CB125T2 from 1980

[K2-K6]

I've not experienced fuel wetted plugs ever being a problem once you get enough air into it. I just get it cranked with everything in place and maximum air to get them firing.

Have you got CV carbs on it,  or just straight slide type?

[peteuk]

The slide slide type.

[K2-K6]

As hairy advised then regarding air into it,  leave choke in hot / normal / non applied position, hold carbs wide open,  completely,  and kick it without dismantling anything.

If you could get someone to push you on it with same settings and in second gear it should fire,  eventually.

Both ways,  just don't close the throttle at all until you hear it trying to fire.

[peteuk]

Hi, still waiting for the new battery to arrive but I've got another problem and it's electrical which as mentioned earlier is definitely not my forte. I've bought a rear tail/stop light. The light works ok (i keep charging the faulty battery) when I press the brake the rear light should get brighter but the light actually goes out. I bought a new rear brake switch but still the same. When I put the parking light on, the brighter light comes on. I've tried different combinations of the rear wiring but it doesn't seem to make a difference. Any ideas anyone
Thanks....Pete

[K2-K6]

Initially I'd look to make sure that the earth to the light is working well. If you are unsure of this,  run a straight wire from the earth on the light unit direct to the battery minus terminal and then try to operate the different lights.

Let us know what happens then.

[peteuk]

I'm at a total loss.....
So the bike was running last week and was sounding pretty good. One day after doing nothing since the last time it ran, the bloody thing would not start, not even fire. I've now changed the battery for a new one, changed the plugs. I've checked the timing.....again. I've got a good spark at the plugs. Petrol to the plugs (you can smell it when they are removed), and yet it wont even fire, no farting, coughing....nothing. what's going on?. Any ideas anyone cos I'm totally out. Cheers

[matthewmosse]

Had similar shenanigans on dad's Rassant junior mower, he was convined it was timing - a stuipidly fiddly job on that mag 10hp engine. Eventually I found two things amiss, slight head gasket leak and a sticky carb controll rod. So glad I avoided the timing setting job. Sounds like you have covered most of the bases. Worth trying yet another set of plugs - I had a problem on my kh125 a few years ago, found 2 out of 4 plugs I had  (all new at the start ) were duds,sparked ok out of the cylinder but wouldn't do fiddly squat in there, a 3rd went like that after flooding it. A bad day for that bike. Worth spraying neat carb cleaner through the air intake and see if it fires to rule out mixture being weak etc, also worth popping the float bowls off to verify no crud in there. If you can get a straw on a carb cleaner can into the jets in situ, worth jetting them out on every visable hole - sometimes removes blockages you didn't know were there. Seems to have worked on a few chainsaws, strimmers and rotovator engines I've got back running. It is rather frustrating after a long slog to get them running that a week later it seems back to square 1, but persist now and once these teething issues are resolved hopefully it will settle to reliable service. Seems to have worked for a Fair few engines I've revived recently.

[K2-K6]

A couple of questions,  was the fuel tap left on? Check the oil level and smell the dipstick,  does it smell of petrol?

It's possible that petrol has been running through the carbs into the engine. If there's any evidence of that it may give you problems. This would happen if floats didn't hold shut too well.

If an engine is flooded,  it's not the spark plug that's a problem,  usually. If cylinder mixture is so rich then a spark,  even a good one,  won't ignite it. They'll go eventually if you get enough air into them.

Similar to what matt says,  I've had two stroke engines I've had to turn upside down to let the fuel out the exhaust port then able to start them.

If the fuel is on, then turn it off and leave it that way until you get a response from the engine initially.

[peteuk]

Thanks for the replies. Checked the dip stick and no smell of petrol. I'll get some more plugs tomorrow and try that although these are new ones and I feel like I'm wasting money but I'll give it a go. I've had the carbs off and cleaned em out. It could be a too rich mixture I guess. How do I reduce the richness, is it by tightening the screw on the side further in or should it be backed out

[K2-K6]

The air screws set at about 1 1/4 turns out from being fully in aren't usually going to be a problem. They only adjust the running at idle anyway and wouldn't usually cause flooding so I'd not worry about those if they aren't in that range.

The main jetting is adjusted by raising and lowering the clip on the needle in the main slide. Lower the needle gets leaner,  raising the needle gets richer. With correct main jet number the needle clip is usually in the centre of 5 clip options.

As carbs came from different bike ( thought that's what you posted) do you know what main jet sizes art installed?

I know others have different experiences,  I allways just use plugs even if they've been flooded,  never really get problems with them and not thought it necessary to buy new if you've got them sparking.

We must be able to get it going one way or another,  they can be frustrating though.

[hairygit]

I assume you've checked the float levels? And the float valves for leaks?

Sent from my X6pro using Tapatalk

[peteuk]

There's no leaks from the carbs and I've not changed anything from when not was last running. It's so frustrating cos now I can't even get it to fire...or even misfire. I have no idea what jet sizes are but like I say, it was running so it doesn't make sense. I'll get some new plugs and try it but I don't hold out much hope for that. Just one thing that may be relevant to someone. Last time it was running, it died then started, then died off again

[K2-K6]

You're right about jet size,  it more me thinking out loud to see if it catches anything odd,  or prompts someone else looking in.

Guess we have to keep thinking things to check that may influence it. It's something of course that if it's in front of you you just go through,  bit more time when relaying it across.

That stalling out may have a significance. You could try and take off the exhausts to see if they have an influence, then try to start it.

Also have you tried like earlier posts with a little carb cleaner down the intakes to see if it'll encourage it.

[peteuk]

Ok thanks. I've tried easy start but still no firing. I've tried it thro the inlet to no avail and then taken plugs out and sprayed it down the plug holes.....still nothing. I will try removing exhausts and giving that a go cos they are new exhausts but it has been running since they were fitted.

[K2-K6]

I suppose as you check through all the known and more obvious things you're left to look at the obscure to maie sure something unaccounted for hasn't happened.

I know it's clutching at straws with the exhaust but it will hopefully eliminate them. They shouldn't really affect it firing but if they somehow had been blocked internally it's worth eliminating them from the investigation.

Next thing I'd look at would be compression to understand what you've got there.