Lack of power Honda cb125tdc

[mygingernut]

Hi all hope that your knowledge will be able to help me
Ive got a 1986 cb125tdc it starts fine and ticks over ok but when I take it out on the road I cant get it over 5000rpm and it will struggle to do 30mph checked timing that looks ok looked at carbs and they look clean changed coils and cdi units from a running bike now im just stumped has any one got any clues as to what could be wrong.
Thanks

[Benson_JV]

Have you replaced the spark plugs?
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Willson - "If you go out on that CBR600 in the winter, you're going to do more miles on your side than on your wheels."
Riding: CBR600FW Driving: Audi A6
Previous Bikes: '96 Bandit 600, '96 GPz305

[Paxovasa]

Check that the brakes are not binding and that the chain is not to tight
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Suzuki GSF600 K3 (in the fastest colour, black).

[Teflon-Mike]

Nice choice of bike.... I have half a dozen of the little buggers!

First of all, you say you've checked the timing....... I PRESUME ignition timing...... which SORT of begs the question 'how'?

CB125TD ignition is fixed.... triggered by a lug on the generator rotor passing an inductor bolted inside the genny cover, there is virtually no way to adjust it......

As for checking it, well, what did you do? Strobe through the top genny inspection hole of LH Spark-plug?

You then say you 'looked' at the carbs.......

I suspect this is where the problem is likely to lie, if you have checked the ignition by substitution of the CDI & Coils....

So, when you say 'looked' WHAT did you look at? Did you actually take them off and strip & clean them?

Not much you can tell about them without looking inside the little blighters!

THOUGH, there are a few suspects. First one is the choke cable.

This comes up from the carb under the tank and should poke out the intrument binicle just beneath the clocks. They have a habbit of siezing.

Next up, there are two carbs, and there are two chokes. Choke cable works the LH carb, RH carb's choke is worked by a bit of tin plate between the two choke spindles.

Quite possible for the plate to get twisted and the choke to only one or close one flap on one carb.

Then theres the throttle slides & throttle cable. Both carbs have thier own idle stops, RH one though is a right royal PITA to get at, so often doesn't get adjusted; idle is set purely on teh LH cylinder.

So, Motor runs lumpy on one pot at idle, and THEN pot luck where the second pot kicks in, depending on whether the cables are adjusted so that the slides pick-up together.

Just a FEW things to have a look at before you start tearing into the carbs themselves.

So, do some more looking, then take the carbs off.......

You'll need to remove battery, battery tray, and BOTH air-boxes to get them out easily, but that will give you access to the NEXT thing to look at, which is the cam-chain adjuster.

Its NOT self adjusting on the 125 Twins, you have to get the carbs off to get at it and give it a little twiddle.

I'm not going to tell you what to do, its in the Haynes..... you DO have one, dont you?

Its in the first section, 'Routine Maintenence' and normally completely ignored, BUT you wantto get one of these things running 'on-song', read it & follow the instructions. Tells you how to adjust the cam-chain tension, then the valve clerances or tappets.

Do those two bits, fit a pair of new plugs, drop the float bowls off the carbs, clan them, blow out the jets, set the float heights, and level the slide stops, before re-fitting, then if you want to be clever, buy, beg or borrow a manomiter (sorry, Vacuum Guages) to balence the carb vaccuum, and adjust the cables so the slides are lifting in unison...

AND the thing SHOULD, provided it hasn't got a terminal case or bore errosion or cam-chain stretch induced cam-retardation, BLUDY SCREAM its head off!

These things should rev WELL beyond the 12K blood-line, and tug almost like a hi-po two-smoke off the line, they are REALLY eager little engines.

ONLY thing they tend NOT to like is bad exhausts, so you might like to have a look around that area. Badly fitted Microns or Motads often result in leaks around the cylinder head studs, and more than occassionally the gasket slupping and actually blocking teh port!

If it has teh OE twin-pipes, though have a look at the balence pipe between the two header pipes. Shoudl sit just ahead of the centre stand.

Here's a pic of a VERY rusty one.
https://i178.photobucket.com/albums/w269/teflons-torque/Little%20Dreams/imag0785.jpg

Balence pipe is the bar between teh two pipes, that looks like its holding them together. If its rotted out, you can get all manner of rough running.

Thinking through other common niggles; Have you had this bike running good before? How has the fault developed, or have you just got the bike & are trying to fettle it up?

Back to those twin air-boxes; theres an air-filter in each, and they are something like £17 each, making services a tad expensive, so old filters often dont get changed. They are also paper, and if they get wet, turn to mush.

Myriad possibilities in that area, from mushed or clogged old filters, to no filters to cheap after-market stubby filters used in substitution.

And the last thing in the ignition, the trigger senders. SOMETIMES the wires crack in the potting to them, and they short out or go dead, bu that will give no spark.

Other one is misaligned genny cover. Geny cover is located on two steel dowels that go through the back-plate that supports the starter motor.

Take the genny cover off, the back-plate CAN slip a bit making fitting the cover back on difficult.

I've opened a couple of motors and the location dowels have either been crushed, becouse they havebn't lined up the cover AND back-plate properly, or they have removed them altogether so they dont have to, or have completely crushed the dowels!

Misaligned cover though can screw the timing a LITTLE and or the clerance gap between the ignition trigger and the rotor, causing a weak signal, and a poor spark or occassional miss, particularly under load.

Cant think of anything more to suggest, but starting point is ALL the service stuff........ think theres plenty fot you to investigate here.

Where in teh country are you BTW?
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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?'

[mygingernut]

this is what i have done so far read the old haynes manuel and followed it to the letter

check the timing checked the valves clearances
2 x new spark plugs
had both carbs off and cleaned them and blown out the jets and fuel lines
2x new air filters

If it is standing on the stand it will tick over and run ok but as soon as you take it on the road thats when it wont rev past 5k

Bike was running spot on until I was out on the bike and used the reserve fuel for about 5 miles then the problems started if I pull back hard on the throttle while it is stood it just dies on me


im just off j30 M1

[Teflon-Mike]

OK, well Sheffield is a LITTLE to far for me to have a look next time I'm out & about!

BUT sounds like you have sucked some shit int the carbs from the sediment in the tank when you went over to reserve.

Get a smakk inline fuel filter. E-Bay, We-Moto or your fabourite, should be about a quid, £2.50 tops even with P&P.

Get a short length of fuel hose; local motor-factors or same emporium as the fuel filter, again £2.50 tops, should be about 99p.

You need to remove the fuel petcock (tap!) and drain the tank.

You probably wont want to try and save any fuel that was in there, it'll be filthy, so get a gallon can & some new petrol ready for when you are done.

Remove fuel-tap. Theres a nylon tube on the inside, and a mesh gauze on the end of it..... well there OUGHT to be.

Chances are that its either rotted or got clogged up. Dont worry about it, your going to fit the inline filter so it shouldn't matter much, BUT sludge and crud from teh bottom of the tank CAN clock the fuel-tap, so you MIGHT want to look for a new one at some point.

For now:-

Drain the tank of old petrol, with teh fuel-tap removed. Good to take the tank off to do tis, so you dont have petrol pissing over paint-work.

Next, wash the tank out.

Hot soapy water, and lots of.

Then rince.

Personally I put a pressure washer inside, and give it a good blast about. Also put it up in the fuel-tap hole to make sure thats clear.

Next inspect the fuel tap, and wash that out.

Bit more tricky, but you can back-flush it, quirting some water or cleaner old petrol through the hose that used to connect to the carbs.

Finally blow it out with an air-line.....

Remember to clean it through in BOTH 'On' and 'Res' possitions.

Now, tank should have dried out, so refit tap, put it in closed possition, and put tank back on bike.

Then add about half your gallon can of petrol, and leave it over-night.

Any water left in the tank will have settled to the bottom of the tank, as petrol is less dense than water.

In the morning, WITHOUT knocking the tank and mixing petrol and water again, hold your finger over bare end of petrol pipe, and turn it STRAIGHT to reserve, then using your finger as a valve, vent it into a jam-jar or something.

Bit of luck not much water will come out, and you'll get clean petrol pretty quick, but let it run for a little to be sure.

THEN switch off the petrol, attach inline filter to tap, and length of hose from filter to carbs.

Top up the tank, & switch to 'On', & try engine.

BIT of luck you should have cleared teh blokage, and engine will run fine again.

If not, you have got some more crap in the carbs and they'll need cleaning again, which should sort it.

If you have kept the old fuel-tap, though, depending on teh state of the tank, might need doing again, periodically, as it'll be the rust and sludge building up in the tank collecting in teh fuel-tap, choking flow to the carb, meaning that theres not enough fuel getting there to keep the engien supplied at revs under load.

Inline fuel filter isn't essential, but saves having to clean the carbs every time you get some shit through the fuel-line!

See if that helps.
____________________
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?'

[mygingernut]

Just a Quick thanks to you all I have now got it running as it should.

I took the cabs Back off cleaned them out AGAIN and blew through with airline.Emptied fuel tank and steam cleaned it out put fuel back in and this morning drained off any exsess water this morning.

Put in fresh fuel this morning amd took it for a spin it is runninng better now than what it was before.

SO A BIG THANKYOU TO YOU ALL

[Teflon-Mike]

GOOD NEWS
Glad your sorted.
& Thanks for letting us know how you got on.
Been wondering whether you sussed it or not.
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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?'