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@Chris :

BEFORE you take it apart, try to use a different compression tester ~ one of my old CT90's was hard to start and the compression test showed about 90 # so I took it apart, bought a new piston & rings, lapped the valves and so on, it still didn't want to start so just for giggles I re-tested the compression, ~ only a FIVE POUND IMPROVEMENT o_O I got another Motocycle compression tester and Lo ! it had plenty of compression (it felt fine by the kick starter pedal before but....) .

So make darn sure before you take things apart .

90% of the time an older engine has tight valves and this causes seriously low compression - NEVER trust the guy who says "oh, I checked that all ready" .
 

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Discussion Starter · #22 ·
Why not one day, :unsure: shouldn't take much longer then that to tear it down, it's just a little motorcycle,
but you are going to need a small torque wrench before you can re assemble it properly if it's not just a valve clearance adjustment. I would adjust the valve clearances first and then compression test again if valves appear out of adjustment, that might save you a lot of work.
The valve clearance is ok, already checked on two different occasions to be sure, however I intended to find a free day to devote to disassembling the cylinder, it would be the first 4-stroke I disassemble but I don't think it's that difficult, I also have the manual so I think there are no problems.

... hey wait, this is a 2 cylinder motorcycle, 75 PSI instead of 175 PSI in one cylinder, what's the other one? or was one 75 and the other one 175 psi
However my CB125x is exactly like the S so it is single cylinder, the 175psi is the standard value written in the manual, the 75 are the ones I found last night.
 

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Discussion Starter · #23 ·
BEFORE you take it apart, try to use a different compression tester ~ one of my old CT90's was hard to start and the compression test showed about 90 # so I took it apart, bought a new piston & rings, lapped the valves and so on, it still didn't want to start so just for giggles I re-tested the compression, ~ only a FIVE POUND IMPROVEMENT o_O I got another Motocycle compression tester and Lo ! it had plenty of compression (it felt fine by the kick starter pedal before but....) .

So make darn sure before you take things apart .

90% of the time an older engine has tight valves and this causes seriously low compression - NEVER trust the guy who says "oh, I checked that all ready" .
in fact with the kick starter pedal the bike seems to have compression, my girlfriend struggles to turn it on and that's why I didn't immediately think about the pressure measurement test ...
in fact yesterday doing the measurements I thought of a loss of pressure, it seemed strange to me it was so little, but leaving the tool under pressure it did not drop ... so, in your opinion, apart from trying with another measuring instrument, what do you say to check?

Thanks for your information, you are giving me hope😂
 

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in fact with the kick starter pedal the bike seems to have compression, my girlfriend struggles to turn it on and that's why I didn't immediately think about the pressure measurement test ...
in fact yesterday doing the measurements I thought of a loss of pressure, it seemed strange to me it was so little, but leaving the tool under pressure it did not drop ... so, in your opinion, apart from trying with another measuring instrument, what do you say to check?

Thanks for your information, you are giving me hope😂
Compression wont happen till the middle of the power stroke, and as it nears its end, it will increase in psi. I have told many people this before, there needs to be a minimum of 170 to 190 psi for good power production, as the compression value drops so does the ability to produce any power.

On a good running engine, you would typically induce more fuel and more air to acheave more power with the same bore and stroke length. hence the use of turbos, and superchargers.

but this is all dependent on the compression within the cylinder in the first place. So i would agree, that the second test with a seond tester would be a good idea. But in the event you get the same result. the info you provided about your girl barely being able to kick it over would suggest two potential things.

one being that your comp test was incorrect, and compression is present. Or, there is a broken ring galging into the cylinder wall. but this last one would be VERRY obvious.

items that can cause a loss in comp- broke/worn rings, bent/broke valves, or a bad head gasket.
 
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...
However my CB125x is exactly like the S so it is single cylinder, the 175psi is the standard value written in the manual, the 75 are the ones I found last night.
So it's a single cylinder engine and the compression is 75psi
... good news is you definitely don't have a bent valve or a big hole in the piston.
 

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My 260cc single cylinder Honda engine has 10.5:1 compression ratio
Service manual says cylinder compression is 460 kPa (4.7 kgf/cm, 67 psi) at – 800 min (rpm)
Make sure you have the right numbers for your engine model.
 

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Discussion Starter · #27 ·
Compression wont happen till the middle of the power stroke, and as it nears its end, it will increase in psi. I have told many people this before, there needs to be a minimum of 170 to 190 psi for good power production, as the compression value drops so does the ability to produce any power.

On a good running engine, you would typically induce more fuel and more air to acheave more power with the same bore and stroke length. hence the use of turbos, and superchargers.

but this is all dependent on the compression within the cylinder in the first place. So i would agree, that the second test with a seond tester would be a good idea. But in the event you get the same result. the info you provided about your girl barely being able to kick it over would suggest two potential things.

one being that your comp test was incorrect, and compression is present. Or, there is a broken ring galging into the cylinder wall. but this last one would be VERRY obvious.

items that can cause a loss in comp- broke/worn rings, bent/broke valves, or a bad head gasket.
today I tried with a new compression tester and the values are always the same.

There is one thing that came to my mind today, when I was adjusting the timing advance and turning the rotor with a wrench, I didn't have much trouble, but turning the rotor so easily I think is another sign for poor compression, the compression test I performed it with a hot engine as per the manual, opening the full trottle and pumping the pressure on the tool.

I have ordered new cylinder and head gaskets and hope to open the engine as soon as possible
So it's a single cylinder engine and the compression is 75psi
... good news is you definitely don't have a bent valve or a big hole in the piston.
hope to open the engine and find no surprises 😂
My 260cc single cylinder Honda engine has 10.5:1 compression ratio
Service manual says cylinder compression is 460 kPa (4.7 kgf/cm, 67 psi) at – 800 min (rpm)
Make sure you have the right numbers for your engine model.
I am sure of the values recommended in the shop manual, and thinking about it is now the only option that makes sense in creating the problem I encountered
 

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Chris ;

Understand that I'm an old Mechanic so I tend to do things maybe differently .

Unless you've adjusted the valves, personally I can pretty much guarantee they're tight as Honda only use .002" clearance and it tends to close up, especially the intake valve .

So, unless you've done that, STOP AND DO IT BEFORE TAKING IT APART .

Also, when doing the compression test , dribble some oil in the cylinder via the spark plug hole, re install the tester and test it again, being sure to operate the kick starter pedal briskly ~ as mentioned the speed at which you rotate the engine makes a lot of difference .

The next fun thing is the old "used bike no tools test" : when you depress the kick starter, does the engine turn over " flup, flup flup with distinct pressure resistances or does the pedal go down : whooshwhooswhoosh ? .

Therein lines an important clue and you're out there diagnosing this thing on your own .

Anything above 74 # or so it should easily start ut won't have much power at any speed .

If this bike has contact points you need to find and pay close attention to the marks on the alternator rotor ~ there will be a "T" mark that's used ONLY to position the engine to TDC for adjusting the valves and on initial assembly .

The ignition must -always- be set using the handy-dandy "
F" (for 'firing point') mark .

Almost 50%
of the time I find the timing set to the "T" (top dead center) mark, this gives you retarded timing that kills the power although it makes the engine run smooth as glass and it also causes the engine to run too hot .

CHRIS asked : "so, in your opinion, apart from trying with another measuring instrument, what do you say to check? ".
 

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Discussion Starter · #30 ·
Thanks everyone for the answers.
Today I measured in a cold engine and gained 5 PSI, I then measured with a teaspoon of 10w40 oil inserted into the cylinder and the pressure increased to 130 psi, so I assume the bands are definitely one of the problems, operate the kick starter pedal briskly as recommended.
In all cases I have checked the valves for the third time and they are correctly calibrated, same for the timing advance (I have various vision problems but luckily I have not confused the T with the F 😂 ).
For next week I should open the cylinder and let's see the conditions, I will probably then take the various pieces to a grinding center to evaluate the various measures with a comparator and then act accordingly.
 

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There you go .

The piston rings are worn .

If you're handy with tools this isn't a hard repair but it's fiddly and usually rather oily/filthy .
 

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BTW Chris :

That factory shop manual is for the 1970's vintage U.S.A. CB125 models, your 'General Export Version' being a 1982 is slightly different but will work fine here .

? Do you have a dynamic timing light or just a bulb that lights up ? .

I ask because the timing advancer often gets sticky / dirty / gummed up and doesn't advance the timing as the engine speed increases .

It's fairly easy to test and if the timing remains fixed at the "F" mark or doesn't move up to the double full advance marks shown on page # 9 (PDF) / 5 (manual) Fig. ! .
 

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Discussion Starter · #33 · (Edited)
There you go .

The piston rings are worn .

If you're handy with tools this isn't a hard repair but it's fiddly and usually rather oily/filthy .
yes, no problem, it's my first time with a 4t but I have a friend who has a lot of experience and he comes to help me
BTW Chris :

That factory shop manual is for the 1970's vintage U.S.A. CB125 models, your 'General Export Version' being a 1982 is slightly different but will work fine here .

? Do you have a dynamic timing light or just a bulb that lights up ? .

I ask because the timing advancer often gets sticky / dirty / gummed up and doesn't advance the timing as the engine speed increases .

It's fairly easy to test and if the timing remains fixed at the "F" mark or doesn't move up to the double full advance marks shown on page # 9 (PDF) / 5 (manual) Fig. ! .
for now I don't have a dynamic timing light but by the time I close the engine I'll try to find it if I have any problems, but can I ask you a question?
How does the timing advancer work to increase the advance as speed increases? I have not found any type of mechanical element to do this, in the plate containing the contact breaker it is a single piece, is it in the rotor? it's probably a trivial question but I can't understand how it works, other motors with contact breaker I've worked on (but always 2T) had a static advance.

Thank you very much
 

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You may ask all the questions you want ~ that's how you learn .

Others here will prolly have good information too .

The spark advancer is underneath the breaker points plate .

It has only a few parts, two weights and return springs , the two pieces ofttimes get stuck because it's in the cylinder head .

Here's a link to how it works and how to clean and lubricate it .

Be aware the two large pieces are easy to assemble 108 degrees (1/2 turn) out, resulting in a no run situation .

This is a robust yet durable engine so don't force things coming apart nor going back together,

I highly suggest printing out the CB125 manual you posted, use both sides of the paper and take the pertinent pages to the work area and use them as "cheat sheets" ~ you can make notations on the, no worries ~ it'll prolly take a few days to get the job done and you will forget some things so don't hesitate to make notes and take pictures as you go along .

Here's a link to dynamic timing lights, I don't live in Ireland (altho I AM Scots/Irish) so I don't know how your local partshaus works : https://www.youtube.com/results?search_query=low+cost+dynamic+timing+light

In the U.S.A. you can get a brandy new one for about $35 U.S., because modern engines use electronic timing control I find I can get good used dynamic timing lights for $5 after i test them, at most Auto Jumbles or estate / tag sales .

I hope you currently have and understand how to use a static timing light ~ it's basically a bulb with two wires that have alligator clips on their ends, in the link I gave you there's a how to make one , use an LED bulb, it is used to time the ignition before you start the engine, they dynamic one is used with a running engine to test and fine tune .

Press on wards ! .
 

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I found a DIY dynamic timing of an old Honda CT90, it's the same thing :
 

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Discussion Starter · #36 ·
You may ask all the questions you want ~ that's how you learn .

Others here will prolly have good information too .

The spark advancer is underneath the breaker points plate .

It has only a few parts, two weights and return springs , the two pieces ofttimes get stuck because it's in the cylinder head .

Here's a link to how it works and how to clean and lubricate it .

Be aware the two large pieces are easy to assemble 108 degrees (1/2 turn) out, resulting in a no run situation .

This is a robust yet durable engine so don't force things coming apart nor going back together,

I highly suggest printing out the CB125 manual you posted, use both sides of the paper and take the pertinent pages to the work area and use them as "cheat sheets" ~ you can make notations on the, no worries ~ it'll prolly take a few days to get the job done and you will forget some things so don't hesitate to make notes and take pictures as you go along .

Here's a link to dynamic timing lights, I don't live in Ireland (altho I AM Scots/Irish) so I don't know how your local partshaus works : https://www.youtube.com/results?search_query=low+cost+dynamic+timing+light

In the U.S.A. you can get a brandy new one for about $35 U.S., because modern engines use electronic timing control I find I can get good used dynamic timing lights for $5 after i test them, at most Auto Jumbles or estate / tag sales .

I hope you currently have and understand how to use a static timing light ~ it's basically a bulb with two wires that have alligator clips on their ends, in the link I gave you there's a how to make one , use an LED bulb, it is used to time the ignition before you start the engine, they dynamic one is used with a running engine to test and fine tune .

Press on wards ! .
Sorry for the delay, thank you very much for everything you explained to me, you can see that you have a lot of passion for engines! today I had the time to check what you explained to me and I found the spark advancer, I have not yet understood how it works in delaying the timing advance, the mechanism seems free and in excellent condition (turning the shaft the gear changes position ), can you give me the link I think you forgot to put in the previous post?
Thanks for the advice, I will do so, as well as many photos of the various stages of processing so as not to make mistakes.

I have already created and used a static timing light with a motorcycle led arrow and I was able to position the contact breaker plate well.

For the dynamic timing light I will get one for when the engine is closed, looking good I found it at 30 Euros, practically the same dollar price that you told me.

In the meantime, for those who know, I would like to ask a question:

The neutral sensor does not work, there is continuity from the side where it connects with the wiring but when I insert the neutral gear the contact to ground does not close, from what I think in neutral gear there is an internal gear to the gear group that should touch the sensor and thus close the contact, but it does not happen, any advice?

Thank you very much
 

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O.K., you have posted a very nice picture of the spark advancer, grasp the smooth center part between two fingers and rotate it, this should / must make the two curved pieces move outwards a little bit then return as soon as you release the moving center part .

very delicate

Either way, every so gently wiggle and pull the plastic contact part out of the engine case, you'll see it has a copper tip that you can polish using any metal polish, just DO NOT ABRADE IT ! no sand paper, steel wool etc. ~ ONLY metal polish may be used .

It touches the end of the rotating shift drum and makes a
very delicate contact so sanding / abrading it will ruin it .

There's a small O-Right there, I suggest you replace it because this thing always leaks oil .

Yes, I have a passion (or foolishness) about old machines and engines etcetera .
 

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Nobody here has said anything about gearing . That is the most likley problem ..
What sprockets do you have ? Front ? Back ?
 

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Discussion Starter · #39 ·
O.K., you have posted a very nice picture of the spark advancer, grasp the smooth center part between two fingers and rotate it, this should / must make the two curved pieces move outwards a little bit then return as soon as you release the moving center part .

very delicate

Either way, every so gently wiggle and pull the plastic contact part out of the engine case, you'll see it has a copper tip that you can polish using any metal polish, just DO NOT ABRADE IT ! no sand paper, steel wool etc. ~ ONLY metal polish may be used .

It touches the end of the rotating shift drum and makes a
very delicate contact so sanding / abrading it will ruin it .

There's a small O-Right there, I suggest you replace it because this thing always leaks oil .

Yes, I have a passion (or foolishness) about old machines and engines etcetera .
Thank you very much for the explanation, the spark advancer seems ok doing the tests by hand, while I am opening the engine I will also change the o-ring you told me about, for now it should be ok so, if it should be needed then I will polish it as per explained by you.

Tomorrow I open the engine if I have no problems at work
Nobody here has said anything about gearing . That is the most likley problem ..
What sprockets do you have ? Front ? Back ?
already checked, everything ok as original
 

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Thank you very much for the explanation, the spark advancer seems ok doing the tests by hand, while I am opening the engine I will also change the o-ring you told me about, for now it should be ok so, if it should be needed then I will polish it as per explained by you.

Tomorrow I open the engine if I have no problems at work

already checked, everything ok as original
Gearing can be changed , you did not mention your weight . Are you a heafty dude ?
 
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