CB125s 1973 - Not starting

[Jock85]

Hi guys, thanks for checking out this post.

I bought a CB125s a few years ago and over the last few months decided to finally get 'er going. It was bought on a whim costing £137 but it had been sitting out for years on a farm (no docs, no real history) This thing was barely rolling, never mind running, when I got it and have slowly replaced the missing or past-it parts to get to a state where I can actually see what it needs to recommission it to a presentable and usable state. My thinking is that, if I know the engine is sorted, thats the acid test met to justify putting more time into it.

Initial major barriers:
No kick start
No oil pump cover
Missing exhaust collets
No power to any of the electrics so no spark
Soiled tank etc (you get the idea)

With the above addressed, the initial kick gave me 40 psi (well short of the 150+ stated in the honda manual) so I took the top end apart. The cylinder and piston are both within tolerance so the cylinder was honed only (remaining at standard bore "). The rings were fine but were replaced anyway. Full new gasket set was fitted (excluding the casing gasket as it hasn't been split - I didn't think there was a need as it turns freely). I then turned my attention to the valves - these were popped out, inspected and the exhaust valve was replaced. Once seated a local bike mechanic confirmed that they were fitted fine (brake cleaner behind the valves with no leaks for a minute equals A-OK). I gave the top end a good visual/double/triple check before reassembly so I am assuming this is fine (including setting the three rings (markings facing up) at 120 degrees rotated from each other, piston orientated right way, new head gasket tightened down by feel (torque not known).
Next I addressed the no spark, no electrics so a new battery was fitted, new ignition switch and fuse checked. When kicked, the old plug now sparks away against the engine casing so to my mind I have spark.
Ignition timing has been checked about 20 times but basically when I crank the engine by hand (i.e. turning anti-clockwise on left hand side) the test light connected to the points spring lights up when the crank passes the "F" so it seems to be timed right.
Carb has been given a good going over, new petcock fitted and fuel tank thoroughly cleaned so the bowl now fills up fine.
Other things I have done is checked tappet gaps, spark plug gap, points gap and points and cam lobe wet & dryed, exhaust fitted, oil filled.
But, my compression is still in the 80 psi mark. The spark plug is sparking and smells of fuel after some kicks but the bike isn't thinking of running. It has maybe "caught" three or four times out of the 100+ kicks over the last few weeks but I don't know what I'm overlooking.

Is it possible that the old spark plug isn't sparking when in the compression chamber? Or could the old electrics maybe not be generating a big enough spark? I know the compression is low but would it not still start at 80psi? I thought it might be lower without the carbon on the valves giving a perfect seal.

Basically I'm at a loss guys and any help would be appreciated. I will also try to get some pics up in case it interests any one.

https://s1348.photobucket.com/user/jock85/media/20160406_180147_zpstu5k60xe.jpg.html

[Rogerborg]

[wodge]

Yes, change the plug. Spark plugs sometimes work fine outside the engine but when put in the harsh enviroment of an internal combustion engine the go all shy and stop working.

Are you sure the valves are opening and closing at the right times?

Is the ignition system working on the "lost spark" principle where it fires at TDC on every stroke or only on the power stroke? If its the latter is it firing at TDC on the power stroke?

Is the exhaust blocked?
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Yamaha RXS100 RS100

[jaffa90]

Is the spark blue with a cracking noise?

[Fizzer Thou]

Many,many moons ago I had a CB125s as my first bike.I learnt a lot of how to fix that bike over the 30 months that I rode that bike in all weathers and on average 13,000 a year.

The one ignition problem that I had was the original metal shrouded plug cap.This caused the spark to jump to earth when wet,caused starting problems when it was damp and if my leg was too close when it rained,I would get the occasional shock.

From what I also remember,there is a condenser under the points cover.There is no way to test this component and so it may well be an idea to replace this,the points,the DR8ESL plug and the plug cap.If it still has the OEM plug cap,replace it with a new NGK plastic cap.

As for checking the compression,this should ideally be done with a hot engine,the choke off and the throttle held wide open.As you have replaced the piston rings and not had a chance to bed them in yet,then,as has been suggested,some engine oil down the plug hole and another kicking session should see the compression test result jump up.
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[Jock85]

Thanks for all your replies chaps.

The compression was checked with the throttle closed and a few drops of oil didnt seem to make any difference. As Fizzer Thou said, the compression should be checked once the engine has been warmed so I don't actually know how it would compare when warm.

The valves are aligned in time with the piston and have been checked by removing the valve caps to feel the valve arms (?) slight movement at power stroke TDC. I can't say what principle the timing system runs by but as the test light illuminates at "F" and remains on for 1 and 1/3 crank rotations I would be inclined to say it sparks at both TDC's and not just the power stroke. On kick there is definite compression coming out the end of the exhaust pipe but I could maybe try with the exhaust removed to see how it reacts on kick.

Although the spark does seem strong and blue and steady, I didn't know the spark plug could act differently when compressed so that's a new piece of info. I was just wondering though, with it only being a 6v system with old wiring could this affect the actual strength of the spark even though it seems strong?

Going on the suggestions made, I think im going to replace the ignition components suggested and redo a compression test to the above method to see how it compares.

It's a more than slight humbling feeling being bested by what I thought was such a simple bike.

[Jock85]

Just thought I would give you a brief update. On the recommendation to replace some of the ignition parts although they seemed to work already I went ahead and ordered them. The spark plug was the first to arrive today so i popped it in and in the first time in years she started on the fifth kick and is purring away nicely.

Thanks very much all. I really didn't know that parts like the spark plug can appear to work and actually not work when needed to. So now If I can just get the final drive to operate I will run her in and then start to resto

[Jmh600]

That's awesome. I've just done a total top end rebuild on my gsxr600..... Not fun.

Have you got some pics??
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Jmhmotorcycles

[Fisty]

inb4 severe autism.
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Quietly and consistently taking the piss.
TL1000R | Hayabusa | ZXR400 | TL1000S | Bandit 400 V
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[Nobby the Bastard]

[Jock85]