cam runs in the alloy head but without bearings. It shouldn't be an issue because there should always be a film of high pressure oil between the steel cam and the alloy head but we are talking about a 125 learner bike here so basically a previous owner has thrashed the tits off it and either run it low on oil or simply not changed the oil enough. The oil feed to the head gets low and the cam starts to eat into it's bearing cups.

Why does it run rough...Well... The ignition points run off the cam so as the cam jumps up and down in it's now worn and sloppy fitting journal, the points gap changes and so the ignition timing changes (quite a lot!) and therefore idle goes up and down. You need to get a new head. Don't much around with trying to sort out the old one...Just stump up the ante!...Unless they are completely impossible to find in which case some proper 'build it up and machine it out again' engineering may be needed. The good thing is that the steel cam is often still OK....

running well they are sweet and increasingly valuable little bikes...

[ayjay]

take the points cover off and see if theres any up/down/sideways movement on the cam

[Jon]

It's easiest to spot by measuring the points gap...It will change as the engine turns. Oil build up in the cam journals can make the cam feel fairly tight when the motor is stopped..

[Jon]

It's easiest to spot by measuring the points gap...It will change as the engine turns. Oil build up in the cam journals can make the cam feel fairly tight when the motor is stopped..

I've just realised how stupid that sounds... Of COURSE the gap will change as the motor turns but if the cam journals are worn the measured gap at TDC or wherever will vary every time the motor gets back there. i had a CB100 with this fault... I shimmed the cam journals with carefully scissor-cut razor blades but that was a really horrible bodge!

there are likely to be machine shops who could built up the surface and line-bore the journals out again and John oldfield in Southampton has quite a lot of stuff if you know the model number of the bike ( you find the code for your bike, put it in the second box of the product number and all the stuff for your bike comes up.

[bub2006]

Hi guys sorry been a wwhile since I been on but work and family eetc. Anyways about the bike it is a cdi model so no points. Now is running sweet after sorting out the problem. All i have done is set Carb needles as per manual complete Carb clean and balanced the carbs. Was owned by a mature chap before me who owned it for 4 years and he fitted new rings pistons gaskets seals etc. Still want a cub tho hope Its ok to come a rride with you guys sometime if anyone riding matlock area?

[Jon]

So much for second guessing the problem! Glad it's OK!

[bub2006]

Its ok thanks for giving your contribution all the same mate. Does any one no of any rides out planned forround this area for me to tag along.?