Topic: CBX125 Issue hight gear no power

[Chris97]

Hi there, I've bought an 1982 Honda CBX 125 (made in Italy) with some electrical problems last year. In my free time I've restore the entire bike: 12V conversion with 350 stator, new electrical system design by me, graphics ecc, the electrical issues are made by stator who burn old rectifier, ignition condender and the old bulbs, now electric parts are ok, it recharge correctly the battery and the engine works well in every RPM.

Now I have a sigle problem, the bike at high revs doesn't have power, in first and second gears the bike run very well at every revs, after third gear the bike turn 9000 Revs/80 km/h very slowly (only in a long road), in 4 gear it is stable at 6000 Revs, 80 Km/h, and when I put the 5 gear the bike lose Revs... Now, I've searched everywhere for the same issue and I can't find nothing, the carburator is ok with correct calibration, same with valves adjustement and ignition timing, spark plug is new, cleaned air filter ecc.

Before somebody ask, the ignition coil is the original with 2 one Ohm resistance in series and it made a good spark.

The strange thing is that the bike behaves correctly at all revs, without noises deriving from a carburetion or electrical problem, the problem occurs only from third gear onwards.
Before opening the engine and checking its status I would like to understand if anyone has had the same problem or any advice.

Thank you very much

[grcamna2]

Hi,
Would you consider looking at the color of the spark plug to determine how the engine is running ?
I would image a compression test will be a good step toward diagnosing a problem of 'low power'.

[Chris97]

hi, I'm waiting for a friend who has to bring me the tool to measure the compression, the spark plug seems ok, the bike had a problem at low revs and it was due to an incorrect minimum jet fitted by the old owner, now the bike reaches high revs without problems in neutral until second gear but the problem persists.

I don't think it is a carburetion problem as it is regular at every throttle opening...

[scottly]

Compression should not affect loss of power in higher gears (greater load on engine). Check the spark plug first.

[Chris97]

Just changed with a precise model taken from the shop manual of honda cb models, Denso X24ESR-U

[scottly]

What was the condition of the old spark plug? How does it look?

[Chris97]

unfortunately the old spark plug as well as being of a model not compatible with the denso one mentioned above, therefore mistakenly mounted by the old owner, was dark like rich carburetion but coincides with what the old owner said, that is, before the bike no longer starts , it had started to go bad for the electrical problem with the alternator and therefore probably did not spark well, moreover it had a wrong minimum idle and in fact the bike was very rich at idle

[scottly]

How long has the bike been run with the new plug? Have you inspected it after it has been running?

[grcamna2]

Chris97,
Does your bike have a resistor spark plug cap on it ? because if so it may only require an X24ES-U non-resistor spark plug.I hope your CDi box has good integrity.

[Chris97]

Chris97,
Does your bike have a resistor spark plug cap on it ? because if so it may only require an X24ES-U non-resistor spark plug.I hope your CDi box has good integrity.

I have no idea, I try to measure and let you know, thank you very much

[Chris97]

How long has the bike been run with the new plug? Have you inspected it after it has been running?

the new spark plug has maybe 20 km and yes, i looked at it and it looked ok, next time i try the bike between today and tomorrow i take a picture and share it. Thank you

[grcamna2]

How long has the bike been run with the new plug? Have you inspected it after it has been running?

the new spark plug has maybe 20 km and yes, i looked at it and it looked ok, next time i try the bike between today and tomorrow i take a picture and share it. Thank you

Do you have a manual/book with the correct specifications or part numbers or possibly a source of information on whether your spark plug and spark plug cap are supposed to be resistor or non-resistor ?

[Chris97]

How long has the bike been run with the new plug? Have you inspected it after it has been running?

the new spark plug has maybe 20 km and yes, i looked at it and it looked ok, next time i try the bike between today and tomorrow i take a picture and share it. Thank you

Do you have a manual/book with the correct specifications or part numbers or possibly a source of information on whether your spark plug and spark plug cap are supposed to be resistor or non-resistor ?

I attach screenshots from the manual and in fact the R is not mentioned, so yes, probably the spark plug is wrong, I have no idea, however, if the spark plug cap has the resistance as standard or not

Thank you

[grcamna2]

How long has the bike been run with the new plug? Have you inspected it after it has been running?

the new spark plug has maybe 20 km and yes, i looked at it and it looked ok, next time i try the bike between today and tomorrow i take a picture and share it. Thank you

Do you have a manual/book with the correct specifications or part numbers or possibly a source of information on whether your spark plug and spark plug cap are supposed to be resistor or non-resistor ?

I attach screenshots from the manual and in fact the R is not mentioned, so yes, probably the spark plug is wrong, I have no idea, however, if the spark plug cap has the resistance as standard or not

Thank you

OK Chris,
Would you possibly take a picture of your spark plug cap along with the numbers molded/etched into the plastic ?

I can also suggest that you check your valve clearances to be sure you have the proper adjustment on them;tight valves can cause some problems with a lack of power when your engine heats-up.

[scottly]

Since this bike has a points type ignition, I doubt this is the problem. My first thought on your problem was that there was an insufficient flow of fuel into the carburetor; at higher loads, the engine needs more fuel, so the engine may perform well in the lower gears, but as the load increases the fuel supply doesn't keep up with demand.

[Chris97]

after work tonight I finally carry out all the recommended tests and I also take a picture of the spark plug cap

I have already measured the valves and they are correctly calibrated

Thank you

Since this bike has a points type ignition, I doubt this is the problem. My first thought on your problem was that there was an insufficient flow of fuel into the carburetor; at higher loads, the engine needs more fuel, so the engine may perform well in the lower gears, but as the load increases the fuel supply doesn't keep up with demand.

I thought exactly the same a few days ago and I checked every component of the carburetor that it was clean and correctly calibrated, the float also, the tank tap is new, the tank has been cleaned and treated with tankerite ... at this point I don't think it is the lack of fuel

[Chris97]

tonight I had little time but I did some tests, before disassembling the whole bike and carrying out the ignition time adjustment perfectly I tried to move the contact breaker plate to anticipate a little more than what was set and in fact now the third gear can be pulled better than before, fourth gear is always bad but certainly better than before.

unfortunately I was unable to make a video as it was already dark and would not have been very useful

I then adjusted the ignition correctly and precisely as per the manual, calibrating the contact breaker plate with a precise reference to the mark on the stator, when the light bulb that was positioned lights up (I enclose the reference page).

I then checked and yes, even the spark plug cap is resistive (I attach photos) and I read that resistive spark plug + resistive spark plug cap creates problems, so I found in the garage a spark plug cap without resistance (measured the Ohms to be sure ) and I mounted it.

For the moment I leave the resistive spark plug and see how the bike goes.

I hope tomorrow to have more time and try the bike in this configuration, perhaps by making a video on board

[grcamna2]

Chris,that's good work.I have a 1981' Honda CB125S US model and 1981' was the model/year it started to have CDi ignition.I made a mistake on yours thinking it also has CDi ign.
The surface of your ignition points need to be smooth and free of grease and oil to fire correctly.

[Chris97]

ah ok but in all cases the resistive spark plug could give problems coupled with the resistive spark plug cap, right?

however certainly, already cleaned during the restoration.

thanks a lot

[grcamna2]

ah ok but in all cases the resistive spark plug could give problems coupled with the resistive spark plug cap, right?

however certainly, already cleaned during the restoration.

thanks a lot

Yes,you only require one single resistor? according to your manual.

[Chris97]

I don't know, I don't think it's written in the manual, in all cases the bike goes the same, now I'll explain why ...

[Chris97]

Unfortunately, given the rain of the past few days, I tried the bike this afternoon after the advance setup etc. but the bike still doesn't have power, in the end it behaves more or less as before, I also made some videos but they are useless considering what I later discovered.

A friend of mine has recovered tonight (here it is 9:00 pm now) the tool to measure the cylinder pressure and yes, problem found, 75 PSI instead of 175 PSI, in the next days and / or weeks I will proceed with the opening of the engine and checking the cylinder and head, I hope to get by with little expense.

Thanks to all of the valuable advice

[grcamna2]

Unfortunately, given the rain of the past few days, I tried the bike this afternoon after the advance setup etc. but the bike still doesn't have power, in the end it behaves more or less as before, I also made some videos but they are useless considering what I later discovered.

A friend of mine has recovered tonight (here it is 9:00 pm now) the tool to measure the cylinder pressure and yes, problem found, 75 PSI instead of 175 PSI, in the next days and / or weeks I will proceed with the opening of the engine and checking the cylinder and head, I hope to get by with little expense.

Thanks to all of the valuable advice

I wonder if you have a machine shop that will bore the cylinder if needed and also check the cylinder head,grind the valve seats,check the valve guides,etc. ? I recommend having a qualified machinist do that type of work.
I may have new or used parts for you if needed.

[Chris97]

Thank you very much for your availability but luckily I am surrounded by qualified machine shops, luckily I also have another engine in pieces and maybe something can come in handy if there are serious problems even if I believe that in addition to a bored cylinder, a new piston and check the head / valves (as well as a compulsory leveling of the head given the age), I should be fine.

Thanks a lot for the advice anyway, I'll update you as soon as I open the engine

[grcamna2]

Thank you very much for your availability but luckily I am surrounded by qualified machine shops, luckily I also have another engine in pieces and maybe something can come in handy if there are serious problems even if I believe that in addition to a bored cylinder, a new piston and check the head / valves (as well as a compulsory leveling of the head given the age), I should be fine.

Thanks a lot for the advice anyway, I'll update you as soon as I open the engine

Certainly.
I hope your machinist follows the factory clearance specifications as the factory Honda manual says